Why XFCC restart during EPU check?

[khandoost33]

Why XFCC restart during EPU check?
It stay normal during transiton from main gen to epu but recycle in transition from epu to main gen.

[huzntz]

Check your bus power transfer relays.

[khandoost33]

Bus pwr relays changed

[yooper65]

First off, I believe tech data states you should not turn on your XFCC prior to EPU Bleed Air Check. I am thinking the most likely reason is because what you are seeing when doing so.
Second, since your issue is only occurring when EPU Bleed Air Check is completed (i.e. – EPU shuts down causing ESS Bus (Blk 15) Xfer Relays to de-energize), I can think of two things…

1. Normal Ops. Reason – the Emer GCU can allow the ESS Bus Xfer Relays to stay energized even if the Emer Gen is only putting out 70 VAC. I believe this drop in voltage, which is occurring during EPU Shutdown, gets to the point of being too low for the XFCC to operate, thus causing it to shut down (restart) before the ESS Bus Xfer Relays de-energize (when the Main Gen takes over).

2. If not item 1 (which is just my theory), then take a good look at whichever ESS Bus Xfer Relay supplies power to your XFCC and replace it.

[khandoost33]

First no restriction for xfcc on for epu check.
Its have no issue with other tail no.
Bus pwr fuctions check they are sat.as relay control many sys not only xfcc.
Wiring check c/w between xfcc and fuel flow transmitter:they are short with each other.
Connector repair of fuel transmiter.
Problem still persist

[yooper65]

I don’t have access to a copy of your -1 T.O. (Flight Manual) but, the ones I have looked at all contain Notes about voltage transients during EPU Bleed Air check that may cause fault light indications and that a power cycle of systems may be required. Also, the same Flight Manuals have the Pilot turn on Avionics after EPU Check is completed.

Now with that being said, you state that this is the only aircraft that this problem is occurring on. Because of this - I would still recommend replacing the ESS Bus Xfer relay (Blk 15 and below) or Emer Bus Xfer relay (Blk 25 and above) that controls power to your XFCC. I understand that these relays control more than just one system but, maybe this system is the most sensitive to power transfer and that your relay is just a little slow in fully de-energizing after EPU Check is completed. Also, if you were willing to replace the Bus Pwr Contactor (mentioned above) which does not get exercised per se during an EPU Bleed Air check, then why not replace one that does.

And last but not least – maybe give your Emer GCU a shot. Hope this helps.

[khandoost33]

Thanx:caught the bug:it xfcc.
But could not understand why it behave like this.

[profpoinz]

khandoost33,

After the EPU bleed air check is completed and the EPU/GEN test switch is released, the 3-phase, 115 vac, 400 Hz output from the EMER GEN drops significantly in voltage and frequency (just like yooper said) due to the bleed air being turned off which makes the EPU turbine start spinning down. At some lower RPM, the EMER GCU allows source power to be transferred from the EMER GEN back to the MAIN GEN. This specific power transition is notorious for restarting F-16 avionics and air vehicle systems that are Mil-Std-704A challenged.

If you have an oscilloscope or data acquisition system, you can monitor the A2 wiper on one of the ESS (A/B) or EMER (C/D) XFER relays to observe that transition. Then you can compare the measurements between your problem jet and one of your known good jets. If you don't get anywhere, I suggest you contact me via the F-16 Product Support help desk at LM Fort Worth. I am a power & control engineer in F-16 engineering.

Hope this helps.
BNO (profpoinz)

P.S. The product support help line is 1-800-316-5672