Continental 737 Off Runway at DEN

[theron]

in these previous events of rudder control reversal were there also reports of strange sounds on the CVR?

if not then how would you explain them in this case to fit with your possible scenario?

does this type of event show up in the FDR? i.e. rudder input graph synchronises with rate of deviation graph?

it's been about 10 days now, has the plane been lifted/moved yet? (i understand its the holiday period, this is not criticism, just curiosity since the results may end some of the speculation here)

and have we heard anything about the weather information recorded by the weather station which was closest to the event? (as mentioned in the NTSB press conference)

im too curious

[snowfalcon2]

in these previous events of rudder control reversal were there also reports of strange sounds on the CVR?

if not then how would you explain them in this case to fit with your possible scenario?

speculation...

The most likely source of the sounds is from the landing gear, which was obviously not involved in the previous incidents. A skid which leaves rubber marks on the runway presumably puts some strong sideloads on the landing gear, exposing any bearing plays and creating unfamiliar new soundpaths.
That said, on the other hand I recall the NTSB spokesman confirming the tires had no flat spots that might have caused the rattling. Well, we'll see.

[Smilin_Ed]

Flat Spots

The comment about no flat spots on the tires would have been only about the main mounts since the nose wheels had not yet been uncovered.

[BOAC]

I may have missed it in all the flak, but has anyone come up with a plausible theory to cover the reported 'nosewheel marks' on the runway, starting just after the mainwheel marks?

[FullWings]

has anyone come up with a plausible theory to cover the reported 'nosewheel marks' on the runway, starting just after the mainwheel marks?

I theorised in an earlier post that the tyre slip angle must have become quite large (as the nose wheel is unbraked) to leave visible rubber deposits on the runway. Therefore, the alignment of the wheels would have been significantly different from the ground track of the aircraft. I can think of two likely scenarios (not excluding any others, of course):

1. The departure to the left wasn't connected to any malfunction of the nose gear/steering, so the slip angle was generated by the rudder fine steering.

2. There was a problem with the nose gear that led to a significant sideways force being generated, enough to overcome the restoring couple from the rudder.

I tend to go more for Nr. 2, as the maximum normal excursion of the nose gear in azimuth is quite limited (unless the tiller is used...). Also, if full nose steering left marks on the runway in normal operation, you wouldn't be able to see the surface in most airports by now!

The marks from the mainwheels confused me for a bit but then I remembered that full RTO braking operates at the limit of traction, so would leave tell-tale anti-skid marks behind as the wheels just started to lock and then were released to start another cycle.

The fact that the nosewheel marks start just after the mainwheel ones could be explained by the forward weight transfer as braking started, increasing the forces acting on the nose gear significantly.

The above speculation doesn't discount any other possible failures but it does seem to roughly fit the facts as we know them at the moment...

[airfoilmod]

Fullwings

Very infrequently the nose gear (or main) oleo can fail; the subsequent excursion of the truck can include 90 degree angle (or any) to track, causing scrubbing and wild heading excitement. It's possible some procuring cause set the stage for nose gear oleo parting its mount, (the nose scrub began after mains). Side pressure on articulating wheels can induce intense loads on a structure (oleo) not intended for such. A theory, to be sure, but it happened to a friend. (All turned out well). .

[FullWings]

I vaguely remember an A32? having a nosewheel/gear problem into LA and landing in a shower of sparks. (About 3 years ago?) I think, luckily, that it failed at 90degs offset, so there weren't large forces trying to push it off the runway. Could have been very bad if it had stuck at 20-30degs...

[BOAC]

(One of the) puzzles here is that the n/w marks began AFTER (presumably) RTO braking on the mainwheels and thus the 'mainwheel marks'. One would therefore deduce that any 'departure' of the n/w alignment occurred after the RTO? So, we wait to hear the cause of the RTO.

How are the injured doing, anyone know?

[snowfalcon2]

The way I understood the NTSB briefing was that RTO was not initiated until the airplane had diverged some way from the centerline, perhaps even at the runway edge. The main proof point is that the airplane continued to accelerate from 90 kts, when the first officer noticed leaving the centerline, to a maximum of 119 kts if I remember correctly. If this is correct, the runway main wheel marks are not from braking but from skidding.

[airfoilmod]

RTO

Max braking associated w/ RTO @ 105 knots puts a hellish load on wheels that are meant mainly to prevent the a/c from curtseying on the ramp. The sequence is suggestive of maingear inducing sideloading on the nose gear, which held on long enough to steer the a/c into the weeds before it failed completely. At 105 knots this machine is an out of control "truck", not quite a bird. Transition from truck to bird or vice versa imposes high forces on equipment that is asked to do much very near its limits.

