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Pitch Attitude During Stall (NW255)  
User currently offlineDr.DTW From United States of America, joined May 2000, 290 posts, RR: 1
Posted (8 years 3 months 1 day ago) and read 5021 times:

I'm currently researching the crash of Northwest 255. The aircraft crashed on takeoff from DTW, due to the crews failure to set the flaps/slats. The MD-80 entered into a stall immediately after rotation, and crashed.

My question revolves around the time of rotation.

When the aircraft rotated, it did so at a very high angle (10-12 degrees), which almost resulted in a tail strike. Why would this happen? Does it have to do with the "clean" wing configuration at the time of rotation? The aircraft reached V1 and Vr wihout any difficulty. Or, did the flight director present false cues, which would result in an over-rotation of the aircraft?

Thanks
Dr.DTW

31 replies: All unread, showing first 25:
 
User currently offlineJoness0154 From United States of America, joined Nov 2005, 667 posts, RR: 0
Reply 1, posted (8 years 3 months 1 day ago) and read 5005 times:

Quoting Dr.DTW (Thread starter):
When the aircraft rotated, it did so at a very high angle (10-12 degrees), which almost resulted in a tail strike. Why would this happen? Does it have to do with the "clean" wing configuration at the time of rotation? The aircraft reached V1 and Vr wihout any difficulty. Or, did the flight director present false cues, which would result in an over-rotation of the aircraft?

It was probably due to the wing producing less lift because the flaps/slats were not extended. In order to produce the same amount of lift, a higher AOA was needed.

Also, leading and trailing edge devices change the chord of the wing, essentially giving the wing a higher AOA at the same pitch attitude (versus without flaps/slats), and therefore a lot of pitch would not be needed to get the plane off of the ground.



I don't have an attitude problem. You have a perception problem
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 2, posted (8 years 3 months 23 hours ago) and read 4993 times:

Quoting Dr.DTW (Thread starter):
The aircraft reached V1 and Vr wihout any difficulty.

Sure it would. It was on the ground, the weight was borne by the wheels, not the wings. Lift might be thought of as drag, but drag in an upward direction. So with slats (and flaps) retracted the wing was clean and offered even less resistance than a normally configured takeoff. It might well have accelerated faster in this clean configuration. Hell, if you removed the wings outboard of the landing gear you'd have the world's fastest tricycle!

There is very good evidence - objections to the contrary aside - that this plane did take off with flaps/slats retracted. It is unfortunate that the human checks on this would have failed but a bigger question is why the takeoff configuration warning did not sound.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineVikkyvik From United States of America, joined Jul 2003, 10254 posts, RR: 26
Reply 3, posted (8 years 3 months 23 hours ago) and read 4992 times:
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Most commercial aircraft will climb out with a pitch angle of at least 10-15 degrees. I'm not sure exactly how rotation works (not a pilot), but I think you generally rotate smoothly up to a bit below this angle (maybe 8-10 degrees?).

I don't know the details of the crash, but here's what I would surmise:

V1, Vr and V2 (if applicable) would have been calculated for some flap/slat configuration (not a clean wing). The plane would have no problem reaching V1 and Vr, because these speeds are reached during the takeoff roll with the plane still on the ground (technically, there would actually be slightly less drag, so the speeds would be reached slightly sooner than normal).

Upon rotation, the airplane would probably still lift off the ground. However, upon reaching a normal climb angle, the airplane would not have enough speed, and hence, enough lift, to maintain this angle (with flaps and slats, you get more lift at lower speeds). This would lead to a stall, and at the low altitude that I assume the plane was at, the stall would lead to the crash.

I'll leave it to those more knowledgeable to make any corrections in my post.

~Vik



How can I be an admiral without my cap??!
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17115 posts, RR: 66
Reply 4, posted (8 years 3 months 23 hours ago) and read 4975 times:

Quoting Dr.DTW (Thread starter):

When the aircraft rotated, it did so at a very high angle (10-12 degrees), which almost resulted in a tail strike. Why would this happen? Does it have to do with the "clean" wing configuration at the time of rotation? The aircraft reached V1 and Vr wihout any difficulty. Or, did the flight director present false cues, which would result in an over-rotation of the aircraft?

