Avionics Abnormals

Gulfstream GVII

Eddie sez:

As we (the collective we) gain experience with the GVII suite of avionics we (the individual we) learn to deal with things that are not what they seem.

What follows is a short (but soon to expand) list of some of these abnormals. There are things from the references shown below, but most of what follows is from experience.

Last revision:

20191206

HUD Comp Fan Fail

What the AFM says:

[AFM, §3C-08-40]

  • Possible Cause: The HUD computer fan has failed.
  • Displayed CAS: HUD Comp Fan Fail
  • System Impacts: HUD computer cooling capability reduced.
  • Corrective Action: avoid extended fan operation

What the FAQ says:

[FAQ, p. 44] If the message continues to cycle on and off, SSPC #3417 (HUD FAN L) may be pulled to prevent nuisance cycling. It is recommended the SSPC be reset for ground operations. Fixed in G500 Block 1.

What I've Learned Elsewhere:

The HUD COMP FAN FAIL message was designed to display when the HUD is on and the fan isn’t working. The system determines that the fan is operating by looking at the fan motor current draw. If the motor’s current draw is zero to x amps and the fan motor is supposed to be running, the CAS message will post because the assumption is the fan motor isn’t operating due to the low current draw.

This plan worked perfectly in the lab but on some installations depending on cabin altitudes the amp draw required to operate the fan drops so low that the system assumes the fan isn’t operating, the CAS message is posted. The fan continues to operate but the system thinks it isn’t working.

The system message anomaly first occurs around FL330 - FL350. The CAS message posts every time the amperage drops to the trigger value. An annoying on and off chime occurs until climbing through an altitude that gives a cabin altitude high enough that the fan current draw is low enough to keep the CAS message displayed constantly. The fan motor continues to run, the message posts but has no effect on the fan motor.

The problem is that you normally do not fly above FL410 so the CAS message cycling doesn’t go steady until approx 41,300 feet, that’s what we experienced. You will have to pull the circuit breaker to stop the flashing and cycling tones at FL410. On descent passing through FL300 or so, reset the breaker.

It’s important to note, the HUD can continue to be operated at altitude without the fan operating.


See Also:

Gulfstream GVII-G500 Airplane Flight Manual, Revision 4, August 29, 2019

Gulfstream G500 Frequently Asked Questions, Revision D, September 2019