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Thermocouple system and slow overheat

Aviation Mechanic Airframe (AMA) practice question 6 of 9 on Airframe Fire Protection Systems. ACS element AM.II.M.K2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONAn engine gradually overheats during a long climb, yet the thermocouple fire warning system gives no indication. Which characteristic of that system explains the result?

  1. AIt signals only when a short circuit develops in it.
  2. BIt signals only when both junctions heat at equal rates.
  3. CIt signals only on a rapid rate of temperature rise.
Show the answer and explanation

Correct answer: C. It signals only on a rapid rate of temperature rise.

  • A — incorrect A short circuit produces no warning from this system either, so this cannot be what the system waits for.
  • B — incorrect Equal heating of the hot and reference junctions produces no voltage at all, which is the condition in which the system stays silent.
  • C — correct A thermocouple system depends on the rate of temperature rise; during a gradual rise both the hot junction and the reference junction heat at the same rate, so no voltage and no warning is produced.

BACKGROUND

A thermocouple fire warning system consists of a relay box, warning lights, and thermocouples wired into detector, alarm, and test circuits. Each thermocouple joins two dissimilar metals, such as chromel and constantan, at a hot junction exposed to the fire zone, while a reference junction sits in a dead air space between insulation blocks. A voltage appears only when the hot junction heats faster than the reference junction, which is what a fire does and a slow engine overheat does not. When the resulting current exceeds 4 milliamperes, the sensitive relay closes, energizing the slave relay and the warning light. Overheat detectors and rate-of-temperature-rise detectors are listed as separate detection methods, and the fire protection system of most large turbine-engine aircraft incorporates several different methods.

STUDY TIP

Remember the phrase "rate of temperature rise" for thermocouple systems, and contrast it with thermal switch and continuous-loop systems that respond to reaching a set temperature.

KEY POINTS

  • The thermocouple system responds to the rate of temperature rise.
  • A slow overheat or a short circuit produces no warning.
  • Current above 4 milliamperes closes the sensitive relay and lights the warning.

SOURCES

  • sources/ama-airframe/ch17.txt "Thermocouple System"
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MORE ON AIRFRAME FIRE PROTECTION SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.