B737 LE FLAPS TRANSIT QRH question?

[The Mad Russian]

The QRH (Classic) calls for a Flap 15 landing with a LE Flaps Transit Light illuminated.

As the trailing edge flaps are still being extended by the normal flap drive if you follow the NNC, this requirement can’t be because of go-around considerations? i.e. slow movement due use of the electric TE flap drive.

Are there other reasons (aerodynamic?) why the TE flaps should not be at F30 or F40 (well beyond 15) with a LE device at the extended, rather than the fully extended position? Or is the check list written for the worst case of a complete LE Flap/Slat failure with the device or devices in the fully retracted position?

I’m sure someone will know the answer, looking forward to your replies.

[Seat1APlease]

I suppose that if the leading edges are not properly extended then the stall speed may be higher, in which case the faster flap 15 approach speed may be more appropriate, but that is just an initial guess.

[LEM]

With asymmetric lift, there will be a tendency to roll.
The higher F15 Vref provides for better control margin (ailerons more effective).

Regarding the higher stall speed, that is covered by the additional 5 knots added to Vref 15.

[theWings]

Notwithstanding the asymmetic considerations, how about this: the LE flap will move the CoP forward and the TE flaps move it back, the two working together ensuring that the CoP doesn't move too far (back). If the TE flaps aren't fully extended and the LE flaps are extended beyond 15deg, the resulting balance of forces of would be outside certification limits. Practically speaking, could an otherwise legally loaded a/c suffer loss of authority in pitch in these circumstances?

tW