747-400 APU Parameters in Flight

[NSEU]

I've been told that, in one of our company simulators, that the APU parameters display on the Status Page by selecting the APU to ON (not start).

This certainly doesn't seem to be the case on the ground. Start is required (or the Ground Test Enable switch on the overhead maintenance panel).

The maintenance manual and wiring schematics offer no solution.

Could this be an error in the sim? Has anyone tried this on the real aircraft?

Thanks
Regards
NSEU

[spannersatcx]

1 don't believe everything you hear
2 don't believe a sim is accurate
3 when you go to 'on' on the ground, without going to start, you see apu oil qty, N1 and N2 will be blank, can't remember about egt, but once you go to start and the ecu is powered they all show.

[NSEU]

1 don't believe everything you hear
2 don't believe a sim is accurate

OK.. good advice, as always, Spanners ;)


3 when you go to 'on' on the ground, without going to start, you see apu oil qty,

I'll try to confirm this. I know with the APU spooling down, N1, N2 and EGT values disappear after the values stop spooling down, but the oil quantity lingers for about 2 minutes.

[cf6-80c2b5f]

Anyone ever follow up on this? Our company manual states on the preflight setup:

APU OIL QUANTITY - Check
Verify RF not displayed. Desired minimum quantity is .55.
NOTE: Oil quantity check is valid only if APU is operating.Does this mean the oil quantity will not appear unless the APU is operating (or APU switch is on) or that the quantity displayed with the APU off is just invalid?

[NSEU]

Anyone ever follow up on this?


Not I... I retired.

There is a significant difference in level with the APU running and not running. Your company simply wants you to check the value when running.

We were given running and non-running acceptable values.

[cf6-80c2b5f]

Thanks and congratulations on your retirement.

Do you know if you will actually see a quantity on the ground with the APU switch Off or do you have to turn the switch to On to see the quantity (albeit inaccurate until actually started and running)?

[JammedStab]

I think it may depend on your version of the 747. Seem to remember something about the pax versions showing no APU info when it was not operating, but oil quantity could be seen on the 747F.

I guess they figured that just because the APU is not operating doesn't mean that you are not ever going to be interested in its oil quantity prior to start so it was changed. Or maybe it is some sort of option.

[cf6-80c2b5f]

Thanks, JammedStab. This is one of those stump-the-chump oral questions that would be good to know.

[NSEU]

Thanks and congratulations on your retirement.

Thanks!

Do you know if you will actually see a quantity on the ground with the APU switch Off or do you have to turn the switch to On to see the quantity (albeit inaccurate until actually started and running)?

Oil quantity on our (pax) aircraft was only visible with the APU running (and for a short period after shutdown) or if the overhead (Maintenance) Ground Test Enable switch was activated. Amongst other things, the Ground Test switch powered up the APU Controller. It's the APUC which provides the data for the EICAS display.

I don't know why the Freighter should be different.

Rgds
NSEU

[cf6-80c2b5f]

Thanks, NSEU. I don't recall ever checking it on the airplane. In the sim, a common question to be asked was how you could check the APU oil quantity prior to an APU battery start (probably a carry-over from the classic days).

Since the lower EICAS is unpowered in this situation (cold/dark), they wanted to see you select the inboard display selector to EICAS, which brings the lower EICAS up to the inboard display, which of course is powered by the APU standby bus. The sim would display the oil quantity, but as mentioned above, "don't believe a sim is accurate."

The whole thing was an exercise in futility because checking for .55 minimum was useless anyway -- the APU was not yet running.

The unintended consequence of this whole exercise though was that a lot of the anal pilots then felt compelled to monitor every battery APU startup -- because they could.

[JammedStab]

It is possible that it is based on newer serial numbers rather than pax or freighter type.