Engine Performance and Matching Questions and Answers

Answer: C

Correct answer: Blade Mach number follows temperature

The parameter that must be preserved for aerodynamic similarity is the blade Mach number, which is blade speed divided by the local speed of sound. The speed of sound depends on temperature alone, so dividing the physical speed by the square root of the temperature ratio is exactly what holds Mach number constant. Pressure does not enter, which is why the speed correction is the simplest of the four.

Answer: D

Correct answer: The control enforces whichever binds first

Different limits bind at different conditions - torque or pressure on a cold day when the air is dense, temperature on a hot one, speed or temperature at altitude, and surge margin during a transient. The control system watches all of them simultaneously and holds the engine back at whichever is reached first, so no single limit is the operative one across the envelope.

Answer: A

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

Answer: B

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. Because the nozzle is choked, its throat area alone fixes the corrected flow the compressor must supply at each pressure ratio, so a small area change shifts the entire working line. That makes it both the most effective and the most economical remedy for an inadequate margin found during development.

Answer: C

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

Both are true and the reason gives the justification. Once speed and flow are expressed in corrected form, sea level static, a hot day take off and a cruise at 11 000 m all fall on the same characteristic, because the corrections remove the dependence on inlet temperature and pressure. One map therefore describes the compressor everywhere in the envelope.

Answer: D

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

Both are true and the reason names the two hazards that make the schedules necessary. Adding fuel too fast raises the working line into the surge line; cutting it too fast leaves the combustor too lean to sustain a flame, and the engine goes out. The control therefore bounds the rate of change in both directions rather than passing the throttle demand straight to the fuel valve.

Answer: A

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. Worn seals leak and growing tip clearances let flow escape past the blades, so component efficiencies fall and more heat must be added to produce the same work. The temperature at which that same thrust is achieved therefore rises, which is why the exhaust gas temperature margin is the standard measure of an engine’s condition.

Answer: B

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 figures must be stated. Bleed air taken for cabin pressurisation and anti-icing never reaches the turbine, so the turbine passes less flow than the compressor delivered and the whole matching shifts. A thrust or fuel consumption figure quoted without saying what bleed was being drawn is therefore not comparable with any other.

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 what makes the technique work. A compressor fault and a turbine fault both raise exhaust gas temperature, but they change fuel flow and the two shaft speeds in different combinations. Gas path analysis compares the measured pattern of deviations against those signatures, so the faulty module can often be identified before the engine is opened.

Answer: D

The reason is true but it refutes the assertion. The engine runs where the three conditions intersect, which is generally not where any individual component would be at its best, and that compromise is what matching means.