Challenger crash at KASE
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Still guessing that after the 'bounce' they ended up high in the air, but below stall speed and with increasing thrust. The thrust seems pretty clear thanks to the IR imagery - could be wrong - but the exhaust heat signature is well formed and stable or increasing.
How that could happen is a mystery - possibly a combination of ground effect, wind shear, and automatic configuration changes?
How that could happen is a mystery - possibly a combination of ground effect, wind shear, and automatic configuration changes?
Last edited by thcrozier; 23rd Jan 2014 at 06:11.
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Not sure where the mystery is. There is such a vast array of permutations of pilot's nervous actions that could account for the whole scenario. This is a repeat of similar accidents in small training Cessnas albeit energies, speeds and altitudes are higher.
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We are almost in agreement. Look at the assumptions I used and see if they are reasonable.
Assumption 1. The stick pusher is there to avoid a locked in stall condition.
Assumption 2. In a locked in stall, the aircraft would stay pitched up or even pitch up somewhat further relative to its flight path.
Assumption 3. Drag would rise, lift would decrease, and the aircraft would settle.
Assumption 4. Ground contact would stop the downward acceleration at a much lower velocity than would a lawn dart type earth entry thus increasing survivability. For this to be true, the stick pusher inhibition altitude would probably have to be on the order of 200 feet agl. It is still possible that a roll off could occur if controls are mishandled.
Do we really need the full 75 lbs of nose down force on the yoke in all flight conditions? Do we need a warning that the stick pusher is about to activate?
Assumption 1. The stick pusher is there to avoid a locked in stall condition.
Assumption 2. In a locked in stall, the aircraft would stay pitched up or even pitch up somewhat further relative to its flight path.
Assumption 3. Drag would rise, lift would decrease, and the aircraft would settle.
Assumption 4. Ground contact would stop the downward acceleration at a much lower velocity than would a lawn dart type earth entry thus increasing survivability. For this to be true, the stick pusher inhibition altitude would probably have to be on the order of 200 feet agl. It is still possible that a roll off could occur if controls are mishandled.
Do we really need the full 75 lbs of nose down force on the yoke in all flight conditions? Do we need a warning that the stick pusher is about to activate?
The various takeoff/icing accidents to befall the type illustrate what would happen without the pusher - a rapid roll and impact with the ground in a heavily banked state off to one side of the runway. (In those cases, the contaminated wings meant the pusher was unable to do its normal job in time)
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The post crash pictures shows the nose gear and elevators missing. Could the first bounce have been so severe that the nose gear came off, taking out one/both elevator(s)?
Isn't this dive what happens if your elevators ice up to the point where they lose lift? You nose over?
Isn't this dive what happens if your elevators ice up to the point where they lose lift? You nose over?
On the stall issue questions...
About the stick pusher...
Yes you can deactivate the stick pusher... There are two toggle switches on either side of the cockpit at each pilot's stations, which can be moved out of their "normal" position to the "OFF" position to disable the pusher. Either one to the "OFF" position will disable the pusher.
Someone asked about the autopilot and stall system...
If the AP is being used, as soon as the stick shaker comes on the AP disconnects.
About the force a pilot would need to override the pusher...
I cannot remember the actual number but a pilot can override it by pulling back on the controls. In the simulator when practicing stalls if you are not paying attention and the pusher triggers depending on your recovery technic, you could have your hands full not to have multiple pusher activations.
About the centre of thrust...
Yes on the Challenger the C of T is high and combined with the huge fans with their high bypass ratio (6.2:1), when the thrust kicks in you will get a major nose down moment. This again is very noticeable during stall practices in the simulator. This is where if you are not paying attention as the thrust comes in you will have a tendency to over react to the nose down moment and pull back too hard or rapidly on the elevator control and go into a second or third pusher activation. It's like riding on a seesaw.
IMO, and this is without additional info on the accident, these guys were probably high and hot. With a very high TAS and GS due to tail wind and turbulence correction factors added in (plus add some windshear) the aircraft probably settled in the ground affect and started to float down the runway.
At one point someone probably pushed on the nose to get it on the runway. The first impact was hard enough to substantially damage the aircraft (in what matter is unknown at this time).
