Engine Start on AC Power without a Break Power Transfer

Gulfstream G450

Eddie sez:

Frequent fliers to Nice, France and other places that strictly control APU operating time often ask this question: how do you avoid a break power transfer going from external AC to the APU?

Answer: you don't.

The only thing that permits a no break power transfer is the IDG. Once you understand that, the procedure is easy to remember.

Everything here is from the references shown below, with a few comments in an alternate color.

Last revision:


Engine Start on AC Power without a Break Power Transfer


G450 AC power system overview, from FlightSafety G450 Maintenance Training Manual, figure 24-10.

Systems Review

[G450/G550/G650 (ATA 24) No-Break Power Transfer System Overview.]

  • Several operators have called into Gulfstream Technical Operations wanting information on the No-Break Power Transfer (NBPT) System, which maintains power on the buses while switching power sources. The Left Bus Power Control Unit (BPCU) selects the frequency reference for the left and right Generator Control Unit (GCU) to control frequency and phase angle of the GCU's Integrated Drive Generator (IDG), located on each engine.
  • The order of frequency references are as follows:
    • APU generator
    • AC External Power
    • BPCU internal 400 Hz
    • GCU internal 400 Hz.
  • When down to the GCU, there will be a Break Power Transfer (BPT).
  • The frequency reference is used to provide frequency control between an IDG power source and a non-IDG power source, which allows the system to make the NBPT. The APU generator and External AC power cart are not frequency controlled by the GCU and the BPCU; therefore, power transfers between these two power sources are BPTs, which causes the momentary power loss during transfer. Simply put, an IDG must be involved in a power transfer to have an NBPT.
  • Many airports are limiting use of the APU from ten minutes prior to passenger arrival to ten minutes after passenger departure, any other time an AC external power cart is required. Nice, France (ICAO: LFMN) requires ready-for-start status for APU operation. Immediate shutdown of the APU is required, with transfer to the AC external power source after engine shutdown.
  • All automatic power transfers will be NBPTs except those between an external AC power source and the APU generator, since the APU generator is not an IDG and the GCU cannot adjust frequency or phase angle without a hydromechanical drive unit (Ref. Aircraft Operating Manual 2A-24-20, 2.D).

More about this: G450 Electrical Control / BPCU.

How to avoid a BPT

[G450/G550/G650 (ATA 24) No-Break Power Transfer System Overview.] The best way to avoid a BPT is to select the APU generator switch to "OFF" before starting the APU. Continue to use the AC external power cart to power the aircraft and the APU as the air source until at least one or both engines are started with IDGs online. There will be no BPT when using this technique. This information has been submitted for inclusion in the Quick Reference Handbook.

See Also:

G450/G550/G650 (ATA 24) No-Break Power Transfer System Overview, The Gulfstream Journal, October 3, 2014.