EO Accel Ht, B737 and VNAV

[ImbracableCrunk]

My airline has been using VNAV on takeoff for a while now, but not much has changed as far as manuals and sim training in regards to VNAV and engine failures.

On TAKEOFF REF 2/2, you can enter the EO ACCEL HT. What does that do? I've looked in the FCOM, FCTM, and my POM, but there isn't much to go on.

The FCOM simply says, ". . . VNAV will be enabled to automatically adjust the target airspeed profile upon engine out detection."

I understand the V2 to V2+20 bit, but does VNAV give you an accel pitch at the EO ACCEL HT?

[repulo]

Hi Imbracable

VNAV conbined with an engine failure will command an acceleration for flap retraction, giving you close to level flight, but never a descent command via the FD or AP.
There are some traps included in VNAV take-offs:

As far as I know Boeing is not reccomending VNAV T/O´s on the NG since it will not give windshear alerts under certain circumstances. This bug might be fixed on the next update.

Also, if an immediate turn is required being one engine out, the turn radius is usually based on V2-V+20 plus some additional addatives, so accelerating in that phase might bust the obstacle corridor.

If using LNAV as well you should have a procedure to cross check the one engine out profile before T/O since it will give additional stress if the special procedure is not in the box.

My company did LNAV/VNAV T/O´s for a while but went back to 400´LNAV, 800`VNAV since there are too many problems with that procedure. But of course, that is only valid for may company and the software we are using, don`t know about the others.

Hope that helps,

regards

repulo

[ImbracableCrunk]

We're still using VNAV, but we were issued a memo about hitting TOGA again for windshear.

What is the pitch mode during the EO? Is it still VNAV SPD? What speed is it commanding? VZF? 230 until clean?

We have several airports that have 180 turns after takeoff for EO SID, and I'm concerned the same way you are.

I don't want to play the "What is it doing now?" game.

[A37575]

I don't want to play the "What is it doing now?" game.

That is easy fixed. Rather than gaze at the FD and wonder what it is trying to get you to do, simply switch it off and hand fly raw data until the aircraft is cleaned up and on the track you require. At your convenience re-introduce the automatics as needed. If you feel underconfident about your raw data manipulative skills then that becomes your problem and needs to be sorted in the next simulator session.

[ImbracableCrunk]

That is easy fixed. Rather than gaze at the FD and wonder what it is trying to get you to do, simply switch it off and hand fly raw data until the aircraft is cleaned up and on the track you require.

Really? Can you turn that stuff off? :rolleyes:

At your convenience re-introduce the automatics as needed. If you feel underconfident about your raw data manipulative skills then that becomes your problem and needs to be sorted in the next simulator session.

Or I could anticipate what the system is going to do. Then I'd know ahead of time whether I want to use the automatics before I'm forced turn them off because I didn't like what it was giving me.

I don't need a lesson on instrument flying, I need a specific answer to a systems question.

Thanks.

[Avenger]

Thıs system ıs descrıbed very well ın the B737 systems manual under navıgatıon. It tells you the speed profıle ıt commands and the logıc behınd ıt. Chapter 12 sectıon 11.41
As for raw data flyıng an EFATO..why would anyone do that

[ImbracableCrunk]

Thıs system ıs descrıbed very well ın the B737 systems manual under navıgatıon. It tells you the speed profıle ıt commands and the logıc behınd ıt. Chapter 12 sectıon 11.41

That's the section I was looking at as well, but I can't find much beyond:

if an engine failure is detected, target speed will be last MCP speed (V2) +20 kts if the airplane is at that speed or greater, or the existing speed if the airplane is between V2 and V2 + 20 kts


I want to know for what the FMC uses the EO ACCEL HT info.

[ninja_turtle]

What I could get from our FCOM is that for VNAV armed for takeoff it commands the following speed schedule :

1) V2 + 20 knots till all engine accleration height

If an engine failure is detected until reaching the EO accleration height , target speed will be
a) V2 + 20 knots if the speed is at that or greater or
b) existing speed if the aircraft is at a speed between V2 and V2 + 20

Thereafter :

Will command an increase to 250 below 10000 if feed in but will limit it initially to

2) Flap placard speed minus 5 knots ( Flaps 15 and 25 take off's)

3) 230 knots till the time LE flaps are not fully retracted

4) Finally the enroute climb speed after FL 100

More importantly to answer your exact question , the FMC used the EO Acleration information , to ascertain at what height it must start the accleration in case of an engine out scenario .

If it is below that height , it will command a climb at a speed discussed above , when it gets to the height it will begin the accleration .

If the all engines accleration and Engine out accleration height is different , it will acclerate at the appropriate height depending on the case .

I hope the answer fulfills your query .

[ImbracableCrunk]

For the one or two interested, I finally found Boeing's answer in the FCTM in the paragraphs "Flap Retraction - One Engine Inoperative" and "Flaps Up - One Engine Inoperative." (Pg. 3.35 in my copy.)

As repulo stated, at the EO Accel height from Takeoff 2/2, the FD commands a near-level climb and after flaps up, it commands a climb at maneuvering speed.