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Military Flight Aptitude Test: Flight Instruments

The Military Flight Aptitude Test will have questions about flight instruments. The early aviators didn’t have much in the way of flight instruments, but today’s pilots have more than a piece of string in the windshield (yes, they actually used that) to give guidance on what their aircraft are doing.

The flight instruments tell you how high you are, which way you’re heading, how fast you’re flying, and if you’re in a turn. The instruments in the following sections come in handy when you’re flying visually at night and punch into the clouds without warning.

Altimeter

You can determine your altitude based on the atmospheric pressure; the greater the altitude, the less the outside pressure. Your aircraft’s altimeter is an instrument that uses the barometric pressure to accurately tell you your elevation in relation to mean sea level (the average surface level of the sea). To do so, it applies a standard (a constant for comparison); the standard atmospheric condition is 29.92 Hg (inches of mercury).

The altimeter, shown here, features an elevation dial and a small pressure window (called the Kollsman window) where the barometric pressure is set. Just about every initial aviation communication includes the barometric pressure; by setting this number in the Kollsman window, you can tell your altitude in relation to sea level. If you know how high the surface elevation you’re flying over is, you know your above ground level (AGL) altitude.

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Say you’re flying in a relatively high-pressure area; approach control has told you the pressure is 30.22. You set the altimeter to reflect the information given to you by air traffic controllers (along with current safety hazards), and you see that your altimeter reads 1,275 feet.

Looking at your approach chart or map, you see that the airfield that you’re over has an elevation of 230 feet, so you know you’re 1,045 feet above the ground (that’s your altimeter reading minus the airfield’s elevation).

Density altitude is the correction based on variations from standard temperature and is used in aircraft performance charts to determine things such as distance required to clear a given obstacle.

Attitude indicator

The attitude indicator, also known as the artificial horizon, gives you an aircraft orientation relative to the horizon of the earth. Before each takeoff, you set the attitude indicator to level flight.

This instrument, in conjunction with others, can then tell you whether you’re climbing or descending (pitch indication) and whether your wings are in a turning attitude (roll indication). This indicator is one of the primary instruments for nonvisual (inclement weather) flying.

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Turn indicators

Turn indicators serve as backups to the attitude indicator (see the preceding section) and utilize gyroscopic inertia (the tendency of a gyroscope wheel to continue rotating in the same plane about the same axis in space). Turn indicators fall into two basic categories, both of which are shown here.

  • A turn-and-slip indicator shows the rate and direction of a turn. The slip portion of the indicator tells you you’re performing a coordinated turn and using all the controls efficiently.

    In a standard turn, with all aerodynamic forces being efficient, you will complete 360 degrees in two minutes.

  • A turn coordinator shows the rate of turn and the roll rate (how fast you enter into the turn at a specified bank angle).

    A turn-and-slip indicator (a) and turn coordinator (b).
    A turn-and-slip indicator (a) and turn coordinator (b).

Heading indicator

The heading indicator, also known by the older term directional gyro, is a mechanical instrument that backs up and serves as an aid to the magnetic compass. The heading indicator (shown in this figure) stabilizes the readings and allows you to navigate without the fluctuation errors found in the magnetic compass.

A heading indicator.
A heading indicator.

Airspeed indicator and vertical speed indicator

The airspeed indicator (shown here) detects and displays a differential pressure that adjusts ram airspeed (pitot pressure) for static pressure. Basically, that means that the airspeed indicator tells you vital airspeed rates critical to achieving a desired flight profile (such as a climb, descent, or level). If you look closely at the indicator, you see various colored lines.

The white arc represents the stall (minimum) and maximum speeds you can operate at with the flaps extended. The green arc shows the normal airspeed operating range of the aircraft. The yellow arc is the caution range in which you can safely operate the aircraft in smooth, non-turbulent air only. The red line at the upper edge of the yellow arc is the velocity at which the aircraft sustains structural damage, so don’t cross it.

An airspeed indicator.
An airspeed indicator.

The vertical speed indicator (shown here) tells you whether you’re climbing or descending and at what rate of speed by detecting the rate of change in static air pressure.

A vertical speed indicator.
A vertical speed indicator.
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