Some random and completely uninformed musings:

Since the wing was clean it produced less lift at rotation speed. My speculation is that this led to less inertia in the rotation and thus over-rotation. However, even the correct pitch angle would have led to a stall. The AA DC-10 at ORD suffered from that problem. Correct pitch angle but the pilots didn't know the slats ad retracted from the damaged wing. Kaboom.

As for the flight director, on the MD-80 it's hardly the kind of instrument present in a 320. It would not adapt to the clean configuration by dynamically lowering the pitch angle presented.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 5, posted (8 years 3 months 23 hours ago) and read 4971 times:
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Have you guys tried viewing the "factual" (more in-depth) report at the NTSB's site? It isn't working for me...




2H4





Intentionally Left Blank
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 6, posted (8 years 3 months 22 hours ago) and read 4956 times:

Quoting Starlionblue (Reply 4):
Since the wing was clean it produced less lift at rotation speed. My speculation is that this led to less inertia in the rotation and thus over-rotation.

I don't follow your reasoning. I think inertia is the old "objects in motion tend to stay in motion, objects at rest tend to stay at rest" simplification of Newton's first law. I don't see the connection between more/less left and more/less inertia.

Even if that was the case, and the rotational inertia (or, more correctly "momentum" since inertia isn't really a quantity per se) of the plane about its center of gravity was lower due to less left being produced, (which would assume the center of lift is in front of the center of gravity,) it would seem to follow that the plane would have under-rotated, not over-rotated.



Position and hold
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17115 posts, RR: 66
Reply 7, posted (8 years 3 months 22 hours ago) and read 4946 times:

Quoting Bri2k1 (Reply 6):
Quoting Starlionblue (Reply 4):
Since the wing was clean it produced less lift at rotation speed. My speculation is that this led to less inertia in the rotation and thus over-rotation.

I don't follow your reasoning. I think inertia is the old "objects in motion tend to stay in motion, objects at rest tend to stay at rest" simplification of Newton's first law. I don't see the connection between more/less left and more/less inertia.

I should have specified even more clearly that I have no reasoning  Wink I was thinking there might be a reason.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAlias1024 From United States of America, joined Oct 2004, 2794 posts, RR: 2
Reply 8, posted (8 years 3 months 21 hours ago) and read 4936 times:

Quoting Dr.DTW (Thread starter):
When the aircraft rotated, it did so at a very high angle (10-12 degrees), which almost resulted in a tail strike. Why would this happen?

I'll take a poke at it. Imagine you are flying this plane. You reach rotation speed and pull back, but the nose doesn't lift off like you are expecting. It seems heavy, and the plane doesn't want to climb. You see those light poles down past the end of the runway approaching quickly. What would be your gut instinct reaction? Probably to pull back harder and try to get the plane climbing. That might explain an overrotation.

Here is another thought on what might cause a near tail strike Plane reaches Vr and pilot pulls back on controls. The airplane isn't going to leap off the ground because the flaps aren't extended. Instead, it's going to take a little more time for the mains to actually leave the ground. During that extra second or two, the pilots are still pitching up at a normal rate, resulting in the tail getting very close to the runway.



It is a mistake to think you can solve any major problems with just potatoes.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 9, posted (8 years 3 months 18 hours ago) and read 4904 times:

Quoting Alias1024 (Reply 8):
You reach rotation speed and pull back, but the nose doesn't lift off like you are expecting.

Actually it probably rotated as normal. No reason it should not. The problem was not in getting to the takeoff attitude but, rather, that the takeoff attitude was not producing the lift it should have.

Given enough runway and no obstructions this plane could have gone along a few degrees nose up, continuing to accelerate until eventually it reached the speeds needed by a clean wing, whereupon it would have taken off and flown. But there was no room for that.

I don't know if they continued the rotation, seeing the end of the runway coming at them or not. Clearly at some point between brake release and striking the first obstruction something would have dawned on them. One of those "that's not right" moments. After passing V1 (accelerate-stop limited) they sort of became passengers. Not much they could have done, or were likely to have done after that, which would have made any difference.

Why didn't the takeoff warning sound?



