The Complete Advanced Pilot: A Combined Commercial and Instrument Course

The Complete Advanced Pilot: A Combined Commercial & Instrument Course

Sixth Edition

by Bob Gardner

Aviation Supplies & Academics, Inc.

7005 132nd Place SE

Newcastle, Washington 98059-3153

Visit the ASA website often, as any updates due to FAA regulatory and procedural changes will be posted there: www.asa2fly.com

© 1994 – 2019 Aviation Supplies & Academics, Inc.

All rights reserved.

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopy, recording or otherwise, without the prior written permission of the copyright holder. While every precaution has been taken in the preparation of this book, the publisher and Bob Gardner assume no responsibility for damages resulting from the use of the information contained herein.

None of the material in this manual supersedes any operational documents or procedures issued by the Federal Aviation Administration, aircraft and avionics manufacturers, flight schools, or the operators of aircraft. The chart excerpts contained in this manual are reproductions for example only, and are not to be used for navigation.

ASA-CAP-6-EB

ISBN 978-1-61954-854-1

Print Book ISBN 978-1-61954-853-4

Original illustrations: Dick Bringloe and Don Szymanski

Foreword

As an aviation educator and journalist, I receive an abundance of e-mail asking aviation questions, often-complex ones. My answers are always prompt and wonderfully informational because an airline Captain knows everything. I confess: Two things I know readily are what bookshelf Bob Gardner’s books are located on for easy reference, and I know Bob Gardner’s phone number. Together with my own 39 years of experience that encompasses knowing just about everything.

In reading the Foreword written several years ago complimenting Bob’s The Complete Advanced Pilot, I’m tempted to underline many comments, but not alter anything. The latest edition amplifies in some areas, updates to current procedures, corrects a couple minor errors and is another outstanding exhibit of Bob’s mastery of aviation academics.

The addition of color pages makes it possible for Bob to just scratch the surface of all that is new with graphic weather reports and forecasts. Like peeling an onion, each new web page exposes more information for planning.

Therefore, from the previous Foreword, I repeat with utmost sincerity:

In a long and rewarding friendship, Bob Gardner has always delighted me with his multitude of talents. Foremost in my inventory, he is a total gentleman, a giving and caring person who is driven to share all the knowledge he’s accumulated throughout a diverse and impressive career.

His aviation experience is abundant: a flight in­structor, charter pilot, corporate and freight Captain, ground school instructor, splendid speaker and dedicated educator. He has again documented decades of accumulated knowledge in another brilliantly organized instructional adventure: The Complete Advanced Pilot. As with all of his books, Bob’s organized mind carves a route for you to reach complex destinations of understanding through simple road maps of educational travel.

To make the complex simple is the accomplishment of the capable teacher. The need to share and give and uplift are attributes of a person of quality. I find Bob’s book exceptionally readable, splendidly sectioned for ready reference for virtually any topic, and supplemented with the caliber of personal ex­perience that integrates technique into procedure with mastery.

Someone once defined for me the specifics of the good and the bad teacher. The latter always held back, denied to others some ingredient of his personal knowledge, retained and guarded some educational asset that set him above the bunch. Bob Gardner has always met the good definition, wanting you to know everything he’s learned, in ways you can employ it, and to gain the confidence that you have become an enlightened and safe pilot by having absorbed all that this good man can give to you.

I’m grateful for the opportunity to review and comment upon the latest educational achievement by Bob Gardner. As my peer, friend and fellow educator, Bob never fails to earn my applause and endorsement.

Capt. Dave Gwinn, TWA-Retired

June 2008

About the Author

Bob Gardner has always been an admired member of the aviation community. He began his flying career as a hobby in 1960, during his time in the U.S. Coast Guard in Alaska. By 1966 Bob earned his Private land and sea, Commercial, Instrument, Instructor, CFII, and MEL. Over the next 16 years he was an instructor, charter pilot, corporate and freight Captain, and served as Director of ASA Ground Schools.

Bob holds an ATP certificate with single- and multi-engine land ratings; a CFI certificate with instrument and multi-engine land ratings; and a Ground Instructor's Certificate with advanced and instrument ratings. He has been a Gold Seal Instructor and has been flight instructing for many years, with an impressive list of additional accomplishments as a well-known author, journalist, and airshow lecturer.

Books by Bob Gardner

The Complete Private Pilot

The Complete Multi-Engine Pilot

The Complete Advanced Pilot: A Combined Commercial/Instrument Course

Say Again Please: Guide to Radio Communications

Introduction

Just what is an advanced pilot? My definition is a pilot with a commercial certificate and an instrument rating. This certificate and rating will allow you to have a long and enjoyable career in aviation. Sure, a multi-engine rating is valuable, but thousands of pilots have flown thousands of revenue hours without ever flying a twin. And if the Airline Transport Pilot Certificate is the Ph.D. of aviation, the commercial and instrument tickets represent the bachelor’s and master’s degrees.

Most private pilots aim for the instrument rating first, knowing they can get started right away while their brains are used to studying. They also know that the ability to fly in the clouds will speed up the progress of acquiring the 250-hour minimum for the commercial pilot certificate (Part 61). Many pilots have no interest in getting a commercial certificate but want the instrument rating so they can free themselves of VFR restrictions.

All of these pilots face knowledge examinations—one in the case of the noncommercial aviator, two for the pilot who wants to fly for money. Some of the required information overlaps; for example, both examinations test your knowledge of weather, weight and balance, and regulations. I’m going to handle that by having two sets of review questions where appropriate—read all of the text and then check your understanding by doing the review questions for the proper knowledge test. All review questions for this edition have been taken from FAA test databases.

Where information is unique to instrument flight or to commercial operations I will make that clear, and of course there will be whole chapters that apply to only one of your immediate goals. I will lead off with the instrument rating information because that applies to all of you, and finish with what you need to fly for hire.

