Flaps also reduce dihedral effect.

I think on B737 winglets can make a difference - when they are fitted I suggest there is less spanwise flow so outboard ailerons more effective.

Pretty sure Batman's got it.

Pretty sure it's not just one thing but a combination of things. :ok:

Makes sense.:ok:

So we are back to PIO due to configuration change to F40 and a gusty wind?

Batman got it wrong. The 'arms' that control the thing are the ailerons. The spanwise mass distribution doesn't change when extending the flaps. The spanwise lift distribution may change, but doesn't affect roll sensitivity.

This is the correct answer.

Sounds like a rigging problem.

But he's not talking about 'control' he's talking about lateral stability. Lateral stability is reduced with flaps.

That, along with other effects of flaps, does increase roll sensitivity.

LSM,

I know nothing of stability, just find these explanations unconvincing. I take it you are talking of lateral-directional stability, i.e. the tendency to raise the low wing in a sideslip?

Another "interesting" ( 738W & non W) phenomenon which I found slightly unsettling the 1st few times I experienced it, is a strange buffeting (almost like pre-stall buffet) whilst rolling wings level at low altitude (say 300' on a circling approach) with F40 & approach power setting.

Noticed this often enough, & on several different aircraft, to put it down to imagination/individual rigging. . . . . and yes , I was keeping an eye on my IAS :rolleyes:

Flaps 40 is a big old barn door. Gusty winds, and the barn door (airplane) is going to roll. For gusty winds use flaps 30, duh.

HN, basically yes. Although I'm sure you appreciate it's a lot more complex.

As flaps are inboard any change in lift due to side slip is created further inboard and the resultant moment is a lot less. This reduces dihedral effect and lateral stability so it is easier to create a larger rolling moment, or displacement from the aircraft's flight path.

Of course, this doesn't only happen with Flap 40 but the effect is, apparently, more noticeable.

Most elementary I thought; pretty much a daily thing on approach in my previous life while flogging the Flogger for a couple of decades...worked well on an ATR too in my present innings..but when I pushed the rudder in the sim during my recent NG endorsement training the guy behind made me promise never to do that again!!
Having just begun as a 73 Capt; would like to know whats the big deal..
All Ears!!!

I think, as often, it's a combination of things. LSM and Batman are both right, more flap makes the effective part of the wing shorter, and with less dihedral (and with a more aft c of g, although not sure if that's an issue). The tip washout is increased, so here's my 2 cents: the ailerons have reduced effectiveness because the down-going aileron is not so much in the high pressure lower surface air than would be the case with less flap. (I may be wrong here but stay with me). So... to get the same roll authority, the spoilers have to do more, hence the increased sensitivity.

You can tell it's a wet and windy Sunday....

full flaps approach to land
 

I believe use of full flaps or avoidance of use due to wind gusts and turbulence is not limited to 737 or Airbus..going back to basics...even on a Piper Tomahawk and Cessna 150 the same rule of thumb applies...unless ofcourse a shortfield dictates full flaps..be they Flaps 40 or whatever config by design. On ATR42-300 series Flaps 45 was normally prohibited- only used strictly for Emergency (it was gate protected!).