A basic aircraft needs four instruments: an airspeed indicator that reads the pressure of the air hitting a forward-facing tube, an altimeter that reads the falling pressure of the air with height, a magnetic compass for heading, and a slip indicator that shows which way gravity and the turn are pulling the pilot. Each one reads a physical quantity indirectly, so each has a known weakness, and none of them teaches a pilot to fly. Add engine temperature and, where the engine allows, oil-pressure gauges so that the engine is monitored as well.
Part of Aerodynamics and Flight
The few gauges that matter in a simple aircraft, what each measures and why each can mislead.
What Each Instrument Actually Reads
| Instrument | What it measures | Typical failure |
|---|---|---|
| Airspeed indicator | Dynamic pressure from a pitot tube against static pressure | Blocked tube or leaking line |
| Altimeter | Static air pressure, converted to height | Weather changes the reading |
| Magnetic compass | Direction of the magnetic field | Engine and steel parts deflect it; swings in turns |
| Inclinometer | Direction of the apparent gravity in the cockpit | Says nothing of bank or heading |
The carrier says these are the minimum for safe flight (Aerodynamics and Flight). Engine gauges (cylinder or coolant temperature, oil pressure where fitted) are not on its list but belong beside them (Cooling Systems).
An Instrument Is Only As Good As Its Calibration
The pressure instruments have scales that must be calibrated against a reference. No scale is supplied here because the scale depends on the instrument you build. The method is to compare each against a known standard in controlled conditions and write the correction card, as the carrier asks of the compass. A scale copied from a drawing without a calibration check is a decoration.
What To Do When An Instrument Is Wrong
Pilots flying by instruments with no outside view trust them completely; pilots flying by eye in daylight use them to check what the aircraft is already telling them: the sound, the wind in the controls, the attitude against the horizon. A first aircraft should be flown by eye in good visibility, with the instruments confirming. If the airspeed reads zero in flight, or the altimeter does not move, that is a blocked line, not a calm; fly the attitude and land.
Mounting
Instruments must be readable in sun and in vibration, away from the engine’s heat, and the pressure lines routed and sealed without kinks. Keep a magnetic compass away from the engine, magnets and steel (Compass Navigation). The carrier’s quick card lists them.
Common Mistakes
- Fitting instruments you have not calibrated.
- Staring at the panel — a first-aircraft pilot looks outside.
- Assuming an instrument that agrees with another is right — two lines drawn from one blocked source agree and are both wrong.
- No engine monitoring.
Safety
Instruments do not teach flying
A person who has not been trained to fly cannot use an airspeed indicator to avoid a stall in the few seconds it takes. Training in a known sound aircraft is the only thing that gives that judgement.
What’s Next
- Learn next: Airspeed Indicator, the most important of the four
- Combine with: Flight Testing, where each one is checked