Some of the information may seem basic. There are two reasons for this: Many prospective commercial pilots earned the private certificate many years ago, so some review is helpful; also, the commercial knowledge exam explores some operational areas in more depth than did the private pilot knowledge exam. However, I am not going to cover all of the private pilot information that you have needed to know in order to fly safely up to this point. If it has been a long time since you reviewed the knowledge requirements of a Private ticket, it might benefit you to review The Complete Private Pilot.

This introduction has implied a heavy emphasis on knowledge exams, but that is not my style as an instructor. What you need to know for the knowledge test represents less than half of the text—the rest is solid information you must have but the FAA doesn’t ask about. To ace the test, use the appropriate ASA test preparation book.

You will also note an emphasis on computers, the internet, and the worldwide web. Most pilots are to some extent technically oriented, and it is estimated that well over half of all pilots use home computers for flight planning, acquiring weather information, maintaining their logbooks, etc. Accordingly, I have included access information wherever it is appropriate. As web surfers know, if you can find one webpage you will find links to dozens of other pages ready to be accessed with the click of a mouse button. You can reach me at [email protected]; I am also active in several internet forums such as AOPA, and Pilots of America.

The world of aviation is constantly changing and new information comes to light between editions of this book. Stay ahead of the game by going to www.asa2fly.com for resources and text updates. When I run across an article that expands on a subject beyond what I have written in this book, I upload it to the Reader Resource page there: www.asa2fly.com/reader/cap.

One final word: I am a flight instructor, and flight instructors love to talk. You should hear my voice in your ear as you read. Also, I know that it aids in understanding if some information is presented in different contexts—so if you see the same material in more than one section it is not due to poor editing but is intended to carry out an instructional purpose.

You and Your Flight Instruments

The FAA will test your knowledge of the flight instruments on both the commercial and instrument knowledge examinations. The material in this chapter applies to both.

When you begin training for the instrument rating you must make a mental commitment to believe the indications of the flight instruments and to ignore physical clues to flight attitude. The days of instrument flight by the seat of your pants never existed. It takes commitment and concentration to sit in a cockpit with nothing to look at but a collection of gauges and to feel comfortable and confident in your ability to control the airplane, to know its position in space, and to guide it safely to your destination. It’s an ego trip. Pilots are a special group, and instrument pilots are the cream of the crop.

It’s difficult to place your faith in an instrument unless you know how it works, where it gets its information, and how to use its indications to control the airplane. We’ll begin with how the flight instruments work and then examine the systems that allow them to function. In the next chapter, we will discuss how to develop the most efficient method of scanning the instruments.

The six basic flight instruments are divided into two groups by source of power or input: pitot-static and gyroscopic. Your knowledge of how each instrument derives its input will help you troubleshoot any erratic indications and isolate the instrument or system which has failed.

It’s a Whole New World

For the immediate future, FAA Knowledge Exams will assume that your trainer has analog instruments, an arrangement known to pilots as a six-pack, and questions will be based on that assumption. Although there are a lot of whiz-bang new airplanes with digital instrumentation, chances are that you will train in an older plane with a six-pack. Your knowledge test will include questions about satellite navigation regulations and requirements but you will not be asked to interpret a display.

An electronic flight display, when compared to the legacy analog instruments, offers new capabilities and simplifies the flying task. The following sections will discuss where and how those instruments get their inputs. (To learn more, go to www.faa.gov and search for the Advanced Avionics Handbook, FAA-H-8083-6.)

Digital instruments get information from an Attitude and Heading Reference System (AHRS), a collection of solid-state or micro-electromechanical systems (MEMS) providing pitch, heading, and yaw signals to an electronic display. The errors inherent in analog instruments, which I discuss in detail, are vanishingly small in digital displays…until the power fails, in which case you must fall back on the required analog instruments: attitude indicator, airspeed indicator, and altimeter, all of which, hopefully, have power sources independent of ship’s power.

The avionics industry has developed a number of battery-powered digital systems, some handheld, to solve this problem. I recommend that you invest in one.

One step up from the AHRS is the air-data reference system (ADHRS), which incorporates positioning information from a Global Positioning System (GPS) navigator plus airspeed and altitude into the basic AHRS.

Pitot-Static Instruments

The pitot-static system consists of a pitot (pressure-sensing) tube, a static (zero pressure) source, and related plumbing and filters. The pitot-static instruments are the airspeed indicator, the altimeter, and the vertical speed indicator; they measure changes in air pressure caused by the airplane’s vertical and horizontal movements in the atmosphere (see Figure 1-1).

Figure 1-1. Pitot-static system

Table 1-1. Pitot-static system failures

Airplanes equipped for instrument flight have pitot tube heaters, virtually identical to the resistance elements in your kitchen toaster, and they soak up a prodigious amount of electricity. Airplanes approved for instrument flight in commuter or on-demand operations must have pitot tube heaters. The pitot heat should be turned on before you fly into visible moisture, so that ice has no opportunity to form on the pitot tube. If water gets into the pitot plumbing it will cause erratic indications or worse. When the tubing connecting the pitot head to the airspeed indicator is blocked by ice, the air trapped between the point of blockage and the diaphragm in the instrument will expand as the airplane climbs, and the airspeed indicator will react as an altimeter, indicating higher airspeeds as altitude increases. A cross-check of the other instruments (especially the altimeter and VSI) will quickly pinpoint the ASI as having failed.

As a professional pilot or as a private pilot with a professional attitude, you should know what airspeed will result from a given pitch attitude and power setting; that is, you should be able to fly the airplane without an airspeed indicator if that becomes necessary in an emergency.

The static port is located where the airplane’s motion through the air will create no pressure at all: on the side (or both sides) of the fuselage or on the back of the pitot tube. The airspeed indicator is calibrated to read the difference in pressure between impact air and still (static) air—both inputs are required.

If either the pitot tube or the static port is blocked the system will be useless, much like trying to get electricity from only one side of an electrical outlet. Blockage of the static system would disable the airspeed indicator, the altimeter, and the vertical speed indicator because no pressure differential would exist.

