Continental 737 Off Runway at DEN
AF;
Small point AF - I think you're intending to refer to V1, not V2?
Agree on the cg. On the tiller, no, I can't either - in fact, the dynamics at speeds close to V1 mitigate against the use of the tiller/nosewheel for DC notwithstanding stab trim, (for others: which I think was in the normal position - see earlier discussion), c-of-g etc, in that the nw will skip unless the nws angles achieved are very small and fed in slowly, (relatively speaking). IOW, I can't see the use of tiller causing further divergence - I can only see it not being effective.If the airplane was not responding to "full" rudder at the speed at which the diversion began then either a rogue gust which exceeded the certification limits (rudder authority at/near V1) occurred or there was a technical/mechanical failure.
Something that hasn't been mentioned yet is the position of the control wheel or any slight roll angle just before the diversion began. Upwind spoiler deflection will tend to pull the nose in that direction, as we know. Not suggesting, just asking.
The 320 usually begins recording the wind at about 70 to 80kts CAS or so - on the ND as well as on the QAR - anyone know what the 735 does?
Small point AF - I think you're intending to refer to V1, not V2?
I still am curious why one might think tillering might be a solution to loss of DC? I don't think cg enters into it;
Something that hasn't been mentioned yet is the position of the control wheel or any slight roll angle just before the diversion began. Upwind spoiler deflection will tend to pull the nose in that direction, as we know. Not suggesting, just asking.
The 320 usually begins recording the wind at about 70 to 80kts CAS or so - on the ND as well as on the QAR - anyone know what the 735 does?
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PJ2
Yes V1, Senior moment. But cheap pancakes at IHOP.
Yes I think people may be neglecting the dynamic nature of T/O. Done well, as is the usual case, it looks effortless, and frequently is. A seeming minor glitch can rapidly cascade into an emergency even with rapid and experienced hands at the controls. 99% boredom interspersed with 1% panic, or whatever that old bromide is. I can't imagine (at the moment) what might have caused this that will remain mysterious, but patience is the foe of a busy website, no? Like I wrote earlier, Boeing figured out BA's short arrival quickly, though the AD (sic) came much later. I still don't understand how the industry can be satisfied with a solution that instructs pilots how to fly safely with corrupted (wet) fuel. Trent or GE? two separate profiles? AF
Yes I think people may be neglecting the dynamic nature of T/O. Done well, as is the usual case, it looks effortless, and frequently is. A seeming minor glitch can rapidly cascade into an emergency even with rapid and experienced hands at the controls. 99% boredom interspersed with 1% panic, or whatever that old bromide is. I can't imagine (at the moment) what might have caused this that will remain mysterious, but patience is the foe of a busy website, no? Like I wrote earlier, Boeing figured out BA's short arrival quickly, though the AD (sic) came much later. I still don't understand how the industry can be satisfied with a solution that instructs pilots how to fly safely with corrupted (wet) fuel. Trent or GE? two separate profiles? AF
but patience is the foe of a busy website, no?
Re 99%, 1%, yeah, you got that right as well! ;-)
The usual takeoff is about 35 to 38 seconds. Over 40, unless one is WAT limited, that's starting to get longish. I figure between the the time things began to go wrong and the a/c coming to rest in the weeds might've been about 30 seconds or so, and maybe 4 to 6 seconds between initial heading divergence and leaving the runway? Just a guess...
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Takeoff roll time is dependent on airport elevation altitude, weight, aircraft type, and power setting. Was just in DEN, forgot to time the takeoff roll.
Had a 757 Max power takeoff recently that took 48 secs from brake release to rotation.
Seemed long, but not as long as sitting in the back of a 727-200 at the same high altitude airport. That took 1:04 from brakes release to the start of rotation!
Had a 757 Max power takeoff recently that took 48 secs from brake release to rotation.
Seemed long, but not as long as sitting in the back of a 727-200 at the same high altitude airport. That took 1:04 from brakes release to the start of rotation!
misd-agin;
Yep - WAT...Weight, Altitude, Temperature ;-)
Never timed the 340-300 on an all-up takeoff out of HKG but it was like the DC8-61 - made you wonder about a few things.
Takeoff roll time is dependent on airport elevation altitude, weight, aircraft type, and power setting.
That took 1:04 from brakes release to the start of rotation! 