Perhaps they were at idle at the first impact and perhaps there was enough wheel spin to deploy the ground spoilers. In any case whether due to pilot input or from the momentum of the aircraft it got airborne again (perhaps 20 to 30 feet). Perhaps full thrust was selected as the aircraft got airborne after the initial impact, causing the ground spoilers to retract but then the aircraft pitches down (steep angle) caused perhaps by the C of T and a pilot that could not compensate for it (damaged aircraft, pilot incapacitation). If they then went back to idle and there was enough wheel spin the ground spoilers could have deployed yet again causing a loss of lift which in their situation would not have helped.
We all saw what happened at the second impact.
Yes you can deactivate the stick pusher... There are two toggle switches on either side of the cockpit at each pilot's stations, which can be moved out of their "normal" position to the "OFF" position to disable the pusher. Either one to the "OFF" position will disable the pusher.
Someone asked about the autopilot and stall system...
If the AP is being used, as soon as the stick shaker comes on the AP disconnects.
About the force a pilot would need to override the pusher...
I cannot remember the actual number but a pilot can override it by pulling back on the controls. In the simulator when practicing stalls if you are not paying attention and the pusher triggers depending on your recovery technic, you could have your hands full not to have multiple pusher activations.
About the centre of thrust...
Yes on the Challenger the C of T is high and combined with the huge fans with their high bypass ratio (6.2:1), when the thrust kicks in you will get a major nose down moment. This again is very noticeable during stall practices in the simulator. This is where if you are not paying attention as the thrust comes in you will have a tendency to over react to the nose down moment and pull back too hard or rapidly on the elevator control and go into a second or third pusher activation. It's like riding on a seesaw.
IMO, and this is without additional info on the accident, these guys were probably high and hot. With a very high TAS and GS due to tail wind and turbulence correction factors added in (plus add some windshear) the aircraft probably settled in the ground affect and started to float down the runway.
At one point someone probably pushed on the nose to get it on the runway. The first impact was hard enough to substantially damage the aircraft (in what matter is unknown at this time).
Perhaps they were at idle at the first impact and perhaps there was enough wheel spin to deploy the ground spoilers. In any case whether due to pilot input or from the momentum of the aircraft it got airborne again (perhaps 20 to 30 feet). Perhaps full thrust was selected as the aircraft got airborne after the initial impact, causing the ground spoilers to retract but then the aircraft pitches down (steep angle) caused perhaps by the C of T and a pilot that could not compensate for it (damaged aircraft, pilot incapacitation). If they then went back to idle and there was enough wheel spin the ground spoilers could have deployed yet again causing a loss of lift which in their situation would not have helped.
We all saw what happened at the second impact.
Last edited by Jet Jockey A4; 23rd Jan 2014 at 14:16.
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The way the nose comes down so aggressively it looks like Stick Pusher activation drove the nose into the ground. Low airspeed with increased wing loading due to bounce could easily activate Stick Pusher. An override feature is available but one would have to be anticipating Stick Pusher and be waiting with their thumb on the button at this low altitude.
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Spoilers and Other Automation in Type
Then among the automatic configuration changes at landing are 1) with thrust at idle, if the wheels begin to spin, the spoilers deploy; 2) if full thrust is subsequently applied, the spoilers will automatically retract, regardless of wheel spin, and; 3) if thrust is retarded again and the wheels are still spinning, the spoilers will deploy again regardless of actual contact with the ground.
In addition, stick pusher issues could come into play depending on airspeed and AoA. Correct so far?
Are there other automatic configuration changes in this phase of flight?
In addition, stick pusher issues could come into play depending on airspeed and AoA. Correct so far?
Are there other automatic configuration changes in this phase of flight?
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Camera 5 seems to show a really abrupt pitch-down before the final touchdown (about 2:16 to 2:17 in the original video, not the slow motion one). It seems to go from slightly nose up to significantly nose down in less than a second. My guess would be 5 to 50 deg nose up to 20 to 30 nose down.
Can the flight controls explain that quick of a rotation, 30 to 45 pitch in around a second? Is there that much elevator authority at that low speed? Perhaps the pilots could have panicked and pushed down hard but would the plane have been able to react that fast?
Any chance the plane did in fact stall after the bounce and the stall caused the pitch down? The bounce might have caused an abrupt pitch up, stalling the wings too quick for the pusher to stop it.
(There was no roll as Mad Sci pointed out is expected in a stall but still)
Can the flight controls explain that quick of a rotation, 30 to 45 pitch in around a second? Is there that much elevator authority at that low speed? Perhaps the pilots could have panicked and pushed down hard but would the plane have been able to react that fast?
