Continental 737 Off Runway at DEN
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RTO When?
News reports are now saying RTO was initiated after the airplane left the runway. Here is one more: Pilot: I tried to control jet - The Denver Post
Fox News just gave the same report. They seem to be based on this NTSB release: Press Advisory, but I do not see it saying they hit the brakes after leaving the runway.
The NTSB release did say "The FDR data also shows that both engines were commanded into reverse thrust following rejection of the takeoff by the flight crew, which occurred after the aircraft had already left the runway."
This statement is a little ambiguous. Does "occurred after the arcaft departed the runway" apply to "engines were commanded into reverse thrust" or to "rejection of the takeoff by the flight crew" i.e. hit the brakes. Brakes are immediate. Take off thrust to reverser operation takes longer. Could that be a red herring being reported regarding RTO after leaving the runway?
If they were doing 119 knots when they hit the brakes, it would have taken at least 4 seconds to cover the 750 feet from the first tire marks to the point where 1404 left the runway. It could be four seconds between "ABOET! ABORT!", brakes applied, and getting the levers to full reverse.
Fox News just gave the same report. They seem to be based on this NTSB release: Press Advisory, but I do not see it saying they hit the brakes after leaving the runway.
The NTSB release did say "The FDR data also shows that both engines were commanded into reverse thrust following rejection of the takeoff by the flight crew, which occurred after the aircraft had already left the runway."
This statement is a little ambiguous. Does "occurred after the arcaft departed the runway" apply to "engines were commanded into reverse thrust" or to "rejection of the takeoff by the flight crew" i.e. hit the brakes. Brakes are immediate. Take off thrust to reverser operation takes longer. Could that be a red herring being reported regarding RTO after leaving the runway?
If they were doing 119 knots when they hit the brakes, it would have taken at least 4 seconds to cover the 750 feet from the first tire marks to the point where 1404 left the runway. It could be four seconds between "ABOET! ABORT!", brakes applied, and getting the levers to full reverse.
Last edited by repariit; 8th Jan 2009 at 15:44. Reason: Correction
Gust from the Right?
So far no mechanical malfunctions, nor any slippery patches on the runway.
A wind gust would have to be a really big one, especially on bare pavement.
Wake turbulence from a heavy departing 34L
The wind side force and velocity can be derived from the resultant path, less any opposing rudder, thrust and wheel vectors. That can be compared to LLWAS data.
Note that there would have to be a sustained side force for some 750'
A wind gust would have to be a really big one, especially on bare pavement.
Wake turbulence from a heavy departing 34L
The wind side force and velocity can be derived from the resultant path, less any opposing rudder, thrust and wheel vectors. That can be compared to LLWAS data.
Note that there would have to be a sustained side force for some 750'
Still seems to point towards something breaking on the TO roll and leaving the aircraft with an overwhelming urge to go left. After all, the 'plane in question is certified to lose an engine on the critical side (left in this case), with a steady 34kt crosswind and stay on the paved surface (or complete a SE TO at >V1). By all accounts this is not deemed to be limiting, just the maximum that has been demonstrated.
Containing deviations in yaw on the runway is something airline pilots get a lot of practice at, both on the line and in the simulator and I'll be very surprised if it turns out to have been mishandling in this case. I'd have thought the sort of *sustained* crosswind needed to take a jet like this into the weeds, especially when accelerating with the rudder becoming more effective, would be in the order of 50kts... Way over what was reported, even for gusts.
Containing deviations in yaw on the runway is something airline pilots get a lot of practice at, both on the line and in the simulator and I'll be very surprised if it turns out to have been mishandling in this case. I'd have thought the sort of *sustained* crosswind needed to take a jet like this into the weeds, especially when accelerating with the rudder becoming more effective, would be in the order of 50kts... Way over what was reported, even for gusts.
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FullWings
Couldn't agree more. I'll I need is the pilot's report that the a/c "veered left" unexpectedly while tracking the centerline. Either there was a mechanical problem, or what he meant to say was "my skills fell apart unexpectedly". At his speed, Rudder was unquestionably adequate, but he mentions tiller input as an effort to correct? Signals an anomaly to me. Was his tillering an effort to correct a slight left tendency of the a/c, or meant to "return" a malfunctioning n/w (and/or a/c) to track,unsuccessfully? Hence the tire marks? The appearance of nw marks on the asphalt after the mains would suggest the TO is already in distress, and the tillering was meant to be corrective of a building problem, not a minor adjustment to track (which is its design).
