Table of Contents

The airspeed indicator in a light airplane shows some of the airspeed limitations of the aircraft by means of colored arcs. On aircraft manufactured prior to 1978, these arcs are calibrated airspeed. The arcs on later aircraft are indicated airspeed.

The white arc is the flap operating range. The low-speed end of the white arc is V_S0, which is the stalling speed, or the minimum steady-flight speed in the landing configuration. The high-speed end of the white arc is the maximum flap extended speed. Flight at airspeeds greater than V_FE with the flaps extended can impose excessive loads on the flaps and wing structure.

The green arc is the normal operating range. The low-speed end is V_S1, which is the stalling speed or the minimum steady-flight speed in a specified configuration. At the high-speed end of the green arc is V_NO (maximum structural cruising speed).

The yellow arc begins at V_NO and continues to the red line, V_NE (never exceed speed). Operations may be conducted only in smooth air and with caution.

Other speed limitations which are not color-coded on the airspeed indicator include:

V_S -- Stalling speed or minimum steady flight speed at which the airplane is controllable.

V_F -- Design flap speed.

V_LE -- Maximum landing gear extended speed.

V_A -- Design maneuvering speed. If severe turbulence (for example, significant clear air turbulence) is encountered during flight, the pilot should reduce the airspeed to the design maneuvering speed. In addition to setting the power and trimming to obtain an airspeed at or below maneuvering speed, the wings should be kept level, and allow slight variations of airspeed and altitude. This technique will help minimize the wing load factor in severe turbulence. Maneuvering speed is also the maximum speed at which full or abrupt control movements may be made. Maneuvering speed decreases as gross weight decreases. See Figure 3-1.


Figure 3-1. Airspeed indicator

Altitude Definitions

Indicated altitude -- the altitude indicated on an altimeter set to the current local altimeter setting.

Pressure altitude -- the altitude indicated on an altimeter when it is set to the standard sea level pressure of 29.92 inches of mercury (29.92" Hg). Above 18,000 feet MSL, flight levels, which are pressure altitudes, are flown.

Density altitude -- pressure altitude corrected for a non-standard temperature. The performance tables of an aircraft are based on density altitude.

True altitude -- the exact height above mean sea level. Calculation of true altitude does not always yield a correct figure. Atmospheric conditions may deviate from the standard temperature and pressure lapse rates used in the computation of true altitude.

Magnetic Compass

The Magnetic Compass is the only self-contained directional instrument in the aircraft. It is affected by deviation error. Magnetic disturbances (magnetic fields) within an aircraft deflect the compass needles from alignment with magnetic north. Each aircraft will affect a magnetic compass differently, and the direction and magnitude of the error varies with heading and with the electrical systems in use. Compensating magnets are used to minimize this type of error as much as possible. Any remaining error is noted on the compass correction card.

Gyroscopic Instruments and Systems

Gyroscopes ("gyros") exhibit two important principles -- rigidity in space and precession. Of the seven basic flight instruments, three are controlled by gyroscopes:

• Attitude indicator

• Turn coordinator/turn-and-slip indicator

• Heading indicator

The turn coordinator/turn-and-slip indicator is the only one addressed on the test. The turn coordinator is designed to show roll rate, rate of turn, and quality of turn. See Figure 3-2. The turn-and-slip indicators are gyroscopically-operated instruments designed to show the rate of turn and quality of turn. The turn- and-slip indicator does not show roll rate. See Figure 3-3.

A single needle-width deflection on the 2-minute indicator means that the aircraft is turning at 3° per second, or standard rate (2 minutes for a 360° turn). On the 4-minute indicator, a single needle-width deflection shows when the aircraft is turning at 1-1/2° per second, or half-standard rate (4 minutes for a 360° turn).

Before starting the engine, the turn needle should be centered and the race full of fluid. During a taxiing turn, the needle will indicate a turn in the proper direction and the ball will show a skid. An electric turn-and-slip, or turn coordinator, acts as a backup system in case of a failure of the vacuum-powered gyros.


Figure 3-2. Turn coordinator


Figure 3-3. Turn-and-slip indicator

Attitude Instrument Flying

The four flight fundamentals involved in maneuvering an aircraft are: straight-and-level flight, turns, climbs, and descents.