B738 Single Engine Fuel Calculations

[carnival30]

Hello,
If one engine fails lets say at FL410, driftdown is required to lets assume level 210 with speed 247 kts. For non Etops operations (B738), lets assume we have a suitable enroute alternate too within 60 mins flying time with single engine. My question is how this sixty minutes is calculated. If one engine fails (ENG no 1), then can Eng no 2 use left main tank fuel? This may sound stupid but this is really bothering me. There is no transfer in air for 738s. We are not opening the crossfeed valve (checklist doesnt say so) so how Eng no 2 uses fuel from tank no 1? Also why fuel burn increases other than the fact of Low Altitude? If Eng no 2 can use fuel from tank 1 then why theres a fuel restriction for single engine operation? Thanks for answering.

Yours sincerely.

[victorc10]

Hello,
If one engine fails lets say at FL410, driftdown is required to lets assume level 210 with speed 247 kts. For non Etops operations (B738), lets assume we have a suitable enroute alternate too within 60 mins flying time with single engine. My question is how this sixty minutes is calculated. If one engine fails (ENG no 1), then can Eng no 2 use left main tank fuel? This may sound stupid but this is really bothering me. There is no transfer in air for 738s. We are not opening the crossfeed valve (checklist doesnt say so) so how Eng no 2 uses fuel from tank no 1? Also why fuel burn increases other than the fact of Low Altitude? If Eng no 2 can use fuel from tank 1 then why theres a fuel restriction for single engine operation? Thanks for answering.

Yours sincerely.


Checklist says Fuel............Balance. How do you propose to do this without opening the crossfeed?

Sixty minutes is not about fuel, ETOPS is about is something else, for example whats the difference between an ETOPS 738 and a non ETOPS 738 with the same engines? Got nothing to do with Fuel, except that you need to have enough fuel to perform the various scenarios. Fuel increases due lower altitude, and increased thrust to MCT on the remaining engine, drag also increases due asymmetry, dead engine and control deflection.

[carnival30]

Checklist says Fuel............Balance. How do you propose to do this without opening the crossfeed?

Sixty minutes is not about fuel, ETOPS is about is something else, for example whats the difference between an ETOPS 738 and a non ETOPS 738 with the same engines? Got nothing to do with Fuel, except that you need to have enough fuel to perform the various scenarios. Fuel increases due lower altitude, and increased thrust to MCT on the remaining engine, drag also increases due asymmetry, dead engine and control deflection.

Balance only if needed. We are not keeping the crossfeed open the entire time right? So how the live engine takes fuel from the dead engine's tank? I dont know am I missing something?

Another question is after engine failure does the FMC fuel prediction for overhead alternate accurate? If so then do we need to open the qrh performance for that after re programming the fmc for alternate destination? Sorry asking too many questions.

[carnival30]

I guess lets just simplify the question.

Can one engine take fuel from both the main tanks with the crossfeed valve closed?

[Brookmans Park]

As with most a/c opening the xfeed enables the fuel from the dead engine side to feed the live engine and imbalance is maintained manually within FCOM limits

[wiggy]

I'm reluctant to add to the above because I don't know the 737 architecture but I'll offer that on the 777, which I'm guessing might possibly have vaguely similar plumbing, the Fuel imbalance checklist ( which you may well have to action post an engine failure) involves opening a crossfeed and then turning off the fuel pumps in the (wing) tank containing the least fuel.. as a result the live engine burns fuel from the "high" quantity tank until such time as you decide to stop balancing.

Any chance the 737 architecture is similar and would allow the same sort of procedure...? It should all be made clear in the FCOM/QRH and Flight Crew Training Manual.

(beaten to it by BPK....)

[NGsim]

Just read the checklist - all your answers are there. It advises you to balance fuel as needed - it’s then so kind as to tell you how to do it.
And as with all 737 checklists, in a non normal configuration you will find the words - Do not use FMC fuel predictions

[gearlever]

Look.....


https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/240x362/fuelpanel_200_eb5824f50215ec77bb2521ae9f90e203c2e7b583.jpg

[Australopithecus]

Sorry...are you a current pilot? Learning to calculate specific fuel consumption and hence range wasn’t taught in the first few weeks of flight training? How do you plan to manage your fuel considerations considering that the FMC does not know drag, and does not learn from the current performance?

[victorc10]

Balance only if needed. We are not keeping the crossfeed open the entire time right? So how the live engine takes fuel from the dead engine's tank? I dont know am I missing something?

Another question is after engine failure does the FMC fuel prediction for overhead alternate accurate? If so then do we need to open the qrh performance for that after re programming the fmc for alternate destination? Sorry asking too many questions.

Yes - once you have balanced the fuel, Keep the x-feed open. If you do not you will definitely get an imbalance. With the x-feed open you will also get an imbalance unless the combined fuel pump pressure between tanks is equal, which is highly unlikely, which means you may need to Balance the fuel as needed.

The live engine can not use fuel from the opposite tank if the x-feed valve is closed (unless the valve has a leak of course). Having the x-feed valve open does not constitute balancing it simply enables the engine to use all available fuel, balancing as per the supplementary procedures involves manipulation of the pumps in order to balance the fuel between main tanks, if you do it correctly.

With an engine out, do not use the FMC. Use the fuel flow to estimate your available endurance, after giving yourself an acceptable reserve. You should review the fuel regularly to ensure your predictions are reasonable, and especially if you change level or speed.

[Sidestick_n_Rudder]

Doesn’t the 737 FMC have Engine Out performance data?!

[wiggy]

TBH I've lost track of whether the OP was looking at the planning case or what to do in the real world post despatch ( as you imply either the FMC will have an eng out option or failing that once stabilised post drift down there should still be enough info available from the FMC and the Fuel Flow gauges (s) to calculate fuel overhead the alternate :ok:).

Back in the initial post the first query seemed to actually be an ask about the 60 minute rule and the ERA, but then we got into Crossfeeds...and here we are...:ooh:

[B737900er]

The burning question is,
Does the OP have a license, or is he/she adding to the statistic of people in that region with having a fake license?

[KRH270/12]

I guess lets just simplify the question.

Can one engine take fuel from both the main tanks with the crossfeed valve closed?




No! That’s what crossfeed valves are designed for.

And one eng. inoperativ cruise speed is 410kts TAS for alternate calculation... so fuel should never be a problem in the event of an eng. failure during cruise and correct flight planning ...

And why don’t you want to open the crossfeed valve?
Checklist states: „balance fuel as needed“

[Dave Therhino]

Just to quickly answer the original question, the radius of the 60 minute circle drawn around the origin, destination, and alternate/diversion airports for flight planning purposes is the distance the airplane flies in still air at an engine-out engine cruise speed selected by the operator and approved by their regulatory authority. The "sixty minute rule" is 14 CFR 121.161. FAA Advisory Circular 120.42B has an excellent discussion of the history and concepts behind this rule and ETOPS.