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One thing is for sure, if the reported comments by the Captain are correct - to need to try n/wheel steering at the speeds we appear to have is a most definite last-ditch effort. It would normally cause such enormous loads on the nose-gear and large sideways forces on the a/c that is never normally used above about 60kts.
Indeed curious, and an urgent early diagnosis by the NTSB for 737 operators is a must.
Indeed curious, and an urgent early diagnosis by the NTSB for 737 operators is a must.
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What's the height of the 735 tail? Note: LLWAS sensors are not on the ground, but up on towers - some of which are > 100 ft. Any of you know the terrain to the W of 34R. Looks like creek drainages.
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misd-agin
"Tire marks might be from the side load on the main gear tires and have nothing to do with brake application.
Nose tire marks might also be from side loads, especially after the nosewheel steering was used."
seems very logical to me.
Nose tire marks might also be from side loads, especially after the nosewheel steering was used."
seems very logical to me.
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Not much else they could be! A simultaneous double lock-up of both wheel bearings is surely beyond the range of possibilites? Certainly IF n/w steering was being used, probably aggressively, the wheels would have been turned well across the motion.
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misd-agin
"Tires not turning would also leave tire marks". Er, why would the n/w tires not be turning? BOAC has it right. There are no brakes on the "front tires" to stop their rotation. They might be cocked @ 90 degrees to track, and some how have ceased rotation, but, come on. Nothing but side load would leave marks on the RW. (nose wheel).
Gust from right or left?
Agreed that a gust from left conventionally works mostly on the fin and tends to point the a/c left.
However a gust, perhaps vortex, from the right working against the fuselage applies a force directing the a/c to the left. If the a/c entered a vortex at a shallow angle, it would act on the front fuselage for a period before reaching the tail.
How much of the MLG skid marks are the result of side loads vs. braking?
If all the a/c components and crew worked correctly as the data so far seem to indicate, there's not much else besides the wind or a vortex to push it off the runway.
However a gust, perhaps vortex, from the right working against the fuselage applies a force directing the a/c to the left. If the a/c entered a vortex at a shallow angle, it would act on the front fuselage for a period before reaching the tail.
How much of the MLG skid marks are the result of side loads vs. braking?
If all the a/c components and crew worked correctly as the data so far seem to indicate, there's not much else besides the wind or a vortex to push it off the runway.
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"How much of the MLG skid marks are from sideloads vs. braking?"
NTSB release stated abort came after the a/c had departed the runway. That would seem to indicate all sideload, or other unknown reasons, vs. braking.
NTSB release stated abort came after the a/c had departed the runway. That would seem to indicate all sideload, or other unknown reasons, vs. braking.
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misd-agin
Some things don't add up here. Excellent point about the call off the hard surface. Forty one seconds from brake release to 119 knots, 1650 feet to the tire marks, (mains), Captain says the a/c veered smartly left, and could not be coaxed back to the centerline, this thread is resembling another guesswork marathon, BA038. The causes of that accident were spoken by Boeings chief safety pilot within a week, and the FAA's AD was a copy of his prediction. When there are multiple possible explanations for an event, Occam says, it is most likely the simplest one. Like 038, it is virtually certain the NTSB and all principals know with certainty what occurred. Out. AF
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Sitting Bull,
Yes, too simple. I understand weathervaning, however except for gusts, this tends to dominate earlier in the takeoff roll. As the relative wind becomes more frontal as the aircraft accelerates and the rudder becomes more effective, this lessens with speed.
Then as the aircraft approaches Vr, side-skipping downwind (not upwind) can be an issue with poor technique (not likely in this case), leaving tire marks and torturing the landing gear. I honestly don't know if larger aircraft can side-skip on the runway (I assume so), but smaller aircraft certainly can.
Weathervaning at 119kts seems a little far fetched, but possible with a large enough gust. However rudder effectiveness should have been good at that speed to counteract.
Edited to add: Side-skipping, if possible, certainly would have been heard and felt, and probably recorded on the CVR. Not likely here I think.
I agree with others, based on available information, side loads on the tires seems a likely cause of the tire marks, but from the right? That's the puzzling part, which does tend to suggest mechanical failure of some kind.
Yes, too simple. I understand weathervaning, however except for gusts, this tends to dominate earlier in the takeoff roll. As the relative wind becomes more frontal as the aircraft accelerates and the rudder becomes more effective, this lessens with speed.
Then as the aircraft approaches Vr, side-skipping downwind (not upwind) can be an issue with poor technique (not likely in this case), leaving tire marks and torturing the landing gear. I honestly don't know if larger aircraft can side-skip on the runway (I assume so), but smaller aircraft certainly can.