Looking only at info. from the NTSB, there are some interesting points:
A directional control problem became evident at high speed on the TO roll; corrective action using rudder was unsuccessful, as was a subsequent attempt at using the tiller and the TO was rejected. The order of these actions (as in when the RTO was commanded) is not completely clear as there is some semantic confusion in the initial bulletins but they seem to imply that the RTO was intiated as/after the aircraft left the runway. This makes the MLG & NW tyre marks on the runway (as reported) look rather odd.
To get rubber left behind you need a) a lockup, which can be discounted as there were no flat spots found on the tyres or b) ABS working at maximum effect, which would imply the RTO came much earlier than stated or c) a large enough slip angle to abrade the tyre. Going with the last option for a moment, that means the aircraft's heading had diverged *significantly* from its track. Something happened, either aerodynamically or mechanically (or both) to introduce yaw to the stage that it was 'drifting' along the paved surface; the forces involved eventually took it off the side.
Examining purely environmental effects, it would take a fierce, prolonged 'gust' to cause this loss of control, given the type of aircraft and a dry runway. Also factor in the speed (>100kts), the veering of the wind towards the runway alignment during a gust and the increasing rudder effectiveness and it seems unlikely (but still a possibility, of course). This is with the primary flight controls functioning normally. There's not much in the METAR/TAF at the time to suggest anything unusual: no CB activity or precipitation/virga; no LLWAS info. and -4/-14 with cloud at 4000'AGL. The crosswind was a known issue.
As far as the (brief) use of the tiller goes, I'd expect that to be along the lines of: 'We're going to crash, try anything that might help!' as to use the tiller you have to let go of something else, i.e. the control column, which is something I'd not want to do close to getting airborne in a crosswind. (I don't know CO SOPs but I assume it'd be P1 hand on TLs up to V1.)
Pure speculation: In a fight, which would win - nosewheel or rudder? Low speed I'd give it to the nosewheel. >100kts, I'd say rudder. Points towards some sort of rudder malfunction, doesn't it? Add in a brisk crosswind... Who knows?
A directional control problem became evident at high speed on the TO roll; corrective action using rudder was unsuccessful, as was a subsequent attempt at using the tiller and the TO was rejected. The order of these actions (as in when the RTO was commanded) is not completely clear as there is some semantic confusion in the initial bulletins but they seem to imply that the RTO was intiated as/after the aircraft left the runway. This makes the MLG & NW tyre marks on the runway (as reported) look rather odd.
To get rubber left behind you need a) a lockup, which can be discounted as there were no flat spots found on the tyres or b) ABS working at maximum effect, which would imply the RTO came much earlier than stated or c) a large enough slip angle to abrade the tyre. Going with the last option for a moment, that means the aircraft's heading had diverged *significantly* from its track. Something happened, either aerodynamically or mechanically (or both) to introduce yaw to the stage that it was 'drifting' along the paved surface; the forces involved eventually took it off the side.
Examining purely environmental effects, it would take a fierce, prolonged 'gust' to cause this loss of control, given the type of aircraft and a dry runway. Also factor in the speed (>100kts), the veering of the wind towards the runway alignment during a gust and the increasing rudder effectiveness and it seems unlikely (but still a possibility, of course). This is with the primary flight controls functioning normally. There's not much in the METAR/TAF at the time to suggest anything unusual: no CB activity or precipitation/virga; no LLWAS info. and -4/-14 with cloud at 4000'AGL. The crosswind was a known issue.
As far as the (brief) use of the tiller goes, I'd expect that to be along the lines of: 'We're going to crash, try anything that might help!' as to use the tiller you have to let go of something else, i.e. the control column, which is something I'd not want to do close to getting airborne in a crosswind. (I don't know CO SOPs but I assume it'd be P1 hand on TLs up to V1.)
Pure speculation: In a fight, which would win - nosewheel or rudder? Low speed I'd give it to the nosewheel. >100kts, I'd say rudder. Points towards some sort of rudder malfunction, doesn't it? Add in a brisk crosswind... Who knows?
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As I'm getting bored. . . . . did we discuss whether there was any chance that they encountered wake turbulence descending from a heavy departure ahead ?. . . . . or how about (if the aircraft was de-iced) Capt's foot slipped off rudder pedal (which due to x-wind he was holding a bit of force on) due to doing walk-round in pools of the slimy stuff. ( happened to me taxying, fortunately I got back on to the brakes in time ) Well may as well get all the strange ideas out the way now before someone actually allows facts to get in the way. 
) Well may as well get all the strange ideas out the way now before someone actually allows facts to get in the way.
