FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsHard
During cruise flight, a pilot notices the ammeter needle pegged toward a full-scale discharge indication and the low-voltage warning light illuminated. What is the most likely cause and appropriate initial action?
- AThe battery is overcharging; the pilot should turn off the master switch immediately
- BThe pitot heat has failed; the pilot should divert to warmer altitudes
- CThe vacuum pump has failed; the pilot should switch to the standby attitude indicator
- DThe alternator has failed; the pilot should reduce electrical loads and monitor battery power
Show answer & explanationAnswer & explanation
Correct answer: D. The alternator has failed; the pilot should reduce electrical loads and monitor battery power
A full-scale discharge reading combined with a low-voltage light indicates the alternator (or generator) is no longer supplying the electrical system, leaving the battery as the sole power source. The pilot should reduce nonessential electrical loads to conserve battery power for essential equipment like radios and flaps, and plan to land as soon as practical.
Why the other options are wrong
- A. Overcharging would show excessive charge, not discharge, and turning off the master would eliminate all electrical power, including essential equipment.
- B. Pitot heat failure would not cause a discharge reading and has nothing to do with altitude or temperature diversion.
- C. The vacuum system is unrelated to electrical or battery indications; a vacuum failure would affect gyro instruments, not the ammeter.
Alternator Failure Indications
A full discharge on the ammeter plus a low-voltage annunciator indicates the alternator has stopped charging, leaving only battery power available.
- Ammeter shows current flow to/from battery
- Discharge = battery supplying all loads, no alternator input
- Response: shed nonessential loads, monitor battery voltage, land soon
- Overcharge (opposite issue) shows excessive charging current, risking battery damage
Memory trick: Needle stuck on discharge = alternator quit, battery's now the only battery in the game.