The Airspeed Indicator (ASI)

1. Principle of Operation

An aircraft on the ground in still air experiences only static (atmospheric) pressure. In flight, the leading edges of the aircraft are subject to an additional dynamic pressure due to relative motion through the air. The pitot head senses the total (pitot) pressure; the static vent senses static pressure only.

Where: ρ = air density; V = True Airspeed (TAS).

The ASI is a differential pressure gauge that measures the difference between pitot and static pressures (i.e. dynamic pressure), displaying the result in knots on a calibrated scale. (1 knot = 1 nautical mile per hour.)

2. Construction

Static pressure is fed into a hermetically-sealed instrument case. Pitot pressure is piped to a thin metal capsule inside the case capable of expansion and contraction.

Static pressure acts on both the inside of the case and the outside of the capsule walls, so it cancels. The pressure differential across the capsule wall is therefore:

Capsule expansion/contraction is proportional to changes in dynamic pressure (= changes in airspeed). The movements are transmitted via a temperature-compensated magnifying linkage to the pointer on the ASI face.

3. Calibration

Dynamic pressure depends on both speed and air density. Density varies with temperature and pressure, and therefore with altitude. The ASI is calibrated to read True Airspeed for the ISA MSL conditions only:

No allowance is made for the change in density with altitude. Flight at any condition other than ISA MSL therefore introduces errors. The ASI accuracy tolerance (CS-25) is ± 3% or 5 kt, whichever is greater.

4. ASI Errors & Airspeed Definitions

4.1 Indicated Airspeed (IAS)

The reading directly off the instrument face. Affected by all errors below.

4.2 Instrument Error

Manufacturing imperfections and wear cause small errors determined under laboratory conditions using a datum instrument. A correction card is produced for the speed range of the instrument and placed near the ASI in the cockpit.

4.3 Position Error (Pressure Error)

Arises mainly from sensing incorrect static pressure due to the probe’s location on the aircraft. Position errors vary across the speed range and are determined during the manufacturer’s flight test programme. In practice, a combined Instrument + Position Error correction card is used.

4.4 Manoeuvre-Induced Error

Associated with changes in angle of attack, producing transient errors and lag in airspeed indication. Primarily affects rapidly changing flight conditions.

4.5 Calibrated Airspeed (CAS)

Also historically known as Rectified Airspeed (RAS) (no longer in general use, but may appear on the Pooley CRP-5).

Example: If combined correction is +2 kt and pilot wants 100 kt CAS, fly at 98 kt IAS.

Why CAS matters: Aircraft stress limits and stall speeds are functions of dynamic pressure. Because the ASI is calibrated in “knots” (true only at ISA MSL), CAS provides a consistent measure of dynamic pressure regardless of altitude and density. Stall and structural limit speeds are expressed as CAS/IAS values.

4.6 Density Error

At altitudes above MSL, air density is less than 1225 g/m³ (the calibration density). Less dense air produces less dynamic pressure for the same TAS. The capsule expands less, and the ASI under-reads the true speed. If density is greater than ISA at MSL, the ASI over-reads.

4.7 Compressibility Error

Air is compressible; at speed the pressure in the pitot tube is higher than for an ideal incompressible fluid (for which dynamic pressure = ½ρV²). The ASI is calibrated for compressible flow at ISA MSL density. At lower densities (higher altitude), the standard compressibility correction becomes incorrect and the instrument over-reads (IAS and CAS too high). A subtractive compressibility correction must be applied.

4.8 Equivalent Airspeed (EAS)

In practice the difference between EAS and CAS is not large unless altitude is significant. However, at high-elevation airports with high-performance aircraft, CAS (and IAS) will be higher than EAS for the same dynamic pressure condition.

4.9 True Airspeed (TAS)

Density must be calculated from pressure (altitude) and temperature together — there is no direct density instrument. The navigational computer combines altitude and temperature to derive the density correction. The major factor is the pressure value.

5. Airspeed Correction Sequence — IAS to TAS

flowchart LR
    A["IAS\n(Indicated Airspeed)\nRaw instrument reading"] -->|"+ Instrument Error\n+ Position Error"| B["CAS\n(Calibrated Airspeed)"]
    B -->|"+ Compressibility\nCorrection"| C["EAS\n(Equivalent Airspeed)"]
    C -->|"+ Density\nCorrection"| D["TAS\n(True Airspeed)"]
    style A fill:#e8f1fb
    style B fill:#e8f1fb
    style C fill:#e8f5e9
    style D fill:#e8f5e9

6. Limiting Speeds & ASI Colour Coding

6.1 Limiting Speed Definitions

6.2 Colour Arc Coding

7. Pitot and Static Blockages

7.1 Pitot Head Blockage

If the pitot head becomes blocked (typically by ice or insects), the pressure locked inside the capsule remains constant.

• Level flight: Any change in airspeed will not be registered — the ASI freezes at its last reading.
• Climb with blocked pitot: Static pressure around the capsule decreases (as altitude increases). With pitot locked, (pitot − static) increases → ASI over-reads. IAS increases during a climb.
• Descent with blocked pitot: Static pressure increases. With pitot locked, (pitot − static) decreases → ASI under-reads. IAS decreases during a descent.

Special case: If both the ram air input AND the drain hole are blocked (static port open), the locked pitot pressure causes the ASI to react like an altimeter — reading increases during a climb and decreases during a descent.

7.2 Static Head/Vent Blockage

A blocked static source freezes the static pressure at the value existing when the blockage occurred.

• Descent with blocked static: True external static pressure is higher than the frozen value. The capsule experiences (pitot − locked lower static) → larger differential → ASI over-reads. Dangerous: aircraft is closer to stall than indicated.
• Climb with blocked static: True external static pressure is lower than the frozen value. ASI under-reads.

If the alternative static source is selected, an error due to position error may occur. Turbulence effects usually result in higher static pressure at the alternate source, causing an under-reading. This error is documented in the Flight Manual.

flowchart TD
    A["Blockage\nDetected?"] -->|Pitot| B["During Climb?\nASI over-reads\n(IAS rises)"]
    A -->|Pitot| C["During Descent?\nASI under-reads\n(IAS falls) ⚠"]
    A -->|Static| D["During Climb?\nASI under-reads\n(IAS falls)"]
    A -->|Static| E["During Descent?\nASI over-reads\n(IAS rises) ⚠ DANGER"]
    style E fill:#fdecea
    style C fill:#fff8e1

8. Pitot and Static Leaks

8.1 Pitot Leak

A leak in the pitot line causes loss of dynamic pressure → ASI under-reads.

8.2 Static Leak — Unpressurised Aircraft

In an unpressurised aircraft, if the static leak occurs where the outside pressure is lower than the sensed static pressure (almost always the case aerodynamically), a false lower pressure is sensed around the capsule. The differential (pitot − leaked static) increases → ASI over-reads (usually not significantly).

8.3 Static Leak — Pressurised Aircraft

9. Serviceability Checks

Practice Questions & Detailed Answers

Master Reference Tables — Chapter 4

Numerical Values

Airspeed Definitions Summary

PUDSOD Reference

Colour Arc Summary

Source Answer Key