Why no VNAV before flap retraction on 737NG?

[pez_man]

Hi!

I've been wondering about this for a while now; how come that you shouldn't engage VNAV before flap retraction on the 737NG?

I've heard that this has something to do with the commonality rating with the classic 737:s and that some airlines actually use VNAV to "walk through" the flap retraction speeds on the NG, just as it can be used on all other recent Boeings and Buses. Is this correct?

rgrds

/fredrik granfors

[Fat Dog]

Never heard of this. In my company we use VNAV before flap retraction on both the NGs and classics. VNAV will command initial climb speed, and flap retraction is based on set speeds in the classic and is aircraft mass dependant in the NG. What's the reasoning behind it?

[pez_man]

Hi again!

Hmm, I've been fishing around some more and this is one reason I've found for Boeing recommending against using VNAV prior to flaps retraction. (Haven't got a clue if it's correct though )

"However, note that on the 737, VNAV engagement before flap retraction is highly discouraged since the FCC does not have any knowledge of the position of leading edge devices. (placard
limiting only looks at the trailing edge flaps) Thus if a leading edge device fails to retract, there is nothing in any of the systems to prevent overspeeding the leading edge device. Once flaps are up and the crew has verified that the LE extended and transit
lights are extinguished, then it is OK to engage VNAV."

Does all the other Boeings and 'buses have more "control" over what's going on with the leading edge devices then? Or is the risk of overspeeding the leading edge device just deemed so unlikely that it's nothing to bother about?

This has gotten me more and more curious!

rgrds

/fredrik

[Fat Dog]

Thats interesting. Must admit I didn't know that was a Boeing recommendation.

Where did this come from?

[wing737]

Although we also use VNAV, I heard an old guys recomendation about closely monitoring the LED's position not to pass the limit speed if they stay extended in case of a failure.I haven't seen if its a Boeing ecomendation though but it makes sense.I saw a THY 737 which seriously damaged the LED's after passing the placard speed during a go around.
We mostly maintain 220kts after N1 selection as we retract the flaps on schedule.Then speed up with VNAV or else as convenient and as the situation requires.

[seat 0A]

IN our company, we only use VNAV after flap retraction, and before flap selection.
Both on PG and NG fleets. Don`t know why, though.