Depending on the location of the static source or sources, structural icing might cause such a blockage, and many all-weather airplanes are equipped with electrical static port heaters to eliminate this hazard.

Although an alternate static source is not required by 14 CFR 91.205 for noncommercial instrument flight, most IFR airplanes are equipped with one. (Part 135 requires an alternate static source for passenger-carrying flights operating under instrument flight rules.)

The alternate static source is a small valve or petcock at the pilot station which, when opened, vents the static system to the cockpit. When it is in use, the altimeter and airspeed indicator read slightly high; the vertical speed indicator will indicate correctly after momentarily reading in reverse. Opening the cabin vents will affect the readings of pitot-static instruments by slightly pressurizing the cabin when the alternate static source is being used.

If you are flying in icing conditions and your airplane does not have an alternate static source, water freezing in the static plumbing will put the pitot-static instruments out of commission. Your only option is to open the system to cabin pressure by breaking the glass on the vertical speed indicator. That will render the VSI pretty much useless but save the day for the airspeed indicator and altimeter. The VSI isn’t a required instrument anyway.

The Federal Aviation Regulations require that the altimeter and static system of any airplane used for instrument flight be inspected every 24 months, and that the logbook endorsement indicate the maximum altitude to which the system has been tested. For unpressurized airplanes this altitude will far exceed the service ceiling of the airplane.

Airspeed Indicator

The airspeed indicator requires input from both the pitot (pressure) and static (unchanging) sources. Air from the static port fills the airspeed instrument case, while air from the pitot tube is led to a diaphragm. As airspeed changes, the pressure exerted on the diaphragm also changes and the movement of the diaphragm in response to these changes is transmitted to the indicator needle. The designer tries to locate the pitot tube so that it registers pressure in free air and is not affected by local airflow around the supporting structure. The airspeed indicator is the only instrument that uses air pressure from the pitot tube.

At the start of the takeoff roll there is no difference in pressure between the pitot and static inputs, and the airspeed indicator will read zero. As the airplane accelerates, the pressure in the pitot tube increases and that pressure is transmitted to the airspeed indicator needle. The designer cannot completely isolate the pitot and static inputs from the effects of airflow around the wing or fuselage, so an airspeed correction table is provided. The needle on the airspeed indicator reads indicated airspeed (IAS); when corrected for installation or position error, it becomes calibrated airspeed (CAS). Note in Figure 1-2 that the greatest difference between indicated and calibrated airspeed occurs at low speeds which require high angles of attack, and that as the angle of attack is reduced and speed increases the difference between IAS and CAS becomes negligible. The colored arcs on the airspeed indicator are usually based on calibrated airspeed; other operating speeds may be based on indicated airspeed. Check the operator’s handbook to be sure.

Figure 1-2. Airspeed calibration

It takes a pressure of about 34 pounds per square foot on the pitot side of the airspeed indicator’s diaphragm to make the airspeed needle register 100 knots at sea level—that’s how the instrument shop calibrates your ASI. As the airplane climbs to altitude, the air becomes less dense. The airplane will have to move much faster through the less dense air at altitude to develop a pressure of 34 psf in the pitot tube, so the true airspeed will be faster than 100 knots when the airspeed indicator shows 100 knots. Your flight computer will allow you to make accurate calculations of true airspeed using IAS, pressure altitude, and temperature, but as a rule of thumb true airspeed increases by 2 percent per 1,000 feet of altitude. At sea level, under standard conditions, indicated and true airspeed will be equal; at 10,000 feet msl, at the standard temperature for that altitude, an indicated 100 knots means a true airspeed of approximately 120 knots. When the ambient temperature rises above standard while indicated altitude is constant, pressure levels rise and both true airspeed and true altitude will increase.

Note: Glass cockpit airspeed indicators (and altimeters) take their inputs from air data computers, and their displays are vertical tapes and digital readouts, not needles. Some glass airspeed indicators provide a trend vector, showing what the airspeed will be in six seconds if acceleration/deceleration does not change.

At airspeeds in excess of 240 knots, heating caused by the compression of the air in the pitot tube must be taken into consideration in calculating true airspeed. Equivalent airspeed is calibrated airspeed corrected for the compressibility of the air, and should be of no concern at speeds less than 240 knots. Equivalent airspeed and calibrated airspeed are equal at sea level on a standard day—it is at high altitude and high airspeed that they differ.

You will use true airspeed in flight planning, but most airspeeds that you will use in actual flight are indicated airspeeds. You will always use the same indicated airspeeds, regardless of altitude. For example, if you are taking your flight training at a sea level airport and find that 110 knots indicated is the correct final approach speed, you will use 110 knots indicated airspeed on final when you fly to an airport at 5,000 feet above sea level as well. Your true airspeed will be 121 knots (2 percent times 5 = 10 percent, 1.1 times 110 = 121). Because the airplane approaching the airport at 5,000 feet is moving faster through the air to have an indicated airspeed of 110 knots, its ground speed will be higher and landing roll will be longer. A pilot who adds a few knots just in case while on final approach at a high altitude airport may have difficulty getting stopped on the available runway, especially if it is wet. Flying at the manufacturer’s recommended airspeed will have predictable results.

A useful memory aid for the various airspeed corrections is Ice Tea upside down. That is,

True (equivalent corrected for nonstandard temperature)

Equivalent (calibrated corrected for compressibility)

Calibrated (indicated corrected for installation or position error)

Indicated

Pilots of light aircraft can safely ignore equivalent airspeed.

Most modern IFR airplanes are equipped with an airspeed indicator capable of being set to indicate true airspeed when the outside air temperature and pressure altitude are set in a window at the top of the instrument. True airspeed is read within the white arc at the bottom of the instrument. The inside calibrations on this type of instrument will still be indicated airspeeds, one in miles per hour and one in knots. See Figure 1-3. Some glass cockpit airspeed indicators display true airspeed in addition to indicated airspeed. These high-end units take input from an air data computer that measures air density and temperature.

