Mastering The Flight Instruments

One of the first things students observe as they begin flight training is their aircraft's "complicated" instrument panel. True enough, even in the most humble two-seat trainer, there is an array of exotic, intricate, perplexing, and intimidating gauges, meters, and dials staring the new aviator right in the face. Upon his first look at the instrument panel one of my students brilliantly observed, "Boy, you won't have any trouble telling what time it is in this thing!" More advanced airplanes such as the Socata Trinidad or Beechcraft Bonanza feature an aggrandized complement, in numbers and elaborate appearance, of instruments spread out across the panel.

Student pilots can certainly be anxious at the thought of having to learn about the display of heretofore-unknown flight instruments in front of them. However, a dose of good flight instruction, backed up with practical and theoretical experience and knowledge, will open the door to this aviation mystery.

Some flight instruments provide the pilot with narrowly confined and specialized information through their limited and/or unique functions. Other instruments give a variety of information and duplicate the functions provided by some flight instruments to offer a backup in the case of an instrument, power, or systems failure. Still, each of the flight instruments on an aircraft's instrument panel provides pilots only a fragment of information when not used in concert. Fixation on a single instrument can lead to confusion and an incomplete picture of the aircraft's attitude or situation. Omission of an instrument in the pilot's "scan" also can lead to an incomplete picture of available information. Pilots learn to understand and consider the information provided by multiple instruments to furnish them with a complete picture of their aircraft's vertical, horizontal, and positional situation.

Because of their multiple functions, flight instruments are divided into different groups based on the information they provide, and/or their source of power or operation. The breakdown of those instrument groupings follows the list of required instruments (in "operable condition") for different flight operations.

The airspeed indicator is a pitot-static instrument that provides pitch and power information. If the airspeed is increasing and the power setting has not been changed, the aircraft is descending; if the airspeed is decreasing and power has not been adjusted, the aircraft is climbing. Knowing the aircraft's performance in specific conditions - check the tables in the pilot's operating handbook - allows you to use this instrument to confirm power. A red line on its face indicates the aircraft's V_NE, or never-exceed airspeed. The yellow arc is a caution range, while the green arc is the normal operating range. V_NO, the maximum structural cruising speed, is the top of the green arc. V_S, the stalling speed "clean" (with flaps and landing gear up) is at the bottom of the green arc. The white arc is the flap operating range, with V_FE - the maximum flap extension speed - at the top of the white arc, and V_SO - stall speed in the landing configuration, with flaps and landing gear fully extended - is at the bottom of the white arc.

The altimeter is a pitot-static instrument that provides pitch information - if the altitude is not changing, the aircraft is not climbing or descending, and therefore is in level flight. Most altimeters have 10,000-foot, 1,000-foot, and 100-foot pointers. The barometric pressure changes 1,000 feet for each inch of altimeter setting. If the air temperature is colder than standard - 15 degrees Celsius (59 degrees Fahrenheit) at sea level - true altitude will be lower than indicated altitude. Likewise, if the air is warmer than standard, true altitude will be higher than indicated.

The attitude indicator is a gyroscopic instrument normally powered by vacuum generated by a pump on the aircraft's engine. It provides bank and pitch information. A warning flag indicates the unit's failure. The heading indicator is another vacuum-powered gyroscopic instrument. In addition to the aircraft heading, it indicates the aircraft's bank - if the heading is constant, the aircraft is not banking left or right and is level.

The turn coordinator, which indicates bank, is another gyroscopic instrument that is electrically driven in most modern aircraft. This provides redundancy to the vacuum-powered attitude and heading indicators.

The tachometer (in airplanes with fixed-pitch propellers) and manifold pressure gauge (in the case of constant-speed propellers) indicate the amount of power being produced by the engine.

Before-Takeoff Instrument Checks

Although flight instrument and systems failures are rare, they do occur. Often they are very subtle, and a pilot may not notice without the utmost vigilance. A turn coordinator can be completely nonfunctioning and, because the inclinometer continues to work - along with the other bank instruments - a pilot can fly for hours and never notice. Having pulled the turn coordinator circuit breaker during flights with both private pilot and instrument students, I know that very few notice when the turn coordinator's little white airplane is completely static, no matter what the actual aircraft's attitude.

