Definition
The order in which the voltage peaks of a three-phase alternating current system reach their maximum value. In aircraft AC systems, the standard phase sequence is A-B-C, meaning phase A peaks first, followed by phase B, then phase C, with each phase offset by 120 electrical degrees.
Plain English
The order in which the three power lines of a three-phase electrical system take their turn reaching peak voltage. Getting that order right matters because it controls which way three-phase motors spin.
Context Anchor
Seen in aircraft electrical system maintenance, especially when checking generators, alternators, motors, or wiring that use three-phase power.
Derivation
Phase comes from the Greek phasis, meaning 'appearance' or 'stage' — here it refers to one of the three staggered AC waveforms. Sequence simply means the order they occur in. Together: the order in which each phase makes its appearance at peak voltage.
Why Pilots Care
If phase sequence is wired incorrectly, three-phase motors will run backwards — fans, pumps, and other rotating equipment will turn the wrong direction. Generators being paralleled onto a bus must share the same phase sequence or severe electrical damage results.
Grounding Statement
Picture three electrical peaks arriving one after another; phase sequence names which peak arrives first, second, and third.
Intuition Check
Phase sequence does not mean the order of maintenance steps or phases of a flight. Here it means the order of electrical phases in an alternating-current system.
Example Sentence 1
Before connecting the replacement generator to the bus, the technician verified the phase sequence matched A-B-C to prevent motors from running in reverse.
Example Sentence 2
Swapping any two leads on a three-phase connection reverses the phase sequence and can damage equipment.