Figure 1-3. Airspeed indicator with TAS window

Every airplane has a design maneuvering speed (VA), which is the optimum speed in turbulence at maximum gross weight. Maneuvering speed is reduced as weight is reduced; get rid of ten percent of your payload weight and maneuvering speed will be reduced by five percent. Flight at or below maneuvering speed ensures that the airplane will stall before damaging aerodynamic loads are imposed on the wing structure.

The manufacturer may designate other speeds, which you will find in the Pilot’s Operating Handbook and possibly placarded on the instrument panel. Landing gear extension and retraction speeds, maneuvering speed, and speeds for partial flap extension will be found in the operating handbook and not on the airspeed indicator.

You will be asked to interpret a velocity/G-load diagram, used to determine V-speeds, on the commercial pilot FAA knowledge examination. That subject will be covered in Chapter 2.

Angle of Attack Indicators (AOA)

The airspeed indicator can be considered a form of angle of attack indicator, since indicated airspeed is dependent on both angle of attack and power setting. Several manufacturers provide actual angle of attack indicators, however, which are calibrated to measure the actual angle between the chord line and the relative wind and provide you with angle of attack information by some form of safe-unsafe or fast-slow instrument reading. One such instrument compares air pressure changes both vertically and horizontally and measures sink rate. See Figure 1-4.

Figure 1-4. Angle of attack indicator

Angle of attack indicator installation requires only a mechanic’s logbook entry. Legally, it does not replace the airspeed indicator; operationally, if you have an AOA indicator you will never look at your airspeed indicator again.

In every case, you need only keep the instrument’s needle in the safe area and no interpretation is required.

Go to the Air Safety Institute’s website (or YouTube and search for margins of safety) and watch videos on angle-of-attack indicators.

Altitude and Altimeters

You may recall Figure 1-5 from your private pilot training. Absolute altitude is your airplane’s height above the ground as it might be measured by a radar altimeter. True altitude is your airplane’s height above sea level; that’s what it reads when you set the Kollsman window to the local altimeter setting. If you set it on the ground, indicated altitude should be within 75 feet of the published airport elevation; if it isn’t, the altimeter needs work. Before you enrich the instrument shop, however, make sure that your airplane is not parked at a spot higher or lower than the airport reference point. Pressure altitude is measured above the standard datum plane of 29.92" Hg and is used at all times above 18,000 feet (Flight Level 180). You also use pressure altitude extensively in making performance calculations.

Figure 1-5. Altitude definitions

Aircraft altimeters are aneroid (dry) barometers calibrated to read in feet above sea level (true altitude). The altimeter gets its input from the static port, which is unaffected by the airplane’s movement through the air. An aneroid barometer contains several sealed wafers with a partial internal vacuum, so as the airplane moves vertically and the outside pressure changes, the wafers expand and contract much like an accordion. This expansion and contraction is transmitted through a linkage to the altimeter needles.

Barometers provide a means of weighing the earth’s atmosphere at a specific location. At a flight service station or National Weather Service office, an actual mercury barometer may be used, and on a standard day the weight of the atmosphere will support a column of mercury (Hg) 29.92 inches high at sea level. Inches of mercury are the units of measure for barometric pressure and altimeter settings. The equivalent metric measure is 1013.2 millibars.

Up to 18,000 feet, altitude is measured above sea level, and sea level pressure will normally vary between 28.50 to 30.50 Hg. The Aeronautical Information Manual contains specific procedures to be followed if cold weather causes an altimeter setting of 31.00 or more. If the barometric pressure is less than 28.50, both you and your airplane should be protected from hurricane-force winds.

Your altimeter has an adjustment knob and an altimeter setting window, and you must enter the sea level barometric pressure (altimeter setting) at your location as received from a nearby flight service station or air traffic control facility (each time you are handed off from one ATC controller to another, you should receive an altimeter setting). You can only use field elevation when nothing else is available, and even then you must get an altimeter setting as soon as possible. The altimeter will, when properly set, read altitude above mean sea level (msl). See Figure 1-6.

Figure 1-6. 3-Needle altimeter

As you increase the numbers in the altimeter setting window, the hands on the altimeter also show an increase: each .01 increase in the window is equal to 10 feet of altitude, each .1 is 100 feet, etc.

Misreading of altimeters has caused several accidents. The indication of the 3-needle altimeter found in many aircraft can be misinterpreted by 10,000 feet if the needle on the outside rim of the instrument is ignored or misread. In Figure 1-7, which instrument depicts 10,000 feet? Instrument R is correct. The 10,000-foot needle reads one, and the 1,000-foot and 100-foot needles read zero. What are the readings of instruments Q, S, and T? Check below for the answers.*

*Q = 1,000 feet; S = 11,000 feet; T = 10,100 feet.

Figure 1-7. Reading 3-needle altimeter

The drum-pointer altimeter is encountered quite often in light aircraft and is the altimeter of choice as you move up to more expensive flying machines. It has a single needle and a drum counter similar to an automobile’s odometer. As the needle rotates, the drum reads the altitude directly in easily understood numbers. Each rotation of the needle causes the counter to increase 1,000 feet. See Figure 1-8.

Figure 1-8. Drum-pointer altimeter

With a glass cockpit, altitude is displayed using a vertical tape with a digital readout. Some high-end units display height above ground level (for example, 2320B on the Chelton display means 2,320 feet above the ground, measured by barometry).

Pressure Altitude

You will need to determine pressure altitude to convert indicated airspeed to true airspeed or to calculate density altitude using your flight computer.