If a vacuum system failure occurs, the attitude indicator will display a red warning flag that, if not expected, can seem invisible. To make matters worse, the attitude indicator will usually begin showing a very slight bank. Because the failure does not usually cause the instrument to display an attitude that seems radical or abrupt to an unsuspecting pilot, he can fly on and on - even in visual conditions - making slight bank corrections based on the false information he is utilizing until the airplane's attitude is unmistakably in conflict with the instrument's display.

Because these and several other flight instrument errors often are so subtle, pilots should make a few useful checks before every flight. Of course, even a satisfactory pre-takeoff check cannot prevent the flight instruments or their systems from failing later in flight, so all pilots should maintain continual vigilance for systems failure flags and inconsistent flight instrument indications, as part of their active scan of all instruments, through the course of every flight.

Start by setting the altimeter to the appropriate barometric pressure. This figure is obtained from the airport's automatic terminal information service (ATIS) at a towered airport or from the automated surface observation system (ASOS) or automated weather observing system (AWOS) at airports with these services. You can find them in AOPA's Airport Directory Online ( www.aopa.org/members/airports ), in the Airport/Facility Directory, and on aeronautical charts. If none of these broadcast weather services is available, then set the altimeter to the airport's field elevation for takeoff and obtain the local altimeter setting from a nearby air route traffic control facility (frequencies can be found on various charts) after takeoff.

Before you begin your taxi, set the heading indicator to the magnetic compass, so your setting is accurate and the compass is not swinging back and forth while you try to figure out which heading you are on. Accomplishing this before you start moving keeps you from being distracted during the taxi and provides accurate information for proper aileron and elevator deflection during taxi. Compare the HI to the compass again before takeoff, as the HI's gyro is vacuum-driven and will not indicate accurately during taxi, when the engine is operating at low power.

During the taxi to the runway, check the compass and HI movement. During left turns, both instruments' heading displays show decreasing headings. During right turns, the numbers on the compass and HI show increasing headings. Also, it is very important to note that the compass and HI show headings in the opposite direction. The HI reads lower to higher from left to right, the way we read in English, while the compass's headings get higher to the left and lower to the right.

The only other flight instrument that should move during the taxi is the turn coordinator (or turn and slip indicator). The miniature airplane shows a right turn when the airplane taxis into a right turn, while the ball in the inclinometer moves to the left. In a left taxi turn the opposite indications should be seen. If the airplane does not move, check its circuit breaker.

As part of your preflight, verify that required instrument and system checks are current. The pitot-static system, as well as the altimeter and Mode C encoder, must be inspected every 24 months with the inspection recorded in the aircraft logbook. And if you're taking off on an IFR flight, the VOR must have been checked within the past 30 days; a notation should include DEPS for date, error, place, and signature.

Before takeoff, check the heading indicator against the magnetic compass and set the altimeter to the local barometric pressure (or field elevation, if the pressure is unavailable). Finally, set the attitude indicator's miniature airplane to the horizon line.

Instrument Errors

Modern flight instruments are very reliable and accurate because they are the products of decades of aerospace technology, knowledge, testing, and actual in-fight operations. All flight instruments provide pilots with accurate information because they are able to gather information from and are powered by various sources. Although rare, each flight instrument is vulnerable to failure and certain errors - another reason to keep a very active scan and avoid fixation on any one instrument.

A pitot tube blockage affects the airspeed indicator, which will drop to zero because no air pressure is entering the pitot tube. This can be very dangerous because the airspeed indicator tells the pilot about the most important piece of information required to keep the airplane in the air - airspeed. Turn the pitot heat on immediately if you see the ASI go to zero, as ice blocking the ram air input is the most common cause of pitot tube blockage.

A pitot tube and drain hole blockage also affects the airspeed indicator only. The ASI acts like an altimeter in this case - when altitude increases, airspeed appears to increase, and vice versa. Instead of dropping to zero, the airspeed indication continues to change with altitude, making it harder for the unsuspecting pilot to figure out that he or she has a problem. The ASI indicates a decrease as you descend, which should help you avoid stalling close to the ground because the decreasing indicated airspeed reminds you not to pull the nose up.

A static port blockage affects the ASI, altimeter, and VSI. The ASI will indicate a lower-than-correct airspeed when the airplane is at an altitude above where the blockage occurred, and a higher-than-correct airspeed when the airplane is at an altitude below where blockage occurred. The altimeter freezes on the altitude where the error occurred, and the VSI settles on zero fpm vertical velocity.

The cure for any static blockage errors is to find an alternate static source. Use the aircraft's alternate static source, if there is one. Otherwise, break the VSI's glass cover. Using an alternate static source provides some errors of its own: The ASI falsely indicates a higher airspeed than actual; the altimeter falsely indicates a higher altitude than actual; and the VSI falsely indicates a climb.