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NTSB preliminary report states early tests establish control continuity for all flight control surfaces, including the rudder, so rudder failure seems out for now.
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The Rudder
FullWings, very nice analysis!
Preliminary NTSB review of rudder system may not have included bench testing and teardown inspection of the Rudder PCU. If true, we may be hearing more about it in a futre NTSB release. It has only been a few days since they have moved the wreckage.
As I recall previous Rudder PCU issues were intermittent conditions relating to temperatures and close tolerances of the slide and sleeve assembly. It may take lengthy time on the bench to duplicate the conditions.
Preliminary NTSB review of rudder system may not have included bench testing and teardown inspection of the Rudder PCU. If true, we may be hearing more about it in a futre NTSB release. It has only been a few days since they have moved the wreckage.
As I recall previous Rudder PCU issues were intermittent conditions relating to temperatures and close tolerances of the slide and sleeve assembly. It may take lengthy time on the bench to duplicate the conditions.
Last edited by repariit; 10th Jan 2009 at 14:34.
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Rudder
I agree with you, the rudder will take a long time to properly analyze.
I flew the 737 and remember all the training we got about rudder hardovers. NONE of it included an on the runway encounter. I did imagine what I might do, and it is making the centerline your friend. AND DOING whatever it takes to stay on the centerline.
I recall the tragedy of USAIR 427 and the United COS crash. I can imagine a scenario where an unusual side load on the rudder, either meteorological wind (cross wind), wake turbulence, or something similiar triggering a rudder event.
HOWEVER, as I mentioned before, staying on the runway is a vital part of the takeoff. AND I know our airline has always admonished pilots against using the tiller above about 20 knots or so.
AS I recall, a reduction in hydraulic pressure happens at 700 feet radio altitude...but no protection on the ground. The first and last minute of flight is always the most challenging. I always imagined what I might do below 700 feet to keep from hitting the terminal or other planes. As I said in my first post, assymetric thrust can be your friend.
I am not ruling out other causes including human error, but nothing is off the table just yet.
I flew the 737 and remember all the training we got about rudder hardovers. NONE of it included an on the runway encounter. I did imagine what I might do, and it is making the centerline your friend. AND DOING whatever it takes to stay on the centerline.
I recall the tragedy of USAIR 427 and the United COS crash. I can imagine a scenario where an unusual side load on the rudder, either meteorological wind (cross wind), wake turbulence, or something similiar triggering a rudder event.
HOWEVER, as I mentioned before, staying on the runway is a vital part of the takeoff. AND I know our airline has always admonished pilots against using the tiller above about 20 knots or so.
AS I recall, a reduction in hydraulic pressure happens at 700 feet radio altitude...but no protection on the ground. The first and last minute of flight is always the most challenging. I always imagined what I might do below 700 feet to keep from hitting the terminal or other planes. As I said in my first post, assymetric thrust can be your friend.
I am not ruling out other causes including human error, but nothing is off the table just yet.
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To turn the question around, what happens if the RPR suddenly reduces my rudder pressure on ground, limiting down to less travel authority and less pressure? A system which should kick in shortly after liftoff suddenly becomes active during takeoff roll (in a stiff x-wind). Anybody able to comment on the controllability of the aircraft?
Would also be interesting to know if there was hydraulic fluid found on rwy and how the reservoirs incl. wheel well appeared.
Would also be interesting to know if there was hydraulic fluid found on rwy and how the reservoirs incl. wheel well appeared.
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I agree, FullWings, very nice analysis.
What I would like to know is, does anybody have any details about the tire marks? Are there any photos on the net?
Where were the tire marks in relation to the runway centerline?
Were they solid, or dashed -- straight or curved -- thick or thin?
Were they symmetrical between left and right MLG?
If the marks started before the abort, then how did the abort change their direction?
This could provide some useful information while we wait for the NTSB to release some relevant FDR data.
What I would like to know is, does anybody have any details about the tire marks? Are there any photos on the net?
Where were the tire marks in relation to the runway centerline?
Were they solid, or dashed -- straight or curved -- thick or thin?
Were they symmetrical between left and right MLG?
If the marks started before the abort, then how did the abort change their direction?
This could provide some useful information while we wait for the NTSB to release some relevant FDR data.
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I have read this entire thread with much interest noting all of the dozens of possible causes of the accident which have been offered. Hydraulic fluid was mentioned which caused me to wonder what fluids (if any) the previous departing aircraft could have leaked onto the runway surface (making it slippery) which would have evaporated from the runway before the inspection of the runway determined that it was "dry"?
