3: Flight Instruments

Airspeed Indicator

Altitudes

Magnetic Compass

Gyroscopic Instruments and Systems

Attitude Instrument Flying

Electronic Flight Instruments

Airspeed Indicator

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 VS0, the stalling speed or the minimum steady-flight speed in the landing configuration. The high-speed end of the white arc is VFE, the maximum flap extended speed. Flight at airspeeds greater than VFE 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 VS1, the stalling speed or the minimum steady-flight speed in a specified configuration. The high-speed end of the green arc is VNO, maximum structural cruising speed.

The yellow arc begins at VNO and continues to the red line, which represents VNE, the never exceed speed. Operations at speeds in the yellow arc may be conducted only in smooth air and with caution.

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

VS—stalling speed or minimum steady flight speed at which the airplane is controllable.

VF—design flap speed.

VLE—maximum landing gear extended speed.

VA—design maneuvering speed. If severe 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

Altitudes

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 nonstandard temperature. The performance tables of an aircraft are based on density altitude.

True altitude—the exact height above mean sea level (MSL). 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 will vary with heading and 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:

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.

Figure 3-2. Turn coordinator

Figure 3-3. Turn-and-slip indicator

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.

Attitude Instrument Flying

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

The attitude of an aircraft is controlled by movement around its lateral (pitch), longitudinal (roll), and vertical (yaw) axes. In instrument flying, attitude requirements are determined by correctly interpreting the flight instruments. Instruments are grouped as to how they relate to control, function, and aircraft performance. Attitude control is discussed in terms of pitch, bank, and power control. The three pitot-static instruments, the three gyroscopic instruments, and the tachometer or manifold pressure gauge are grouped into the following categories:

Pitch Instruments:

  • Attitude indicator
  • Altimeter
  • Airspeed indicator
  • Vertical speed indicator

Bank Instruments:

  • Attitude indicator
  • Heading indicator
  • Turn coordinator

Power Instruments:

  • Manifold pressure gauge
  • Tachometer
  • Airspeed indicator

When climbing and descending, it is necessary to begin level-off in enough time to avoid overshooting the desired altitude. The amount of lead to level-off from a climb varies with the rate of climb and pilot technique. If the aircraft is climbing at 1,000 feet per minute, it will continue to climb at a descending rate throughout the transition to level flight. An effective practice is to lead the altitude by 10 percent of the vertical speed (500 fpm would have a 50 foot lead; 1,000 fpm would have a 100 foot lead).

The amount of lead to level-off from a descent also depends upon the rate of descent and control technique. To level-off from a descent at descent airspeed, lead the desired altitude by approximately 10 percent. For level-off at an airspeed higher than descending airspeed, lead the level-off by approximately 25 percent.

When making initial pitch attitude corrections to maintain altitude during straight-and-level flight, the changes of attitude should be small and smoothly applied. As a rule-of-thumb for airplanes, use a half-bar-width correction for errors of less than 100 feet and a full-bar-width correction for errors in excess of 100 feet.

When recovering from an unusual attitude without the aid of the attitude indicator, approximate level pitch attitude is reached when the airspeed indicator and altimeter stop moving and the vertical speed indicator reverses its trend.

The following procedures are accomplished to recover from a nose-low attitude:

  1. Reduce power.
  2. Level the wings.
  3. Raise the nose to the horizon.

The following procedures are accomplished to recover from a nose-high attitude:

  1. Add power.
  2. Apply forward elevator pressure.
  3. Level the wings.

Electronic Flight Instruments

Electronic flight instrument systems integrate many individual instruments into a single presentation called a primary flight display (PFD). Flight instrument presentations on a PFD differ from conventional instrumentation not only in format, but sometimes in location as well. For example, the attitude indicator on the PFD is often larger than conventional round-dial presentations of an artificial horizon. Airspeed and altitude indications are presented on vertical tape displays that appear on the left and right sides of the primary flight display. The vertical speed indicator is depicted using conventional analog presentation. Turn coordination is shown using a segmented triangle near the top of the attitude indicator. The rate-of-turn indicator appears as a curved line display at the top of the heading/navigation instrument in the lower half of the PFD.

Figure 3-4. A typical primary flight display (PFD)

[02-2025]