Air Data Instruments Questions and Answers

Answer: A

Correct answer: The aircraft is lower than the altimeter indicates

The setting of 1030 hPa is still in the subscale, but the actual sea level pressure is now 995 hPa, 35 hPa lower. At roughly 30 ft per hectopascal that is about a thousand feet of error, and the error is in the dangerous direction: the instrument overreads, so the aircraft is a thousand feet lower than the dial suggests. High to low, look out below.

Answer: B

The ram rise grows with the square of Mach number and is tens of degrees at high subsonic cruise. The air data computer removes it to recover the static temperature that true airspeed and speed of sound calculations require.

Answer: C

True airspeed is equivalent airspeed divided by the square root of the density ratio, and that ratio falls throughout the climb. This is why a climb schedule switches from constant indicated airspeed to constant Mach number at the crossover altitude.

Answer: D

Correct answer: static

The airspeed indicator works on the difference between total pressure from the pitot tube and static pressure from the flush ports, and that difference is the dynamic pressure. Note that dynamic pressure is the output of the subtraction rather than one of its inputs - which is why no probe senses it directly and why the instrument needs both sources to function.

Answer: A

With QNH set, the altimeter shows the aerodrome elevation when the aircraft is on the ground there. QFE reads zero on the aerodrome and the standard setting is used above the transition altitude for mutual consistency.

Answer: B

The chain runs indicated, calibrated, equivalent and then true, each step removing one physical effect. The compressibility step is negligible at low speed and grows quickly above about two hundred knots and at altitude.

Answer: C

The pitot line vents through the drain to static pressure, so the differential across the capsule collapses. If the drain is blocked as well, the trapped pressure instead makes the instrument behave like a crude altimeter.

Answer: D

Bringing the air to rest at the probe compresses it adiabatically and raises the sensed temperature. The rise goes with the square of Mach number, so it is trivial in the circuit and substantial in high subsonic cruise.

Answer: A

Only a static failure drives the vertical speed indicator to a steady zero, because only then does the instrument see no pressure change at all. Airspeed misbehaves in both cases and is therefore the weaker discriminator.

Answer: B

True airspeed is equivalent airspeed divided by the square root of the density ratio, and that ratio falls steadily with height. It is why a climb schedule changes from constant indicated airspeed to constant Mach at the crossover.