[FullWings]

So, we wait to hear the cause of the RTO.

Yes. There's not been much mention of that, officially, as far as I can see.

Candidates *could* be:

Loss (or impending loss) of control as full corrective input failed to rectify a deviation. - Fairly likely, IMHO.
Engine fire/failure. - No reports of that, yet.
Config. warning. (Including reverser unlock.) - Again, no reports.
Noise/vibration. - Possible but most SOPs are against rejecting at high speed for these kind of things, unless accompanied by some sort of indication.

A clue must be in the fact that the aircraft continued to exit stage left, even after the initial RTO actions had been accomplished (assuming they were, which I think we must at this stage). This would rule out most things to do with the engines. I don't feel the weather had much to do with it (dry runway, x-wind within limits, etc.) so that leaves some sort of mechanical failure leaving the aircraft uncontrollably tracking left: not pleasant to have to deal with...

[sleeper]

A couple of years back one of our 737's had a birdstrike just after t/o. The bird had hit the nosewheel and unbeknown to the crew had severed a cable. Upon landing at destination they found out that, after nosewheel landing, the aircraft veered sharply to the left and was uncontrollable. It left the runway in a similar manner as the denver one. No casualties, but the aircraft was written of. It turned out that the nosewheel was locked somewhere between straight and 90 degrees.

PPRUNE thread:
http://www.pprune.org/rumours-news/1...unway-bcn.html


Final report
http://www.fomento.es/NR/rdonlyres/0..._070_A_ENG.pdf

[theron]

BOAC

I may have missed it in all the flak, but has anyone come up with a plausible theory to cover the reported 'nosewheel marks' on the runway, starting just after the mainwheel marks?

the distance between the marks left by the nose and main gear was about 100 feet as stated by the NTSB, the wheel base of 737 is shorter than 100feet, so my logical conclusion (and as you mentioned in your next post) is the main gear started making marks on the runway first, but not by a large amount of time.

what would induce this to happen?

the wait goes on...

[misd-agin]

airfoilmod - RTO at 105 kts isn't anywhere near the loads experienced at max weight at much higher speeds.

Have to be an engineer to figure the difference between the loads they experienced and what a max gross weight RTO from 140+ kts would be.

But I wouldn't be surprised if the loads that the accident aircraft experienced were less than 50% of the max capability of the brakes.

[airfoilmod]

Misd-agin

You are absolutely correct. My 105 knots is a guess at the speed the a/c went off pavement, at which time all known parameters lose some relevance. Having reached 119 I think my point was more about side loading as the heavy jet slewed left. (unabated). The point about the wheelbase and onset of skidding vis a vis mains and n/g is important, since it suggests that the tires and wheels were more likely reacting to events as a whole, rather than independently, or as a cause per se. Any of a number of bad things could have precipitated the need for RTO (or the perceived need).The noises described as thumping and banging may be important, but as has been suggested here before and on another thread, noise isn't sufficient to abort necessarily (Brussels, Kalitta?). Everything wanted to go left, there was a reason, assuredly. Regards, AF

[etesting2000]

Rudder PCU

Saw a reference to an AD requiring compliance by 11-08. Forgot how much fun it was to research AD Notes. Any comments on applicability and compliance date?

Boeing Model 737 Airplanes

Quote from AD

"SUPPLEMENTARY INFORMATION:

Discussion

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to include an AD that supersedes AD 2002-20-07 R1, amendment 39-12940 (67 FR 67518, November 6, 2002). The existing AD applies to all Boeing Model 737 airplanes. That NPRM was published in the Federal Register on November 15, 2006 (71 FR 66474). That NPRM proposed to continue to require installation of a new rudder control system and changes to the adjacent systems to accommodate that new rudder control system. For certain airplanes, that NPRM proposed to add, among other actions, repetitive tests of the force fight monitor of the main rudder power control unit (PCU), repetitive tests of the standby hydraulic actuation system, and corrective action; as applicable. For those airplanes, that NPRM also proposed to add, among other actions, replacement of both input control rods of the main rudder PCU and the input control rod of the standby rudder PCU with new input control rods, as applicable, which would end the repetitive tests. For certain other airplanes, that NPRM proposed to add installation of an enhanced rudder control system in accordance with new service information.
Actions Since NPRM Was Issued

Since we issued the NPRM, we have received a report of a fractured rod end on the input control rod of the standby rudder PCU on a Model 737-700 series airplane. This condition was discovered during accomplishment of an operational test of the standby hydraulic actuation system in accordance with Boeing Alert Service Bulletin 737- 27A1280, dated May 25, 2006 (one of the actions specified in the NPRM). Investigation revealed that, although the input control rod had an existing crack of significant size, it ultimately fractured due to fatigue damage. Fatigue damage is caused by repetitive forces being applied (i.e., cyclic loading).

