Dual-loop automatic self-interrogation result

Aviation Mechanic Powerplant (AMP) practice question 4 of 7 on Engine Fire Protection Systems. ACS element AM.III.E.K2 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a dual-loop fire detection system with automatic self-interrogation, the control circuit of one loop signals fire and the self-interrogation circuit then tests the other loop and finds it operative. What does the system do?

AOutputs a fire warning based on the one loop alone
BSuppresses the fire signal from the single loop
CDisconnects both loops and shows a fault message

Show the answer and explanation

Correct answer: B. Suppresses the fire signal from the single loop

A — incorrect A fire signal is output from a single loop only when the interrogation finds the other loop inoperative.
B — correct If the other loop tests operative, the circuit suppresses the fire signal, because an operative loop would also have signaled if a fire actually existed.
C — incorrect Self-interrogation exists to eliminate the fault indication and keep detection available; it does not shut the system down.

BACKGROUND

Dual-loop systems are essentially two complete fire detection systems whose outputs are connected so that both must signal before a fire warning is given. This "AND" logic greatly increases reliability against false fire warnings. Automatic self-interrogation performs the loop switching and decision making that would otherwise fall to the flight crew. When one loop signals fire, the circuit tests the other loop: if that loop is operative it would have signaled too, so the lone signal is treated as false and suppressed; if that loop tests inoperative, the circuit outputs a fire signal. The interrogation and decision take place in milliseconds, so no delay occurs if a fire really exists.

STUDY TIP

Work the logic in two steps: first ask whether the other loop is healthy, then decide whether the lone signal is believable. A healthy silent loop means no fire, so the signal is suppressed.

KEY POINTS

Dual-loop systems use AND logic: normally both loops must signal to give a fire warning.
Self-interrogation tests the second loop when only one loop signals fire.
An operative second loop causes the lone fire signal to be suppressed; an inoperative one lets it through.

SOURCES

sources/amp-powerplant/ch09.txt "Dual-Loop Systems"
sources/amp-powerplant/ch09.txt "Automatic Self-Interrogation"

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