Engine Performance and Matching Questions and Answers

Answer: C

Components are sized for one condition, and everything else is off design by definition. An engine matched only at that point starts badly, accelerates slowly and surges readily everywhere else, which is unacceptable in service.

Answer: D

Corrected mass flow explicitly divides by the pressure ratio, which is how the pressure dependence is removed. Saying it is independent of pressure confuses the corrected quantity with the physical one it was derived from.

Answer: A

Correct answer: No, it lowers the working line

Opening the throat lets the choked nozzle pass more corrected flow at any given pressure ratio, so the compressor is required to deliver more flow at less pressure - and that point lies below and to the right of where it was, further from the surge line. This is why enlarging the nozzle area is the standard remedy for an inadequate margin.

Answer: B

Calorific value alters the fuel flow needed to reach a given temperature but not the geometry that governs the flow balance. The other three all change how much air the compressor must deliver at a given pressure ratio.

Answer: C

Correct answer: No, it risks weak extinction of the combustor

The two transient hazards are opposite in direction. Rapid acceleration risks surge, because fuel is added faster than the flow can rise. Rapid deceleration cuts fuel faster than the airflow falls, so the mixture goes lean and the flame may be extinguished. The deceleration schedule sets a minimum fuel flow to keep the combustor alight.

Answer: D

Correct answer: Falling exhaust gas temperature at a fixed thrust setting

Deterioration can only raise the temperature required for a given thrust, because every wear mechanism - opened clearances, worn seals, eroded profiles - reduces component efficiency and demands more heat for the same work. A fall at fixed thrust would indicate an improvement, which wear cannot produce.

Answer: A

Both compressor and turbine efficiency losses raise the temperature, and only the accompanying changes in shaft speed and fuel flow separate them. Reading the pattern rather than one instrument is the basis of gas path analysis.

Answer: B

Correct answer: The aircraft's cruise altitude

Cruise altitude is a flight management decision taken by the crew and the air traffic system, and the engine control has no part in it. What the control does enforce are the engine’s own limits and schedules: the acceleration schedule that protects surge margin, the rotational speed limit that protects the discs, and the turbine temperature limit that protects blade life.

Answer: C

Air removed after compression never reaches the turbine, so the flow the components were matched for is altered. Performance figures are quoted with the offtakes stated precisely because the operating point itself shifts.

Answer: D

Correct answer: Increasing the rate of acceleration

A faster acceleration adds fuel more quickly relative to the rise in flow, so the working line climbs further towards the surge line - it consumes margin rather than creating it. The other three all improve matters: opening either nozzle lowers the working line, and reducing bleed restores the flow balance between compressor and turbine.