FMC FAILURE

[Hardy Heinlin]

Modifications are now implemented in 10.0.8-beta8:

http://aerowinx.com/board/index.php?topic=3462.0


|-|

[emerydc8]

Thanks, Hardy!

[hellowk]

Thanks!

[hellowk]

Hi Hardy,

Just downloaded your update.
Under the VNAV of the update, you mentioned this :
0.8.0039. Master FMC failure: If current altitude is more than 50 feet from MCP altitude, VNAV word gets amber line, and AFDS maintains pitch (unless in alpha protection); otherwise ALT mode engages.

However, I did not experience this in the SIM, VNAV will get a strikeout as well. I went through Peter's video and this is the case as well. Also, one of our instructor pilot had a write up on the failure and I will share the picture.
https://drive.google.com/file/d/0B8u4g8QNcHNPNFIxYmNSWU81OEU/view?usp=sharing

Regards,
Ray

[Hardy Heinlin]

Hi Ray,

I referred to this quote:

During this time, A/T mode shows blank, roll and pitch show mode fail [I assume yellow lines], with FD bars biased out of view, but a footnote says VNAV may change to altitude hold (ALT) if the airplane is near the MCP altitude.

"May". -- Perhaps you may see this in your sim as well -- sometimes -- just not every time?

I don't know the conditions behind "may" ...

Maybe +/-50 ft is not the final condition?

Maybe it's similar to the V/S mode's altitude capture: ALT may engage only during vertical motion when coming from outside the ALT bug width of +/-50 ft? I think the faulty VNAV mode (pitch hold submode) is an AFDS mode (not an FMC mode) and therefore is able to capture the MCP altitude; it would be a bad design, in my opinion, if the pitch hold submode would shoot through the MCP ALT, although it is technically able to monitor it.


Regards,

|-|ardy

[Hardy Heinlin]

Hi,

in update 10.0.8 I implemented the stuff with the yellow lines through the LNAV/VNAV mode words etc. when the master FMC fails. In previous versions LNAV/VNAV changed to HDG HOLD/ALT etc. in this case.

This implementation has a side effect: Other relevant failures keep LNAV/VNAV now engaged and operative. In previous versions they too caused LNAV/VNAV to change to HDG HOLD/ALT etc.

These other relevant failures are:

• Less than 3 engines running
• IRS not aligned
• Any probe heat failure

I think these 3 conditions too should have the same effect on the LNAV/VNAV modes as a master FMC failure.

As usual, when any of these 3 failures exist, armed LNAV/VNAV cannot engage. That's OK. But once they are engaged they won't disengage anymore. That's not OK.


Regards,

|-|ardy

[Britjet]

Hi Hardy,

I have been having a discussion with Jon recently that I seemed to recall that LNAV and VNAV didn't work with two engines failed. He quoted a colleague who tested it and said that they did work.
So you are saying that you think they should be inoperative?

Peter

[Hardy Heinlin]

Hi Peter,

well, if he tested it, then I believe him. I didn't remember that discussion.

I'll remove the E/O condition from those 3 criteria ...

What remains is the probe heat criterion (IRS alignment loss will disengage the whole AFDS anyway). I'm sure that this will inhibit the LNAV/VNAV engagement (real life incident, and QRH notes), so it must also disengage it, respectively draw that yellow line etc.


Cheers,

|-|ardy

[Hardy Heinlin]

I just re-discovered a source in an 744 FMC engineering book from 1989 which says VNAV will disconnect when "more than one engine failure is detected".

(Ref 6-123)

[emerydc8]

Hi Hardy and Peter,

I am 100% sure that Atlas has its pilots arm LNAV and VNAV on a 2-engine approach. They want you to wait until the VPP is 1000' low before turning your base. When you turn base, they want you to do the LVSA procedure (Arm LNAV, Arm VNVAV, Speed Intervene, Set Decision Alt). They use 500' as a nominal decision altitude. So, although you have no A/T, they still want you to use LNAV, VNAV and A/P to start the approach and then disconnect just prior to 1000' when you center the rudder trim.

Jon D.

[Hardy Heinlin]

Hi Jon,

that book from 1989 might be outdated.

If they want you to arm LNAV/VNAV, are you sure that they expect an engagement, and that they don't want to demonstrate any non-engagement?


|-|ardy

[emerydc8]

They definitely expect an engagement. The idea is that they take away your ILS and radar when you lose a second engine on your 3-engine missed approach. This leaves you with no lateral or vertical guidance and you are totally own your own trying to get back to the airport (no vectors either) You tell the NFP to select the ILS for the runway you want and extend from the runway fix (threshold). Even though there's no ILS, this gives you lateral and vertical guidance to the runway and it also activates the on-approach logic because you are within 25 miles of the threshold. When you are on a downwind, you turn your base when the VPP shows 1000' low (fly up), then do the LVSA and the AFDS will intercept the final and start down in VNAV PTH. At 1000', you disconnect the A/P and center the rudder trim, but you still have F/D guidance all the way down to the runway using VNAV and LNAV.

