Instruments

Instruments

ALT - Altimeter
An instrument that indicates an aircrafts altitude or (FL) flight level depending on the barometric pressure the pilot has entered into the instrument.

In order to indicate the altitude correctly, the altimeter needs to be adjusted to the surrounding barometric pressure by using the knob at the lower left. This one is set to a barometric pressure of 29.9. The altimeter is read like a clock, the longer hand reads hundreds of feet, and the shorter hand reads thousands of feet. The instrument shown is indicating 1635 ft. Some altimeters, such as the one shown, have a third “indicator” (the small window below the zero at the top of the gauge) which reads tens of thousands of feet.

ASI - Air Speed Indicator
Fairly obviously, this is an instrument that indicates what your current airspeed is.

Here's one reading 103kts. This one has a knob at the bottom that allows you to set the temperature (in a window at the top of the dial) and the sliding scale between 100kts and 140kts will allow you to compensate for the difference in air pressure as the temperature changes.

CHT - Cylinder Head Temperature

Compass
The compass is the only instrument that can't mislead you or suddenly break down. Unfortunately, it's also the most useless instrument in an aircraft.

An aircraft compass is made from a small plastic sphere, floating in oil. This means that it is always parallel to the floor, and the oil damps its movement so it doesn't wobble about. Unfortunately the oil also means that it's a bit slow to react. Also, the earth's magnetic field isn't evenly spread through the atmosphere, and in England it slopes down dramatically into the floor. This means that the compass has to have a small lead weight attached to it to move the scale down so that the pilot can see it.

The lead weight means that the compass' center of gravity isn't in the center of the sphere, meaning that any acceleration will make the compass move from the correct position. If you are heading east or west and you accelerate, the compass will swing northwards. If you turn through south, the compass will lag behind significantly, whereas if you turn through north the compass will race ahead.

It isn't uncommon for an aircraft to be heading south-east and to turn through south while accelerating, and no one can remember exactly what the compass will do under all circumstances. Because of this the compass is only used during straight and level flight and the aircraft is flown by reference to the DI.

EIS - Engine Information System

EGT - Exhaust Gas Temperature

Fuel Gauge

GPS - Global Positioning by Satellite
A collection of satellites have been placed in very low earth orbit by the American government to allow their military to locate their vehicles. A complex bunch of computer equipment receives the signals from several satellites and works out how far it is away from each one. Since it knows where all the satellites are supposed to be, it can work out where it is in relation to them. This used to allow the receiver to pin down its position to about 30 feet because the American military used a feature called Selective Availability to turn satellites on and off so that 'the enemy' couldn't use them accurately. They've recently realized that this is a stupid idea and turned off SA so now a receiver can pin down its position to about 5 feet.

GPS is a godsend to pilots because it gives you an exact position (ie. location and altitude) thus rendering some of the other navigational equipment in airplanes redundant.

Turn Coordinator
The turn coordinator allows properly balanced turns to be made. There is a depiction of an airplane which shows roll angles, and a small ball in a tube which shows the current yaw rate. The secret to smooth flight is simply to keep the ball in the center.

This instrument is showing a (nearly) rate 1 roll to the right, with a slight yaw to the left.

Quad Gauge - CHT, RPM, EGT, Hour Meter