Boeing uncommanded nosewheel steering events

[JammedStab]

TSB Board concerned that runway overruns may continue if more is not done to understand uncommanded nosewheel steering events on Boeing aircraft

Montreal, Quebec, 5 November 2013 – The Transportation Safety Board of Canada (TSB) today released its investigation report (A10Q0213) into the 30 November 2010 runway excursion of an American Airlines Boeing 737-800 at the Montreal/Pierre Elliott Trudeau International Airport.

The aircraft was on a flight to Montreal, Quebec from Dallas, Texas with 113 people aboard. The aircraft landed normally on the wet runway in a crosswind. As the aircraft was slowing down, it veered, uncommanded by the crew, towards the left side of the runway. The captain, the pilot flying, tried using the rudder and the nosewheel steering tiller to steer the aircraft back to the runway centerline. Although the aircraft heading started to return back towards the runway heading, the aircraft continued to travel towards the left and exited the runway surface. There were no injuries, and damage to the aircraft was minor.

Investigators found that the uncommanded veer to the left of runway centerline was likely due to a jam in the nosewheel steering system. Following the occurrence, investigators did not find any anomalies with the nose-gear wheel system, as the jam likely cleared up. The flight crew did not receive any indication that a problem existed with the nose-gear steering system, nor are there any written procedures to manage a nosewheel steering problem of this kind. The aircraft’s flight data recorder (FDR) does not record parameters from the nosewheel steering system, thus making it difficult to determine when and how problems with nosewheel steering occur.

There have been 11 similar occurrences reported involving various Boeing aircraft types in the past 21 years throughout the world. The cause of these uncommanded nosewheel steering occurrences remains uncertain, despite post-event examinations and other efforts to analyze them. The manufacturer’s safety review process has deemed these occurrences to be an acceptable risk given their remoteness, and the manufacturer has not taken further action to correct them. As their cause is uncertain, and because little is being done to better understand the problem, the Board is concerned that there remains a risk for runway excursions to occur.

Following the accident, American Airlines now discusses this occurrence as part of recurrent training for its flight crews. This training is given to the flight crews to raise awareness of the possibility of a runway excursion due to a nosewheel steering problem after landing.


The TSB is an independent agency that investigates marine, pipeline, railway and aviation transportation occurrences. Its sole aim is the advancement of transportation safety. It is not the function of the Board to assign fault or determine civil or criminal liability.

For more information, contact:
Transportation Safety Board of Canada
Media Relations



Transportation Safety Board of Canada - Aviation Investigation Report A10Q0213 (http://tsb.gc.ca/eng/rapports-reports/aviation/2010/a10q0213/a10q0213.asp)

[JW411]

Unlike some runway departures on the BAe146 which were caused by one of the pilots leaving a Coca Cola can underneath the nosewheel steering handle which then got jammed......................However, I am sure that this could never happen in a Boeing.

[Prober]

Being presented with my lunch on a very quick turnround, fortunately at Base, with so many other matters requiring my attention, sloppy eating resulted in my fork slipping from my hand and bouncing across the control column, hitting something else and slithering, handle first, down the very slim gap between the nosewheel steering gear and the surround. It promptly disappeared. Talk about four candles.....!
So it seems as if it could, indeed, happen on a Boeing.

[captplaystation]

Lost a very nice (metal/engraved) pen in a 737 cockpit one day. Spent the whole of the next turnaround (45 min ) trying to find it. . . . finally, a nice gift for the engineers on the next C check.

