Definition
In a three-phase alternating current (AC) electrical system, the order in which the three voltage phases reach their peak values. The standard sequence is A-B-C, meaning phase A peaks first, followed by phase B, then phase C. This order determines the direction of rotation of three-phase AC motors and the correct paralleling of AC generators.
Plain English
In an aircraft's three-phase AC power system, this is the order the three power lines take turns peaking. Get the order right and motors spin the right way; get it wrong and they spin backward.
Context Anchor
Seen in aircraft electrical maintenance when checking three-phase generator output, external power connections, or equipment driven by three-phase motors.
Derivation
Phase' comes from the Greek 'phasis,' meaning 'appearance' or 'stage.' In electrical work, it refers to the timing stage of a waveform. 'Sequence' and 'rotation' both describe the order in which those stages occur, one after another, around the cycle.
Why Pilots Care
Incorrect phase sequence can cause motors to run backward, leading to equipment damage or loss of critical systems such as fuel pumps or hydraulic pumps.
Analogy
Picture three people taking turns raising their hands in order: A, then B, then C. If the order changes to A, then C, then B, the pattern is reversed.
Intuition Check
Do not read “rotation” here as only a spinning shaft or propeller. Here it means the repeating order of the electrical phases.
Example Sentence 1
Before connecting the external power cart, the technician verified phase sequence to make sure the aircraft's three-phase motors would rotate in the correct direction.
Example Sentence 2
Reversing any two leads changed the phase rotation and reversed the pump direction.