Uncommanded slat deployment

[DeltaTango]

....ok, What happenes?

Will the nose pitch up?
Will the pilot have to to pitch up due to cord change?

I'm just reading a book and the authors is'nt keeping to the facts. However, before I trash it completely I want to be sure.....


Thanx for ya help pprunies

:)DT :)

[Stroppalot]

That'd be Airframe then...

[Hung start]

Howdy´ DeltaTango, how´s life in sunshineland?

Well, my aircraft pitches UP when deploying the slats. Whether "commanded" by me or "uncommanded", wouldn´t make any difference :)

Flaps puts the nose down!!

Now, slats only extending on ONE side, is a totally different different ballgame!

Vi tales ved. Rgds. Hung

[DeltaTango]

HUNG!!!!!!!
How goes?
I'm fine, and have a nice trip on the 28'th...vi ses.

Stroppalot - bingo...any good?

cheers
DT

[747FOCAL]

Depends on what speed you are at. If you are in cruise like that 727 the Pilot and Co-Pilot were screwing around on with passengers in the back, you will end up doing a Split S and loosing about 14,000 feet of altitude and a few of the slats themselves.

[Hung start]

Talking to DeltaTango or me 747FOCAL??

[Lu Zuckerman]

I can’t say what would happen if the uncommanded signal used the full power of the power control unit forcing the slats out against the oncoming air loads. However if it were an A310 that had an internal leak inside the power control unit a single hydraulic motor would drive at a very slow speed and as a result would be incapable of moving the slats against the oncoming air loads at cruise speed. If however the aircraft slowed below cruise speed where the developed force could move the slats the aircraft would be in a speed range where the slats would normally be deployed. If the command sensor unit were in the flaps and slats retracted position you would get an indication of the uncommanded deployment but the computer could not stop the slat movement.

If however you had the same condition on the flap system the computer would not stop the movement. When the flaps were sufficiently deployed the following air loads would assist the flaps in deploying and when they got far enough deployed at cruise speed you have just bought the farm along with the SLF in the rear end.

If you don’t believe me just respond to this post.


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The Cat

[Hung start]

Well thanks, but the question was: will the aircraft pitch up or down in an (un)commanded slat deployment!!!
UP is my answer. Would be interested to know if this is any different in other aircraft.

If hyd. power is enough or not enough to deploy the slats, was not the question here, unless I misunderstood something. I that case I apologise.

[pterodactyl]

The DC9-30 max slat extend speed was 280kts as I recall. Extending slatat high speed (not recommended) caused pitch DOWN.
Extending slat at the recommended speed of about 210kts resulted in pitch UP.
As noted before it depends on the speed.

[cb747]

Take a look at http://www.airdisaster.com/cgi_bin/view_details.cgi?date=04061993&airline=China+Eastern+Airlines

[Airbanda]

DT,

Read the book, can't vouch for absolute technical accuracy but it was at least plausible. I'm only a hobbyist so not qualified to say too much but judging from the comments of professionals effect of uncomanded slats deployment depends on the a/c type, speed etc. Were they assymetric as well?. The journo types rang some bells as well!!

Stick with it,you need to get ot the end to fully appreciate the plots.

[HOMER SIMPSONS LOVECHILD]

I believe it was the MD11 that had to redesign the flap/slat actuation lever after a few incidents of accidental deployement in the cruise.The position of the FO's coffee cup holder and the wearing of long sleeved shirts were all factors if I remember correctly.Regardless of that a violent pitch up is the typical airliner response.This may well be followed by a high altitude stall with all the fun and games involved in that.These type of incidents are not uncommon and will almost certainly result in death or injury in the cabin and major structural damage to the airframe.

[DeltaTango]

Thanx for all the info.

:)DT :)

[Flight Safety]

OK, here's a guess.

When slats are deployed (without flaps) two changes occur that affect the wing, there's a chord line change and a center of lift change.

The chord line change (from new leading edge to old trailing edge) will result in a small reduction in the angle of incidence of the wing, relative to the fuselage. In high speed cruise where the AOA is low, extending the slats lowers the overall AOA and would normally result in a pitch down movement.

The center of lift change occures because the wing's leading edge is extended forward when the slats are extended. This moves the center of lift forward by a small amount. As the AOA increases and the stagnation point (the point where the airflow divides above and below the wing) moves farther down the leading edge, the center of lift change becomes somewhat more pronounced. In low speed flight where the AOA is higher, then the center of lift change is normally more pronounced than the small reduction of the angle of incidence. Thus you get a pitch up movement when the slats are extended under these conditions.

Each individaul aircraft will vary as to what airspeed and AOA these two opposing forces would cancel each other out, so that above this airspeed and below this AOA you get pitch down, and below this airspeed and above this AOA you get pitch up.

I hope this makes sense...

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Safe flying to you...

[john_tullamarine]

came in late ... not aware of the book to which members are referring. However the comments seem to be a little simplistic ... the problem comes down to one of what effect the slat/slot combination has on the wing upwash/downwash and the tailplane downwash field ... in addition there will be some changes in the drag distribution ... these .. with the thrust line and cg position ..will determine the overall effect on pitching moment ..which is the concern .. too complex a story to come up with simple outcomes ..

[DeltaTango]

FS-wount the stagnation point go up and back......thus creating the down pitch?!??!

...or maybe I just missed something in your explanation which is seems otherwise correct.

The thing is that the book describes a sim flight in which the slats are deployed with an autopilot failure.
it describes the nose to go up and then the pilot pulles the nose up further which is supposed to be the right thing to do.

Why is that? is the nose up only the first reaction to be followed by a nose down secondary reaction?


DT

[kabz]

Hope this isn't dumb, but won't the primary factor affecting the aircrafts behavior be the change in AOA of the wing after slat deployment ?

If the slats effectively lower the AoA, then won't there be a pitch up as the airplane tries to fly back to trim... There may be another pitch up factor as the wing is generating more lift (?) and will want to slow down even further...

Maybe these last two are the same thing...