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.