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Hazard of high resistance in the EGT circuit

Aviation Mechanic Powerplant (AMP) practice question 8 of 13 on Turbine Engines. ACS element AM.III.B.K10 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONWhy is a turbine engine thermocouple circuit whose resistance is out of tolerance on the high side considered dangerous?

  1. AThe indicator reads low while the engine runs hot
  2. BThe indicator reads high while the engine runs cool
  3. CThe indicator freezes at the last reading shown
Show the answer and explanation

Correct answer: A. The indicator reads low while the engine runs hot

  • A — correct A high-resistance condition gives a low indication on the EGT indicator, so the engine can be operating with excess temperature while the gauge conceals it.
  • B — incorrect High circuit resistance drives the indication down, not up, and a high indication would prompt the crew to reduce power rather than hide a problem.
  • C — incorrect Excess resistance changes the amount of current flow and therefore the displayed temperature; it does not lock the pointer in place.

BACKGROUND

Exhaust gas temperature is one of the most important factors affecting turbine engine life, and excess EGT of only a few degrees reduces turbine component life. Because the operating temperature is set using the EGT gauge, that gauge must be accurate. The resistance of the thermocouple harness is held to very close tolerances, since a change in resistance changes the amount of current flow in the circuit and gives erroneous temperature readings. A high-resistance condition produces a low indication on the aircraft EGT indicator, which is dangerous because the engine may be running with excess temperature while the gauge reads low. An analyzer's resistance and insulation check circuits are used to isolate this kind of error in the aircraft system.

STUDY TIP

Reason from the circuit: more resistance means less current, and less current means a lower displayed temperature. The danger is always the error that hides an overtemperature, not one that exaggerates it.

KEY POINTS

  • EGT harness resistance is held to close tolerances because it changes circuit current.
  • High resistance makes the EGT indicator read lower than the true temperature.
  • The engine can then be operating over temperature with no indication to the crew.

SOURCES

  • FAA-H-8083-32B ch10 "Resistance of Circuit Out of Tolerance" (Troubleshooting EGT System)
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MORE ON TURBINE ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.