In a landing gear warning circuit, an open in which wire will prevent the warning horn from sounding when the landing gear are up and the throttles are retarded?

• A. No. 9.
• B. No. 2.
• C. No. 4.
• D. No. 6.

Answer: (C)

The warning horn is energized only when two conditions are met: the landing gear is up and the throttles are retarded. These conditions are detected by two switches wired in series, so current can reach the horn only if both switches are closed.

An open in the wire that carries the throttle-retard signal to the horn breaks that series path. Even though the gear is up, the throttles being retarded will not complete the circuit to the horn, so no current flows and the horn cannot sound. In this setup, the throttle-retard signal path is the critical leg that, when open, prevents the horn under those conditions.