737 driftdown altitude - "Lt / Rt eng out"
 

FMC eng out cruise page has a slightly different driftdown altitude for Left or Right eng out.

The only info I can find on this is from Bill Bulfer:-

"This tells the FMC which engine has failed so it can account for the actual bleed configuration. Accuracy of the maximum altitude calculation is improved."

If someone could expand on this it would be much appreciated.

Obviously, the engine wing anti ice are the same, as is the pack.

But I believe the bleed requirements for the hyd, water, aux fuel tank pressure and aspirated TAT probes account for the difference.

Not forgetting you can mix engines!

maybe because....
 

if you loose #2 than #1 bleed will have to supply BOTH sides wing AI.
but
if you loose #1 you can use APU bleed for that side and keep the isolation valve closed.

Thanks for your thoughts guys.

Cough - TAT probes could be the answer ....not sure how much air they use...the hyd and water would be minimal unless someone locked the water tap on. Is aux fuel tank pressure BBJ or am I missing something obvious.

BOAC - Pls explain

FCS - My initial thought was use of APU could be in there somewhere...but it goes against the QRH..and we can't use the APU for wing anti ice. Also...I suspect the FMC output would not be allowed to make an assumption that APU was avaiable and/or selected.

hmmm....maybe it simply alters the N1 page so there's no thrust limits for the dead engine?

The max alt is based on the current gross weight and engine anti-ice off. It also assumes pack switch is Auto above 17000 ft and engine bleed air if off below 17000ft. The difference could simply be the MCT values for each engine, as BOAC mentioned, they may be different

LH engine failed - Apu providing Bleed for LH pack below 17,000
RH engine operating - providing bleed for EAI

RH engine failed - No bleed available to systems
LH engine operating - providing bleed for LH pack and EAI

Maybe? - Beer and another public holiday is not helping though!

Bulfer also describes the ANALOG DISC pages which can alter the options your carrier can choose.
ECS PACK
ECS PACK H/L
ISOL VALVE
COWL A/ICE
WING A/ICE
OLEO SWItch

Vol 2 Sect 11.42.27 9 (4) Also states

"....After page selection, the FMC accounts for wing and engine anti-ice, air conditioning, and engine bleed of the operating engine"

You are thinking like Pilots and not FMC! The FMC makes the assumptions I highlighted above.;)

As the Max Alt EO figure is advisory would not the 'assumptions' of max Alt be comming from the aircraft performance database within the FMC rather than engine specific (and supplied) figures?
I'm still struggling to see why there would be a difference between the two engine out figures from your post.

And the disparity has always been there when I've looked.

BOAC - Good point engine thurst output is rarely identical.Even when I used to test new aircraft we had to set a specific N1 which meant the engine delivered the thrust it was certified to deliver, to meet the Production Flight Test Schedule, rather than its maximum...out of the crate figure.

I need to speak to the engineers though as I am fairly certain the FMC does not have this info. As far as I know there is no engine serial number/thrust info which has to be re-entered when there is an engine change. I can only assume the FMC driftdown altitude is based on a worst case thrust output for a generic engine.

Could it not be that the FMC is wired to the EEC and therefore ......

EEC might know about bleed config

Here's a spanner in the works, if the EO CRZ page is executed, this becomes the active N1 limit and takes the actual bleed config into account, now, the resulting stabilisation altititude may change, slightly. Apart from the aquisition of advanced technical knowledge, what is the point to this thread? we may be talking a couple of hundred feet, and we are not going to be anywhere near that with MORA. Obviously, depending on the software level, you will loose VNAV and all performance predictions and the EO max CRZ will not be accurate as the system cannot calculate the residual rate of climb ( default is 100fpm, company rate from CI may be greater).

Kirk the ENG OUT CRZ page is for 'info only' - Not executable so the new active N1 idea is not possible.

Though you are correct that when the EO CRZ page is selected 'the FMC accounts for wing and engine anti-ice, air conditioning, and the engine bleed of the operating engine'

Just to highlight the original posters question - he was questioning why the difference between

LT ENG OUT MAX ALT - say FL187
and
RH ENG OUT MAX ALT - say FL190

The couple of hundred feet is not the major issue but the L/R reasoning is.

Hope I didn't misunderstand your post.
Thanks

AdAstra - I'm very confused!

How do you configure the bleeds to get the RH engine feeding the left wing A/I whilst the APU is feeding the pack?

ref. AA's quote from Vol 2

"....After page selection, the FMC accounts for wing and engine anti-ice, air conditioning, and engine bleed of the operating engine"

This suggests the FMC get inputs from these systems and adjusts accordingly. Two questions.

1. Nothing like a bit of empirical evidence so any chance someone could put the eng anti ice on and see if it reduces the altitude. Would do it myself but my company went into administration 17/12/09.:{

2. If there is normally a 300' difference when both bleeds / packs are on/APU bleed off.What is the cause of this?

Goes without saying putting engine AI on will reduce altitude as it reduces available thrust and the residual rate of climb. But why there is a 300ft diff between L and R, don't know, cant change it, does it matter!

Cough

I meant Engine anti-ice. Amended accordingly.
Without aircraft or books so I may be on a tangent here.

KG - I should have been clearer with my question.

When you put the eng anti ice on (and therefore reduce the drift down altitude) does the FMC reduce the drift down altitude displayed.

Just trying to prove empirically the theory that the FMC is able to assess the aircraft's actual bleed configuration.

Does it matter...does to me. Like I say just lost my job ..got an interview coming up and this is one of the tech questions that gets asked.

Pint of beer for anyone willling to explore the system.