FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsEasy
An outside air temperature (OAT) gauge is installed with a probe extending through the aircraft skin into the airstream. The gauge requires no electrical power source to operate. Which physical principle allows this gauge to indicate temperature?
- AA capacitor's dielectric constant changes with temperature
- BA Ni-Cd battery inside the probe powers a digital display
- CA thermocouple junction generates a voltage proportional to temperature
- DA bimetallic strip inside the probe bends with temperature changes, moving a pointer
Show answer & explanationAnswer & explanation
Correct answer: D. A bimetallic strip inside the probe bends with temperature changes, moving a pointer
Simple, non-electrical OAT gauges use a bimetallic strip that bends as temperature changes, mechanically moving a pointer across a dial. This requires no electrical power, unlike thermocouple-based EGT systems.
Why the other options are wrong
- A. Capacitance change with temperature is not how simple OAT gauges work.
- B. Simple OAT gauges are non-powered mechanical instruments.
- C. Thermocouples generate voltage but require an electrical indicating circuit, typically used for EGT.
Bimetallic OAT Gauge
A non-electrical outside air temperature gauge that uses a bimetallic strip which bends with temperature change to move an indicator needle.
- No electrical power required
- Bimetallic strip made of two metals with different expansion rates
- Common on light aircraft for simple temperature indication
Memory trick: Bimetal bends like a bent thermometer needle