APU air inlet door

[John H Watson]

If stuck at 15°, shutdown will occur some seconds after APU start.

You mean automatic shutdown?
Autoshutdown will occur only if the APU door is closed. Since the APUC only sees the 13 degree switch (not the 15 or 45 switches), then I have to assume "closed" is less than 13. "Open" is 13 or more.

The DDG specifies that the if the door is stuck open, the APU should not be started unless the door is at least 15 degrees.

[JRBarrett]

We have been fighting issues with APU door faults on a CRJ-200 I maintain. Finally found the issue to be an intermittent failure of the actuator. It's a completely different aircraft of course, but like the 747, the APU door will normally transition from about 45 degrees on the ground, to about 15 degrees in flight. On the CRJ, it is standard practice to land and takeoff with the APU running to supply the packs - shutting it down when climbing through 10,000 feet in climb, and restarting it during descent at 10,000 feet.

Although the normal in-flight position of the door is half open, if the actuator fails before flight, it is permissible under MEL procedures to manually crank it fully open, pin it in that position, and fly. The proviso in the M procedure of the MEL is that the APU must NOT be shut down in flight under these circumstances. Having the door fully open is not the issue from an aerodynamic standpoint, but if the APU is shutdown, the ram air coming in the inlet will cause the APU compressor to windmill at high speed, which could eventually cause bearing damage due to lack of pressure lubrication.

Flight with a running APU has a maximum altitude restriction, but no airspeed restriction. If the APU fails in flight with the door pinned fully open, there is then a maximum IAS restriction in effect - again, not for aerodynamic loading on the door, but to minimize APU compressor windmilling speed.

I know that the 747 APU can't be started in flight, but it is permissible to takeoff with it running. Curious what the QRH might say regarding airspeed restrictions if the APU door fails to close after shutting the APU down in flight.

[John H Watson]

Curious what the QRH might say regarding airspeed restrictions if the APU door fails to close after shutting the APU down in flight.

I think I need help interpreting this....

APU DOOR

Condition: The APU door is not in the commanded position

Objective: One of the following:
*In flight, to ensure accurate fuel burn calculations
*ON the ground, to attempt an APU restart [presumably because the door didn't reach 13deg for APU start logic]

1. Choose one:
In flight>>> Go to step 2
On the ground>>> Go to step 3

2 Choose one:
APU selector is OFF: Apply the APU door open fuel burn penalty of 2%. End
APU selector is ON: APU selector to OFF. End

3 Choose one:
APU selector is OFF: End
APU selector is ON: Go to step 4

4 APU selector .... OFF, then ON

5 Choose one:
APU Door message blanks: A restart may be attempted. End.
APU DOOR message stays shown:
APU selector ... OFF
Do not start the APU... End.

[Hardy Heinlin]

5 Choose one:
APU Door message blanks: A restart may be attempted. End.
APU DOOR message stays shown:
APU selector ... OFF
Do not start the APU... End.

To check whether the APU Door message blanks, you need to wait 1 minute as the message incorporates a delay, doesn't it?

If the door is closed, the message will blank by cycling the selector to OFF, right?

So, shouldn't the text read like this?

APU DOOR message remains blank for 70 seconds: ...
APU DOOR message re-appears after 70 seconds: ...


|-|

[John H Watson]

If the door is closed, the message will blank by cycling the selector to OFF, right?

If the door is stuck completely closed, and an APU start is attempted, because the 13 deg switch isn't activated, the APU won't start (Note: no cooldown period and the rpms will be at zero). So... the Advisory message should disappear and not reappear if the selector is put back to OFF (Commanded position = actual position). The Status message may be latched however. I'm not 100% sure, but you may also get some kind of APU message because of the failed start.

If the APU selector is simply moved from OFF to ON on the ground, the open relays will be energised. If the door doesn't reach the 45 deg position, the open relays will remain energised and the messages will appear after 60 seconds. If you then go straight back to OFF, the close relays will try to engage. If the door is already (stuck) fully closed, the close relays won't activate and the EIU earth will be lost and the Advsy message will disappear. If the door is stuck slightly open and the selector is moved back to OFF, the close relays will activate and reset the EIU message timer (assuming there are a few milliseconds between open and close relay switching and that the EIU doesn't have a small capacitor to keep the earth signal active during the switchover period). If the door doesn't close, the message will reappear. 

