MD80-83 series stall protection design philosophy question.

[Northbeach]

I am looking for somebody who has experience with the MD80 series of aircraft from a design perspective to answer a question of mine.

I am no longer current on the series, so the question is born out of curiosity, it concerns the automatic deployment of the slats from mid to fully extended position. For the discussion I will call this the auto slat function (I don’t remember if that is the correct terminology).

By design, if you had the slats extended to the mid range and then went into a high angle of attack the stall warning computer would detect the high AOA and drive the slats to the fully extended position, without any pilot input, it would do this automatically. However this “protection” was only available if the slats were already in the mid position; not from a clean wing. And that is my question; why was the auto slat function of the stall warning computer/protection on the McDonald MD80/83 series not designed to deploy from a clean wing?

For you “Magic Bus” drivers who may be reading. Does Airbus stall protection design automatically deploy, whatever high lift devices your wing may have, automatically from a clean wing to prevent a stall?

Looking forward to your responses,

Northbeach

(I never flew prior versions of the DC-9 type; only the MD80, so I don’t know the earlier history of this system.)

[411A]

...why was the auto slat function of the stall warning computer/protection on the McDonald MD80/83 series not designed to deploy from a clean wing?



Because, according to a design engineer who worked on the type, natural stall warning was noticeable/sufficient in the clean configuration, whereas, with partial slats, it was not.
Don't shoot the messanger...that's what he said.

This sort of arrangement (natural stall buffet, not aircraft systems-generated warning) is present on other types as well.
Example.
With the L1011, natural stall buffet will be noticed well before stick-shaker activation in the clean configuration, whereas, with slats extended, it will not...IE: shaker first, then stall buffet.
I know this because, I have completed full stalls in the type, long ago....and not something I desire to repeat.:}

[727gm]

Probably also because, from my experience in other aircraft (I don't know the DC-9/MD-80 series), extension of leading-edge devices or trailing-edge flaps is typically prohibited above 20000 feet.

[Mad (Flt) Scientist]

Another speculative suggestion.

The max speed for flight with spoiler deployed, partially deployed or in transit is probably about the same. So there's no structural reason why you can't autodeploy from partial to full.

But flight slats-in could be all the way up to Vmo. If autodeploy was capable of initiating from zero slat, you'd have to consider both the case of the accelerated/high-g stall near Vmo and the failure cases causing inadvertent autodeploy at Vmo. The hazard involved would perhaps bias against auto-deploy from clean, especially if the characteristics were already reasonable without it.

[Northbeach]

Thanks for the responses everybody!

411A

Don't shoot the messanger...that's what he said.

Thought never occurred to me. Since this animal goes back to the late 60s I’m sure there are lots of “interesting” details.


I know this because, I have completed full stalls in the type, long ago....and not something I desire to repeat


Now that would be a fascinating read! If you ever care to write up that experience I would love to read it (names & dates deleted to protect the guilty:}). Feel free to PM me if you care to pass on that little experience.

Mad (Flt) Scientist:
But flight slats-in could be all the way up to Vmo. If autodeploy was capable of initiating from zero slat, you'd have to consider both the case of the accelerated/high-g stall near Vmo and the failure cases causing inadvertent autodeploy at Vmo. The hazard involved would perhaps bias against auto-deploy from clean, especially if the characteristics were already reasonable without it.

............. Fascinating analysis - thank you.

Northbeach

[galaxy flyer]

Funny, 411A, the L1011 design study, the C-5 had similar stall characteristics.

GF

[stilton]

It's been a long time since I flew the MD80 and I can't answer your question.



However I would not want to go into a full stall in that Aircraft under any circumstances.

[ZeroThreeLeft]

Just a wild guess...

I would think that the engineers would have thought at the time, that you are most vulnerable to stall the plane with the initial climb when also having slats extended for best rate of climb. I know on the B727 even 2° extended slats makes a big difference with the climb. Maybe they thought that you wouldn't ever stall at cruising Alt / speed, thus only have them fully deploying if they were already deployed. You wouldn't want the slats to deploy from a clean wing confg on a high speed stall for instance...

Im no expert on the MD-80 but in my opinion this makes sense???

[md-100]

AS an MD80 driver I can say:

1. Stalling in a clean configuration is very rare. You have ALFA SPEED protection with the A/T that would not let you stall. But if you flight it without A/T, then you would be not such a good pilot to stall it in a clean configuration.

2. and the most important reason. Mostly take offs and go around are made with midslat, and in the case of a w/s and a sudden lost of speed, the auto slat will give you extra protection (It happened to me in the real world, taking off and encounter w/s at 300 ft, the speed drops and the autoslat actuated, giving me margin till recovering.)

I guess this case is what the autoslat was thinking for. ( like ZeroThreeLeft posted about the 727)

STAY AT SCHOOL
WINNERS DON'T STALL PLANES:D;)

[Northbeach]

IGh,

Thank you for taking the time to share your obvious extensive background in the industry. I greatly appreciate your willingness to shed light on my question.


Stilton

However I would not want to go into a full stall in that Aircraft under any circumstances.


You are of course 100% correct; few of us would want to go into a full stall. Unfortunately human beings do the most imprudent things at inopportune times. As IGh pointed out in the above post (the 727 clean wing takeoff and the MD82 slow speed cruise pitch upset) pilots sometimes put their aircraft or allow the aircraft to reach an undesirable state. And I was questioning, from a philosophical design standpoint, why the system under discussion was designed in the way that it was.

I am still waiting for an Airbus pilot to contribute. If your Airbus were to get into a slow speed stall with a clean wing, would the high lift devices automatically deploy?

[stilton]

You are completely correct Northbeach.



Incidentally, I have an old videotape 'building and flying the 727 and 747' which covers a lot of the test program on these two superb Aircraft.


Some of the highlights I remember from the documentary were the high speed testing of the 747 up to .99 Mach and the 727 a mere .96 !



Each Aircraft was found to be vice free in that part of the flight envelope. Just as interesting was the slow speed testing of the 727, which, even with it's T Tail had a very benign, straight ahead nose drop at full stall.



It says a lot for the Boeing aerodynamic engineering department.

[Avante]

Hi North,

Altough I am not a MD-80 pilot, I think that not extending slats automatically from a clean configuration could make sense in a coffin corner context.

Imagine you are flying close to the aircraft celing at high speed and, due to any reason (turbulence, pilot error, etc.), speeds dropes down and the stall protection gets activated. If this automatically changes your configuration from clean to slats deployed at high mach speed, this could drive quite a serious issue, much more problematic than correcting a stall initiated at FL 370.