High Altitude Loss of Thrust on both.

[framer]

For the 737-800 pilots.
If you had Loss of Thrust on Both Engines at FL 400 would you aim for 275kts immediately even though that would put you through MMo while you carried out the checklist ( memory checklist that is), or would you sit on MMo while you did it?
I had always thought I would sit at MMo but some Boeing stuff I'm reading seems to indicate that it would be a good idea to prioritise the re start and exceed the speed. In my mind I can't think of much worse than setting off the clacker in that situation.
Your thoughts appreciated.

[Bula]

I don't fly CFM engines nor do i fly a 737. however, i gather you a talking about a failure of both engines. What seems to be common is the scope of the restart/relight envelope.

Firstly, Mmo exceedance..... Really? A. What's the Md on the 737, B. why would you want to be anywhere near there? But any case I think you will find a Mach/IAS limit on the relight envelope.


Secondly, what is the upper limit of the restart/relight envelope? I would suggest you may need to be below FL300 before the initiation of a successful relight.

[nitpicker330]

You NEVER intentionally operate the Aircraft outside its normal operating envelope. :eek:

So, no you would ALWAYS respect the Barbers pole. ( unless you are too young to know what a Barbers pole is !! )

[framer]

Heh heh, cheers for the replies.
I didn't expect quite so much emotion but that's good fun :)
To explain a bit, I've been reading a document about re lights in the case of loss of thrust on both engines and it went into some detail about the importance of initiating the relight immediately in order to take advantage of any rotation and in a later paragraph touched on the importance of achieving the 275kts for airflow.
It made me wonder if I had mis interpreted the intention of the checklist. A few hours later and looking back I agree that going 5 or 10 knots into the barbers pole in order to achieve the IAS is a silly idea, much more sensible to wait a bit until the available IAS opens up a bit while carrying out the restart procedure anyway. I feel suitably chastised and will revert to plan A and slink away to my corner for an hour or two:O
The barbers pole call was a bit tough though Nitpicker, I've spent my share of time hugging them with the power levers up in turbo props.
Have a good one,
Framer

[de facto]

So, no you would ALWAYS respect the Barbers pole. ( unless you are too young to know what a Barbers pole is !! )


....true,never intend to deliberately exceed MMO,however the FAR 25 gives a .07 margin with i think a 1.5 G for an aircraft to be certified...so going into barber pole all the way to M.89 in a 1.5 G maneuver and you are still within the certifiaction envelope...no need to sh:mad: in your pants either.

The barbers pole call was a bit tough though Nitpicker, I've spent my share of time hugging them with the power levers up in turbo props.
Me too however the older i get the slower i tend to fly...getting more and more economical in a sense:E

[vilas]

Framer
I am not 737 pilot but I suppose CFM engine on Airbus A320 and 737-800 is same. Your priority is to get a relight but the relight envelope for CFM starts from 25000ft. So above that is no relight zone. Bursting the MMO at 400 is meaningless, you won't achieve relight.

[de facto]

Bula,your sensible answer is sooo refreshing,however above 25 000 ft restart are not guarenteed but possible...

[fleigle]

Would you not need to descend sharpish anyhow to preserve the cabin pressure?:ooh:
f

[de facto]

You would anyway with 2 engines out at 40000 ft.no?:E

[Denti]

@vilas, the 320 series uses the CFM56-5 series whereas the 737NG uses the more modern CFM56-7 series. So no, they are not the same engine. The last variant for the NG (CFM56-7BE) was only introduced in 2010 and among other things it brought a change to the inflight start envelope and therefore relighting with a loss of thrust on both engines.

The checklist calls for 275kts above FL270 and 300kts below, however as others have pointed out that is not meant to exceed MMO.

[WeekendFlyer]

Don't!

Or to put it in more detail, at MMo you will probably already have shock waves forming on the upper surfaces of the wings and tail. You might even have shock waves forming on the undersides as well. If you exceed Mmo they will start to move back towards the trailing edge and grow larger. As this happens the aircraft will experience a nose-down pitching moment that could eventually become too large to counteract; this phenomenon is know as Mach Tuck and can be lethal if an aircraft is not designed to deal with it, because the nose down attitude leads to a speed increase, leading to a greater moment, leading to greater speed and so on. It can become "unrecoverable".

You can also experience effects such as reduced effectiveness of flight controls, or even reversal of their effects, as the shock waves approach the ailerons and elevators.

So, if the choice is some or all of the above vs a loss of altitude to achieve engine relight IAS, I know which I would choose! And pressurisation loss should not be a factor; there are one way valves (NRVs) in the ECS to stop air going the wrong way and out through the engines, and on a modern aircraft it should not leak out anywhere else at a rate that would be cause for concern. Having said that, I was once on a VC10 K3 that was completely maxing out both cabin compressors just to maintain pressurisation at FL380. If we had lost one we would have been descending sharpish!

[framer]

Well I think we've fairly well confirmed that it was a ridiculous notion to begin with. Lets forget about the original question quickly so that I don't look even more foolish. Some other points worth discussing have come up;

Your priority is to get a relight but the relight envelope for CFM starts from 25000ft. So above that is no relight zon

It's worth dispelling that for any 737 NG pilots. This from the FCTM
The inflight start envelope defines the region where windmill starts were demonstrated during certification. It should be noted that this envelope does not define the only areas where a windmill start may be successful. The Loss Of Thrust On Both Engines NNC is written to ensure that Flight Crew take advantage of the high RPM at engine failure regardless of altitude or airspeed. Initiate the memory portion of the LOSS OF THRUST ON BOTH ENGINES NNC before attempting an APU start for the reasons identified above.

[lomapaseo]

Follow SOP if it covers the situation, else;

Assess the situation and the likelihood of restart in the published restart envelop. All engine models are not equal. The only cert requirement is that they publish a restart envelop as certified.

Achieve that envelop ASAP but do not damage your engines by continued un

successful restart attempts outside that envelop[ (EGT exceedences). If and only if you run out of restart options in the published envelop then configure for optimum glide that has a high degree of chance of a dead engine landing

[framer]

The Boeing books have you attempting relight 17,000ft above the published restart envelope.

[Brian Abraham]

The Boeing books have you attempting relight 17,000ft above the published restart envelopeMight possible core lock be an influencing factor?

[vilas]

Framer
What I quoted is from Airbus presentation. I stated before that I have not flown 737 so may not be applicable to 737 aircraft but applies to Airbus. There are no memory items for dual engine fail in Airbus.
Vilas

[Kefuddle]

It's worth dispelling that for any 737 NG pilots. This from the FCTM
Just to add to that, Boeing seems to be quite clear with specific guidance of a loss of thrust from both engines, that prompt action is necessary regardless of airspeed or altitude in order to capitalize on high engine RPM.

And pressurisation loss should not be a factor
I think one should expect the cabin to climb at around 1500fpm. So not an immediate concern and I certainly would not delay a restart attempt to deal the pressurisation. But the cabin altitude situation probably will need some management soon, preferably after the initial start attempts.