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Boris_The_Spider

EO ACCEL HT not observed by AFDS

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Hi,

 

In the 737NGX SP1c, with FMC U10.8A I am seeing behaviour that I don't believe is correct, regarding the speed commanded by the FD with an engine out. I also have a separate question regarding the behaviour of the autothrottle in this scenario.

 

With VNAV and LNAV armed on the ground, I set the sim up to give me a V1 cut, roll, have the engine failure, get airborne and the FD commands V2 correctly (rather than V2+20 for an all-engines takeoff). At this point, the active pitch mode is TO/GA, with VNAV armed.

 

At 500' AGL (baro) or 400RA (I'm not sure which, perhaps someone can clarify?) two things happen, and I have a question about both:

 

1. The active pitch mode changes from TO/GA to VNAV SPD, as this happens the speed bug moves to V2+20 - this seems reasonable (since we are now at a "safe height"), but can anyone confirm that VNAV SPD should be commanding V2+20 (rather than V2) with an engine out?

 

2. At the same time as the pitch mode changes, the autothrottle trips out. That doesn't seem right? Nothing has changed on the engine DU at this point - the N1 bugs are still at TO. I am aware that Boeing do not recommend the use of autothrottle with an engine out, but should it automatically disconnect (and remain disabled so it cannot be re-selected)?

 

Now, the thing I am pretty much sure is wrong is this: At EO ACCEL HT as set in TAKEOFF REF page 2 (set in this case to 800') I don't get an acceleration commanded, instead the speed bug remains at V2+20 (and that's what the FD pitch bar keeps commanding). It remains that way until 3000' AGL (the all-engines acceleration height, as set on TAKEOFF REF page 2) when the command airspeed bug moves up to clean manoeuvering speed and the acceleration is commanded.

 

The FCTM says:

 

On airplanes with FMC U10.8 and later, at engine out acceleration height, if VNAV is engaged, a near-level climb segment is commanded for acceleration. Retract flaps on the takeoff flap retraction speed schedule.

 

I believe on the real airplane, this was a bug (the FD behaviour at EO accel ht) but that it had been fixed in U10.8, so since the 737NGX is simulating U10.8A, I believe with an engine out, acceleration for flap retraction should be automatically commanded by the FD at the set EO acceleration height.

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