I suspect though, that this is something more mechanical/hydraulic . . . Boeing have kept this buried almost as well as the rudder problems (BTW B737 pilot since 1989 & no desire to fly an A320 thanks := ) but DO wish Boeing could be less "solicitor friendly" & more "pilot friendly" . . .well, I can dream. :rolleyes:

[dash6]

Once had to apply an excessive force to the tiller on a 767 to overcome the plastic water bottle FO had stowed behind his tiller.Bottle exploded,no de-brief required..lessons learnt.....blah:)

[cockney steve]

Wet runway, sidewinds, looong fuselage forward of MLG which has lots of wheels and weight.....could the relatively small, lightly-loaded (in comparison) nose gear have possibly aquaplaned?...especially as directional control began to re-establish, coincidental with reducing-speed.

when one looks at the complex tread-pattern of motorcars and the somewhat simpler, but deeperpattern on HGV tyres and compares them with aircraft tyres which are expected to maintain an adequate contact-patch and directional stability, at more than twice the road-vehicle's speed, it's not a huge leap of the imagination. :hmm:

[captplaystation]

Which leads me to a question I have asked myself of late. . . all Scandi countries (& Germany ) recommend, or mandate by law , the fitting of Winter tyres on cars , yet. . . .I use exactly the same tyres to land a 737 in Malaga in Summer @ 37c as I do to (try to ) land one in February in Tromso with braking action that is (as advised by the Norwegian CAA ) at best "unreliable" am I missing something here :hmm:

[deptrai]

If you add tread and use softer rubber snow/ice/slush, you might have too much friction on other surfaces, ie generating excessive heat, then if you land your snow tires somewhere with hot temperatures, possibly more blowouts...just thinking out loud. Cars rarely travel to different climates. Cars also don't have reversers. The added complexity in managing spares etc...it's probably a mix of logistical and cost issues that outweigh possible (small) benefits.

[bluedonk]

I had this happen many years ago on a 757. On take off, fortunately, at low speed, the aircraft veered left. The takeoff was aborted and we stopped before we transited into the boondocks. Full right rudder had been applied to no effect. There were no obstructions to the controls and no obvious reason why it happened. We returned to stand and engineering found no fault. We departed with no further problems.
I am sure there are experts here who will tell me, how we could have done better and that our beer cans got trapped in the rudders. I bow to their superior knowledge as I only have in excess if 10,000hours on the airplane and it must have been my imagination.

[captplaystation]

bluedonk, hell yeah, what the :mad: did you know ? ? :ugh:


deptrai, yep, it is not an easy one, not at all, particularly when flying (lets say) South of Spain to North of Norway ,+25/-25 is possibly the extremes, but, if we look worldwide in LH operation that is probably chickenfeed, but it is a problem, like aerodynamic deterioration after de-icing, that has been conveniently "ignored" for decades.)

[tdracer]

On take off, fortunately, at low speed, the aircraft veered left.


Over the years, I've investigated a number of such events during takeoff - without exception they were due to thrust asymmetry - clearly obvious in N1/EPR splits when we checked the DFDR.
I worked 737 classics for a while - with the hydromechanical engine controls there is often miss-match in the engine accel rates which will give a thrust asymmetry during the takeoff throttle push. If memory serves, we determined it was appropriate to move the engine stabilization N1 higher prior to engaging the A/T (70% N1?) and the problem largely went away.
FADEC engines accel very uniformly, so that part of the issue is pretty much gone on the newer airplanes.

Since I've been working 747, we sometimes see "idle stagger", where the pilot - while gripping all four throttles - inadvertently staggering the throttles enough that one or more engines are above idle. When they advance the throttles, the engine(s) that are already above idle accel much quicker. The procedure of an A/T takeoff is to 'stand up' the throttles to obtain ~ 1.10 EPR or 40% N1, allow the engines to stabilize, then hit TOGA (or manually advance to TO). What I've seen is the pilots are occasionally careless about the 'stabilize' part, hitting TOGA before all four engines have caught up.


All that being said, it's unlikely to have anything to do with the Montreal event. I'm with Cockney - suspect crosswind/aquaplaning rather than a mechanical problem. I to have wondered about the lack of meaningful tread on aircraft tires...

Bluedonk, just curious, Rolls or Pratt on your 757?

[BluSdUp]

Capt Play.
You just invented inflight tire service, crewchange, next,,,?
To somone that has an idea about winter ops, maybee ,wellcomm to my world.