Any chance the plane did in fact stall after the bounce and the stall caused the pitch down? The bounce might have caused an abrupt pitch up, stalling the wings too quick for the pusher to stop it.
(There was no roll as Mad Sci pointed out is expected in a stall but still)
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You can see the same thing at the end of Camera 2, as well as what appears to be the heat signature of high thrust which, as Jet Jockey points out, could cause a powerful downward pitch moment.
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45 degree pitch change? And engine heat signature
YRP,
Don't forget that the video from camera 5 is substantially foreshortened. The angle of the climb after the bounce isn't that extreme. Look at camera 2 as the aircraft goes back up: from frame to frame as it leaves the shot, it appears to be at a much more natural angle. The same goes for the sharpness of the nose down before impact, which is caught on camera 3.
thcrozier,
Any exhaust leaving the engine, even at idle, will appear bright on an IR image. I don't think the visibility of the jet exhaust means anything more than that the engines are running.
On camera 1 there's still a clear hot wake from the engines as the aircraft floats into the first touch down, with no obvious change as the aircraft leaves the frame (I think still floating just off the ground - no tire heat signature). On camera 5, it's plausible that the engine glow rises after the first touch down as the "go around" starts, but it's not absolutely clear - the aircraft is approaching the camera, and naturally getting brighter anyway. I'd guess that's consistent with an intention to go around at the initial float, but for the likely damage/loss of the nose wheel in the first burst of heat underneath the aircraft (most visible on camera 2): that heating looks more dramatic than the tires heating on contact.
Don't forget that the video from camera 5 is substantially foreshortened. The angle of the climb after the bounce isn't that extreme. Look at camera 2 as the aircraft goes back up: from frame to frame as it leaves the shot, it appears to be at a much more natural angle. The same goes for the sharpness of the nose down before impact, which is caught on camera 3.
thcrozier,
Any exhaust leaving the engine, even at idle, will appear bright on an IR image. I don't think the visibility of the jet exhaust means anything more than that the engines are running.
On camera 1 there's still a clear hot wake from the engines as the aircraft floats into the first touch down, with no obvious change as the aircraft leaves the frame (I think still floating just off the ground - no tire heat signature). On camera 5, it's plausible that the engine glow rises after the first touch down as the "go around" starts, but it's not absolutely clear - the aircraft is approaching the camera, and naturally getting brighter anyway. I'd guess that's consistent with an intention to go around at the initial float, but for the likely damage/loss of the nose wheel in the first burst of heat underneath the aircraft (most visible on camera 2): that heating looks more dramatic than the tires heating on contact.
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awblain,
Since you are physically closer to JPL than I am, I'll take your word for it, unless of course you are in Pasadena, TX
You are correct that my perception of increasing thrust could well be due to the changing view angle and increasing proximity in 2 and 5. It may also be confirmation bias; i.e. I'm trying to understand how it could get so tail high so quickly, and the leverage of high thrust well above CG, close to or below stall speed, might partially account for it - so I might just be seeing what I want to see.
Since you are physically closer to JPL than I am, I'll take your word for it, unless of course you are in Pasadena, TX
You are correct that my perception of increasing thrust could well be due to the changing view angle and increasing proximity in 2 and 5. It may also be confirmation bias; i.e. I'm trying to understand how it could get so tail high so quickly, and the leverage of high thrust well above CG, close to or below stall speed, might partially account for it - so I might just be seeing what I want to see.
Last edited by thcrozier; 23rd Jan 2014 at 18:28.
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So many are looking for airplane problems or uniqueness to account for this accident.
I think it boils down to this:
Plane was too fast (indicated airspeed)
Plane was too fast (ground speed because of above and tailwind, and high pressure altitude/higher TAS)
Plane hit nose wheel first. Best correction would have been a normal go around.
PILOTS attempted to salvage landing instead of going around and:
Either two pilots didn't work together and worked to make things worse
or, possibly there was a slightly forward CofG impacting elevator authority.
I think it boils down to this:
Plane was too fast (indicated airspeed)
Plane was too fast (ground speed because of above and tailwind, and high pressure altitude/higher TAS)
Plane hit nose wheel first. Best correction would have been a normal go around.