APU DOOR message remains blank for 70 seconds: ...
APU DOOR message re-appears after 70 seconds: ...

If the door does not open (i.e. stuck completely closed), moving the selector back to OFF will remove the Advsy message completely.

Any time there is a door angle change command, one of the relays will engage (and send an earth to the EIU) until the door reaches the commanded position. If the door doesn't reach the commanded position, the message will appear after 60 seconds. The message timer begins at command change.

[Hardy Heinlin]

I'm not sure if we are on the same page ... :-)

I'm referring to the appearance delay of the APU DOOR message.


APU DOOR

Condition: The APU door is not in the commanded position

Objective: One of the following:
*In flight, to ensure accurate fuel burn calculations
*ON the ground, to attempt an APU restart [presumably because the door didn't reach 13deg for APU start logic]

1. Choose one:
In flight>>> Go to step 2
On the ground>>> Go to step 3

2 Choose one:
APU selector is OFF: Apply the APU door open fuel burn penalty of 2%. End
APU selector is ON: APU selector to OFF. End

3 Choose one:
APU selector is OFF: End
APU selector is ON: Go to step 4

4 APU selector .... OFF, then ON

My comment: The OFF selection will blank the message because it commands the door to close. The door is already stuck closed, so the agreement will blank the message.
Then the ON selection will command the door to open, as far as I understand it (the start sequence is initiated and waits for a response from the 13° sensor switch). But the door remains stuck closed. So when this disagreement lasts for 60 seconds, the messsage re-appears.

5 Choose one:
APU Door message blanks: A restart may be attempted. End.

My comment: It blanks because the OFF selection caused an agreement. But the ON selection thereafter will keep it blank only for 60 seconds. The blanking due to the OFF selection doesn't mean that the door system is now OK. Only if it remains blank for more than 60 seconds the door system is OK, and a restart may be attempted, ... as far as I understand it :-)

You may try to start it anyway. But the door will not open. However, if you try it instantly (not waiting 60 sec) you will instantly see that the N2 will not rise. So you will instantly see that the system is not OK, and you need not wait 60 sec to see whether system is OK. Is this the purpose of that QRH line? Don't wait for the message, just check whether N2 rises, and you will promptly see whether the system is OK or not.


Cheers,

|-|ardy

[John H Watson]

ah.. yes.. your interpretation may be correct.... but I don't know for sure.

4. APU selector .... OFF, then ON

5. APU Door message blanks: A restart may be attempted. End.

This is confusing. When is the message supposed to blank? In the OFF or ON position?

Scenario: The door fails as it moves towards open during the first "ON" after landing and gets stuck at 5 degrees. Message appears 60 seconds after ON selection. OFF is selected. Door attempts to close, but remains stuck at 5 degrees. Does the message remain in view because capacitors in the EIU hold the earth signal from the open relays (until the close relays replace the earth signal)?
Reselecting ON may just give the extra push to the actuator (e.g. break up a build-up of ice on the actuator/mechanism) and make it move to the fully open position (where the position = command).

Pilots really need to be aware of the timing of the messages. Otherwise they might take the wrong course of action.

[Hardy Heinlin]

This is confusing. When is the message supposed to blank? In the OFF or ON position?

I would say in the OFF position. -- OFF=Close the door. -- Door is stuck below 13°=Closed signal. -- Close=closed. -- Agreement. -- Blank the message.

Does the message remain in view because capacitors in the EIU hold the earth signal from the open relays (until the close relays replace the earth signal)?

I can't imagine that the EIU incorporates capacitors for certain messages. If the EIU really latches a message, it certainly latches it by digital memory rather than by analog capacitors, I would say. But anyway, the question is whether it does latch under specific conditions.

Pilots really need to be aware of the timing of the messages. Otherwise they might take the wrong course of action.

That's why I mentioned the "70 second" test above :-)


Cheers,

|-|ardy

[John H Watson]

I would say in the OFF position. -- OFF=Close the door. -- Door is stuck below 13°=Closed signal. -- Close=closed. -- Agreement. -- Blank the message.

Just to clarify....  The EICAS message is responding to the auxilliary contacts of the close and open relays (R460/R461). These relays control the actuator-motor. On the ground, the close relay will not deactivate until the door is 100% shut. i.e. zero degrees. If the door is stuck below 13° (but above 0°) it can still trigger the door message. The 13° switch (S1598) seems to be mostly for APU start and run (to ensure enough airflow to the APU).

