Slats deployment near critical AoA with flaps set to 0

[MaverickSu35S]

Hello,

I am an aerospace engineer, aerodynamicist and glider pilot. We all know what the slats are there for right? For increasing the AoA (angle of attack) at which the wing meets airflow separation, or for short to increase the stall AoA. Of course that every benefit has it's own deficiency! Never in this world you'll find something that does only what you need. The slats produce two more unwanted effects which are profile drag and a higher nose down pitching moment which will require a higher pitch trim that also slightly increases drag. Another inconvenient would be that of a structural load limitation on the slat itself during in flight operations, limitation which may not allow for the extension of the slats above a certain dynamic pressure that may damage it.

The question is: Why do all airliners, both old and new designs, only allow the activation of the autoslats when the flaps setting is different than 0 (at least set to 1 unit). The "autoslats" is a term for the deployment of the slats to full extension once the DFCS (digital flight control system) senses that the alpha (AoA) has gone past a certain limit (which is still below critical anyway). This system is designed to not allow the autoslats to operate, no matter the AoA, if the flaps have not already been set to a unit different than retracted.

I am not personally convinced why this happens! Fighter aircraft are using autodroops (similar to slats but are simply a plane leading edge flap and don't travel forward like a fowler (which is a slat)) whenever the angle of attack has passed above a certain value at whatever airspeed, which leads me to think that the structural stress on the droops isn't of concern even if the fighter might be flying supersonic (Mach > 1.2). The slats indeed extend forward (being a fowler type high lift leading edge device) and downward in order to give better aerodynamic performance than a simple leading edge droop, and yes, because of this feature it's structural links and driving mechanism might not withstand the aerodynamic loads (structurally) as easy as a droop, but is this difference really that high in order not to allow the slats to be deployed above a certain IAS?

Maybe I've answered the question myself, but I'm not sure until someone who really knows it, can actually confirm.

For example, an airliner flying at FL 370 at Mach 0.85, by calculation has an IAS of around 300 knots. At 300 IAS, the dynamic pressure is around 51156.61 N/m^2 or 7.4196 lbf/in^2. The structural force suffered by the slat would be this value multiplied by a reference surface. Can that force be so great that could lead to a structural failure in the linkage between the slat and the wing's structure, thus not being allowed to deploy in such conditions, or is there another reason for it?

Let's remember that there were many cases when the aircraft found themselves stalled (by pilot error, system error or both) at more than 270 knots IAS because the pilots pulled the alpha way beyond stall, where an extended slat could've very nicely increase the controllability margin of the airplane beyond stall or simply keep the wing slightly away from an aerodynamic stall.

Thank you!

[vilas]

There is no auto slat on airliners as you have mentioned.

[Goldenrivett]

For example, an airliner flying at FL 370 at Mach 0.85, by calculation has an IAS of around 300 knots.

Above FL 200 slats / flaps deployment is outside the operating envelope. (Limitations).

See what happened to TWA 841 in 1979. TWA_Flight_841_(1979) (https://en.wikipedia.org/wiki/TWA_Flight_841_(1979))

[TURIN]

Fighters are designed to fly at high alpha throughout their speed range. Agility is key. Airliners aren't. The autoslat function ON AN AIRLINER is there to stop something silly happening, not to enhance its capability. Have look at the 737 autoslat logic.

[Amadis of Gaul]

Let's remember that there were many cases when the found themselves stalled (by error, system error or both) at more than 270 knots IAS because the pulled the alpha way beyond stall, where an extended slat could've very nicely increase the lability margin of the beyond stall or simply keep the wing slightly away from an aerodynamic stall.

Thank you!

Well...it occurs to me that pilots who managed to stall airliners without slats extended would probably find a way to do so WITH slats as well.

Sort of reminds of when nail clippers got banned from airplanes. At that time it occurred to me that someone who can take over an airplane with nail clippers can probably do it WITHOUT nail clippers also...

[blind pew]

BEA nearly lost another Trident 1 in the Clacton hold in the 70s.
We all knew the droop limitations...out by 225 knots and in by 250 knots but most didn't know the Mach limit. The Clacton hold had a max speed of 210, the skipper had gone for a leak and the new boy had selected the droop out entering the hold somewhere around fl250.
The wing went supersonic locally and they descended somewhere around 5000ft before they regained control.

[vilas]

I still don't understand what is auto slat and which airliner has it?

[john_tullamarine]

Auto slat (extension) is a system facility whereby the LE slats extend without specific pilot command as alpha gets to a critical value ie provides something in the nature of a get-out-of-jail-free card ...

The B737 (http://www.b737.org.uk/flightcontrols.htm#Leading_Edge_Devices), for instance, has an autoslat function in some configurations.

There will be other aircraft Types with similar arrangements but I'll leave it to you to do the searches ...

[vilas]

Thanks JT
I was wondering if it was so common how did I miss it. The three airbuses and two Boeings I flew didn't have them. I don't think any of the major or popular airliners like Airbus or Boeing FBW has them. The common functions are auto retraction and slats alpha lock in airbus.

[Capn Bloggs]

The autoslat system will fully extend the slats for stall protection whenever flap is selected and the slats are not already at the full extend position
Hardly autoslat? If Bloggs selects the flap so fast the slats aren't out yet, they come out automatically? Me no comprehendo.

[stilton]

B757 has autoslats, they extend from midrange to landing position with stall warning.

[skkm]

Thanks JT
I was wondering if it was so common how did I miss it. The three airbuses and two Boeings I flew didn't have them. I don't think any of the major or popular airliners like Airbus or Boeing FBW has them. The common functions are auto retraction and slats alpha lock in airbus.