These are the check airman notes from last year:

 2 engines inoperative.
 Do not set the MCP below 500'.
 Back up the approach with an ILS if available. Use the glideslope if it is available.
 Visual approach techniques are encouraged and may be used if desired.
 If no ILS glideslope is available, use VNAV. Either of the following methods may be used and selected from the DEP/ARR page:
 A non-ILS approach. Preferred method.
 Any approach and extend the runway fix on the LEGS page. Alternative method.

[Britjet]

I would say from my experience, I do seem to recall, that on the BA sim VNAV and LNAV are not available. Jon and I had a private email conversation about it recently)
Unlike Atlas - in BA we didn't try to make a very difficult situation even more difficult by removing ILS etc).
More intrigue!
Peter.

[emerydc8]

Thanks, Peter. I wonder if there was some rationale at BA behind intentionally disabling LNAV and VNAV when you lose two engines. Probably whoever might have made that decision is long retired/passed on by now.

On my type-ride, my airline was a bit nicer on the 2-engine approach -- They gave me vectors for the RNAV Y 04L at JFK. Like Atlas, they don't allow use of the ILS on a 2-engine approach. Instead of extending from the runway fix, as Atlas teaches, I had my FO extend from the FF. I'm not a big fan of using the A/P when down to 2 engines, so I hand flew the approach and I am 100% certain that the F/D gave me VNAV and LNAV guidance all the way down. So, the ex-ANA sim also allows LNAV/VNAV on a 2-engine approach.

If you pop an engine at say 30 West and it takes a second engine with it, when you select the E/O prompt, VNAV displays ACT 2 E/O LRC CRZ. We know the A/T wouldn't work on two engines, but why would Boeing not want you to at least have VNAV and LNAV in this situation? At least you wouldn't have to use HDG SEL in Class 2 airspace for several hours or use FLCH or V/S to maintain the D/D speed. On BA's oceanic contingency procedures for loss of two engines, was there any mention that LNAV and VNAV would not be available? 

Jon D.

[John H Watson]

Regarding any probe heat failure, we may need to revisit this thread

http://aerowinx.com/board/index.php?topic=3643.0

TAT, engine P2T2, etc, may or may not be similar.

[emerydc8]

Hi John,

Do you think the discrepancy between the BA sim and the Atlas/ANA sims (LNAV/VNAV availability with two engines inop) could have something to do with the engine type? RR v. GE?

[John H Watson]

Not sure. One of the differences between RR's and GE's, I recall, is that if any engine EEC is in Alternate mode on the RR, the A/T cannot be re-engaged. I assume any fault which creates EEC alternate mode would stop A/T engagement. I can't remember if (fuselage mounted) pitot/static probe heat faults cause EEC failures.

[Hardy Heinlin]

Hi Peter,

is there any chance to test in the BA sim whether LNAV/VNAV can be engaged if 2 engines are out? If it can't I need to add another airline option.


Hi Jon ...

ACT 2 E/O LRC CRZ

I've never seen this page title in any books. I assume this was a software update. In the books I always read "E/O", and since this term doesn't say anything about the number of engines, I assumed it always means 1 E/O. On the other hand, the PSX FMC does automatically display lower MAX and OPT altitudes when more than one engine is out. It just doesn't indicate the number in the page title.

Now ... what if 3 engines are out? Can LNAV/VNAV still be engaged? It's certainly impossible when 4 engines are out because probe heat is then unavailable due to AC bus power loss.


|-|ardy

[Britjet]

Hi Hardy,

I will try and do a test if I get chance. It will be a week or two..
Peter

[John H Watson]

Do you think the discrepancy between the BA sim and the Atlas/ANA sims (LNAV/VNAV availability with two engines inop) could have something to do with the engine type? RR v. GE?

Here are a few clues on how the RB211 EEC responds to probe heat failures (ref to link below):

If the engine "P2T2" probe heat fails it seems that the EEC switches automatically to Alternate mode irrespective of what the fuselage pitot-static probe heat is doing.

The aircraft has to validate the P2T2 probe data (generally) by comparing it with fuselage mounted probe data from 2 ADCs. If one fuselage pitot-static probe heat fails, I think the EEC has the chance to switch to the other fuselage pitot-static probe. If only one fuselage pitot-static probe heat fails, just one of the two EEC channels fails (i.e. no ALTN mode, I think)

http://www.iinet.net.au/~b744er@ozemail.com.au/PSX/ProbeHeatEECAlternate.GIF

I don't know if the PW4000 follows these lines. The GE doesn't have a P2T2 probe because it uses N1 as the primary parameter.