Altimeters & airspeed

022 InstrumentationChapter 4

Pressure altimeter

Key fact

Aneroid capsule — sealed, partially evacuated; expands on climb (static pressure outside decreases). Linkage converts movement to display. D-spring / leaf spring corrects cockpit temperature. Sub-scale change always varies by 30 ft per datum step at sea level reference.

Key fact

Indicates vertical distance from selected datum (height, altitude, flight level). Indicated = true altitude only in perfect ISA. QNH = equivalent sea-level pressure at the airfield.

Altimeter errors

Error	Cause
Instrument	Mechanical; larger with height
Pressure	Manoeuvre + position (static sampling)
Temperature	Corrective calculus needed
Barometric	Wrong datum set
Lag / hysteresis	Dangerous in rapid descent
Orographic	Lower pressure leeward, higher windward
Blockage/leak	Static system faults

Key fact

Servo altimeter: reduces lag; ±30 ft at sea level, ±50 ft at 20 000 ft, ±100 ft at 40 000 ft. Mechanical sensitive type: ±50 / ±175 / ±600 ft at same levels.

Vertical speed indicator

Key fact

Measures rate of change of static pressure. Metering unit (orifice + capillary) creates lag. Climb: capsule contracts · Descent: expands. Static blocked: VSI returns to zero. IVSI: dashpot/accelerator pump — lag practically eliminated; steep banked turns may cause false reading.

Key fact

VSI accuracy ±200 ft between +50°C and −20°C. Time lag at start/end of climb or descent. Pressurised leak: rapid descent indicated (higher pressure enters).

Airspeed indicator

Key fact

Measures dynamic pressure. Static in casing cancels static in capsule → dynamic. Inside capsule = total (pitot) pressure; casing = static.

Formula — Airspeed corrections (ICE-T)

IAS → CAS/RAS (+ position & instrument error)
    → EAS (+ compressibility above ~200 kt / 10 000 ft)
    → TAS (+ density error)

TAS ≈ EAS × √(ρ₀ ÷ ρ)     [approx rule: TAS ≈ IAS + 10% at altitude]

Exam trap

Pitot leak (not blocked): dynamic pressure reduces → ASI under-reads. Static leak (pressurised): capsule contracts → under-read. Pitot leak (unpressurised): capsule contracts → under-read. Static leak (unpressurised): no change.

ASI markings

Arc	Range
White	VSO – VFE (flap range)
Green	VS1 – VNO (normal operating)
Yellow	VNO – VNE (caution, calm air only)
Red	VNE (never exceed)

Key fact

VRA — max turbulent air (not on ASI). VLO — max gear operation (≤ VLE). VMO — from EAS, displayed as CAS. VNO — max structural cruising; not exceeded except in still air. Mnemonic arcs: SOFE SINO NONE.

Machmeter

Formula — Mach number

M = TAS ÷ LSS
LSS = 38.95 × √OAT (Kelvin)
M = √(2/γ × [(Pt/Ps)^((γ−1)/γ) − 1])   [simplified forms in flight manual]

Key fact

Uses ASI (Pt − Ps) and altimeter (Ps) components. Compressibility, density, temperature errors cancel in ratio. Tolerance ±0.01 M. Climbing constant IAS: Mach increases. Climbing constant Mach: IAS must decrease. OAT affects TAS only — Mach and CAS not affected by temperature.

Key fact

Critical Mach (MCR): free-stream Mach when local Mach first becomes sonic anywhere on aircraft. MMO slightly higher — absolute limit. Max TAS at CAS = VMO and M = MMO (changeover point).

Density altitude

Key fact

Density altitude — altitude in standard atmosphere at which density equals actual atmosphere density. ADC does not determine true altitude.