View Full Version : 737 driftdown altitude - "Lt / Rt eng out"

31st Dec 2009, 19:32
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.

1st Jan 2010, 09:23
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.

1st Jan 2010, 12:22
Not forgetting you can mix engines!

FCS Explorer
1st Jan 2010, 16:19
if you loose #2 than #1 bleed will have to supply BOTH sides wing AI.
if you loose #1 you can use APU bleed for that side and keep the isolation valve closed.

2nd Jan 2010, 18:54
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.

FCS Explorer
3rd Jan 2010, 22:43
hmmm....maybe it simply alters the N1 page so there's no thrust limits for the dead engine?

3rd Jan 2010, 23:00
BOAC - Pls explain - engines do not have to be identical.

Kirks gusset
3rd Jan 2010, 23:16
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

4th Jan 2010, 07:39
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.

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"

Kirks gusset
4th Jan 2010, 10:21
You are thinking like Pilots and not FMC! The FMC makes the assumptions I highlighted above.;)

4th Jan 2010, 10:52
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.

4th Jan 2010, 18:39
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.

Pitch Up Authority
4th Jan 2010, 19:06
Could it not be that the FMC is wired to the EEC and therefore ......

EEC might know about bleed config

Kirks gusset
4th Jan 2010, 20:10
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).

4th Jan 2010, 21:29
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


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

Hope I didn't misunderstand your post.

4th Jan 2010, 23:29
H engine failed - Apu providing Bleed for LH pack below 17,000
RH engine operating - providing bleed for AI
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?

4th Jan 2010, 23:31
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?

Kirks gusset
5th Jan 2010, 00:26
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!

5th Jan 2010, 02:25

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

5th Jan 2010, 10:01
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.

Kirks gusset
5th Jan 2010, 11:33
RMC, I doubt they would ask Cabin Crew this question, Update your profile! I'm doing an LPC Sat eve so will see if this works in the sim, don't really want to do this in the cruise as the temp will be outside icing limits and it's never good to "play" with things, sets a bda example. If you have an interview, they are looking for the thought process so I wouldn't worry too much. If you know the differences between classic and NG, especially electrics, take off warnings, wing anti ice differences, autoslats, and auto shutdowns, you should be fine.. good luck.

6th Jan 2010, 19:11
Thanks KG..look forward to the answer at the weekend.

22nd Apr 2012, 15:27
I can see itīs 2 years since last post in this thread but Iīm currently doing the 737 NG rating and this ENG OUT LT og RT got me wondering about why the FMC would like to know.

It obviously does make a difference as stated by the first post but why.
If itīs just current AC TAI config it should also be able to se which eng is out.

Some good points in this thread but does anyone have the hard facts..


23rd Apr 2012, 08:59
The QRH performance inflight suggest reducing max altitude based on anti-ice on, with wing and engine anti-ice being 5500' so nothing like the figures you are suggesting.

I would also be interested to know this, as its a regular question I get asked during type rating sessions.

23rd Apr 2012, 22:58
I keep on hearing those "cruise briefings" where pilots keep on hammering on the correct drift down speed from the fmc, selecting MCT etc, like if they could prohibit the aircraft from decending. But this is off topic.
Another part is the FMC drift down altitude, like this is now your biggest concern. (apart from over the Himalaya`s)
Anybody asked himself the question why the max single engine etops cruise alt is calculated at level 170, and the FMC at level 410, shows you something like level 220? The FMC takes the actual bleed config into consideration. That means if you will need engine A/I at lower altitudes, start subtracting about 1200 ft from this number. You additional need wing A/I? wow, another 4900ft down the drain...
back to the original question. difference between the engines,well I do not know, never noticed.

24th Apr 2012, 14:41
Had a look in the a/c today:

No difference between the engines.

737-800 winglets

2nd May 2012, 21:00
Good old times ...ever heard about engine "intermix"? . Flying the 727 it was very common to have diferent rated engines mounted on .
Been in the NG for a while and have never seen it , in theory I guess you can have,let's say a 24 K engine on one side and a 27K on the other...

3rd May 2012, 00:01
I'm doubting my sanity whilst posting this, but I am sure during a discussion years ago on this subject, someone told me that the aircraft was acting like a prop driven twin, & one engine was more "critical " than the other. So, something aerodynamic caused by these big multi-bladed CFM "props".

7th May 2012, 15:20
took a look these days , For b737NG, two altitude is the same.

15th Dec 2012, 20:27
Tried it yesterday. If u turn on the eng anti-ice of one engine, the values differ significantly. Now, imagine that u fly with eng anti-ice valve inoperative open and... get the eng failure. Isn't that an answer for the question itself?

Michal Black
29th Jun 2015, 17:59
The Answer is, by selecting L ar R eng out in FMC Cruise Page you just tell FMC from whitch engine should take informatiion for performace calculation.
Signals from (death) engines are false.
Good luck Mike