View Full Version : Flap speed protection on VNAV (767 Pegasus) ?

30th May 2007, 08:15

Need information on what kind of flap speed protection exists on the Pegasus version of the FMC (B767/757) when flying a VNAV coupled approach. (or even outside approach sub mode)

Does the protection exist in the normal VNAV modes (PTH and SPD), and if so, what is the margin of protection to present flap setting maneuvering speed (or is there any margin) ?

Or does the protection only apply for the VNAV "approach sub-mode", and if so, are the margins the same as in the normal VNAV modes?

Any and all information of this is welcome!

(I seem to remember that VNAV should only drive the CAB to present flap man. speed + 5, and continue slowing (obviously only if requested by the FMS, to for example the next hard spd+alt restriction) only after the next flap setting is out)


1st Jun 2007, 16:34
Please. Could someone help with this one?

Or doesn't anyone know :E ?


Fatter Bastard
2nd Jun 2007, 08:24
VNAV - In all modes will command flap limit speed minus 5 knots if commanded speed is above flap limit speed.

e.g. you are taking off at 185T in a GE flap 5. Assume you have entered a climb speed on the Climb page in the FMC of 290KIAS. When VNAV is engaged, say at 1000ft, the speed bug will increase to 225KIAS (our 767s have a 230KIAS flap 5 limit speed and a 250KIAS flap 1 speed - your variants may well have different speeds). Flap 5 to 1 is selected at 215 knots (again, in our GEs at this weight, maybe different in yours). The speed bug will remain at 225KIAS until flaps are at 1. Then speed bug moves to 245KIAS (Flap 1 limit minus 5). Flap 1 to UP selected at 235 and until flaps are up speed bug remains at 245 until aircraft is clean and then increases to 290 (or whatever is in climb page).

2nd Jun 2007, 09:05
Thanks FT, but I knew this about the protection in the climb case (as you very well it describe). And also the CLB case of flap speed protection is of course about not exceeding the structural flap speed limits. Nothing to do with monitoring the maneuvering speeds and flap retraction schedule.

However, I need information on, how the VNAV handles the situations where a speed restriction (coupled with an alt restr. of course, since the box doesn't accept spd only in this bird) present on LEGS would fly the aircraft to a speed that is below the maneuvering speed of the flap setting the aircraft at that time has.


Plane flying on FLAP1 at 200kts (CAB is set at 200kts). FLAP1 maneuvering speed (in this case) is at 190kts. It's on a descent towards a wpt PPRUN where we have 170/2200 in the LEGS page. In a couple of miles, the VNAV should command the CAB to 170kts to agree with the upcoming restriction. However, our flap setting would allow us to fly at only 190kts minimum.

How will the VNAV handle this situation?

- will it command 190kts (minimum)
- will it command 195kts (min+5kt buffer)
- will it command 185kts (min-5kt "leeway") <-- I doubt it! :=

There is NOT this kind of "maneuvering speed" related protection in the CLB phase, as you know. It just lets the pilot handle the flaps according to the schedule, and only protects from exceeding structural limits.

Is it so, that there is NOT this kind of "maneuvering speed" related protection in the descent phase either? Could it be, since I would think that it's extremely hazardous if VNAV commanded speeds below minimum maneuvering speeds... It's not so hazardous if the structural speed limits were not obeyed during the climb (of course it's really bad, but it doesn't subject the aircraft to disastrous situations like an impending stall), but I would think this issue is much more serious during deceleration.

If there isn't this kind of protection available, then the fourth alternative is:

- will it command 170kts, and assume the pilots are handling the flap extension as per the schedule ?

So.. I am not sure, if this kind of protection even exists. Wise men of PPRUNE, tell me how it is :).

Thanks for your comment!


flite idol
2nd Jun 2007, 10:23
I think the Boeing philosophy is similar on the 747. In any integrated mode ie FLCH, VNAV either an FMC constrained speed or a intervened speed (target speed) that is below the current configuration flap maneuver speed will not be met. These modes have stall protection and a higher speed (command speed) will be flown. The exact margin I do not know. However in a non-integrated mode such as V/S there is no such protection. ie if you set 4000 fpm climb and target speed of 300kts the autopilot will honour the V/S, eventually you will be at reference thrust and the speed will decay while the ship pitches for the V/S and a stall will surely follow!

2nd Jun 2007, 10:32
Thanks flite idol.

Maybe this flap man speed protection is annunciated in "ALPHA", but the VNAV would attempt to fly the plane below the min speed?

Or maybe it doesn't even attempt this, and stays above the min speed with a certain margin.

Probably if it went into ALPHA protection, that would be a little foolish, that would indicate that one system is driving the plane outside of limits, and another is protecting from it :).

So in all likelyhood there will be a margin. I wish I knew what it was, and how it was annunciated in VNAV (if such ann. even exists).


flite idol
2nd Jun 2007, 11:59
The only annunciation that springs to mind is the AIR SPEED LOW caution and amber boxing of the A/S window if you happen to venture below maneuver speed for the current configuration. I am not aware of any annunciation associated with the stall protection function. I`m sure someone will chime in with the correct answers eventually.

3rd Jun 2007, 16:50
How do the A/T and pitch limiting modes:


and the autothrottle state "F/S"

fit into this picture ?