Engine Instrumentation Questions and Answers

Answer: C

Correct answer: False, it reads ambient because it measures absolute pressure

The gauge is referenced to vacuum, not to ambient, so with the engine stopped and the manifold open to atmosphere it indicates ambient pressure - some 29 or 30 inches of mercury near sea level, not zero.

That reading is the pre-start serviceability check, and it should agree with the local altimeter setting. Closing the throttle changes nothing, since the manifold still communicates with atmosphere past the butterfly. A gauge reading zero before start is faulty, not correctly indicating an absence of pressure.

Answer: D

Correct answer: It requires an external excitation supply

A thermocouple is self-generating: the Seebeck effect produces a small electromotive force at the junction of two dissimilar metals, so no excitation current is needed. That is one of its practical advantages, and it distinguishes it from a resistance thermometer, which must have a current passed through it to be read.

The other three statements are true. The output depends on the temperature difference between hot and cold junctions, the device generates its own voltage, and cold junction compensation is required to turn that difference into an absolute reading.

Answer: A

Correct answer: No, it is a chosen datum

The 100 per cent point is a reference speed selected by the manufacturer for convenience, typically near maximum continuous, not the maximum permitted. Many engines are certificated to exceed it, and take-off ratings above 100 per cent are common.

The limit is whatever the engine's own red line marks, which may lie above or below the datum. The percentage scale exists so that one instrument and one familiar set of markings can serve engines whose absolute speeds differ widely, and so that the two spools of a twin-shaft engine read on comparable scales.

Answer: B

Correct answer: Reading the pressure ratio more carefully

The suspect indication cannot validate itself. If the probe is blocked, the ratio is corrupted whatever care is taken in reading it, and it may show the target value exactly.

Diagnosis depends on parameters sensed independently of the EPR system: rotor speed, fuel flow and exhaust gas temperature. If all three read low for the setting shown, the setting is wrong and the indication is at fault. That is the general principle - a faulty instrument is exposed by comparison with independent ones, never by closer attention to itself.

Answer: C

Correct answer: False, it is a caution range usable for limited periods

Yellow marks caution, not prohibition. Operation within a yellow arc is permitted with the duration watched, which is exactly what distinguishes it from red.

A red radial line marks a limit not to be exceeded and a red arc a range never to be entered; neither carries any time allowance. Green marks the range in which the engine may be operated continuously. The conditions yellow covers are typically transient ones - high power on take-off, or a temperature acceptable in the climb but not in the cruise.

Answer: D

Correct answer: Mass is easier to measure than volume

Mass is in fact the harder measurement. A volumetric flowmeter is a simple turbine or positive-displacement device, while mass flow requires either a density measurement to correct the volume or a Coriolis-type meter. Convenience is not the reason.

The other three are. Energy content goes with mass, density changes with fuel temperature so a volume means different things at different times, and indicating mass makes range and endurance calculations independent of that temperature - and directly comparable with the fuel loaded, which is weighed.

Answer: A

Correct answer: No, a blocked cooler or a hot day raises it without any loss

Oil temperature responds to the balance between heat rejected into the oil and heat removed from it, so it rises for several reasons that have nothing to do with quantity - a restricted or blocked cooler, high ambient temperature, sustained high power, or a bypassing thermostatic valve.

Diagnosis needs the pressure indication with it. Rising temperature together with falling pressure points to a loss of quantity, since less oil sustains neither. Rising temperature with pressure normal points instead to the cooling side of the system.

Answer: B

Correct answer: Rotor speed continues rising normally to idle

That is precisely what a hung start is not. The engine lights but stalls at some sub-idle speed, unable to accelerate further, so rotor speed stops rising short of idle - the defining symptom.

The other three are all true of it. Exhaust gas temperature climbs steadily, because fuel continues to burn while the airflow needed to carry the heat away is insufficient at the low speed. The engine fails to reach idle. And cutting the fuel is the correct response, before the temperature reaches its limit and turns a hung start into an overtemperature as well.

Answer: C

Correct answer: False, power is torque times shaft speed

Power is the product of the two:

\[ P = T\omega \]

so torque alone determines power only when the speed is held constant. On a turboprop that condition is normally met, because the propeller governor holds rpm at a selected value, and torque then serves as a direct power indication. Change the rpm selection and the same torque represents different power. Feathering the propeller is irrelevant to the relation, and power certainly does not depend on rpm alone.

Answer: D

Correct answer: Exhaust temperature rising before any fuel flow

Temperature can only rise once fuel is burning, so a rise before fuel flow is indicated has no source. In a normal start the sequence is rotation first, then fuel and ignition, then light-up and a temperature rise.

The other three are all expected. The starter turns the engine and rotor speed rises; exhaust temperature peaks shortly after light-up and then settles as airflow increases with speed; and oil pressure rises with rotor speed as the engine-driven pump comes up. A temperature rise without fuel flow would point to an instrumentation fault or a residual condition rather than a start.