What Is a Transformer Rectifier Unit?

A transformer rectifier unit — abbreviated TRU — converts alternating current from the building's mains supply into regulated direct current suitable for powering and starting aircraft. The "transformer" component steps the input voltage to the required level; the "rectifier" component converts AC to DC. The result is a high-current, continuous DC power source that can both start engines and supply maintenance power without the capacity limitations of battery-based systems.

TRUs are fixed installations. They are not portable in the way battery start packs are, and they are not intended for ramp or field use. Their natural environment is the hangar, the maintenance facility, and the military flight line — locations where high-current DC power is needed reliably and repeatedly throughout the working day.

When Does a TRU Make Sense?

The decision to invest in a TRU rather than using portable or semi-portable equipment is driven by three factors: volume, diversity, and infrastructure.

Volume

If your facility processes more than 10 aircraft starts per day on a sustained basis, the logistics of charging, rotating, and maintaining multiple battery packs becomes a significant operational overhead. A TRU eliminates this entirely — it delivers power from the mains on demand, without charge cycles or battery degradation.

Aircraft diversity

Facilities that service a wide range of aircraft types — from light piston to medium turboprop to business jet — need a power source that can deliver both low continuous current for avionics work and high peak current for engine starts across the full range. A single TRU rated at 600A continuous and 3,000A peak covers the entire spectrum.

Infrastructure

TRUs require dedicated electrical infrastructure — typically a 3-phase mains supply with adequate capacity, properly rated cabling, and sometimes facility modifications for mounting. This makes them impractical for temporary or remote operations but ideal for established hangars and maintenance facilities where the installation cost is amortized over years of daily use.

How to Specify a TRU

When specifying a TRU for your facility, the critical parameters are continuous output, peak starting capability, input power compatibility, and physical form factor.

Continuous output should be sized to the most power-hungry maintenance task you perform. If you run cabin air conditioning while performing avionics work, you may need 100A or more sustained. If your work is limited to avionics testing, 50A may suffice. As with portable GPUs, size with 20–30% headroom above your calculated maximum draw.

Peak starting current should be matched to the largest aircraft you service. If the largest aircraft in your fleet needs 2,000A to start, specify a TRU rated for at least 2,400A peak. The cost difference between a 2,000A and a 3,000A TRU is typically modest relative to the total installation cost, so sizing generously for future fleet expansion is often prudent.

Input power must match your facility's electrical supply. Most TRUs designed for commercial/military use require 3-phase power. Verify that your facility has the required phase configuration and that the main panel has adequate capacity for the TRU's draw on top of existing loads.

TRU vs Portable GPU vs Continuous Supply

Key Manufacturers

The TRU market is smaller and more specialized than the portable GPU market. The primary manufacturers include Red Box Aviation (RBC5000 range, up to 600A continuous / 2,000A peak), Powervamp (IP55 all-weather TRU, 600A continuous / 2,400A starting), and several industrial power supply manufacturers who produce aviation-rated units on a custom basis.

For a broader view of the portable and semi-portable alternatives, see our Best Aircraft GPU 2026 comparative review.