Airspeed Indicator (ASI)
The Airspeed Indicator (ASI) is one of the three primary pitot-static instruments. It displays Indicated Airspeed (IAS) — the speed at which air flows onto the aircraft nose.
Operating principle
The ASI measures the difference between total pressure (pitot) and static pressure:
code
Dynamic pressure = Total (pitot) − Static
= ½ × ρ × V²
Where:
- ρ = air density (kg/m³),
- V = true airspeed (TAS) in m/s.
The dynamic pressure is measured inside the instrument by an aneroid capsule — the membrane moves proportional to the pressure differential. Through a lever mechanism the pointer moves over the scale.
Speed definitions
| Term | Meaning |
|---|---|
| IAS (Indicated Airspeed) | as read directly on the instrument |
| CAS (Calibrated Airspeed) | IAS corrected for instrument and position error |
| EAS (Equivalent Airspeed) | CAS corrected for compressibility effects (relevant > ~200 kt / FL 100+) |
| TAS (True Airspeed) | EAS corrected for air density (altitude, temperature) |
| GS (Ground Speed) | TAS corrected for wind |
Chain: IAS → CAS → EAS → TAS → GS, each step adds one correction.
When do IAS and TAS differ?
- At standard conditions at sea level (ISA, 1013.25 hPa, 15 °C): TAS ≈ IAS.
- With increasing altitude: TAS > IAS (lower air density requires higher TAS at the same IAS for equal dynamic pressure).
- Rule of thumb: TAS ≈ IAS × (1 + 0.02 × altitude in 1000 ft) for low altitudes.
- Example: at FL 100 (10 000 ft) → IAS 100 kt ≈ TAS 120 kt.
ASI markings
| Marking | Meaning |
|---|---|
| White arc | Flap operating range: Vs0 (stall, full flap) to Vfe (max flap) |
| Green arc | Normal operating range: Vs1 (stall, clean) to Vno (max structural cruising) |
| Yellow arc | Caution range: Vno to Vne, smooth air only |
| Red line | Vne (Never Exceed Speed) — must not be exceeded |
| Blue line (rare) | Vyse — best single-engine rate of climb (on multi-engine types) |
Pre-flight check
Before every flight:
- Remove pitot cover (common error!).
- Pitot tube clear of blockage, insects.
- Drain hole at the bottom clear?
- Pointer indicates 0 (stationary on ground).
Before every take-off:
- ASI must show an indication by ~30 % of acceleration — if not → abort take-off.
Common failures
- Blocked pitot, static open → ASI behaves like an altimeter: rises with climb (Pitot trapped, static falls).
- Blocked pitot + drain → ASI stuck at last value.
- Blocked static → ASI reads too high on descent, too low on climb.