Engine Instrumentation Questions and Answers

Answer: C

Correct answer: An instrument can be wrong while appearing entirely normal

The last row is the awkward case. Engine pressure ratio reads exactly the target value, so nothing about that indication looks wrong - yet rotor speed, fuel flow and exhaust temperature all say the engine is producing less.

A blocked probe corrupts the ratio without any flag, and it need not read zero to do so. The lesson is the opposite of the first option: the secondary parameters are essential precisely because they are independent of the sensing system in question. When one indication disagrees with three that concur, suspect the one.

Answer: D

Correct answer: Manifold pressure and rpm must be set as a pair

Read the table both ways. Holding rpm at 2400 and dropping manifold pressure from 23 to 21 inches takes power from 75 to 65 per cent; holding manifold pressure at 23 and dropping rpm from 2400 to 2200 takes it from 75 to 70 per cent.

Each variable moves power on its own, so neither determines it alone, and the same 23 inches gives 75 or 70 per cent according to the rpm set with it. That is why the power tables are entered with both values, and why a constant-speed installation gives the pilot two levers and two gauges.

Answer: A

Correct answer: With rpm governed constant, power is proportional to torque

Shaft power is \( P = T\omega \). The propeller governor holds rpm at 100 per cent, so \( \omega \) is fixed and power varies with torque alone - which is exactly what the table's relative power column shows, tracking torque point for point.

That proportionality is what makes torque a workable single power indication for the pilot. It is not that torque is easier to measure than rpm, nor that rpm is unmeasurable: both are indicated, and the relation holds only because the governor is keeping one of them constant.

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 precisely why the cross-check is made. A blocked or iced pressure probe corrupts the ratio without any warning, so the instrument can read the target value while the engine produces something quite different.

Rotor speed, fuel flow and exhaust gas temperature are sensed independently of the EPR system, so agreement between them and the setting confirms it, and disagreement exposes the fault. On take-off the consequence of not checking is thrust materially below that intended, discovered only when the acceleration proves poor.

Answer: C

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

Both are true and the reason accounts for the practice. Gas temperature around the annulus varies considerably - from combustor pattern factor, from differences between burners, and from local cooling flows - so a single probe would report one point rather than the state of the turbine.

Several thermocouples spaced around the annulus and wired in parallel average their outputs and give a figure representative of the whole. A consequence worth knowing is that one failed probe shifts the average rather than removing the indication, producing a drift easily mistaken for a change in engine condition.

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 check. Because the gauge is referenced to vacuum, it indicates whatever pressure exists in the manifold - and with the engine stopped that is ambient, reached through the open induction system.

So a reading equal to the local altimeter setting confirms the instrument is working. A differential gauge would read zero in the same conditions, which is why the check works only for an absolute instrument. A manifold gauge reading zero or well away from ambient before start should be treated as faulty.

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 why the annunciation is set at warning level. The oil film is what keeps the journal off the bearing shell; without pressure to maintain it, metal contacts metal at full rotational speed and the bearing fails in minutes or less.

A warning demands immediate action, a caution merely awareness, and the timescale here leaves no room for the latter. It is also why oil pressure is read with oil temperature: falling pressure together with rising temperature points to a loss of quantity rather than to a sensing fault.

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 exactly why mass is used. The heat released per kilogram of fuel is essentially constant, while the mass in a given volume falls as the fuel warms, so a volumetric indication would mean different things at different fuel temperatures.

Indicating mass per hour removes the dependence entirely and makes fuel used directly comparable with the fuel loaded, which is weighed. That comparability matters in flight: a growing discrepancy between fuel used and tank contents is a standard leak indication.

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 the practice. Baseline vibration differs between individual engines of the same type, so an absolute reading carries little information; change is what signifies.

A rising trend appears as soon as an imbalance begins to develop - blade damage, a shifted balance weight, a bearing starting to wear - well before the loss of efficiency shows in exhaust temperature or fuel flow. That early warning is the whole point of trend monitoring, allowing a borescope inspection to be scheduled rather than forced.

Answer: D

Correct answer: A is false but R is true

The reason is correct: the combustor outlet is the hottest station in the gas path, and turbine inlet temperature is the parameter that genuinely limits the engine.

That is precisely why the assertion fails. Gas there is hotter than any practical probe would survive for long, so the temperature is inferred rather than measured: a thermocouple is placed further downstream, at the exhaust or between turbine stages, and the relationship to inlet temperature established during development. The cockpit therefore shows exhaust or turbine gas temperature, with limits set against the hotter station it represents.