This finding of fatigue damage is not consistent with the results of our investigation that led to actions specified in the NPRM. The actions and compliance times specified in the NPRM were based on our finding that, while the input control rod may have been cracked during assembly, no significant loading was present to further degrade the integrity of the input control rod over time, causing it to fracture.

Therefore, we have determined that, for certain Model 737-600, - 700, -700C, -800, and -900 series airplanes on which the suspect input control rod of the standby rudder PCU was installed during production, the compliance times for the following actions in the NPRM will not detect and correct failure of the input control rod of the standby rudder PCU in a timely manner:

The initial compliance time and repetitive intervals for the operational tests specified in paragraph (g)(1), and

The threshold for the replacement of the input control rod of the standby rudder PCU specified in paragraph (g)(3).

Failure of one of the two input control rods of main rudder PCU, under certain conditions, could result in reduced controllability of the airplane; and failure of any combination of two input control rods of the main rudder PCU and/or standby rudder PCU could cause an uncommanded rudder hardover event and result in loss of control of the airplane. "

"Requirements of AD 2002-20-07 R1

Installation

(f) Except as provided by paragraphs (h) and (i) of this AD: Within 6 years after November 12, 2002 (the effective date of AD 2002-20-07), do the actions required by paragraphs (f)(1) and (f)(2) of this AD, in accordance with a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA.

(1) Install a new rudder control system that includes new components such as an aft torque tube, hydraulic actuators, and associated input control rods, and additional wiring throughout the airplane to support failure annunciation of the rudder control system in the flight deck. The system also must incorporate two separate inputs, each with an override mechanism, to two separate servo valves on the main rudder PCU; and an input to the standby PCU that also will include an override mechanism.

(2) Make applicable changes to the adjacent systems to accommodate the new rudder control system. "

I realy hope they did not defer this action.

[repariit]

More Facts Soon

9NEWS.com, in Denver, reports that the aircraft might be moved tommorow. We should hear about the Nose Gear condition shortly thereafter.

[repariit]

Cranes to Move in January 2nd

Today's report in the Denver Post says the airplane will be moved tommorow, Friday.

Also, that all on board are now out of the hospital, and that NTSB states xwind gusts were below limits.

Wind gusts cited before DIA crash - The Denver Post

[airfoilmod]

Supposition

Hypothesis is interesting and can be instructive.

1. Thank God no one was killed.

2. What some passengers said was "flying" may have been the Captain's (or FP) attempt to get the nosewheel off the deck to restore steering by diff brakes.

3. When the elevator lost authority as the a/c slowed, the nose dropped on a n/g that was other than "on track" damaging or destroying it.

4. Since no one found it on the runway or on the new "high speed taxiway", it must not have sheared.

5. The crosswind component likely had nothing to do with the accident, but reaction to it may have

6. Looking at the ground track off runway, the a/c probably broke its back where the ruts were interrupted at the dropoff the a/c encountered.

Some things I've been considering. There was a reason for the tailwheel lock on the DC-3. Same for castering nosewheels on some a/c. (swivel, not steer). There is a time for eliminating nosewheel steering at certain speeds taking off or landing. Either by lightening the nose or adding aerodynamic authority.

For the rest of you sailors, ever seen the result of trying to steer with a fully deflected rudder at speed? Sideload.

A very expensive toboggan ride, it would seem.

[just.n.av8r]

So, we wait to hear the cause of the RTO.

I may have missed it in all the flak, but has anyone come up with a plausible theory to cover the reported 'nosewheel marks' on the runway, starting just after the mainwheel marks?

I thought the initial CVR report at the second NTSB breifing was quite telling. It indicated the abort was announced/initiated approximately 5 seconds after a distinct noise ( I beleive referred to as loud vibration but can no longer find that reference to verify.) Could have been a nosewheel problem that did not fully manifest until significant force placed upon nosewheel by the abort.

Again, thankful all survived, hope to learn from this.