Engine InstrumentationNavigation — Instrumentation — DGCA CPL practice questions

All 6 questions — Engine Instrumentation

Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.

1. Q1. Which of the following are classified as "Performance Indicators" on a turbofan engine?

   • A.EGT and N2
   • B.Oil pressure and oil temperature
   • C.EPR and N1 (fan speed)✓
   • D.Vibration level and fuel flow

   Why: Performance Indicators measure thrust output. The two primary performance indicators on turbofan engines are EPR (Engine Pressure Ratio) and N1 (fan speed). The text states: "N1 and EPR are the parameters used to measure thrust in turbojets." See Section 1 . — For turboprops, the performance indicator is the Torquemeter (since turboprops produce power/torque, not direct jet thrust). Always distinguish between the engine type when identifying the primary thrust/power instrument.

2. Q2. During the take-off roll, an EPR gauge indicates a reading lower than the value set at brake release. The most likely cause is:

   • A.A genuine loss of engine thrust — throttles should be advanced to restore EPR
   • B.An apparent drop caused by ram rise in engine intake pressure with increasing airspeed✓
   • C.An instrument malfunction requiring abort of the take-off
   • D.Atmospheric pressure change during the take-off roll

   Why: As the aircraft accelerates on the runway, forward airspeed causes a relative increase in engine intake pressure (ram effect). Jet pipe pressure is initially unaffected. Therefore EPR (jet pipe ÷ intake) falls — this is an apparent drop only. Standard procedure: EPR must be set before 60 kt — no increase in power after that speed except in emergency. See Section 3 . — This is one of the most important exam topics in engine instrumentation. The trap answer is (a). The safe action is to set EPR before 60 kt and monitor other parameters (N1, N2, EGT) for trend. Never advance throttles based on…

3. Q3. The thermocouple temperature system used in gas turbine engines requires:

   • A.An external power supply to generate the EMF at the hot junction
   • B.No external power for temperature indication; the thermocouple generates its own EMF✓
   • C.AC power at 115V/400Hz to energise the measuring circuit
   • D.28V DC power supply to the hot junction probes

   Why: The source text states: "Note: The system requires no power supply to indicate temperature." The hot junction (in the gas stream) generates a millivoltage proportional to temperature via the Seebeck effect — no external power is needed for this. However, if the signal must feed a temperature limiting system (which requires amplification), the aircraft's electrical system is then used. See Section 5 . — This is a frequently tested fact. Thermocouples are self-generating (Seebeck effect). The millivoltmeter/galvanometer in the cockpit measures the tiny voltage — no power input required. Power…

4. Q4. In a capacitance fuel gauge system, what is the effect of water contamination in the fuel tank?

   • A.The indicator reads zero, indicating an empty tank
   • B.The system automatically compensates and indicates the correct fuel quantity
   • C.The capacitors are effectively shorted, driving the indicator beyond full scale✓
   • D.The indicator fluctuates rapidly between empty and full

   Why: Water has a relative permittivity (Er) of ~81 (distilled) or even higher (impure), compared to kerosene at 2.10. Water's extremely high dielectric value effectively shorts the capacitors in the sensing units. The resulting anomalously high capacitance drives the indicator beyond full scale — a dangerously misleading overread. See Section 8 . — Water contamination causes an over-reading (beyond full scale) — this is a critical safety fact. Fuel drains and checks before flight detect and drain accumulated water from tank sumps. This is a mandatory pre-flight check.

5. Q5. Which pressure sensing element is most suitable for measuring high pressures such as engine oil pressure?

   • A.Aneroid capsule
   • B.Corrugated diaphragm
   • C.Bellows
   • D.Bourdon tube✓

   Why: The Bourdon tube is specifically described as "normally associated with higher pressures such as engine oil pressure." It is a C-shaped metal tube with an elliptical cross-section; applied pressure tends to straighten the tube, driving an indicator pointer. It is manufactured to handle high pressure ranges. See Section 6 . — Remember: Diaphragm/Capsule = Low. Bourdon = High. Bellows = Variable. This is a classic DGCA pattern-matching question. The Bourdon tube is the oldest pressure sensing element and the primary one for high-pressure systems.

6. Q6. Total Air Temperature (TAT) is related to Static Air Temperature (SAT) as follows:

   • A.TAT = SAT − ram rise
   • B.TAT = SAT (they are identical in level flight)
   • C.TAT = SAT + full ram rise (approximately 100% recovery factor)✓
   • D.TAT = SAT + 80% of ram rise at all times

   Why: The text states: "For use at high Mach numbers, Total Air Temperature (TAT) is measured. The air is brought to rest (or nearly so) without addition or removal of heat. The temperature probes used have a high recovery factor (approximately 100%). TAT is equal to SAT + Ram Rise." See Section 5 . — TAT = SAT + ram rise. For OAT (Outside Air Temperature) use on an aircraft cruise display, TAT is measured and then corrected to SAT using Mach number. The ADC performs this correction automatically. Below Mach 0.2, SAT ≈ RAT ≈ TAT.