Engine Instrumentation Questions and Answers

Answer: C

Correct answer: Mass per unit time

Turbine fuel flow is indicated as a mass flow, in kilograms or pounds per hour, because it is mass rather than volume that determines the energy delivered to the engine.

Fuel density falls appreciably as temperature rises, so a given volume carries different energy at different temperatures and a volumetric reading would mislead. Mass flow avoids that entirely, and it makes fuel used directly comparable with the fuel loaded, which is also weighed. Nozzle pressure and tank contents are different quantities and neither states a rate at all.

Answer: D

Correct answer: Shaft torque

A torquemeter measures the twisting moment the engine applies to the propeller shaft, usually from the axial displacement of a helical gear against a hydraulic piston, or from the strain in the shaft itself.

Torque is the turboprop's thrust-setting parameter because thrust cannot be measured directly and, with rotational speed, torque gives shaft power from \( P = T\omega \). Since a turboprop is normally flown at constant rpm with the propeller governing, torque alone then varies with power, which is what makes it a workable single indication for the pilot.

Answer: A

A blade shedding material or a bearing degrading changes the vibration signature before any other instrument responds. Because a rising trend matters more than an absolute value, the parameter is usually monitored over time rather than judged on one reading.

Answer: B

Correct answer: A change in resistance or capacitance of the sensing element when heated

A continuous loop detector is a length of sensing element routed around the zone to be protected. Heating any part of it changes the electrical property of its core - resistance falls in a thermistor-type loop, capacitance changes in others - and the control unit reads that change as a fire signal.

The advantage over discrete detectors is coverage: the whole loop is sensitive along its length, so no hot spot can occur between sensors. It responds to heat rather than smoke, which suits an engine nacelle where airflow would disperse smoke before an optical detector saw it.

Answer: C

Correct answer: A limit that must not be exceeded

A red radial line or red arc marks a limiting value: exceeding it risks damage, and any exceedance must be recorded and reported for maintenance action.

The colour code is consistent across engine instruments. Green marks the normal operating range, yellow a caution range acceptable only for limited periods, and red the limit itself. Because red admits of no time allowance at all, it is quite different from yellow, which is the trap in most questions on the subject.

Answer: D

Correct answer: A caution range acceptable only for limited periods

Yellow is a caution marking, not a prohibition. Operation within a yellow arc is permitted but should be brief, and it is often associated with transient conditions such as high power during take-off or a temperature that is acceptable while climbing but not in the cruise.

The contrast with red is the point. Red marks a value that must not be exceeded at all, whereas yellow marks a range that may be used with the duration watched. Green is the normal continuous range, in which the engine may be operated indefinitely.

Answer: A

Correct answer: The sensing element stays at the engine

In a remote indicating system the sensor remains at the engine and sends an electrical signal to a display in the cockpit. A direct reading instrument instead brings the measured medium itself forward - oil or fuel through a pipe, or a capillary filled with fluid.

The gain is threefold: no flammable fluid enters the cockpit, no long capillary introduces lag or breakage risk, and an electrical signal can be recorded, processed and displayed on a shared screen. A remote system still needs a sensor and is still calibrated; what changes is what travels along the wire.

Answer: B

Normal values are presented compactly while an abnormal parameter is brought forward with its associated checklist. The design principle is dark cockpit, where an absence of indication is itself the message that all is well.

Answer: C

Damage from an overtemperature depends on both magnitude and duration, so the recorded trace determines what inspection is required. Relying on a crew report alone would lose the duration that governs the engineering decision.

Answer: D

Correct answer: Manifold pressure and rpm together

Power output depends on how much charge reaches the cylinders and how often they fire. Manifold pressure fixes the mass of charge per induction stroke, and rpm fixes the number of strokes per minute, so neither alone determines power.

This is why a piston engine with a constant-speed propeller is set with two controls and read on two gauges, and why the power charts are entered with both values together with altitude and temperature. The same manifold pressure at 2100 and at 2400 rpm gives quite different power. Oil pressure and cylinder head temperature are health parameters, not power settings.