FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsEasy
A technician is teaching a trainee about switches used in aircraft electrical systems. The trainee points to a switch with one common terminal that can be connected to either of two other terminals, allowing it to control two separate circuits from a single lever, such as a landing/taxi light switch. This is best described as which type of switch?
- ASingle-pole, single-throw (SPST) switch
- BMomentary-contact push-button switch
- CDouble-pole, double-throw (DPDT) switch
- DSingle-pole, double-throw (SPDT) switch
Show answer & explanationAnswer & explanation
Correct answer: D. Single-pole, double-throw (SPDT) switch
An SPDT switch has one common (pole) terminal that can be thrown to either of two other terminals, allowing selection between two circuits (e.g., on-off-on or two-position selection) from one control point.
Why the other options are wrong
- A. An SPST switch only connects or disconnects one circuit, with no second position.
- B. A momentary switch returns to its original position when released and is not defined by pole/throw count.
- C. A DPDT switch controls two separate poles simultaneously, not a single common terminal.
SPDT Switch
A switch with a single common terminal that can be connected to one of two other terminals, used to select between two circuit paths.
- Pole = number of common terminals
- Throw = number of positions/contacts per pole
- SPDT allows selecting between two circuits from one switch
Memory trick: Pole = paths in, Throw = paths out.