There are two ways of accomplishing this: first, note your indicated altitude and altimeter setting, then turn the altimeter setting knob to 29.92; the altimeter needles will read pressure altitude. Write down the pressure altitude and return the altimeter setting knob to its original position. The second method requires some mental gymnastics: determine the difference between your present altimeter setting and 29.92 and add a zero. This will give you the difference in feet between your indicated altitude and the pressure altitude. Then add or subtract this value to (or from) the indicated altitude to get pressure altitude, remembering that the altimeter needles always move in the same direction as the numbers in the setting window.

For example, assume that you are cruising at an indicated altitude of 7,000 feet with the altimeter set to 30.15, and you need to know pressure altitude for a flight computer calculation.

The difference between 29.92 and 30.15 is .23, or 230 feet. If you turned the altimeter setting knob to lower its setting to 29.92, the needles would move counterclockwise 230 feet, so the pressure altitude is 6,770 feet. The advantage of this method is that there is no danger of resetting the altimeter incorrectly.

Above 18,000 feet the altimeter must be set to 29.92 Hg; you will be reading your altitude above the standard datum plane. By international agreement, a standard day at sea level is defined as having a barometric pressure of 29.92 (with the temperature 15°C), and by setting your altimeter to 29.92 it will read altitude above that standard level. Below 18,000 feet, having the correct altimeter setting will keep you out of the trees, while above 18,000 feet (where there are no trees or mountains in this part of the world), the common altimeter setting of 29.92 provides altitude separation for IFR flights. Pressure altitudes of 18,000 feet and above are referred to as Flight Levels: I’d like to file for flight level 220." See Figure 1-9.

Figure 1-9. Change to pressure levels above 18,000 feet

msl

14 CFR 91.121 establishes minimum allowable IFR altitudes when surface barometric pressure is 29.91 or lower. The intent is to avoid conflict between IFR airplanes operating in Class A airspace and VFR or IFR airplanes operating at an indicated altitude of 17,500 feet (or below, depending on how low the surface pressure is). Assume that you set your altimeter to the reported surface setting of 28.92 before takeoff and maintain that setting as you climb. When you climb through an indicated 18,000 feet, you reset the altimeter to 29.92, changing its reading to 19,000 feet. If you then descend to an indicated 18,000 feet with that altimeter setting, you will be below airplanes flying at 17,500 feet with the surface altimeter setting. The FAA guards against this by not clearing any airplane to a flight level below 190 when the surface altimeter setting is 28.92 or below, and you guard against being assigned an altitude you didn’t file for by checking 14 CFR 91.121 when the surface pressure is low. See Figure 1-10.

Figure 1-10. 14 CFR 91.121 excerpt

Effects of Temperature and Pressure on Altimeter Indications

When you fly into an area of lower barometric pressure (while maintaining a constant altimeter setting) your true altitude will be lower than your indicated altitude, and that can be dangerous.

Avoid this by frequently checking with ground stations to use an altimeter setting received from a station within 100 miles. If you fly into an area of colder temperatures, where air density is increased, indicated altitude will again be higher than true altitude. From High to Low, Look Out Below applies to both pressure and temperature. You have no means of adjusting for temperature changes, so remember that pressure levels rise on warm days and descend on cold days, and if you are flying a constant pressure level (altimeter setting), you may be dangerously low on a cold day.

Bernoulli’s Theorem, which states that air pressure decreases when its velocity increases, comes into play when strong winds blow over mountain ranges. Barometric pressure can change rapidly and drastically under these conditions, so don’t trust your altimeter when in turbulence near a ridge line.

Encoding Altimeter

The Federal Aviation Regulations require that an airplane be equipped with an altitude reporting transponder (Mode C) when operating within 30 nautical miles of Class B airspace from the surface up to 10,000 feet msl, when in Class E airspace above 10,000 feet msl, and when in Class C airspace or above its horizontal boundaries up to 10,000 feet msl. Selecting ALT on your transponder will not do a bit of good unless your airplane has an altitude encoding altimeter or a blind encoder, however.

An encoding altimeter sends information to your transponder which enables it to transmit altitude information to the ground when interrogated by ATC radar. It reports altitude above the standard datum plane (29.92" Hg.) and is unaffected by any change in the altimeter setting. Encoding altimeters are usually identified as such on the face of the instrument.

A blind encoder is located remotely and has its own internal altimeter; the airplane you are flying may have a blind encoder and you won’t be aware of it unless you check the airplane’s logs—and you should. The blind encoder is also set to report altitude in relation to the standard datum plane, and equipment on the ground makes the necessary correction based on the local sea level pressure. There is nothing you can do in the cockpit that will affect the altitude readout seen by the controller.

Because these encoding devices are occasionally inaccurate or defective, the ATC controller may ask you to verify your altitude as indicated by the altimeter and, if the error is excessive, may ask you to turn off the Mode C. Do not change altitude unless directed to do so by the controller. It is not uncommon for one controller to tell a pilot that his or her encoder is reading incorrectly and should be checked, while controllers in adjacent sectors report no ­problem.

Vertical Speed Indicator

The vertical speed indicator (Figure 1-11) is a static-pressure instrument which reflects rate of climb or descent by detecting rate of change in air pressure. Its internal construction is similar to that of the altimeter, but the aneroid wafer in the vertical speed indicator has a calibrated leak which allows its internal pressure to stabilize when altitude is not changing. During changes, there is a pressure differential between the air in the wafer and the air surrounding it, and the instrument indicates the change in this differential as climb or descent rate in feet per minute. The needle will lag actual changes in altitude until the rate of change stabilizes. However, if your instrument is marked IVSI, it is an Instantaneous Vertical Speed Indicator and the lag has been eliminated. The VSI is not required for either VFR or IFR flight, but you won’t find many airplanes without one. If the static port is clogged or frozen the VSI will be unusable—select the alternate static source if one is available. If the needle does not read zero when the airplane is on the ground, use whatever it indicates as zero until you can have the instrument calibrated by an authorized instrument repairman. For example, if it indicates a 100-foot-per-minute descent when parked, use the negative 100 calibration as zero.

