B737-800 with Flap 40: what causes increased roll sensitivity?
 

On approach with the -800 today the winds on approach were gustier than would have been anticipated from the tower wind given. In these conditions the -800 is noticeably more sensitive in roll and seems to be a lot easier to over-control. I noticed that the -700 does not exhibit this trait to the same degree. The aircraft sets up an oscillating roll similar to Dutch Roll if the pilot is overcompensating for the twitchiness. Does anyone know the explanation for this aircraft characteristic (either design-related or aerodynamic?)

Thanks!

Can't remember the tech side of the B737 very well but could it be pilot induced oscillation brought about by over correcting and breaking out the spoilers on the down going wing?

Agree. Have seen this (PIO) countless times in the simulator and not necessarily only when gusts fed in. One way the instructor can demonstrate this is by setting a visibility of 4 kms for an ILS. As soon as the approach lights appear in the gloom and no visible signs of a horizon I estimate 90% of pilots start the wing waggling which only get worse. The fix is to momentarily relax the death like grip on the control wheel and the problem goes away.

Had one very experienced pilot caught in the simulator with PIO and eventually he was pedalling the rudders as well and he crashed in the go-around. He accused the instructor of putting in un-briefed windshear. But the wind was zero. Tried it again and sure enough as he saw the approach lights he was off into severe PIO. He was quite convinced the instructor was pulling his pisser and sneaking in wind-shear.

Now this bloke had a real problem with temperament and he began to lose his cool. So the instructor put him on a ILS five mile final CAVOK and lo and behold no signs of POH. When the instructor then told him the wind was unchanged - that is no wind at all - the pilot refused to believe it was his own fault. You get those personality types occasionally.

I've seen guys who make very, very few mistakes (and been appropriately jealous) but I can't recall anyone who never made any at all ...

No point in arguing the toss as to how great one is .. the other guys have already seen one's screw up(s) from their relaxed non-PF seats of comfort.

Surely the only sensible way ahead is to admit the deficit to oneself, fix it, and move on ?

And we all had a bit of trouble getting on top of the brawny arm PIO during endorsements and early training flights ...

Aerodynamic 'twist' moves the aileron neutral point 1.5 degrees to one wing(we'll say it's the right wing). If the roll spoilers would typically come up at 4 degrees it's now shifted. Neutral roll is now 1.5 degrees to the right so a roll input of 2.5 degrees to the right starts the right roll spoilers to deploy. A roll input to the left doesn't deploy the roll spoilers until 5.5 degrees(4 plus 1.5).

Numbers aren't exact but that's the basic concept.

Flaps 40 can be humbling or an opportunity to show off. And sometimes the attempt to demonstrate your skill ends up being very humbling. :sad:

Jeez, I've never noticed any significant difference :O

That is new to me too.

I've never noticed a difference, about one in five of our landings are flap 40 due runway length.

I noticed it when I was doing my IOE. I even got myself into a short but noticeable PIO.

It was probably in a -400, but it was at F40.

Yep, I've experienced this as well.

Following the F/D too abruptly makes things interesting. I just put the track bar on the ND between the runway edges with the ND range on minimum with LOC in the middle, and let the F/D come to me instead :E

I was visual and on short final with roll oscillations of maybe a bit less than a second.

Are you talking about PIO, or just over-correcting on a LOC.?

I find very small rudder inputs to be much more effective in this situation. I land flaps 40 almost every time.

Great info, thanks guys

:ok:

That was given to me as a tip in simulators generally, squeeze a bit of rudder in to regain a drifting LOC !:)

Quantifying these oscillations and corrections in flight and simulator is worthful. To learn, to correct, to report, and to improve air safety in and out the airline .

I'm fairly sure this is an aerodynamic effect (not just the 737 affected by flap) and was taught to us during the QFI course at RAF CFS. Damned if I know where my notes are now though, and as that was over a quarter of a century ago my brain has long dumped the explanation from the frontal lobe.

I use flaps when runway exit/length warrants its use,to reduce AB use.
I dont use flaps 40 when airport has close in habitation(noise sensitive),possible bird strikes(engine failure) and gusts...
Yes i feel the ailerons to be much more responsive with flaps 40,a gust correction(wing low)can be corrected with quick input but with flaps 40,a slower gentle input is required to avoid some overcontrolling,the controls do feel mushier with flaps 40.

I think the reason is similar to the explanation that the aircraft is more likely to drop a wing at the stall with flap than without...

...something just moved in my old, ex-QFI brain...

Shy, something to do with lift distribution (lift polar) along the wing and lateral stability?

:ok:If you were a weight lifter you would be far more stable if you held the weights with the arms, above your head, as far apart as possible. If the arms are brought closer together it becomes harder to stop the weights rocking. when flap 40 is selected the centre of lift moves along the wing towards the fuselage, therefore it becomes move sensitive to lateral displacement.
There are notes in Airbus manuals advising using Flap 3, instead of Full flap, in turbulent cross winds.

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!).