B747-400 shift in Flap manoeuvre speed.
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B747-400 shift in Flap manoeuvre speed.
Greetings to all the B747 crew! Seeking your wisdom with this question:
Why do all the flap manoeuvre Speeds (From Up speed through each flap speed ) Jump or Shift when choosing Flap 25 as approach REF as against Flap30? -B747-400s (I have not come across this on other Boeings)
Appreciate your response. cheers
Why do all the flap manoeuvre Speeds (From Up speed through each flap speed ) Jump or Shift when choosing Flap 25 as approach REF as against Flap30? -B747-400s (I have not come across this on other Boeings)
Appreciate your response. cheers
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From: France
To keep the increments from Vref the same?
(Vref + 80, etc.)
I seem to remember that we were told that it was ok to hold below the UP bug, if Flap 25 Vref had been selected, as the “real” Vref increment was based on Flap 30. My last 744 flight was in 2011, so might be mistaken.
(Vref + 80, etc.)
I seem to remember that we were told that it was ok to hold below the UP bug, if Flap 25 Vref had been selected, as the “real” Vref increment was based on Flap 30. My last 744 flight was in 2011, so might be mistaken.
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Thanks for the response, I agree.
If one is flying at up speed and selects flap 25 as the REF flap, one automatically ends up flying below the FLAP manoeuvre speed. There is no real danger of stalling, but it is a bit unsettling.
I am looking for reasons for such a design. ( early FMC design limitations, perhaps)
I am not sure if this is a carryover from the earlier 200/300 models.
If one is flying at up speed and selects flap 25 as the REF flap, one automatically ends up flying below the FLAP manoeuvre speed. There is no real danger of stalling, but it is a bit unsettling.
I am looking for reasons for such a design. ( early FMC design limitations, perhaps)
I am not sure if this is a carryover from the earlier 200/300 models.
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From: Germany
The FMS is programmed not very sensible in this. The cleanest way would be to have the pilot monitoring to select F30° when it helps you to maintain a smaller flap setting for longer, and then have him or her reselect F25° when extending them further. My company did make a request for Boeing changing the flap speed schedule to always stay on F30° reference except for landing flaps. Don‘t think it’s gonna happen, though.
And yes: on the -100/-200/-300, the schedule was always based on Vref30°, except for the Vref itself when landing with F25°.
And yes: on the -100/-200/-300, the schedule was always based on Vref30°, except for the Vref itself when landing with F25°.
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@boeing747430 Thanks for that reply.
Here is what I managed to recover from one of the Boeing Flight Ops symposium. Oct 2002 Q&A session
Q:Flaps 25 vs. 30 landing, why do reference speeds change? In 767 all speeds are reference to F30, inconsistent for 747.
A: When the 747 Classic was developed, it was a Vref airplane. There was only one Vref chart. When the Vref was increased because of non-normal conditions, all the flap speeds, which were based on the Vref, changed also.
When the 767/757 was developed, Vref30, Vref25, and Vref20 were developed. The maneuvering speeds are referenced to Vref30. If the landing speed is increased because of a non-normal, the various flap speeds do not, because they are still based on Vref30.
When the 747-400 was developed, to maintain as much commonality with the 747 Classic as possible, the flap speeds were based on the reference speed. If the reference speed, whether Vref30, Vref25, is increased because of a non-normal, the flap speeds increase the same amount. This copies the 747 Classic, instead of the 757/767.
Here is what I managed to recover from one of the Boeing Flight Ops symposium. Oct 2002 Q&A session
Q:Flaps 25 vs. 30 landing, why do reference speeds change? In 767 all speeds are reference to F30, inconsistent for 747.
A: When the 747 Classic was developed, it was a Vref airplane. There was only one Vref chart. When the Vref was increased because of non-normal conditions, all the flap speeds, which were based on the Vref, changed also.
When the 767/757 was developed, Vref30, Vref25, and Vref20 were developed. The maneuvering speeds are referenced to Vref30. If the landing speed is increased because of a non-normal, the various flap speeds do not, because they are still based on Vref30.
When the 747-400 was developed, to maintain as much commonality with the 747 Classic as possible, the flap speeds were based on the reference speed. If the reference speed, whether Vref30, Vref25, is increased because of a non-normal, the flap speeds increase the same amount. This copies the 747 Classic, instead of the 757/767.
