A pilot experiences a complete electrical system failure during night flight, including the alternator and battery. Which of the following instruments would become inoperative?
- ACommunication radio and navigation lights.
- BMagnetic compass and vertical speed indicator.
- CAttitude indicator and heading indicator.
- DAirspeed indicator and altimeter.
Show answer & explanationAnswer & explanation
Correct answer: A. Communication radio and navigation lights.
A complete electrical system failure means no power from either the alternator or battery. Communication radios and navigation lights are electrically powered and would cease to function. The airspeed indicator, altimeter, VSI, and magnetic compass are all pitot-static or self-contained mechanical instruments that do not require electrical power. The attitude indicator and heading indicator are typically vacuum-driven, though some newer aircraft have electrically driven versions; however, the most universally affected items by a complete electrical failure are radios and lights.
Why the other options are wrong
- B. Magnetic compass is a self-contained mechanical instrument, and the VSI is a pitot-static instrument, neither requiring electrical power.
- C. Attitude and heading indicators are typically vacuum-driven (gyroscopic) in light aircraft, not solely electrical, although some modern aircraft use electrical gyros. This is not the *most* definitive answer for *all* light aircraft.
- D. Airspeed indicator and altimeter are pitot-static instruments and are mechanically operated, not electrically dependent.
Electrical System Failure
The loss of electrical power from both the alternator (or generator) and the battery, rendering all electrical components inoperative.
- Affects electrically powered systems only.
- Pitot-static instruments and magnetic compass are unaffected.
- Gyroscopic instruments may be vacuum or electrically powered.
Memory trick: Electric, Vacuum, Pitot-Static are the Three Instrument Power Types.