The airspeed indicator

Navigation · Chapter 21 · DGCA CPL and ATPL

What the ASI measures, why it reads low up high, the speed ladder IAS to TAS, the coloured dial, and a quick TAS estimate.

Remember

The ASI shows D = pitot − static; scale set for ISA sea level.
Same TAS higher up: IAS lower. Constant CAS = constant dynamic pressure.
IAS → CAS (instrument + position) → EAS (compressibility, > 300 kt) → TAS (density) → GS (wind).
White VSO–VFE, green VS1–VNO, yellow VNO–VNE, red VNE, blue VYSE.
TAS ≈ CAS × (1 + 1.75 % per 1000 ft). Tolerance ± 3 % or 5 kt, the greater.

Inside the ASI

A sealed case • Static (teal line) fills the whole sealed case • Pitot (pink line) goes only into the capsule
The capsule swells • Static is on both sides of the capsule wall: it cancels • Only D is left to swell it
Link, sector, pinion • The capsule pushes a link • The link turns the sector gear; the sector spins the pinion on the pointer spindle
The pointer • The pinion turns the pointer across the dial • Drag the airspeed and watch every part move together
D grows with the square • D = ½ ρ V²: twice the speed, four times D • Drag the airspeed: the capsule swells four times as much for twice the speed

Calibrated for sea level

Same TAS, two heights • Both fly at TAS 200 kt: the same real speed through the air • Up high the air is thinner: less density (ρ)
Less D, lower IAS • Less ρ → less D → the needle shows 146 kt • The dial assumes ISA sea level: 1013.25 hPa, +15 °C, 1.225 kg/m³
Lower than sea level • In denser air (below sea level, or very cold) it is the other way round: the ASI reads more than the TAS