737-3/4/500 Flap load limiter

[eng1170]

A question I couldn't get a good answer for!

If the flap load limiter is U/S our MEL has 3 restrictions, I can remember 2 of them and it's one of these I wanted to ask the question about. They are a) don't use flap 40 and the one I wasn't too sure about the reason for is, b) RVR must be 300m or greater.

I know that the limiter is tied into airspeed in that with flap 40 selected and an increase beyond a set airspeed the flap retracts to 30 units until the aircraft is slowed down again

I can see the logic of not using flap 40, but how is the flap load limiter tied in to a requirement for a specific RVR. I've not been able to run this one by any of our crews.

Answers on a postcard!!

Eng

[IRRenewal]

RVR less than 300m means Cat 3A. Cat 3A is done with flap 40 in 737 classic to improve visibility over the nose and be able to see the lights.

So, the RVR requirement is really the same as the 'don't use flap 40' requirement.

Gerard

[Human Factor]

Just a guess, but your Vref is higher for flap 30, as is your nose attitude which means the visibility over the nose will be less. It's a few years since I flew a 737 but iirc, the nose attitude is significantly different. Hence you need to be able to see further to stop safely.

However, the above could be complete so hopefully someone can confirm!

[eng1170]

I'd be none the wiser either way but I'm understanding the logic here. One of the guy's who had spoken to Maintrol said he thought it was tied into the autopilot some how and in a round about way I guess it is if the requirement is for full Cat 3A.

Thanks

Eng

[alexban]

I didn't find anything regarding flap40 and cat 3A in the fctm or fcom.Did someone?
Maybe the improved visibility,is a point though.But we ,usually,land with flap30,so why use a not so trained configuration during a cat 3A approach?
Brgds Alex

[FlightDetent]

Sorry, but I understand that flap 30 does not provide for the mandatory 90m visual segment at min DH and min RVR for CAT IIIA. Whereas flaps 40 do through lower pitch by cca 2deg - compare to the standard 3deg GP. Therfore this selection is required just to get approval(!) for full CATIIIA and should be included in the appropriate section of your manual.

On the other hand, autoland capability is not tied to specific landing flap selection.

To sum up selecting flaps 40 will not give you more slant visibility but stretch the visual segment, giving you greatly improved ground reference. This can be a great help on any marginal MDA / DH / CAT. On a standard cat I you are already above the lights so seeing below will make great difference on counting the dreaded three lights.

[alexban]

fctm 5.23 :" Fail operational airplane is certified for Category 3B operations with two engines operating for flaps 30 or 40 landing,or,when certified,with one engine operating for flaps 30 landing"

I know this concerns the NG,but ,it's almost the same as classic regarding flaps 30 or 40 influence on deck angle.
Also ,the ils app/ldg geometry diagrams from Boeing are made considering flaps30 .vref30+5
i.e. 737-300 GP 2.5 airplane body attitude 3.7
GP 3.0 BODY ATT 3.2