Compass Errors

Our aircraft are equipped with heading indicators because magnetic compasses are very prone to error and movement (and deviation and variation) while in flight. There is a Catch-22, however, because the HI is corrected with reference to the magnetic compass.

Magnetic dip errors are negligible at the equator and are more pronounced as you move toward the poles. Acceleration/deceleration errors occur during speed changes and are most apparent on headings of east and west, decrease as your heading moves toward north and south (and increases as your heading moves toward east and west), and do not occur when heading directly north or south. When on a heading of east or west and you accelerate, the magnetic compass swings and falsely indicates a turn to the north; when heading east or west and you decelerate, the magnetic compass swings and falsely indicates a turn to the south.

Northerly turning error is most apparent when turning to or from a heading of north or south. This error increases as you near the poles and decreases as you near the equator. When you turn from a northerly heading the compass initially indicates a turn in the opposite direction; it then begins to show the correct direction but lags behind the correct heading; and finally, it shows the correct heading. Turning from a southerly heading the compass initially indicates a turn in the correct direction but it exceeds the actual heading of the aircraft. This error dissipates as the heading approaches east or west.

If you want to roll out on a specific heading using only your magnetic compass, you have to adhere to the following:

• When making a compass-only turn, do not exceed 15 degrees of bank.
• Lead your roll-out on turns to the north (undershoot).
• Lag your roll-out on turns to the south (overshoot).
• Remember the Civil War: north leads, south lags (for rolling out on heading, undershoot the north and overshoot the south).
• Make all turns standard rate at 3 degrees per second (30 degrees per 10 seconds) and time for rolling out on desired heading. The standard rate turn is noted by the little "ticks" below wings-level on the turn coordinator.

All pilots, including student, recreational, and private pilots who won't be flying under instrument meteorological conditions, will greatly improve their competency and confidence with an understanding of the different flight instruments required in the aircraft, along with their functions and their common errors. With the current requirement to train a minimum of three hours in simulated and/or actual IMC, all private pilot aspirants get a head start on their IFR education.

Three hours of instrument training is, of course, a bare minimum. All pilots, no matter what their level of training or certification, should continue to improve their understanding of flight instruments, their functions, power sources, errors, and actions required in the case of a failure.

Flight Instruments By Group

Pitch Instruments

1. Altimeter
2. Airspeed indicator (ASI)
3. Attitude indicator (AI)
4. Vertical speed indicator (VSI)

Bank Instruments

1. Heading indicator (HI)
2. Attitude indicator (AI)
3. Turn coordinator

Power Instruments

1. Tachometer (RPM)
2. Manifold pressure (MP-aircraft with constant-speed propellers)
3. Airspeed indicator (ASI)

Flight Instruments By System

Pitot-Static Instruments

1. Pressure altimeter
2. Vertical speed indicator (VSI)
3. Airspeed indicator (ASI)

Gyroscopic (Vacuum) Instruments

1. Attitude indicator (AI)
2. Heading indicator (HI)

Gyroscopic (Electric) Instruments

1. Turn coordinator (many aircraft)

Required Flight Instruments And Equipment

VFR Day: ATOMS LEFR

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• AA Altimeter, airspeed indicator
• TT Tachometer, temperature gauge
• OO Oil temperature, oil pressure gauges
• MM Magnetic compass, manifold pressure gauge
• SS Seat belt, shoulder harness
• L Landing gear position indicator (retractable-gear aircraft)
• E Emergency locator transmitter
• F Fuel quantity gauge for each tank
• R Rotating beacon

VFR Night: Atoms LEFR + PLFA

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• P Position lights (green � right wing, red � left wing, white � tail)
• L Landing light (only if used for hire, not flight instruction)
• F Fuses � a spare set of each required type accessible to the pilot in flight
• A Anticollision light (red and/or white) � rotating beacon

IFR: Above VFR instruments and equipment for either day or night, plus RRSACAPD

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• R Two-way radio
• R Gyroscopic rate of turn indicator (turn coordinator)
• S Slip/skid indicator (inclinometer)
• A Altimeter adjustable for barometric pressure
• C Clock with digital or sweep hour, minutes, and seconds display
• A Alternator or generator
• P Gyroscopic pitch and bank indicator (attitude indicator)
• D Gyroscopic direction indicator (heading indicator)