Figure 1-11. Vertical speed indicator

Glass cockpit displays use vertical tapes or digital readouts.

Gyroscopic Instruments

The attitude indicator, the turn indicator, and the directional gyro or heading indicator operate on the principle of gyroscopic rigidity in space (unless yours is a glass cockpit with solid-state gyros). A spinning body, such as a bicycle wheel, will maintain its position in space as long as a rotational force is applied—riding your bike no hands is an example of this.

Precession, or turning, occurs when any external force is applied to the spinning body, which will react as though the force has been applied at a point 90° away from the point of application in the direction of rotation. Solid-state gyros do not have this error.

When you lean your bicycle to the right, the top of the rotating wheel moves to the right; the resultant turning force is as though pressure has been applied to the left front of the wheel—90° in the direction of rotation—and the bike turns to the right. Actually turning the wheel to the right instead of leaning may cause the bicycle to topple over to the left, again the result of gyroscopic precession.

Solid-state devices, which use the Attitude Heading Reference System (AHRS), are subject to the same errors as mechanical devices, but the forces involved are so small, and errors are so easily detected and corrected, that for practical purposes pilots can ignore precession—except when taking FAA knowledge exams.

Turn and Slip Indicator

Your airplane may have either a turn needle or a turn coordinator, but in either case a ball instrument will be included. Both the turn needle and the turn coordinator indicate the rate of turn of the aircraft: when a turn needle is deflected a single needle-width, or when the turn coordinator’s airplane wing is on the index, the airplane is turning at the rate of 3° per second, and a complete circle will take 2 minutes. Note: If a turn needle instrument is marked 4 MIN TURN, a single needle­width turn is one-half standard rate and it will take 4 minutes to complete a 360° circle; these are found in high speed airplanes. See Figure 1-12.

Figure 1-12. Turn and slip indicator; half-standard rate turn

The turn indicator (needle) only shows rotation around the yaw axis, while the turn coordinator, with its gyro tilted 45 degrees from the vertical, reacts to both roll and yaw forces. That is, it shows both turn rate and roll rate.

Either instrument will deflect during turns while taxiing, because the airplane is rotating around the yaw axis. You can force the turn coordinator to give misleading indications in flight: push one rudder pedal while holding the wings level with aileron and the turn coordinator will obediently indicate a turn; bank the wings while maintaining a constant heading with rudder and it will indicate a turn while you are rolling into the bank. When the bank is established and you have stopped rolling, the TC will return to a wings-level indication. You are neither rolling nor yawing at that point. Note that the turn coordinator doesn’t lie in normal, coordinated flight—you have to work at it. In either of the situations described the ball would be deflected well out of the center. See Figure 1-13.

Figure 1-13. Turn dynamics

Neither instrument indicates bank angle directly, although bank angle can be inferred if you understand the relationship between airspeed and rate of turn. The short, purple line at the top of the heading indicator is the glass cockpit equivalent on a Garmin display.

Consider a light trainer and a jet, both banked 20°: the trainer would complete a 360° turn in a much shorter time than the jet and with a much smaller radius. Conversely, if both airplanes maintained a 3° per second turn rate they would both complete the circle at the same time—but the jet would be at an extreme bank angle. The bank angle for a 3° per second turn is approximately 10 percent of the true airspeed plus 5, so the trainer at a TAS of 80 knots would bank only 13°, while the jet flying at a TAS of 400 knots would have to bank 45°.

The ball indicates the quality of the turn, with respect to rudder-aileron coordination. The force that causes an airplane to turn is the horizontal component of lift. If the rate of turn is too great for the angle of bank, the ball rolls toward the outside of the turn. This is termed a skidding turn, and either a steeper bank angle (increasing horizontal component) or less rudder pressure on the inside of the turn will return the ball to the center.

Finding the ball instrument on a glass indicator can be difficult. When you find it, it will not be a ball but a little line underneath the heading indicator’s arrow.

The reverse situation has the ball falling to the inside of the turn in a slip, caused by too much horizontal lift component. Less bank angle or more inside rudder will return the ball to the center. A rule of thumb is to step on the ball—apply pressure to the rudder pedal on the side of the instrument that the ball is deflected toward. Turn needles or turn coordinators are almost always electrically driven, but you may find a turn needle driven by vacuum from an engine-driven vacuum pump or a venturi. Be sure that you know what makes your turn indicator operate.

If you are flying an airplane equipped with an autopilot, it is especially important that you know whether its roll input comes from the attitude indicator or the turn coordinator. If the vacuum pump fails, will you lose the autopilot as well? Will the electric turn coordinator work with the autopilot to keep the airplane rightside up while you sort the situation out?

The turn coordinator or turn needle should indicate turn direction correctly during taxi turns, and checking for proper operation should be a part of your IFR preflight check. The ball should move to the outside of any turns while taxiing.

Note: Advisory Circular 91.75 contains information on replacing the turn instrument with a second, battery-powered, attitude indicator.

Attitude Indicator

The attitude indicator is the only instrument on the panel that reacts instantaneously to pitch and roll inputs. The gyroscope and its linkage cause the horizon disc to move in both pitch and roll behind the miniature airplane. If you compare the movement of the horizon line in the attitude indicator with the movement of the natural horizon, you will see that the instrument instantly and accurately reflects changes in the pitch and roll attitude of the airplane. It is this instantaneous representation that makes the attitude indicator the most valuable instrument on the panel when reference to the natural horizon is lost. The bank angle markings at the top of the instrument are 10°, 20°, 30°, 45° (some instruments), 60° and 90°. Your instrument may or may not have pitch markings—those that do have them in increments of 5° above the horizon. If your instrument does not have pitch markings you can estimate pitch attitude by bar widths; the bar which represents the wings of your airplane is 2° thick. See Figure 1-14.