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineAlias1024 From United States of America, joined Oct 2004, 2794 posts, RR: 2
Reply 10, posted (8 years 3 months 18 hours ago) and read 4902 times:

Quoting SlamClick (Reply 9):
Why didn't the takeoff warning sound?

IIRC, the breaker for the takeoff warning was pulled. There was a maintenance issue and I think it was MEL'd.

Quoting SlamClick (Reply 9):
Actually it probably rotated as normal. No reason it should not. The problem was not in getting to the takeoff attitude but, rather, that the takeoff attitude was not producing the lift it should have.

Good catch. I wrote that wrong and it is too late to edit. I meant to say the aircraft doesn't lift off like you are expecting. Attaining the normal pitch attitude wouldn't be a problem. Getting the airplane out of ground effect would be.



It is a mistake to think you can solve any major problems with just potatoes.
User currently offlineLucky42 From , joined Dec 1969, posts, RR:
Reply 11, posted (8 years 3 months 15 hours ago) and read 4870 times:

Quoting Alias1024 (Reply 10):
IIRC, the breaker for the takeoff warning was pulled. There was a maintenance issue and I think it was MEL'd.

Can't MEL a Takeoff warning horn...There is speculation the crew pulled the C/B to silence "Bitchin Betty". The affectionate name for the warning.


User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1656 posts, RR: 1
Reply 12, posted (8 years 3 months 14 hours ago) and read 4858 times:

Simple.

You can try this in your favorite C172: at a safe altitude, set up the 20-degree flap setting takeoff airspeed in level flight but do not use flaps. Now, try to "rotate," at full power, to a normal takeoff attitude and you will see that the airplane will not climb but will, instead, begin a mushing descent. As this happens, try to compensate for the descent by raising the nose. Make sure that you are conversant with power-on stall recovery technique before you do this.


User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 13, posted (8 years 3 months 13 hours ago) and read 4843 times:

Quoting ThirtyEcho (Reply 12):
You can try this in your favorite C172: at a safe altitude, set up the 20-degree flap setting takeoff airspeed

Which model 172 permits 20 degree flap takeoffs? The only ones I am familiar with (N, P, Q, and S models) are only approved for 0 or 10 degrees. However, if I follow what you are describing, I would slow to rotation speed for a short-field takeoff, which is 59 knots (slightly higher than normal rotation speed of 55 knots), apply fully power, and point the nose up. As long as I don't over-rotate, why in the world wouldn't I climb? In fact, how else would I ever climb but to apply full power and increase the angle of attack? Sure, continuing to increase to some rediculous angle of attack would eventually result in a power-on stall, but at a normal rotation attitude, I would climb nicely. There is a huge difference between a C172 and an MD-80, and I'm afraid this might be oversimplifying it a bit.



Position and hold
User currently offlineAlias1024 From United States of America, joined Oct 2004, 2794 posts, RR: 2
Reply 14, posted (8 years 3 months 9 hours ago) and read 4775 times:

Quoting Lucky42 (Reply 11):
Can't MEL a Takeoff warning horn...There is speculation the crew pulled the C/B to silence "Bitchin Betty". The affectionate name for the warning.

I found the report and you are right. NTSB never determined the cause for the power failuer to the CAWS system. Either the breaker was intentionally pulled, it popped and the crew didn't notice, or the breaker failed and didn't pop after it interruped electrical power.

If anyone is interested, here is the report.
http://amelia.db.erau.edu/reports/ntsb/aar/AAR88-05.pdf



It is a mistake to think you can solve any major problems with just potatoes.
User currently offline3DPlanes From United States of America, joined Apr 2006, 167 posts, RR: 0
Reply 15, posted (8 years 2 months 4 weeks 1 day 23 hours ago) and read 4687 times:

Quoting Bri2k1 (Reply 13):
As long as I don't over-rotate, why in the world wouldn't I climb?

Well part of the difference, I think, is that 55 knots Vr for the Cessna is a good ways above the 40 knot (ish) Vso speed. For an MD-80, I'd bet the margin is a bit tighter.

Perhaps a better anology would be soft-field. What's minimum un-stick speed with flaps 20? (Being a Piper guy, I'll have to assume you can t/o with flaps 20.) Say 42-45 knots... Care to try taking off at that speed with no flaps and getting out of ground effect. Especially at anywhere near max gross?