Weathervaning at 119kts seems a little far fetched, but possible with a large enough gust. However rudder effectiveness should have been good at that speed to counteract.
Edited to add: Side-skipping, if possible, certainly would have been heard and felt, and probably recorded on the CVR. Not likely here I think.
I agree with others, based on available information, side loads on the tires seems a likely cause of the tire marks, but from the right? That's the puzzling part, which does tend to suggest mechanical failure of some kind.
Last edited by Flight Safety; 9th Jan 2009 at 06:04.
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Report cites steering in Denver jet crash - USATODAY.com
Report cites steering in Denver jet crash
By Alan Levin, USA TODAY
The captain of a Continental Airlines jet that skidded off a Denver runway and burst into flames last month attempted to steer the jet using a method linked to runway accidents in the past, federal accident investigators reported Wednesday.
The National Transportation Safety Board has not said what caused Flight 1404, a Boeing 737-500 headed for Houston, to skid off the runway on Dec. 20 while attempting to take off in a brisk crosswind. But a preliminary report released by investigators offers the first glimpse of what might have triggered the crash.
All 115 people aboard escaped as jet fuel burned through the right side of the jet. The crash injured 38 people, five of whom were hospitalized, the NTSB said.
As the jet accelerated toward takeoff, the captain attempted to keep it rolling straight by turning the small pair of wheels under the jet's nose, the NTSB said the pilot told investigators. The nose gear is turned with a device called a tiller. The captain is not named in the report.
Two former accident investigators, Kevin Darcy and John Cox, who are not connected to the government's probe, said that using the tiller could cause the front tires to lose traction and start to skid. Pilots typically use the tiller to turn while taxiing at slow speeds, but once a jet accelerates, they steer with rudder pedals, they said.
Nose-wheel steering is something investigators will likely focus on, said Darcy, a former Boeing accident investigator who now works as a safety consultant. Darcy said he investigated a 737 accident in Mumbai in the 1990s which was partly blamed on the pilots' attempt to steer with the tiller during takeoff.
The NTSB cited "excessive nose-wheel steering" as part of the reason for a 1995 accident at John F. Kennedy International Airport involving a 747.
"It is unusual to need a tiller in a 737 on a runway, regardless of wind," said Cox, a former US Airways pilot who is also a safety consultant. "In all my years, I never needed the tiller. The rudder steering was always sufficient."
The winds at Denver were gusting up to 37 mph from the west as the jet attempted to take off to the north. Jets naturally tend to turn into the wind, just as a weather vane does. The Continental jet turned into the wind.
************************************************************ ***************************************
If anyone has done some car performance/handling/racing schools you quickly learn that a tire gives you the greatest traction right before it gives you none. In cars that means it will give you a combination of cornering, braking, and acceleration traction until it can't handle it, at which point it abruptly stops doing anything. Watch F1 or motorcycle racing, in particular, to see the sudden transfer from traction, and tracking, to loss of traction and spinning, sliding, or highsiding. The transfer point can be very sudden.
As related to this accident the investigators have to figure out exactly at what speed, and with what input, the pilot used the tiller. FDR was operative while the a/c was on the runway so that information should be available.
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NTSB release -
Both pilots remarked that all appeared normal until the aircraft began to deviate from the runway centerline. The Captain noted that the airplane suddenly diverged to the left, and attempts to correct the deviation with the rudder were unsuccessful. He stated that he briefly attempted to return the aircraft to the centerline by using the tiller to manipulate the steering of the nose gear but was unable to keep the aircraft on the runway. Bumping and rattling sounds audible on the Cockpit Voice Recorder (CVR) have been time- correlated with the Flight Data Recorder (FDR) and were found to have occurred as the airplane exited the runway and travelled through the grassy areas adjacent to the runway. The aircraft reached a maximum speed of 119 knots, and it was traveling at 89 knots when the CVR and FDR stopped recording.
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The report doesn't indicate that the tiller was used prior to the deviation to the left starting, which is a critical point.
As in all accidents the exact sequence of events is important - the early use of the tiller possibly creating the loss of tracking ability vs. the use of the tiller after the loss of tracking ability.
It is possible for a tiller input to create enough sideload that would overcome the traction ability of the tire. That would remove most, if not all, side load(ie tracking) capability of the nose wheels and the wind on the vertical stabilizer would swing the nose into the wind. At slow speeds it's not uncommon on a/c with light nose wheel weighting (747, DC-10, S80), with large steering inputs on slippery surfaces. It can also occur on 747 and other aircraft with large vertical stabilizers (side loading) and light nose wheel footprints on takeoff roll. Loss of nosewheel traction and the plane starts weathervaning.