Figure 1-14. Attitude indicator

In most light airplanes, the gyroscopes in the attitude indicator and heading indicator are vacuum operated. An engine-driven vacuum pump draws air into the instrument case, and as the air passes over turbine wheels it imparts a rotational force. When the gyroscope rotors are up to speed, they become fixed in the plane of rotation and the airplane moves around them. The instrument presentations are so designed that the pilot has an accurate indication of airplane attitude and heading. Not all gyro installations are entirely vacuum operated—you may find that your heading indicator is electric and the attitude indicator is vacuum, or both may be electrically operated. Check the power source for each instrument in your airplane so that you can better deal with failures.

The attitude indicator has two types of error that are of interest to instrument pilots. The first is acceleration error: when you apply takeoff power the horizon line tilts down, giving you the misleading impression that your climb attitude is too steep. If you react to this with forward stick pressure you may find yourself coming out of the overcast nose down.

Cross-checking with the altimeter, airspeed indicator, and VSI will keep you from being fooled. When your vertical speed has stabilized the error disappears.

The second error develops in vacuum-operated attitude indicators during steep turns. The flow of air passing over the rotor is controlled by pendulous vanes which normally act to keep the gyro erect. They are affected by centrifugal force in steep turns, however, and introduce an error which reaches its maximum value after 180° of turn. If you turn exactly 180° and roll the wings level solely by reference to the attitude indicator’s horizon bar, you will be in error by 3°–5°, and there will be a slight pitch up indication—this error corrects itself quickly, but a check of the turn coordinator to confirm wings level will help.

The error cancels itself out if the turn is a full 360°. If the turn is a skidding turn, the attitude indicator will show a slight bank in the opposite direction when the wings are leveled by visual reference or by checking the turn coordinator.

The attitude indicator should be fully erect within 5 minutes after engine start. Any tendency of the horizon line to tilt during taxi turns indicates that the instrument is not up to speed and may not be reliable.

Heading Indicator

You shouldn’t always trust your heading indicator (directional gyro—see Figure 1-15) to tell you which way you are heading. The heading indicator is not a very smart instrument: it only repeats the heading that has been set into it. For that reason, it has an adjustment knob, and must be set to correspond to the magnetic compass (or to the runway heading) before it can be used for navigation. If you fail to set the heading indicator properly before takeoff and do not notice that it disagrees with the magnetic compass, you can be many miles off course in a surprisingly short time. Jeppesen approach plates give actual magnetic runway headings, while government plates use only the runway number.

Figure 1-15. Heading indicator.

Because the gyro’s rotor spins at a velocity of about 18,000 rpm, its bearings must have a minimum of friction. As the bearings wear, or as dirt and contaminants (such as tobacco tar) collect at these critical points, the gyro will begin to slowly precess (drift) away from the heading you have set. You should check the heading indicator against the magnetic compass at least every 15 minutes, and more often if the instrument is showing signs of age such as grinding noises or rapid precession. You can learn a lot about the condition of your gyro instruments if you sit in the cockpit and listen to them spin down after shutting down the engine.

Reset the heading indicator to the magnetic compass only when the airplane is in straight and level, unaccelerated flight. The magnetic compass develops errors during banks, climbs, and descents, and you do not want to set these errors into your heading indicator.

Pilots have lined up for takeoff on the wrong runway in extremely poor visibility conditions, so you should check both the magnetic compass and heading indicator to ensure that you are in position on the correct runway for takeoff.

Magnetic Compass

The only instrument in your airplane that does not depend on some source of external power is the magnetic compass. All navigational procedures are based on magnetic information.

Unfortunately, the magnetic compass is subject to more errors than any other instrument. The wet magnetic compass consists of a card floating in a liquid, pivoted on a needle point, and having affixed to it small permanent magnets that align themselves (and therefore the card) with the earth’s magnetic field. The magnetic compass is subject to several errors: oscillation error, acceleration error, and northerly turning error. Because of the single-point suspension, the compass card swings in even the slightest turbulence, and you must average its swings to approximate a constant heading. If your compass does not swing when the air gets rough, check to be sure that the fluid hasn’t leaked out!

Northerly turning error is caused by the fact that the lines of force of the earth’s magnetic field are parallel to the earth’s surface at the equator, but bend downward toward the surface as latitude increases and are almost vertical at the magnetic poles.

This force, which pulls the ends of the compass magnets downward, is called magnetic dip: when the airplane banks and the compass card tilts, the compass magnets, affected by dip, introduce a compass error. When you turn from a generally northerly heading, the compass will momentarily turn in the opposite direction, slow to a stop, and then follow the progress of the turn—but lagging the actual heading change. The amount of lag diminishes as the heading approaches east or west. Conversely, when you turn from a southerly heading the compass jumps out ahead of the turn, and leads the heading change, with the amount of lead again diminishing as east or west is approached. The amount of lead or lag which is attributable to northerly turning error is approximately equal to the airplane’s latitude.

Acceleration error is evident on headings of east or west. If the airplane is accelerated, without changing heading, the compass will indicate a turn to the north, while deceleration on an east or west heading will cause the compass to indicate a turn to the south. Don’t trust your wet compass during departure climbs or descents into the terminal area. Use this memory aid: A N D S—Accelerate North, Decelerate South. See Figure 1-16.

Figure 1-16. Magnetic compass errors

It is because of northerly turning error and acceleration error that you must set the heading indicator to agree with the magnetic compass only in straight and level, unaccelerated flight.

If you must rely solely on your magnetic compass for navigation, the most effective way to change heading is the timed turn. Note the magnetic compass heading, determine how many degrees you want to turn, and turn at 3° per second (using the index on the turn coordinator or turn needle) for the appropriate number of seconds. If you complete the turn within 5° or 10° of your target heading, you are within acceptable limits.

