737NG flaps speed VNAV departure
 

Hi,

It has come to my attention that if VNAV is used for takeoff, the autothrottle will command a speed of 230kts, at accel height, if flaps are 5.

My question is, why is that? What is this speed based on, since the max speed for flaps 5 should be 250kts?

Also, what speed will the autothrottle (VNAV) command for the other flap settings (e.g. flaps 1/2/10/15/25), and what are these speed based on?


Thanks in advance!

The search function is your friend.

230 kts for flaps 1 also.

No idea why. Perhaps it wouldn't be prudent to command 250 kts when the flap limit is 250?

Climb profile speeds targets
With VNAV armed for the climb phase, VNAV commands acceleration to :
- last MCP speed (V2) +20 kts until acceleration height
- the flap placard speed minus 5 kts
- 230 kts or less when leading edge flaps are not fully retracted
- 250 knots with flaps flaps retracted
- the active target speed
- waypoint speed constraints, or
- the speed restriction associated with the origin airport, whichever is more restrictive.

In your case 230 kts.

FCOM -> Chap 11

This was posted by Skyjob in 2015.


The LEDs are important in both scenarios.
However the takeoff with flaps greater then 5 selected allows departures with full extended LEDs.
Retraction failures are possible, mechanical problems or an environmental "impact" could result in a situation where one or more LEDs may not retract to partial, subsequently preventing a complete retraction, something which can be verified on the overhead indicators in LE stay in transit.
On departure, there is thus a risk that there could be one or more fully extended LEDs which you wil not find out about until retracting flaps up as I am sure you will not look during flap retraction on departure at the overhead panel to confirm partial retraction has been achieved...
Thus to prevent an inadvertent acceleration beyond 230kts with the potential that a fully extended LED is present, Boeing decided to prevent VNAV from accelerating above 230kts until LE TRANSIT/EXTENDED lights are extinguished.

On approach, the logic is in reverse.
The LE are retracted and extend to partial with the initial fall selections. They will only be fully extended with selection of flap 10, thus below 210kts, again preventing full extended LED in flight at speeds above 230kts.

Hope this helps

Thanks for posting it RAT5, just to add that this is valid for non SFP aircrafts only.

I'm not familiar with SFP a/c, where I believe there is a rescheduling of the LED's to go FULL EXT earlier than standard; I assume at F5 instead of F10. In either case the 230kts is to protect the LED's. SFP a/c LED's are at FULL EXT and standard a/c might have malfunction. In any case there's no rush on departure.

Agree on the no rush. SFPs have full ext LEDs only at Flaps 30&40 or Flaps 15 in case of alternate extension. The FMC logic for VNAV departures is still the same as non SFPs though.