Definition
An electrical component used in aircraft power systems that performs two functions in sequence: a transformer steps the alternating current (AC) supply down to a lower voltage, and a rectifier then converts that AC into direct current (DC). The output is a stable DC supply used to power DC equipment and to charge the aircraft battery on aircraft whose primary electrical system is AC.
Plain English
A box that takes the high-voltage AC power from the aircraft's generators, lowers the voltage, and turns it into DC so that DC-powered equipment and the battery can use it.
Context Anchor
Seen in aircraft electrical system descriptions, cockpit electrical failure checklists, and maintenance discussions about power for direct-current equipment.
Derivation
The name describes the two stages inside the unit. 'Transformer' (from Latin transformare, 'to change shape') changes the voltage level. 'Rectifier' (from Latin rectus, 'straight' or 'right') 'straightens' the alternating current into one-direction current -- DC. Together, the term tells you exactly what the unit does and in what order.
Why Pilots Care
It supplies the steady DC power needed to charge batteries and run essential cockpit instruments and lights when the engines are driving AC generators.
Analogy
Think of a TRU as the aircraft equivalent of a phone charger: it takes the wall's AC, lowers the voltage, and outputs steady DC for the device to use.
Intuition Check
Do not think of it as a power source by itself. It does not create electricity; it converts electricity supplied by another part of the aircraft.
Example Sentence 1
After the number two TRU failed, the crew followed the checklist to isolate the affected DC bus and continued on the remaining unit.
Example Sentence 2
If the transformer-rectifier unit fails, the battery alone must supply all DC loads until the pilot can land.