You may read about compass turns in other texts. A compass turn is one in which the pilot attempts to outsmart the lead and lag errors and factors latitude into the calculation. The FAA does not require instrument pilots to demonstrate the ability to perform compass turns because the concept defies reality. Visualize yourself flying in the clouds, in darkness, turbulence and rain, relying solely on the magnetic compass because your vacuum-operated heading indicator and attitude indicator have failed. Do you think that you would have the presence of mind to deal with things like latitude and applying one-half the bank angle to the lead or lag to figure out when to stop a turn? Unlikely. Use the turn instrument and sweep second hand for timed turns instead.

External forces that affect magnetic compass accuracy are variation and deviation. Variation, as you recall from your private pilot training, changes with geographic location and is independent of heading. Deviation is caused by the interaction of magnetic fields within the airplane itself and the earth’s magnetic lines of force. Because deviation is affected by the airplane’s heading, a compass correction card must be used. It is especially important that you refer to the compass correction card when using the automatic direction finder (ADF) for navigation.

Vertical Card Magnetic Compasses

You may find an airplane with a vertical card magnetic compass. These relatively new devices contain no liquid, and have little or no oscillation error. Their stability approaches that of the directional gyro. Acceleration error and northerly turning error are present, but are quickly damped by internal electrical currents. Variation and deviation must still be accounted for. Vertical card compasses are considerably more expensive (but easier to use) than liquid-filled compasses. If you own your own airplane and plan to fly instruments seriously, have a vertical card compass installed. See Figure 1-17.

Figure 1-17. Vertical card compass

Slaved Gyro Systems

If your airplane has a horizontal situation indicator (HSI) as a heading indicator, it may derive its directional information from a remote magnetic compass called a flux detector mounted in the tail or wing tip, away from electrical influences. Like the liquid-filled compass in the cockpit, it derives information from the earth’s magnetic field (flux); unlike the wet compass, it does not have oscillation error and is relatively unaffected by magnetic dip. The detector does not rotate but develops a variable error signal as the airplane’s relation to the lines of magnetic flux changes. The weak electrical signals from the magnetic flux detector must be amplified before being transmitted to the heading indicator, which then rotates or slaves into agreement with the directional information from the remote source.

Your system may provide two amplifiers, with a selector switch to allow you to change amplifiers if you suspect that one has failed.

The system includes compensating devices to minimize or eliminate any deviation error. The exact location of the amplifier(s) and compensating devices varies between manufacturers, and the location of the magnetic flux detector is determined by the installer. Figure 1-18 illustrates a possible installation with optional flux detector locations.

Figure 1-18. Slaved compass system

These installations include a slaving meter and buttons used to bring the directional gyro card into agreement with the magnetic compass in the cabin, just as you set your heading indicator to agree with the wet compass. The SLAVE-IN button is a toggle—push it in to activate slaving, push it again and it pops out to the FREE GYRO position. The slaving meter indicates the amount and direction of any difference between the heading displayed on the heading indicator and the actual magnetic heading, and it will deflect to one side or the other while the airplane is in a turn, just as the wet compass does. Accordingly, you should never attempt to adjust the slaved gyro unless the airplane is in straight and level flight or on the ground.

When you are in an airplane with a vacuum-operated heading indicator, you note any difference between the reading of the heading indicator and the indication of the wet compass, then use the heading indicator’s adjustment knob to rotate its dial into agreement with the wet compass. The slaved gyro system is electric, and operates only when the airplane’s electrical system is activated with the master switch. If the airplane has been moved since the master switch was turned off, there will probably be a difference between the slaved gyro heading indicator’s reading and that of the wet compass, and the slaving meter’s needle (slightly left of zero in Figure 1-19) will be deflected in the direction of the error. Numbers are really not important: Toggle the SLAVE-IN button to the popped-out FREE GYRO position, and if the slaving needle is deflected to the left, push the clockwise button on the left to zero it; if the needle is deflected to the right, use the counterclockwise button. The heading indicator dial will rotate accordingly. Then push the SLAVE-IN button.

Figure 1-19. Slaving control

FAA knowledge test writers thrive on numbers, so their questions about slaved gyro systems will not simply let you look at a deflected slaving meter needle; for their purposes, you need to know what makes the needle deflect as it does.

When you turn on the master switch in your sophisticated airplane, you note that the magnetic compass reads 110° but the heading indicator reads 125° as in Figure 1-15—ignore the slaving meter for the moment. Which way must the heading indicator card be rotated to bring 110° under the index? Clockwise, correct? You would put the slaving system in FREE GYRO mode and push the clockwise button on the left in Figure 1-19 to rotate the heading indicator card into agreement with the magnetic compass, then activate the slaving system by toggling the SLAVE-IN button to its IN position. The slaving meter needle? Oh, it was deflected to the left, because the desired heading was to the left of 125°; that’s left compass error.

Systems

Your flight and navigation instruments will rely on the airplane’s vacuum and electrical systems for power, and you need an understanding of how these systems operate to assess and correct failures.

Vacuum Systems

In some airplanes, the use of the term vacuum system is inaccurate, because their pumps provide a positive pressure instead of a vacuum. The gyroscope in the flight instrument will operate in exactly the same way in either case, so this discussion will deal with vacuum pumps as though they are all the same.

Vacuum pumps can be either wet or dry. A wet pump is lubricated with engine oil, and is known for leaving black streaks on the bottom of the fuselage. This is the price the pilot pays for the relatively long service life of a wet vacuum pump. A dry vacuum pump consists of carbon vanes rotating in an aluminum housing, and therein lies the seeds of its destruction.

If the vanes were made of metal and a failure occurred which stopped rotation, damage to the gearing in the accessory case which drives not only the vacuum pump but the magnetos might result. The pump manufacturer avoids this by using relatively soft carbon material for vanes and also by purposely weakening the pump’s drive shaft so that it will shear, just like an outboard motor’s shear pin.

If the shaft shears, your vacuum operated gyro instruments will gradually spin down and stop, giving you misleading information as their gyros slow. The suction gauge on the panel will give no warning of impending failure.

If the carbon vanes wear away at their outer edges, you may notice a gradual drop in the indication of the panel vacuum gauge. Small carbon