(only about 10-15 rewrites, and an hour+ of time, editing and trying to avoid misspeaking, speculating, etc, etc...)
And I probably still failed.
Report cites steering in Denver jet crash
By Alan Levin, USA TODAY
The captain of a Continental Airlines jet that skidded off a Denver runway and burst into flames last month attempted to steer the jet using a method linked to runway accidents in the past, federal accident investigators reported Wednesday.
The National Transportation Safety Board has not said what caused Flight 1404, a Boeing 737-500 headed for Houston, to skid off the runway on Dec. 20 while attempting to take off in a brisk crosswind. But a preliminary report released by investigators offers the first glimpse of what might have triggered the crash.
All 115 people aboard escaped as jet fuel burned through the right side of the jet. The crash injured 38 people, five of whom were hospitalized, the NTSB said.
As the jet accelerated toward takeoff, the captain attempted to keep it rolling straight by turning the small pair of wheels under the jet's nose, the NTSB said the pilot told investigators. The nose gear is turned with a device called a tiller. The captain is not named in the report.
Two former accident investigators, Kevin Darcy and John Cox, who are not connected to the government's probe, said that using the tiller could cause the front tires to lose traction and start to skid. Pilots typically use the tiller to turn while taxiing at slow speeds, but once a jet accelerates, they steer with rudder pedals, they said.
Nose-wheel steering is something investigators will likely focus on, said Darcy, a former Boeing accident investigator who now works as a safety consultant. Darcy said he investigated a 737 accident in Mumbai in the 1990s which was partly blamed on the pilots' attempt to steer with the tiller during takeoff.
The NTSB cited "excessive nose-wheel steering" as part of the reason for a 1995 accident at John F. Kennedy International Airport involving a 747.
"It is unusual to need a tiller in a 737 on a runway, regardless of wind," said Cox, a former US Airways pilot who is also a safety consultant. "In all my years, I never needed the tiller. The rudder steering was always sufficient."
The winds at Denver were gusting up to 37 mph from the west as the jet attempted to take off to the north. Jets naturally tend to turn into the wind, just as a weather vane does. The Continental jet turned into the wind.
************************************************************ ***************************************
If anyone has done some car performance/handling/racing schools you quickly learn that a tire gives you the greatest traction right before it gives you none. In cars that means it will give you a combination of cornering, braking, and acceleration traction until it can't handle it, at which point it abruptly stops doing anything. Watch F1 or motorcycle racing, in particular, to see the sudden transfer from traction, and tracking, to loss of traction and spinning, sliding, or highsiding. The transfer point can be very sudden.
As related to this accident the investigators have to figure out exactly at what speed, and with what input, the pilot used the tiller. FDR was operative while the a/c was on the runway so that information should be available.
--------------------------------------------------------------------------------------------------------------------------
NTSB release -
Both pilots remarked that all appeared normal until the aircraft began to deviate from the runway centerline. The Captain noted that the airplane suddenly diverged to the left, and attempts to correct the deviation with the rudder were unsuccessful. He stated that he briefly attempted to return the aircraft to the centerline by using the tiller to manipulate the steering of the nose gear but was unable to keep the aircraft on the runway. Bumping and rattling sounds audible on the Cockpit Voice Recorder (CVR) have been time- correlated with the Flight Data Recorder (FDR) and were found to have occurred as the airplane exited the runway and travelled through the grassy areas adjacent to the runway. The aircraft reached a maximum speed of 119 knots, and it was traveling at 89 knots when the CVR and FDR stopped recording.
---------------------------------------------------------------------------------------------------------------------------
The report doesn't indicate that the tiller was used prior to the deviation to the left starting, which is a critical point.
As in all accidents the exact sequence of events is important - the early use of the tiller possibly creating the loss of tracking ability vs. the use of the tiller after the loss of tracking ability.
It is possible for a tiller input to create enough sideload that would overcome the traction ability of the tire. That would remove most, if not all, side load(ie tracking) capability of the nose wheels and the wind on the vertical stabilizer would swing the nose into the wind. At slow speeds it's not uncommon on a/c with light nose wheel weighting (747, DC-10, S80), with large steering inputs on slippery surfaces. It can also occur on 747 and other aircraft with large vertical stabilizers (side loading) and light nose wheel footprints on takeoff roll. Loss of nosewheel traction and the plane starts weathervaning.
(only about 10-15 rewrites, and an hour+ of time, editing and trying to avoid misspeaking, speculating, etc, etc...)
And I probably still failed.