FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesEasy
A technician needs to determine N1 fan speed for cockpit indication on a turbofan engine. Which component generates the electrical signal used for this indication?
- AA thermocouple mounted in the turbine exhaust duct
- BA torquemeter measuring propeller shaft twist
- CA tachometer generator (speed sensor) geared to or sensing the N1 shaft
- DA pressure transducer sensing compressor discharge pressure
Show answer & explanationAnswer & explanation
Correct answer: C. A tachometer generator (speed sensor) geared to or sensing the N1 shaft
N1 speed is measured by a tachometer generator (often a magnetic speed sensor) coupled to or positioned near the low-pressure (N1) shaft, producing an AC signal whose frequency is proportional to shaft speed for cockpit RPM indication.
Why the other options are wrong
- A. A thermocouple measures temperature (EGT), not rotational speed.
- B. Torquemeters measure shaft torque on turboprops, not turbofan N1 speed.
- D. Pressure transducers measure pressure (e.g., for EPR), not shaft speed.
N1 Tachometer Generator
A speed sensor geared to or positioned near the N1 (fan/low-pressure) shaft that generates an electrical signal proportional to shaft speed for cockpit N1 RPM indication.
- Measures low-pressure spool (N1) speed
- Produces frequency signal proportional to RPM
- Separate sensor exists for N2 (high-pressure spool)
- Distinct from EGT thermocouples and EPR transducers
Memory trick: Tach generator = a tiny speedometer riding the N1 shaft