On the B777, as alluded to by the OP, if the slats are extended to mid-range and a stall warning occurs, they will automatically extend to the full position. This does not occur when the slats are fully retracted.

[FCeng84]

On Boeing 777 and 787 the flap lever on the flight deck has positions of UP followed by 1 - 30 with various detents along the way. The leading edge slats have three positions (retracted, extended, and gapped in that order). With the lever in the UP position the leading edge slats are retracted. With the lever in any of the other positions the leading edge slats are at least in the extended position. For normal landing flap lever positions (25 and 30) the leading edge slats are deployed fully to their gapped position.

Whenever the leading edge slats are in the extended (but not gapped) position, high AOA will result in the auto slat function moving them to the gapped position. As stated by others earlier, if the flap level is in the UP position and the leading edge slats are in the corresponding retracted position there is no automatic slat extension functionality.

[galaxy flyer]

EA had a B727 crew fiddle with the emergency extend switches at cruise. The idle time argument was, "if I select retract, nothing will happen, right?" Well, the armed the system, selected retract and out came the slats, all snaggled and damaged, a few parts falling to Earth. Another C-5 crew ignored the Mach limit and also damaged them pretty badly. Point is, high lift devices on Part 25 planes are for low altitude regimes.

The Falcon 10, perhaps other Falcons,, auto deployed the slats approaching the stall and, IIRC, auto retracted them, if deployed, to force a pitch over at the stall. Then again, the Dassault pilots performed all kinds of acro in 10s.

[MaverickSu35S]

You're perfectly right, but still, it's better to have slats deployed whenever needed than not, as fighter aircraft have, but as I found out about the TWA 727 accident where the slat broke loose at high speed, I have my answer for my question.

Cheers!


BEA nearly lost another Trident 1 in the Clacton hold in the 70s.
We all knew the droop limitations...out by 225 knots and in by 250 knots but most didn't know the Mach limit. The Clacton hold had a max speed of 210, the skipper had gone for a leak and the new boy had selected the droop out entering the hold somewhere around fl250.
The wing went supersonic locally and they descended somewhere around 5000ft before they regained control.

Wow, at just 210kias, the droop changed the camber so much that the airflow became rushed to supersonic? I wouldn't have believed it if I wouldn't learn about it.

Thanks!


Fighters are designed to fly at high alpha throughout their speed range. Agility is key. Airliners aren't. The autoslat function ON AN AIRLINER is there to stop something silly happening, not to enhance its capability. Have look at the 737 autoslat logic.

I know the exact reason of the above. The "something silly" thing is normally to be avoided without the use of slats, but if it may happen for whatever reason, they should be there to help, but for being limited due to structural stress, as I later and conclusively found it, they are to remain stowed until below a given airspeed.


Above FL 200 slats / flaps deployment is outside the operating envelope. (Limitations).

See what happened to TWA 841 in 1979. TWA_Flight_841_(1979) (https://en.wikipedia.org/wiki/TWA_Flight_841_(1979))

Bingo! So a catastrophic structural failure might occur when they deploy above a certain EAS.

Thank you!


Auto slat (extension) is a system facility whereby the LE slats extend without specific pilot command as alpha gets to a critical value ie provides something in the nature of a get-out-of-jail-free card ...

The B737 (http://www.b737.org.uk/flightcontrols.htm#Leading_Edge_Devices), for instance, has an autoslat function in some configurations.

There will be other aircraft Types with similar arrangements but I'll leave it to you to do the searches ...

I din't ask what an auto slat is or what it does, I've already covered this. All that I asked was why are the slats not allowed to deploy at higher IAS/MACH. I have already done my search, thank you...!


..."if I select retract, nothing will happen, right?" Well, the armed the system, selected retract and out came the slats, all snaggled and damaged, a few parts falling to Earth...

I don't know how the emergency extend switches operate (what they do), but it's rather curious, why selecting retract would "out bring" the slats? Don't know how that system works, so it would be interesting to learn.


Hardly autoslat? If Bloggs selects the flap so fast the slats aren't out yet, they come out automatically? Me no comprehendo.

Comprehend your own quote, there's the answer! The question was if the autoslat system could extend the slats even with no flaps. Still no comprehendo?

[BleedingAir]

Hardly autoslat? If Bloggs selects the flap so fast the slats aren't out yet, they come out automatically? Me no comprehendo.

The key term here is "full extend", the 737 slats have two deployed positions, extend and full extend (confusing, yes). The autoslat system will drive them to full extend from extend, not from the retracted position.

[Capt Scribble]

The AB does not have auto slat but does have a alpha/speed slat lock system to protect the aircraft against high alpha. If the Flaps/Slats are selected to Zero (up) from Flap 1 position and the alpha is greater than 8 or the speed less than 165, the slats will not retract until certain values are reached. Sort of Auto slat but the other way round.

[Capn Bloggs]

Thanks Bleeding. Bloggs comprehendo now. :ok:

[galaxy flyer]

I don't know how the emergency extend switches operate (what they do), but it's rather curious, why selecting retract would "out bring" the slats?

The B727 switches were placarded emergency EXTEND for a reason--their purpose was to extend both the LED and TED to LAND the airplane. Since the slats, on type, must be extended before any flap was extended, the switches allowed for adjustment of TED extension, but always extended the LEDs. No LEDs, clean wing landing was required.

[flajjer]

I might be wrong here.
But if you are at a critical AoA with the aircraft barely flying.
Extending the slats only will actually lower you effective AoA.

[john_tullamarine]

Extending the slats only will actually lower you effective AoA.

Not quite. Plenty of info available on the net. A typical LED effect graph (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_fig3.html) from Boeing Aero 12 (http://www.boeing.com/commercial/aeromagazine/aero_12/) gives a simplified story of what happens.