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View Full Version : F30 or F40 in B737 strong xwind and short runways.


Paulm1949
20th Jun 2023, 13:38
What are peopleís preferences when it comes to short runways and strong gusty crosswinds? Do you except F30 with wider margins but possibility to float and land long or outside of touchdown zone or F40 but with tighter margins and possibility of blowback or flap exceedence with gusts etc?

BraceBrace
20th Jun 2023, 17:05
Flaps 30 doesn't float with the correct technique in crosswind landing. Trying to smooth out the landing does.

Flaps 40 only for steep approaches (or light ferry flights... maybe...)

Paulm1949
20th Jun 2023, 17:18
Would speed control be easier with F40? Even if the limits were closer ?

excrab
20th Jun 2023, 18:50
Flaps 30 doesn't float with the correct technique in crosswind landing. Trying to smooth out the landing does.

Flaps 40 only for steep approaches (or light ferry flights... maybe...)

Just had a quick check on our OPT before posting this. At the UK airfield Iím based at a fairly typical winter combination of low pressure and wet runways, given a ninety degree crosswind with a 15 knot gust factor (so as to need a speed increment of 15 knots), at max landing weight flap 30 exceeds the LDA even with max manual braking, whereas flap 40 can just be done, even a couple of tonnes below MLW it can be hard to get flap 30 figures to work in those conditions. Thatís for an 800 without short field performance kit.

So whilst the comment about flap thirty quoted above is correct the comment about flap 40 isnít entirely true. As a purely personal opinion Iíd prefer flap 30 if itís really gusty because of the greater margin between approach speed and flap limiting speed, but if you have to use flap 40 the technique is the same, land it firmly and donít mess around floating down the runway with one wing down trying to make a smooth touchdown, it just increases the chance of striking the flap track fairing.

Thats just a personal opinion, by the way, and others may disagree.

BraceBrace
20th Jun 2023, 22:02
Just had a quick check on our OPT before posting this. At the UK airfield Iím based at a fairly typical winter combination of low pressure and wet runways, given a ninety degree crosswind with a 15 knot gust factor (so as to need a speed increment of 15 knots), at max landing weight flap 30 exceeds the LDA even with max manual braking, whereas flap 40 can just be done, even a couple of tonnes below MLW it can be hard to get flap 30 figures to work in those conditions. Thatís for an 800 without short field performance kit.

Yes, valid points and indeed Boeing will refer to short field performance. However, if you compare the numbers, I'm not always sure what to think if the difference, especially since the values are "advisory". From the head: 2000m at max landing weight and a flaps 40 will give you 100m less?

When it comes to speed stability, there are two sides: a flaps 30 approach with gusts is safe to be controlled within tolerances. I never chase the speed down to the knots precise, I don't think it makes sense when gusts are playing around with the aircraft. The speed does not have to be exact, you get a gust correction to keep you safe and at the end of the "game" the aircraft dances around the average (Vcmd) with small corrections.

Flaps 40 might be a little bit more stable, and easier if you really want to fly Vcmd, but a hard gust can still send your speed all over the place and the corrections become harder. I find myself doing bigger throttle displacements. Low speed requires a lot more thrust, high speed and you might have flap load protection changing your config and there's this mental reaction to almost idle it to avoid it from happening. Don't really like that feeling.

Guess it's a style preference...?

Paulm1949
21st Jun 2023, 06:42
Would F30 float then if you canít keep the speed exact?

alf5071h
21st Jun 2023, 08:05
From a safety viewpoint - the risk of overruns is a significant statistic.

Use the configuration which gives the better distance margin; if wet, how wet, add additional distance margin for crosswind, turbulence, float.

Consider the effect of crosswind on stopping performance. See FSF ALAR notes on thrust reverse, the possibility of having to reduce reverse thrust. n.b. assumptions in timing in use of lift dump, thrust reverse, and braking.

Any reference to advisory stopping distance suggests use of inappropriate performance data.
See thread on Touchdown Point below: Operational Landing Distance.

Fly the aircraft; being aware of possible float, and difficulties in crosswind.

BraceBrace
23rd Jun 2023, 09:37
Floating risks are present flaps 30 and 40. Wet and contaminated runway require positive landings. Not hard, not smooth, but correct ítouchí. I have seen people float with flaps 30 and 40 because of trying to be too smooth with the excess speed, which happens quickly in strong winds and gusts. You float too much, you go-around. Do what you feel most comfortable with. Flaps 40 landings in heavy winds is not the easiest approach if not done regularly.

Cough
28th Jun 2023, 08:48
Yes, valid points and indeed Boeing will refer to short field performance. However, if you compare the numbers, I'm not always sure what to think if the difference, especially since the values are "advisory"

Advisory - Is this because you're using an app which is advisory and if things are tight then you've got to do some maths from the QRH? Just curious as landing performance shouldn't be advisory!

BraceBrace
28th Jun 2023, 10:12
Advisory - Is this because you're using an app which is advisory and if things are tight then you've got to do some maths from the QRH? Just curious as landing performance shouldn't be advisory!

In "planning" all data are based on regulatory data with specified factors. If you would take a look into the FPPM, you would see "planning" wise, we don't really talk about distances, we talk about maximum weight values (field limit, climb limit, obstacle limit,...).

If you look into the FPPM, you will also see that for "enroute" or "inflight", there are landing distance tables available with (other - lower) factors applied or not. These tables clearly mention "advisory", even in the FPPM, because they are meant to be easy to use "inflight" (not in planning). If you look in your QRH, chapter PI (performance inflight), the name of the chapter including all distances (normal and non-normal) is literally called "Advisory Information".