The ASI is not perfect — six error sources are in the PPL syllabus.

Error	Cause	Treatment
Instrument error	Manufacturing tolerance	tabulated in AFM (usually small, few knots)
Position / pressure error	Local flow at static port differs from free-stream static	AFM correction table: IAS → CAS
Density error	IAS reflects only dynamic pressure; actual density differs from sea-level standard	Correction via CRP-5/E6B: CAS → TAS
Compressibility error	Compression effects at high speed	At PPL speeds (<200 KIAS) negligible

CAS = IAS corrected for instrument and position error. Correction is in the AFM; CAS is the "true" indicated airspeed.

At higher altitudes TAS is higher than CAS. Rough rule:

TAS ≈ CAS + (2 % × altitude in 1000 ft)

Example: at 6000 ft pressure altitude with CAS 100 kt → TAS ≈ 100 + (2 % × 6 × 100) = 100 + 12 = 112 kt.

The rule is approximate but useful for a first cockpit estimate. Precise TAS via CRP-5/E6B (with temperature).

Operationally critical. Memorise!

Blockage	ASI behaviour	Altimeter	VSI
Pitot blocked (static free)	ASI behaves like altimeter: reads too high in climb, too low in descent	normal	normal
Static blocked (pitot free)	ASI reads too low in climb, too high in descent	frozen at blockage altitude	zero
Both blocked	all three instruments unreliable	unreliable	unreliable

Covering the pitot tube or static port causes an erroneous ASI reading. Special case insect on the ground:

If an insect on the ground blocks the pitot tube (typically a wasp nest in summer), the pressure inside the pitot line is too high (residue pressure of the insect deposit).
On take-off the true dynamic pressure difference is measured too small (because the pitot internal pressure is already raised) → the indicated speed will be too low.
Pilot notices: Vr is reached without ASI reading Vr → risk of stall or runway excursion.

→ Pre-flight: remove pitot cover! Visually inspect the pitot (tube clear, no insect visible).

The pitot tube is usually heated electrically — a heating element around the inside of the tube prevents icing in flight.

Pilot switches on pitot heat when icing conditions are expected (cloud, precipitation at < +5 °C).
Current draw typically 5-10 A — see ausfallanzeigen for function check via ammeter.

If the ASI is inoperable, the aircraft may only be operated once the ASI has been replaced (or repaired). Reason:

ASI is part of the minimum equipment per CS-23.1303 and EASA Part-NCO.IDE.A.120.
Without ASI, stall warning and speed monitoring are impossible.

→ A faulty ASI makes the aircraft not airworthy for VFR day, VFR night, or IFR.