"Simplicate and add lightness." - Ed Heinemann
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 16, posted (8 years 2 months 4 weeks 1 day 22 hours ago) and read 4674 times:

Quoting 3DPlanes (Reply 15):
For an MD-80, I'd bet the margin is a bit tighter.

Perhaps a better anology would be soft-field. What's minimum un-stick speed with flaps 20?

I'll have to let my academic superiors flesh this out for you but swept wings are MUCH more sensitive to wrong angles of attack than straight wings.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineMeister808 From United States of America, joined Jan 2000, 973 posts, RR: 1
Reply 17, posted (8 years 2 months 4 weeks 1 day 8 hours ago) and read 4571 times:

Quoting SlamClick (Reply 9):
After passing V1 (accelerate-stop limited) they sort of became passengers. Not much they could have done, or were likely to have done after that, which would have made any difference.

Couldn't they have put in the flaps?

-Meister



Twin Cessna 812 Victor, Minneapolis Center, we observe your operation in the immediate vicinity of extreme precipitation
User currently offline113312 From United States of America, joined Apr 2005, 574 posts, RR: 1
Reply 18, posted (8 years 2 months 4 weeks 1 day 3 hours ago) and read 4521 times:

All DC9 series, including the MD-80 and DC-10/MD-11 have allowable takeoff configurations using flaps of Zero through approximately 22 degrees. However, all takeoffs must have leading edge slats extended. The calculated takeoff rotation speed will allow for sufficient angle of attack to achieve liftoff without a tail strike as well as achieving the calculated V2 speed by 35 feet.

The final climb pitch attitude is a function of flap angle, weight, temperature, altitude and thrust. This final attitude is typically between 15-20 degrees. With a normal pitch rotation rate of 3 degrees per second, liftoff is achieved prior to a tail strike and prior to achieving the final pitch attitude.

In the case of the accident in DTW, the flaps and slats were never set to the calculated takeoff setting. The rotation to takeoff attitude was begun at the speed calculated as if they were extended. Thus, there was insufficient aerodynamic lift available for a normal climb although there was enough to stagger off of the ground. Becoming airborne in that mode, a very high attitude and angle of attach would be required to sustain flight and at that angle, there would also be abnormally high drag making it nearly impossible to accellerate to a safe speed. The wing was basically in a stall condition the whole time from liftoff until the aircraft crashed on the highway.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17115 posts, RR: 66
Reply 19, posted (8 years 2 months 4 weeks 1 day 2 hours ago) and read 4501 times:

Quoting Meister808 (Reply 17):
Quoting SlamClick (Reply 9):
After passing V1 (accelerate-stop limited) they sort of became passengers. Not much they could have done, or were likely to have done after that, which would have made any difference.

Couldn't they have put in the flaps?

I think it was too late. The flaps don't just fall out. Also they would have added some drag, lowering the speed, maybe beyond their own ability to compensate for that lower speed.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineBobster2 From , joined Dec 1969, posts, RR:
Reply 20, posted (8 years 2 months 4 weeks 1 day 1 hour ago) and read 4496 times:

If you read the report, the captain could have avoided the crash by easing back pressure on the control column to maintain the angle of attack at 11 degrees. They were going fast enough to fly with clean wing configuration, just not fast enough for more than 11 degrees. So they could have cleared the light pole by 80 feet instead of hitting it. However, the captain apparently thought the stall warning was caused by windshear, he didn't know the flaps and slats were not extended. So that brings us back to the missing configuration warning.

User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4224 posts, RR: 37
Reply 21, posted (8 years 2 months 4 weeks 1 day 1 hour ago) and read 4485 times:

I remember my dad mentioning that some of the dc-9 pilots would pull the CB for the autoploy function on the spoilers to make the touchdown a bit smoother, which also shutoff the config alarm somehow. This carried over to the MD-80 series aircraft. It was apparently one of those things that guys did that they really didnt talk about elsewhere. He was a check airmen and mentioned getting on to guys for asking if he wanted the "long landing."

As was previously mentioned, the instinctive pull-up can be a killer even though you were trying to save the aircraft with all the best intentions. This is another one of those situations where you wish that transport aircraft had direct AOA indications in the cockpit.