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Rudder
Rudder hardover evidence is the pedal displacement and hardness .Nothing about this is declared for pilots.
From Boeing about RPR: Valves like this have proven to be reliable in-service. However,service history has shown that if the valve does fail the regulating valve tends to jam in the nearly closed position,resulting in zero output pressure.Therefore , the predominate failure mode of the RPR is ZERO pressure.
This failure is indicated for A FLT CONTROL LOW PRESSURE light in P5 and MC
Boeing add : An RPR failed low not significantly effect flight characteristic whith both engines operating.
However on all classic models the amount of rudder available is reduced whith the RPR failed to low pressure .This affect the rudder available to counter hight crosswind conditions .However sufficient rudder and lateral control is available to safely counter the x wind levels demostrated during certification testing.
Prtectthehornet: the low press condition ( 1000 psi) is to 1000 ft in takeoff and 700 ft in aproach.
From Boeing about RPR: Valves like this have proven to be reliable in-service. However,service history has shown that if the valve does fail the regulating valve tends to jam in the nearly closed position,resulting in zero output pressure.Therefore , the predominate failure mode of the RPR is ZERO pressure.
This failure is indicated for A FLT CONTROL LOW PRESSURE light in P5 and MC
Boeing add : An RPR failed low not significantly effect flight characteristic whith both engines operating.
However on all classic models the amount of rudder available is reduced whith the RPR failed to low pressure .This affect the rudder available to counter hight crosswind conditions .However sufficient rudder and lateral control is available to safely counter the x wind levels demostrated during certification testing.
Prtectthehornet: the low press condition ( 1000 psi) is to 1000 ft in takeoff and 700 ft in aproach.
rmiller774
your words "making it slippery" seem to imply a loss of friction. I don't see where this fits into a takeoff event where the aircraft should be .under rudder control
Hydraulic fluid was mentioned which caused me to wonder what fluids (if any) the previous departing aircraft could have leaked onto the runway surface (making it slippery) which would have evaporated from the runway before the inspection of the runway determined that it was "dry"? .
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lomapaseo
To be a bit more specific, if the a/c is having steering problems, the preferable surface would be black ice. It is the traction of the tires that at least contributed to the off R/W. If the a/c was skidding (tire skid marks) ice would have allowed the a/c to continue on the R/W, albeit a bit "sideways".
AF
This isn't about brakes, remember? max. brake would come simultaneously with "Abort", or a bit sooner. Would the pilot have applied brakes at an effort to induce marks and wait 5 seconds to RTO? Or did you think in the back of his mind he's thinking "I can save this TO?"
Hindsight muse. If I knew beforehand the a/c was destined to be damaged and the pax put at risk, at the "irretrievable loss of DC" I would pull the right throttle, stand on the right brake and pray for a ground loop. Off the rw is fire and death, on the runway is foam and a cup of coffee. AF
AF
This isn't about brakes, remember? max. brake would come simultaneously with "Abort", or a bit sooner. Would the pilot have applied brakes at an effort to induce marks and wait 5 seconds to RTO? Or did you think in the back of his mind he's thinking "I can save this TO?"
Hindsight muse. If I knew beforehand the a/c was destined to be damaged and the pax put at risk, at the "irretrievable loss of DC" I would pull the right throttle, stand on the right brake and pray for a ground loop. Off the rw is fire and death, on the runway is foam and a cup of coffee. AF
Last edited by airfoilmod; 11th Jan 2009 at 16:28.
Slippery liquids: De-icing fluid comes to mind, especially the newer types which are designed to come off during the TO roll. I can't see them evaporating very quickly at -4C, though, so heavy contamination would have been noted by the investigators. If it was really causing problems, I'd have thought other aircraft might have noticed something?
You can get a build up of de-icer on the runway surface but my experience of KDEN in the winter is that the dewpoints are so low that de-icing is not normally needed unless there has been some sort of frozen/freezing precipitation - don't think there was during the day of the accident.
Brakes: I'm not sure about the specific fit on the CO 737-500 or their SOPs but RTO autobrake is normally triggered by closing the thrust levers above 85kt(?), which should also deploy all the spoiler panels. This leaves you free to select reverse (or have someone do it for you) and concentrate on keeping the aircraft on the runway. It's possible to brake manually using either or both pedals while power is still on - don't know what that does to the autobrake logic (if anything) when you do close the taps. You can knock out the AB after engagement but you have stand on them pretty hard to do it.
