FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsHard
A technician is troubleshooting a cylinder head temperature (CHT) indicating system that uses a resistance-bulb temperature sensor wired into one leg of a Wheatstone bridge circuit. The technician finds a corroded connector adding extra resistance in the sensor leg of the bridge. What effect will this have on the cockpit indication?
- AThe gauge will read higher than the actual cylinder head temperature
- BThe gauge will read lower than the actual cylinder head temperature
- CThe gauge will fluctuate randomly but average correctly over time
- DThe gauge will show no reading at all until the connector is repaired
Show answer & explanationAnswer & explanation
Correct answer: A. The gauge will read higher than the actual cylinder head temperature
A resistance-bulb sensor increases resistance as temperature rises. Added resistance from corrosion mimics an increase in sensor resistance, unbalancing the Wheatstone bridge in the same direction as a real temperature rise, causing the indicator to read falsely high.
Why the other options are wrong
- B. Added resistance mimics increased temperature, not decreased, so this is backwards.
- C. Corrosion resistance is a steady offset, not a random fluctuation.
- D. A fully open circuit (not just added resistance) would typically be needed to lose the reading entirely.
Wheatstone Bridge Temperature Indicator
A bridge circuit using a resistance-bulb (RTD) sensor in one leg; as sensor resistance changes with temperature, the bridge becomes unbalanced and deflects a galvanometer-type gauge proportional to temperature.
- RTD resistance increases with temperature
- Bridge imbalance current drives the temperature gauge needle
- Added resistance from corrosion or bad connections mimics higher temperature reading
Memory trick: More resistance, bridge tips toward 'hot.'