Chicks dig winglets.
User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 22, posted (8 years 2 months 4 weeks 1 day ago) and read 4478 times:
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DATABASE EDITOR




Quoting XFSUgimpLB41X (Reply 21):
This is another one of those situations where you wish that transport aircraft had direct AOA indications in the cockpit.

I had an aero instructor once who swore each and every fixed-wing aircraft should come equipped with AOA and stall margin indicators. He's got my vote...




2H4





Intentionally Left Blank
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 23, posted (8 years 2 months 4 weeks 7 hours ago) and read 4409 times:

Quoting XFSUgimpLB41X (Reply 21):
I remember my dad mentioning that some of the dc-9 pilots would pull the CB for the autoploy function on the spoilers to make the touchdown a bit smoother

Is your dad an MD-80 pilot?

I ask because I am typed in DC-9 but never flew the MadDog. However in the nine, it is not necessary to pull any circuit breakers when you don't want autospoiler - you just don't pull the handle up to the 'armed' position. The MD-80 pilot handbook I wrote, and one from another airline both say that the MD is the same. The handle must be pulled up to arm the spoilers for automatic deployment.

Quoting Meister808 (Reply 17):
Couldn't they have put in the flaps?

Highly theoretical but MAYBE if they'd done it soon enough. As was suggested, it is the slats rather than the TE flaps that were critical here and they do take some time to extend and then it does take some time to make the eighty tons or so of mass start to change direction.

Probably the extension would have to have been commanded while still on the ground - in which case maybe an RTO would have been a better move. All really theoretical here.

Quoting Bobster2 (Reply 20):
If you read the report, the captain could have avoided the crash by easing back pressure on the control column to maintain the angle of attack at 11 degrees. They were going fast enough to fly with clean wing configuration, just not fast enough for more than 11 degrees.

I think there are a lot of us here with some flying credentials who sort of cringe at the idea: Lowering the pitch attitude while wallowing along unable to climb. It is very counter-intuitive and it will require some finesse. I'm reminded of US 1016 at CLT, The Concorde crash at CDG, and the F-86 that crashed into Farrell's ice cream parlor across from Sacramento Executive Airport (KSAC) back in the early 70s.

We understand the concept - you kind of unload the wing a bit by doing this, trust ground effect, and accelerate. Enough speed and your worries are over but too little deck angle and you descend. Like I said, it is a very uncomfortable regime of flight.

So is the moment just after striking the light pole, I suppose.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offline3DPlanes From United States of America, joined Apr 2006, 167 posts, RR: 0
Reply 24, posted (8 years 2 months 4 weeks 4 hours ago) and read 4373 times:

Quoting SlamClick (Reply 23):
Like I said, it is a very uncomfortable regime of flight.

Not sure you'd want to do it in an airliner... And I'm not sure the sim could give the right feel...

But, during my training, I had trouble with soft field takeoffs. I kept climbing too high, out of ground effect. We went to a nearby (little used) field with a 12,000 runway. 3 miles is a -long- time to fly along in ground effect, and I did -much- better after that...



"Simplicate and add lightness." - Ed Heinemann
25 Wannabe : If I remember correctly, the pitch angle was not the problem on the AA DC-10 disaster. When the left engine left the wing, it took out the hyraulics
26 Vikkyvik : I'm not disputing what you stated as the cause of the accident, but lowering the pitch angle sufficiently would have prevented the left wing from sta
27 ALbyDAL : Wasn't this same issue the cause of the DL 727 crash at DFW back in 1988? IIRC the pilots were held on the ground for an extended period, during which
28 Starlionblue : Yes. As I recall, they even said words to the effect of "you're on tape now in case we crash" to the F/A.
29 3DPlanes : As I recall, it wasn't a problem of the left wing stalling without slats... The problem was that, not knowing the had -really- LOST an engine, the cr
30 Lucky42 : Adding to that The DC-10 at that time had an option of having only a stick shaker on one side. AA had taken this option and it was on the F/O's side.
31 N737AF : Speaking of the DL crash at DFW. How were they able to avoid a total disaster liked what transpired at DTW? Luck? Obviously, both involved fatalities,
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