I can't imagine that the EIU incorporates capacitors for certain messages. If the EIU really latches a message, it certainly latches it by digital memory rather than by analog capacitors, I would say. But anyway, the question is whether it does latch under specific conditions.

I was thinking that most (non-latched) EICAS messages take at least a second or two to disappear after the triggering signals disappear. I don't know if this is due to signal delays or if a logic circuit or capacitor is keeping the message in view for this period. If it only takes a couple of microseconds for the open relay R460) to de-energise and the close relay (R460) to energise, the microseconds loss of the earth signal to the EIU (provided by the auxilliary contacts of the relays) might not be enough to register a change at the EIU. If no change (loss of earth) is registered, the timer wouldn't restart and the message would remain in view until the door was fully closed (and the close relay relaxed).

[John H Watson]

In PSX version 10.20, I failed the door with the APU running on the ground during taxi-out. The door froze at 45 degrees. After takeoff, the door failed to move to 15 degrees and the APU DOOR messages appeared as expected. I didn't turn off the APU and I returned to the airport (assuming the 2% penalty was too great to get me to my destination). On landing, the door remained at 45 degrees (as expected), but I expected the Advisory msg to disappear (because the target door angle matched the current door angle), but it remained in view.

Is there something I've overlooked?

Cheers
JHW

[Hardy Heinlin]

I can't reproduce this problem. My APU DOOR messages blank after landing.

However, as expected, the APU DOOR messages will reappear when the APU is shutdown (door commanded closed but stuck open). Maybe that is what you saw?


Cheers,

|-|ardy


By the way, I'll implement the 2% fuel penalty in the next update (increased drag, more thrust required to maintain cruise speed).

Does anybody know whether the FMC gets APU door position data?

[Jeroen Hoppenbrouwers]

Does anybody know whether the FMC gets APU door position data?

How many other things make it to the FMC in general? Flaps stuck down; gear stuck down; partial gear; gear doors; gear tilt; APU door; bleed stuck open; packs doors; cabin doors; what else can ruin the performance calculations but is not normally required for the FMC to make decisions on?

Related... if the FMC takes these things into account, can the pilots check this to be sure? I don't think there is a page enumerating all currently active performance penalties. So you need to completely rely on books and checklists.

Hoppie

[Hardy Heinlin]

I don't think that the FMC has a performance database for gear-down-cruise (at least not the old 744 FMC). I think when you're cruising with gear down, you should ignore the FMC fuel predictions.


|-|ardy

[John H Watson]

Does anybody know whether the FMC gets APU door position data?

Sound unlikely. The FMC doesn't even know if the APU is running (and using fuel), except via fuel tank quantity. Nothing found in the schematics.

However, as expected, the APU DOOR messages will reappear when the APU is shutdown (door commanded closed but stuck open). Maybe that is what you saw?

No, I didn't try shutting off the APU. I'll try to reproduce this.

Is the status message latched in PSX? It makes sense to latch the message as the fault can hide if the door position matches the command.

[Hardy Heinlin]

Not latched in PSX.

[John H Watson]

I'll try to reproduce this.

Strange, it worked ok the second time I ran the situ.

[Avi]

We need to have someone see what parameters are available on the EICAS APU Maintenance Page with the overhead Ground Test switch ON and the APU OFF. If the door position is available (showing CLOSED), we need someone to time how long the door indication takes to show OPEN when the APU selector is moved to ON. This will be the 13 degree switch being activated.

Anyone in the sim soon and who knows how to access the APU Maintenance Page?

I found this and this (but probably not a big help)

Edit: maybe a bingo at 3:35
In PSX it looks too slow.

Edit 2: we may have a winner at 1:55

[Hardy Heinlin]

Thanks, Avi. The second one is helpful.

My guess: Assuming the 767 door goes to 45°, the video starts at 27°, and it takes 8 seconds to 45°. All in all 20 seconds from 0° to 45°. This is about the same as in PSX 10.30.


|-|ardy

[Avi]

I found another one two. Check above.

[John H Watson]

It does look quite fast in those last two videos, but it's hard to say when the door starts and stops moving.

In the first video, it looks like the APU parameters are appearing as a result of the APU selector going to START. The door may already be open. The door display appears later than, say, the EGT, probably because of the way the APUC boots up.