Engine Instrumentation Questions and Answers

Answer: A

Percentage rpm lets one scale serve engines of very different physical speeds, and 100 per cent is a chosen datum rather than a maximum. Many engines are certified to run briefly above 100 per cent, which surprises those reading the scale as a limit.

Answer: B

Correct answer: Absolute pressure in the manifold

Manifold absolute pressure is exactly what the name says: the absolute pressure of the charge in the induction manifold, downstream of the throttle. Being absolute, it is referenced to vacuum rather than to ambient, which is why the gauge reads ambient pressure with the engine stopped rather than zero.

That reading is a useful serviceability check before start. In flight the quantity matters because it fixes the mass of charge each cylinder receives, and therefore, with rpm, the power the engine is developing. Compression pressure and exhaust back pressure are quite separate quantities, and no manifold gauge measures a difference across the throttle.

Answer: C

Because the gauge measures absolute pressure, a stationary engine leaves the manifold open to ambient. A reading far from local pressure at rest is one of the simplest pre-flight checks that the instrument is serviceable.

Answer: D

It correlates closely with thrust, which cannot be measured directly in flight, so it serves as the thrust setting parameter on many engines. Blockage or icing of either pressure probe corrupts the ratio and therefore the thrust actually set.

Answer: A

Several thermocouples wired in parallel give an average around the annulus, which matters because the temperature is far from uniform. The chromel-alumel junction generates its own small voltage and so needs no external excitation.

Answer: B

Correct answer: The temperature difference between the hot and cold junctions

A thermocouple is a junction of two dissimilar metals, and the Seebeck effect generates an electromotive force proportional to the temperature difference between that junction and the reference junction at the instrument.

It follows that the absolute temperature of the hot junction cannot be known from the output alone - the cold junction temperature must be known too, which is why cold junction compensation is built into every installation. Note also that the device is self-generating: it needs no excitation current, unlike a resistance thermometer, which is measured by passing a current through it.

Answer: C

Conditions immediately downstream of the combustor would destroy a thermocouple quickly, so the measurement is taken further downstream and related back by calibration. The limiting temperature still refers to the turbine, since that is what the limit protects.

Answer: D

Excessive head temperature accompanies detonation and leads to valve and piston damage. It is managed with mixture, cowl flaps and airspeed rather than by reducing power alone, which is why the gauge informs several different actions.

Answer: A

Correct answer: Leads rapidly to bearing failure and engine destruction

The oil film is what keeps the journal off the bearing shell. Lose the pressure that maintains it and metal contacts metal at full rotational speed, so the bearing overheats and fails in seconds to minutes rather than in hours.

That is why oil pressure is a red-line parameter demanding immediate action, and why it is watched with oil temperature: a falling pressure together with a rising temperature points to a loss of quantity, since less oil is carrying the same heat. Oil pressure has no bearing on fuel consumption, and a high indication does not signify excess quantity.

Answer: B

A reduced quantity of oil both circulates less readily and carries away less heat, so the two indications move in opposite directions together. Either reading alone is ambiguous, which is why the pair is read as a combination.