Rudder: A 'hardover' would have taken the aircraft off the runway much more rapidly, probably ending up in a groundloop. 'Loss of authority' sounds better. If you just lost movement on the control surface, a 25-30kt crosswind would probably gently twist you off the runway... Just thinking out loud.
You can get a build up of de-icer on the runway surface but my experience of KDEN in the winter is that the dewpoints are so low that de-icing is not normally needed unless there has been some sort of frozen/freezing precipitation - don't think there was during the day of the accident.
Brakes: I'm not sure about the specific fit on the CO 737-500 or their SOPs but RTO autobrake is normally triggered by closing the thrust levers above 85kt(?), which should also deploy all the spoiler panels. This leaves you free to select reverse (or have someone do it for you) and concentrate on keeping the aircraft on the runway. It's possible to brake manually using either or both pedals while power is still on - don't know what that does to the autobrake logic (if anything) when you do close the taps. You can knock out the AB after engagement but you have stand on them pretty hard to do it.
Rudder: A 'hardover' would have taken the aircraft off the runway much more rapidly, probably ending up in a groundloop. 'Loss of authority' sounds better. If you just lost movement on the control surface, a 25-30kt crosswind would probably gently twist you off the runway... Just thinking out loud.
"Negligently aborted"....
Maybe this should become its own thread, but the first lawsuit has now been filed in the Continental crash:
First lawsuit filed in DIA crash : Local News : The Rocky Mountain News
Basis: the crew "negligently aborted" the takeoff.
So if you abandon an unsafe takeoff, that is negligent - and if you continue an unsafe takeoff, that is also negligent....?
First lawsuit filed in DIA crash : Local News : The Rocky Mountain News
Basis: the crew "negligently aborted" the takeoff.
So if you abandon an unsafe takeoff, that is negligent - and if you continue an unsafe takeoff, that is also negligent....?
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I am only surprised at how long it took to file suit.
Continental is also facing a huge trial in France regarding its POSSIBLE role in the crash of Air France Concord.
IF the plane in Denver was mechanically sound at the start of the takeoff roll, and didn't suffer a mechanical problem during the takeoff roll, a lawsuit is certainly justified. As we all know , an aborted/rejected takeoff at high speed is a very demanding maneuver and may not end in a pretty picture. It seems so little was said during the takeoff roll, I am greatly concerned about CRM in this cockpit. I am very interested to find out which crewmember actually commanded ABORT or REJCT...there are many questions still unanswered here.
I've flown with people, in jets mind you, that really didn't care about being on the darn centerline throughout the takeoff or landing. It wasn't as important as being smooth!
The moment a plane diverges from centerline is time to do something...by the time the main wheels are no longer stradling the centerline, things are going to hell in a handbasket (can I say hell?) Aggressive action might have saved things in this situation. Perhaps a better training scenario is due at many airlines.
I know of one case on a 737 300 that the engines didn't spool up at the same rate...the pilot didn't hold the brakes until the engines were spooled up and he had a heck of a time keeping the thing on the runway.
Was this a rolling takeoff? Were the flight controls properly set for Xwind?
It is too soon to be sure, but it is not too soon to have lots of questions. The one area of flying that really hasn't been fully researched is the mind of a pilot.
IF the plane in Denver was mechanically sound at the start of the takeoff roll, and didn't suffer a mechanical problem during the takeoff roll, a lawsuit is certainly justified. As we all know , an aborted/rejected takeoff at high speed is a very demanding maneuver and may not end in a pretty picture. It seems so little was said during the takeoff roll, I am greatly concerned about CRM in this cockpit. I am very interested to find out which crewmember actually commanded ABORT or REJCT...there are many questions still unanswered here.
I've flown with people, in jets mind you, that really didn't care about being on the darn centerline throughout the takeoff or landing. It wasn't as important as being smooth!
The moment a plane diverges from centerline is time to do something...by the time the main wheels are no longer stradling the centerline, things are going to hell in a handbasket (can I say hell?) Aggressive action might have saved things in this situation. Perhaps a better training scenario is due at many airlines.
I know of one case on a 737 300 that the engines didn't spool up at the same rate...the pilot didn't hold the brakes until the engines were spooled up and he had a heck of a time keeping the thing on the runway.
Was this a rolling takeoff? Were the flight controls properly set for Xwind?
It is too soon to be sure, but it is not too soon to have lots of questions. The one area of flying that really hasn't been fully researched is the mind of a pilot.