Engine Instrumentation Questions and Answers

Answer: A

A blade shedding material or a bearing degrading changes the vibration signature before anything else responds. Because baseline levels differ between engines, it is the threefold rise against this engine's own history that matters.

Answer: B

Each action increases cooling: a richer mixture carries heat away, cowl flaps raise the airflow and a shallower climb raises the speed through the cooling fins. Sustained high head temperature invites detonation and cylinder damage.

Answer: C

The same loop supports two thresholds, so an overheat can appear without a fire warning. A bleed duct leak is the classic cause and is a genuine defect, even though it is not the fire the higher threshold would signal.

Answer: D

Correct answer: absolute

An absolute pressure gauge is referenced to vacuum, so it reads the true pressure of the charge rather than its excess over ambient. With the engine stopped the manifold is open to atmosphere through the induction system, so the gauge indicates ambient pressure.

That gives a useful pre-start check - the reading should agree with the local altimeter setting. A gauge measuring differential or gauge pressure would read zero instead, and would also lose its meaning as altitude changed, since the same manifold pressure at sea level and at height represents the same charge density either way only on an absolute scale.

Answer: A

Correct answer: compressor inlet

Engine pressure ratio compares the pressure at the back of the engine with that at the front:

\[ \mathrm{EPR} = \frac{P_{t7}}{P_{t2}} \]

where \( P_{t7} \) is turbine discharge and \( P_{t2} \) compressor inlet total pressure. It is used as a thrust-setting parameter because thrust itself cannot be measured in flight, and the overall pressure rise across the engine tracks the momentum change it imparts to the gas. Both probes matter: icing of either corrupts the ratio and lets a crew set a thrust other than the one intended.

Answer: B

Cold junction compensation inside the indicator adds the reference temperature back, turning that difference into an absolute reading. Any uncompensated dissimilar metal junction in the circuit adds an error of its own.

Answer: C

Correct answer: yellow

Yellow marks a caution range: operation there is permitted but should be brief, and the duration is watched.

The distinction from red is what the colour code exists to convey. Red admits of no time allowance at all - a red radial line marks a limit that must not be exceeded, and a red arc a range never to be entered. Green marks the range in which the engine may be operated continuously. Blue, where used, marks a specific setting such as best-power mixture rather than a range.

Answer: D

Correct answer: mass

Turbine fuel flow is indicated in kilograms or pounds per hour, because the energy the engine can extract depends on the mass of fuel burnt, not on its volume.

Fuel density falls appreciably as temperature rises, so a volumetric reading would mean different things at different fuel temperatures and would need correcting at every reading. Mass flow avoids the problem and makes fuel used directly comparable with the fuel loaded, which is also weighed - so the two can be cross-checked in flight to detect a leak.

Answer: A

Correct answer: Rotor speed together with exhaust temperature

Neither indication settles it alone. Exhaust temperature rises in both cases - steadily in the hung start, briefly beyond the limit in the hot start - and rotor speed rises in the hot start but also rises before stalling in the hung one.

Taken together they separate cleanly. A hung start is rotor speed stopping below idle with temperature climbing, the engine lit but unable to accelerate. A hot start is rotor speed rising normally while temperature exceeds its limit. The distinction matters because both call for the fuel to be cut, but for different reasons and with different subsequent inspections.

Answer: B

Correct answer: Yellow is time limited while red is forbidden entirely

The table states it directly: yellow permits operation for limited periods, red permits none at all.

That is the whole purpose of having two markings. A yellow arc covers conditions that are acceptable transiently - high power on take-off, a temperature tolerable in the climb but not in the cruise - and the pilot manages the duration. A red radial line or red arc marks a value or band that is never to be exceeded or entered, and an exceedance must be recorded and reported for maintenance action rather than merely noted.