Guide to transformer rectifier units for aircraft ground power. High-amperage 28V DC TRUs from ITW GSE, Powervamp, and AXA Power for engine starting and maintenance. 1,500A to 3,000A+ peak.
What Are Transformer Rectifier Units (TRU)?
A transformer rectifier unit — TRU — converts AC mains power directly into the high-current 28V DC needed to start aircraft engines. Unlike battery GPUs that store energy and deplete it with each start, a TRU draws from the grid and delivers unlimited starts as long as the power stays on. This makes TRUs the workhorse of FBOs, MRO shops, and military flight lines where aircraft are started dozens of times daily and battery replacement costs would be prohibitive.
How They Work
A TRU takes standard AC mains power (typically 230V or 480V, 50/60Hz) and converts it to 28V DC through a power transformer and multi-phase rectifier circuit. The transformer steps down the voltage, and the rectifier converts AC to DC with filtering to reduce ripple. High-output TRUs use 12-pulse or 24-pulse rectification for cleaner output and add SCR-controlled voltage regulation to maintain 28V under varying loads — from the 200A of continuous avionics power to the 2,400A+ peak of a turbine engine start.
Manufacturer Comparison
Choosing the Right Transformer Rectifier Units (TRU)
TRUs are the right choice when you're starting aircraft regularly from a location with reliable AC mains power. The math is straightforward: a high-quality TRU costs more upfront than a battery GPU but has no consumable battery packs to replace, no charging downtime, and no degradation over time. For an FBO that starts 10 aircraft a day, the battery replacement costs of an equivalent battery GPU exceed the TRU's purchase price within a year.
The practical limitation of TRUs is portability: they require an AC mains connection, which means running power cables to wherever the aircraft is parked. Fixed-position TRUs at hangar doors and ramp positions solve this with permanently wired installations. Mobile TRUs on carts extend the reach with long power cables, but they're still tethered to an outlet. For starts at remote locations, unpaved strips, or anywhere without mains power, a battery GPU or diesel GPU is the only option.
Sizing a TRU follows the same logic as any GPU: match peak current to your most demanding aircraft, with 20 percent headroom for cold weather. An FBO serving a mix of King Airs and Citations should spec at least 2,400A peak. A facility that sees Challenger and Gulfstream traffic needs 3,000A+ or paralleled units.
Reliability is the TRU's strongest selling point. With no batteries, minimal moving parts (some units are fully solid-state), and components rated for industrial duty cycles, a well-maintained TRU lasts 15 to 20 years. Annual maintenance is typically limited to cooling fan checks, cable inspection, and output calibration verification.
For a step-by-step approach to selecting the right GPU for your aircraft, see our GPU Sizing Guide. For a comparison of lead-acid vs lithium technology, read Lead Acid vs Lithium GPUs.
Recommended For These Aircraft
- Beechcraft King Air
- Cessna Citation (CJ/M2/XLS/Sovereign)
- Bombardier Challenger 300/350
- Gulfstream (G280/G500/G650)
- Bell 412
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Frequently Asked Questions
When should I choose a TRU over a battery GPU?
Choose a TRU when you have reliable AC mains power at your starting location and you start aircraft frequently (daily or more). The unlimited start capacity and zero battery maintenance make TRUs more economical for high-volume operations. Choose a battery GPU when portability and independence from mains power are priorities.
Can a TRU provide continuous power for maintenance?
Yes, most TRUs provide continuous output alongside their peak start capability. The ITW GSE 2400 delivers 150A continuous, sufficient for most avionics maintenance. Some Powervamp models deliver up to 500A continuous.
What happens if power drops during a TRU-powered start?
A momentary power dip during the start sequence can cause the TRU's output voltage to sag, potentially aborting the start. High-quality TRUs include energy storage capacitors to ride through brief dips. For critical operations, consider a TRU with a battery backup module or ensure the mains supply is on a stable circuit.