Engine Instrumentation Questions and Answers

Answer: C

Correct answer: 5.9 inches

The depression is the amount by which manifold pressure lies below ambient:

\[ 29.9 - 24 = 5.9\ \text{inches of mercury} \]

It arises because the throttle butterfly restricts the induction flow, so the manifold sits below atmospheric at any setting short of fully open. At full throttle on a normally aspirated engine the depression falls nearly to zero and manifold pressure approaches ambient, which is why maximum available manifold pressure falls steadily with altitude unless the engine is supercharged.

Answer: D

Correct answer: A factor of three

The increase is expressed as a ratio:

\[ \frac{3.6}{1.2} = 3 \]

What matters operationally is not the absolute number but the trend. Vibration levels differ between individual engines of the same type, so a single reading means little; a threefold rise over ten flights with nothing else changed points clearly to a developing rotor imbalance - blade damage, a shifted balance weight, or bearing wear. Trend monitoring exists precisely to catch such a change while it is still small.

Answer: A

Correct answer: Rises by 25 per cent

Shaft power is \( P = T\omega \). Rotational speed is held at 100 per cent, so \( \omega \) is constant and power is directly proportional to torque:

\[ \frac{75 - 60}{60} \times 100 = 25\% \]

The trap is to read the fifteen-point rise as 15 per cent; it is fifteen points on a base of sixty, which is a quarter. This proportionality is exactly why torque serves as the turboprop's power indication - with the propeller governing at constant rpm, torque and power move together.

Answer: B

Correct answer: 6800 kg

Fuel remaining is what was loaded less what has been used:

\[ 11\,000 - 4200 = 6800\ \mathrm{kg} \]

In practice the figure is cross-checked rather than taken alone: fuel used is derived by integrating flow, while contents gauges measure what is in the tanks, and the two are compared. A growing discrepancy between them is one of the standard indications of a fuel leak, which is why both are displayed and why neither is relied on by itself.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. The gauge is referenced to vacuum rather than to ambient, so with the engine stopped and the manifold open to atmosphere through the induction system it reads whatever the ambient pressure happens to be.

That makes a useful pre-start check: the indication should agree with the local altimeter setting. A gauge reading zero or well away from ambient before start is faulty. Once running, the throttle restriction brings the manifold below ambient, and the difference is the depression.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the reason for the first. There is no practical way to measure the thrust an engine is producing in flight, so a parameter closely related to it must be used instead.

Engine pressure ratio serves because the pressure rise from compressor inlet to turbine discharge tracks the momentum change the engine imparts to the gas. Fan rotor speed is the alternative on engines without EPR sensing. Both are proxies, which is why an EPR probe blocked by ice can cause a crew to set a thrust quite different from the one intended.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. Temperature around the gas path annulus varies considerably, from combustor pattern factor, from burner-to-burner differences and from local cooling flows, so a single probe would report one point rather than the condition of the turbine.

Several thermocouples spaced around the annulus and wired in parallel give an average, since the parallel connection averages their electromotive forces. That averaging is also why the failure of one probe shifts the indication rather than removing it - a drift that is easily mistaken for a change in engine condition.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the reason. Turbine inlet temperature is the parameter that actually limits the engine, but gas there is hotter than any practical probe would survive for long.

So it is inferred instead: temperature is measured further downstream, at the exhaust or between turbine stages, where a thermocouple can live, and the relationship to inlet temperature is established during development. That is why the cockpit indication is exhaust or turbine gas temperature rather than turbine inlet temperature, and why its limits are set against the hotter station they represent.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is precisely why the first is done. Fuel density falls as temperature rises, so a litre delivered warm carries less fuel than a litre delivered cold; but the energy released per kilogram is essentially constant.

Indicating mass flow therefore gives a quantity that means the same thing whatever the fuel temperature, and it makes fuel used directly comparable with the fuel loaded, which is weighed. Volumetric indication would require a density correction at every reading to be useful.

Answer: D

Correct answer: Statement I is incorrect but Statement II is correct

The second statement is right: a red radial line marks a limiting value that must not be exceeded, and any exceedance is recorded and reported.

The first misstates yellow. A yellow arc is a caution range, which may be used for limited periods - it is not prohibited. That distinction is the whole point of having two colours: red admits of no time allowance at all, while yellow permits operation with the duration watched. Green marks the range in which the engine may be operated continuously.