Air Data Instruments Questions and Answers

Answer: C

Correct answer: So that every aircraft shares one common reference

Above the transition altitude every aircraft sets 1013.25 hPa, so all of them are measuring from the same datum and their indicated levels can be compared directly. Vertical separation depends on that consistency, not on accuracy: the standard setting does not give true altitude, and on a day far from standard pressure every aircraft is displaced together, which preserves the separation between them.

Answer: D

Correct answer: The rise grows with the square of Mach number

Read the last column against the Mach numbers: doubling from 0.2 to 0.4 takes the rise from 2 K to 8 K, a factor of four, and doubling again to 0.8 takes it to 32 K, another factor of four. That is the signature of a square law, and it follows directly from the relation \( \Delta T = 0.2M^2T_s \).

Answer: A

Mach number comes from the pressure ratio alone, but turning it into a speed requires the local speed of sound, which depends only on static temperature. That is why a machmeter needs no temperature probe and a true airspeed readout does.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The instrument responds to dynamic pressure, \( \tfrac{1}{2}\rho V^2 \), but its dial is graduated on the assumption that \( \rho \) is the sea level standard value. Where the actual density matches that assumption the reading is the true airspeed; anywhere else it is not, and the discrepancy grows steadily with altitude.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. Mach number can be expressed purely as a function of the ratio of total to static pressure, and in forming that ratio the density and temperature terms cancel out. The machmeter therefore works from the two pressures already present in the pitot-static system, which is why it can be a purely mechanical instrument.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason explains the first exactly. The instrument displays the rate at which static pressure is changing, so if the port is blocked the trapped pressure stops changing and the indication falls to zero regardless of what the aircraft is doing. A zero vertical speed during an evident climb is therefore a direct pointer to a static blockage.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the justification. A wing stalls at a particular angle of attack, essentially independent of weight, load factor or bank. The speed at which that angle is reached varies with all of them - it rises with the square root of weight and with the square root of load factor. Protection based on the invariant quantity is therefore correct in every configuration.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The altimeter converts the pressure it senses into height on a fixed assumption about the sea level value, so a genuine fall in ambient pressure is indistinguishable from a climb. The aircraft ends up lower than indicated, which matters most on approach and over high terrain.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason explains why the card is needed. Position error is not a constant: it depends on the flow field around the fuselage, which changes with speed, angle of attack and flap configuration. It therefore has to be measured for the type and tabulated as a correction against airspeed and configuration rather than applied as a single number.

Answer: D

Correct answer: A is false but R is true

The reason is true and it is precisely why the assertion is false. Because the instrument works by metering air through a calibrated restriction, the case takes several seconds to equalise after a change, so the reading lags. The lag is inherent to the principle, which is why a pilot establishes a climb or descent on the attitude indicator and uses the vertical speed indicator only to trim the rate.