E/O climb logic in NG FMC

[Hardy Heinlin]

Good evening,

as we know, when the engine out acceleration starts, the legacy 744 FMC sets VREF30+100, and the CON thrust limit will activate when reaching VREF30+98.

In the 744 NG FMC, however, when the engine out acceleration starts, the FMC sets VREF30+80 (flaps up speed).

That means, the NG FMC -- climbing at VREF30+80 -- will not reach VREF30+98 for the CON activation.

Does anybody know whether the NG FMC uses +78 instead of +98 for the CON activation?

Maybe it's company specific?

Thank you!


Regards,

|-|ardy

[andmiz]

VNAV commands flaps up manoeuvre speed + 20kt.  CON thrust occurs at flaps up manoeuvre speed +18kt.

[Hardy Heinlin]

Thank you. So Cathay uses the old logic?

Here's a quote from Boeing's Differences Document:

3.1. Engine Out
(NG FMC) If an engine failure is detected during the takeoff flight phase, the VNAV speed target will change at the Engine Out (EO) Acceleration Height to the flaps up full maneuvering speed limited by configuration. The VNAV page title will be ACT XXXKT CLB.

(747-400 Legacy) If an engine failure is detected during the takeoff flight phase, the VNAV speed target will change at the Engine Out Acceleration height to VREF30+100kts, limited by configuration. The VNAV page title will be ACT VREF+100KT CLB.

Or is "flaps up full maneuvering speed" the same as "VREF30+100"?
I.e. is "flaps up full maneuvering speed" not the same as "flaps up speed"?
If so, what's the point in the above quote?


Regards,

|-|ardy


Similarly, on the missed approach climb, they say the NG FMC will set VREF+80 (the legacy FMC sets 250). To me this looks like the NG designers want to get the aircraft more quickly away from the ground in case of a go-around or E/O takeoff.

[andmiz]

Recall on the 747-400, at heavy weights that full flap up manoeuvrability isn't achievable at weights above approx 315 ton unless flown at UP + 20.  Perhaps this wording is delineating this need.

Our books for both the -400 and -8 with the NG FMC's describe flap up + 18kt for thrust reduction.

[Hardy Heinlin]

I've seen VREF30+100 in other NG FMC operator's FCOMs as well. Do you think it might be possible that most FCOMs haven't been updated yet?

If I recall correctly, a couple of years ago some airlines wanted Boeing to release an NG Differences Document because lots of the NG details had been unknown to the pilots. It looks like most of these details are still not mentioned in the FCOMs ...


Regards,

|-|ardy

[dhob]

Again, this is the problem with the differences document that Boeing published back in 2012, in that it doesn't distinguish between the NGFMC on the 747-8 and the NGFMC on the 747-400.

The 747-8 flaps up speed, displayed as UP on the speed tape, is VREF +100.

The 747-400 flaps up speed is VREF+80, but VNAV accelerates to UP+20 knots.

For both aircraft, CON is automatically selected at VREF +98 knots with an engine failure.

[Hardy Heinlin]

The 747-8 flaps up speed, displayed as UP on the speed tape, is VREF +100.

Aha! Thank you.

Here's another NG reference to "flaps up speed":

8.2. Speed Target
(NG FMC) VNAV will target the flaps up maneuvering speed while in a missed approach climb. Upon reaching the cruise altitude for the missed approach, and sequencing all 'at or above' altitude constraints, VNAV will transition to CRZ. The speed target will become 250kts (or the best hold speed if a hold is in the flight plan).

So on the missed approach climb the NG FMC will set VREF+100?

What about the "flap up speed point" on the ND (green donuts on the descent legs)? Is that VREF+80 or VREF+100 on the 744 -- according to the language in that document?

[emerydc8]

On a somewhat related subject, the F/D on the 744ERF (Legacy) will not command more than 15 degrees of bank on all-engine takeoff until it reaches Vref30 + 100 when heavy (above 680,000 lbs).

Jon

[Hardy Heinlin]

Yes, that's a general 744 feature, and it's included in PSX. It also depends on the flap setting.


|-|ardy

[dhob]

For missed approach, VNAV accelerates to flaps up maneuver speed (UP), VREF plus 80 knots, until it transitions to cruise phase.

The green UP deceleration circle indicates VREF + 80.

The bank limit on takeoff is a function of the FCC's, and limits bank to 15 degrees when flaps are commanded too UP, and airspeed is between UP and UP plus 20.