The altimeter measures the height of the airplane above a given atmospheric pressure level by measuring atmospheric pressure at the level of flight. Since it is the only instrument that gives altitude information, the altimeter is one of the most important instruments in the airplane. Its principle of measuring pressure is similar to that of an aneroid barometer. The altitude is presented on the instrument by multipointers and the dial is usually calibrated in units of 20, 100, and 1,000 feet.

The airspeed indicator is a sensitive, differential pressure gauge which indicates the speed at which the airplane is moving through the air (not the speed along the ground). It measures and shows promptly the difference between (1) pitot, or impact pressure of the air as the airplane moves forward, and (2) static pressure, the undisturbed atmospheric pressure at the level of flight. These two pressures will be equal when the airplane is stationary on the ground in calm air. When the airplane moves through the air, however, the pressure in the pitot line becomes greater than the pressure in the static lines. This difference in pressure is registered by the airspeed pointer on the face of the instrument, which is calibrated to show the pilot the speed of the airplane in statute miles per hour (MPH) or nautical miles per hour (Knots), or both.

The attitude indicator (artificial horizon), with its display of a miniature or representative airplane and a bar representing the natural horizon, is the one instrument that will give a clear picture of the flight attitude of the real airplane. The bank attitude is graduated in 10, 20, 30, 60 and 90 degrees. The pitch attitude may or may not show the actual degree of pitch. When properly adjusted, the relationship of the miniature airplane to the horizon bar is the same as the relationship of the real airplane to the actual horizon.

The turn and slip indicator shows the direction and rate at which the airplane is turning and at the same time whether the airplane is slipping sideward. It is actually a combination of two instruments, a broad needle and a ball. The turn needle depends upon gyroscopic properties of precession for its indications and the ball is actuated by gravity and centrifugal force.

The turn indicator, being a gyroscopic instrument, indicates the direction and rate at which the airplane is turning about its vertical axis. It responds only to the rate of yaw. (The more modern "turn coordinator" also indicates the rate of roll about the airplane's longitudinal axis since it responds to both roll and yaw.) Unlike the attitude indicator, neither of these instruments gives a direct indication of the degree of bank of the airplane. However, for any given airspeed, there is a definite angle of bank necessary to maintain a coordinated turn at a given rate. The faster the airspeed, the greater the angle of bank required to obtain a given rate of turn. Thus, the turn indicator and the turn coordinator give only an indirect indication of the airplane's banking attitude or angle of bank during a coordinated turn.

The ball or slip indicator is a simple inclinometer consisting of a sealed, curved glass tube containing a steel ball which is free to move inside the tube. The tube is curved so that during coordinated flight centrifugal force or gravity causes the ball to rest in the lowest part of the tube centered between reference marks. When the effects of centrifugal force and gravity become unbalanced, as in a slip or skid, the ball moves away from the center of the tube.

The magnetic compass is a direction seeking instrument whose basic component consists of two magnetized steel bars mounted on a float, around which is mounted the compass card. The compass card includes the letters N, S, E, and W to show the cardinal azimuth headings, and each 30 degree interval is represented by a number, the first and last zero of which is omitted. For example, 030 degrees would appear as a 3, and 300 degrees would appear as 30. Between the numbers the compass card is graduated in 5 degree segments.

Since the magnetic compass is the only direction seeking instrument in most airplanes, the pilot should be able to turn the airplane to the desired compass heading and then maintain it. There are certain inherent errors in the magnetic compass, making exact straight flight and precise turns to specific compass headings difficult to accomplish, particularly in turbulent air. The pilot should be familiar with the inherent errors and how to compensate for them. These are briefly explained in Chapter 13.

The heading indicator (directional gyro) is fundamentally a mechanical instrument designed to help in maintaining a magnetic heading since its indications are much more stable than the magnetic compass. However, because it has no inherent direction seeking characteristics, it must first be set to the heading shown on the compass. The calibration of its dial, though presented in an upright position for easier reading, is similar to that of the magnetic compass. The heading indicator, being gyroscopically operated, is not affected by the forces that cause the errors in the magnetic compass.

The vertical velocity indicator is a sensitive differential pressure gauge that senses the rate of change in static air pressure to indicate the rate at which the airplane is climbing or descending. The dial is graduated in hundreds of feet per minute.
Engine Instruments

The tachometer is an instrument for indicating the speed at which the engine crankshaft is rotating. The dial is calibrated in revolutions per minute (RPM). The pilot controls the RPM by use of the throttle and/or the propeller control.

The oil pressure gauge indicates the pressure under which oil is being supplied to the internal moving engine parts by the lubricating system. Thus, it warns the pilot of impending engine failure which may result from an exhausted oil supply, failure of the oil pump, broken oil lines, etc.

The oil temperature gauge indicates the temperature of the oil entering the engine so that the pilot can determine if the oil has reached the proper temperature for applying takeoff power and whether oil temperature is becoming excessive in flight. With a very low temperature the oil may be so thick that it is not able to lubricate the internal moving engine parts. With an excessively high temperature the lubricating oil tends to thin out and decreases the lubricating qualities of the oil.

Fuel quantity gauges enable the pilot to determine the amount of fuel in each of the tanks to ensure that sufficient fuel is available for the flight.

The fuel pressure gauge measures the difference in pressure between the fuel and the air being supplied to the engine. Thus, the instrument indicates whether fuel is being supplied and the actual pressure at which it is being forced into the engine.

The manifold pressure gauge indirectly indicates the power output of the engine by measuring the pressure of the air in the fuel/air induction manifold. The higher the manifold pressure, the greater the power being developed by the engine. By means of the throttle, the pilot can control the pressure in the manifold, and thus the power. By reference to this gauge, which is calibrated in inches of mercury, the power can be adjusted to specific values within the capabilities of the engine. The gauge is required on airplanes equipped with a supercharger or a constant speed propeller.

The cylinder head temperature gauge is an important instrument for engines capable of high compression and/or high power. It indicates the temperature of the cylinder head of the hottest cylinder (usually one of the rear ones in a horizontally opposed or flat engine) and gives the pilot a means of determining whether the engine is operating at normal or excessive engine temperatures. Operating the engine at a temperature higher than it was designed for will cause loss of power, excessive oil consumption, and damage to the cylinder walls, pistons, and valves.