PILOTS attempted to salvage landing instead of going around and:
Either two pilots didn't work together and worked to make things worse
or, possibly there was a slightly forward CofG impacting elevator authority.
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thcrozier
no offense was meant sir. I've looked at so many crashes for so long. At one time someone could point to a failure in the structure or something really dramatic and come up with a reason for a crash.
BUT more and more its the boys in the front end that do something wrong.
Stick pushers usually have some sort of warning that they will push, eg a stick shaker. One plane I flew had a series of lights that would illuminate when a pusher was going to fire and you could disable (temporarily) the pusher by pushing one of the lights.
The challenger that crashed on takeoff way out of CG was not the plane's fault, it was the pilot's fault for being out of CG.
I had a math teacher in the 9th grade. A quote of his that has stayed with me for decades was/is: DON'T FALL FOR TRAPS.
Landing at a mountain airport with a major tailwind is a trap.
Landing a plane with swept wings and no leading edge device/s may be a trap.
no offense was meant sir. I've looked at so many crashes for so long. At one time someone could point to a failure in the structure or something really dramatic and come up with a reason for a crash.
BUT more and more its the boys in the front end that do something wrong.
Stick pushers usually have some sort of warning that they will push, eg a stick shaker. One plane I flew had a series of lights that would illuminate when a pusher was going to fire and you could disable (temporarily) the pusher by pushing one of the lights.
The challenger that crashed on takeoff way out of CG was not the plane's fault, it was the pilot's fault for being out of CG.
I had a math teacher in the 9th grade. A quote of his that has stayed with me for decades was/is: DON'T FALL FOR TRAPS.
Landing at a mountain airport with a major tailwind is a trap.
Landing a plane with swept wings and no leading edge device/s may be a trap.
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Glendalegoon, I think you're completely right.
As you say, no obvious airplane problems until landing very fast, on the nose wheel, and seeming to break things off around and about.
After making those sparks, there was a very sharp pull up (I don't think it would bounce off its nose, it would need to be hauled off it), leading to an initial climb - all shown on camera 2 - until presumably a stall or near stall before it appeared on camera 3, leading to a big push into the ground, whether initiated by man or machine, and perhaps helped along by the increasing thrust moment, and bad luck with gusts.
Perhaps streaking to a stop on the nose after the first impact might have ended better too - it certainly doesn't look like that would have smashed the starboard wing and caused that large prompt fire.
thcrozier, It was CA, although no longer.
The pitch did quickly tip down, perhaps by 10 degrees. From an instinctive push to avoid a stall? A stick pusher firing? A pitch-down moment from full power coming on? All work the right way. If you'd been standing between cameras 2 and 3, then I suspect that the pitch down would have been striking, but appear rather less dramatic than it does from camera 5.
As you say, no obvious airplane problems until landing very fast, on the nose wheel, and seeming to break things off around and about.
After making those sparks, there was a very sharp pull up (I don't think it would bounce off its nose, it would need to be hauled off it), leading to an initial climb - all shown on camera 2 - until presumably a stall or near stall before it appeared on camera 3, leading to a big push into the ground, whether initiated by man or machine, and perhaps helped along by the increasing thrust moment, and bad luck with gusts.
Perhaps streaking to a stop on the nose after the first impact might have ended better too - it certainly doesn't look like that would have smashed the starboard wing and caused that large prompt fire.
thcrozier, It was CA, although no longer.
The pitch did quickly tip down, perhaps by 10 degrees. From an instinctive push to avoid a stall? A stick pusher firing? A pitch-down moment from full power coming on? All work the right way. If you'd been standing between cameras 2 and 3, then I suspect that the pitch down would have been striking, but appear rather less dramatic than it does from camera 5.
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No offence taken, Glendalegoon. I agree that in all likelihood the cause of the accident will be placed primarily with the unfortunate crew. Nevertheless, the ballistic events subsequent to the first impact fascinate me.
From awblain:
If only we had the luxury of turning the clock back...
Personally, I don't think I would have even tried the second approach after missing the first one some minutes before; but we all like to think that about ourselves, don't we?
From awblain:
Perhaps streaking to a stop on the nose after the first impact might have ended better too - it certainly doesn't look like that would have smashed the starboard wing and caused that large prompt fire.
Personally, I don't think I would have even tried the second approach after missing the first one some minutes before; but we all like to think that about ourselves, don't we?
Last edited by thcrozier; 23rd Jan 2014 at 22:07.