Sentence deleted .. perhaps we can be a little less confrontational ? JT

[Centaurus]

Back in the 80's we had severe nosewheel shimmy problems on our 737-200's. On a take off from Manila very severe shimmy took place just as we reached take off thrust. There was a loud cracking noise in the nose-wheel area, we momentarily lost steering control and lost A system. Came to grinding halt from a maximum speed attained of about 30 knots.

The shimmy was so severe it broke the nosewheel scissors linkage which in turn snapped the A System Hydraulic line to the nose-wheel.
A few weeks later a similar severe shimmy occurred to another of our 737-200's soon after touch down. Both nose wheel tyres burst. If I recall correctly the defects were traced to faulty retreads on the nosewheels.

[jimjim1]

The ground loading is much higher in aircraft than with cars.

737-800
Nose 185 psi
Main 205 psi[1]

Car 30 psi

This may well make fine tread features irrelevant.

Also the width of the tyres may well be even smaller proportionately to the load reducing the volume of water to be displaced. This presumably matters too.

Wet grip would probably not be a problem if your car had 1cm wide tyres running at 200 psi even if they were slicks.


[1] http://www.boeing.com/assets/pdf/commercial/airports/acaps/737sec7.pdf

[cosmo kramer]

Bluedonk, it sound more like a uncentered nose wheel steering, as in you may have centered the tiller, but didn't allow for enough time for the nose gear to drive back to center. Was in a rolling takeoff after a 90 deg lineup? Maybe little uphill during the turn with 40% already set, immediately when lined up toga-push?

Been there, done that. Took full rudder pedal and some tiller to keep it from going off to the side. Was close to rejecting too. Been extra careful since, to ALWAYS allow the aircraft to roll straight for some meters, AFTER releasing the tiller (to ensure the nose gear is in the same position commanded by the tiller), before pressing toga.

[BARKINGMAD]

Wet runway with a crosswind brings to mind immediately the diagram of the reverse thrust vectors in such conditions, pretty little diagram if I recall.

Did they depart form a field with slushy deposits on the runway? Is it possible the nosewheel steering cables froze inflight in the U/C retracted position and the lowered gear then put a spurious off-centre demand to the NLG steering jacks?

A NG engineer should be able to state whether the cable rigging changes between the lowered and retracted positions.

The steering bypass handle has been known to stick in the partially disconnected position due to lack of lube and a very tight setting of the handle on the spindle. Try it on a number of your fleet during the walkround and report it as a snag, it should spring positively into the "normal" position once the bypass pin is removed.

Just a few thoughts from 9 years on the type, but I wait to be corrected as we're all still learning I hope. :)

[bluedonk]

As I recollect it was a perfectly normal Departure. There were no adverse weather conditions. The engines were spooled up to 1.2EPR and balanced before EPR engaged. It was quite a violent left turn and full right rudder had no effect. I cannot remember what the tiller input was. I take the point of asymmetric power application, but that was not the case here. I have seen the effects of unbalanced power application and that was not the cause in this case. The EPR as I remember were normal, the aircraft path was not. It all happened very quickly but at low speed. There was no runway excursion but a sobering experience none the less, it gets your attention, when, with full right rudder the aircraft turns left.

I am sure there are sky gods out there who have an answer in the blame culture we are now part of, I do not. For many years I have asked what caused it, I have no idea. The rudder control problems on the 737 I think have some similarities. It took a long time to figure out there was in fact a technical problem. This is such a rare event, I have never seen it again with my 10000 hours on the airplane, but it did happen. It happened with an expierenced crew,familiar with the airplane, current, well trained, and well rested and in good ambient conditions. I post this hoping that someone will benefit. I am very circumspect now when power is applied and a reject even at low speed is something that requires attention. I was lucky but it had the potential for a very different outcome.

[cosmo kramer]

I didn't try to assign blame, just to give one possible explanation. As I said I have had it happen to myself. And, like you, now am much more careful, before setting t/o thrust... i.e. let the aircraft roll straight without tiller input, before pressing TOGA.

[grounded27]

11 events on all Boeing aircraft over 21 years?

[STSIFCX.SATIS]

How about - A couple of fuselage drains blocked & fwd toilet sink drain hose leaking causing a puddle of water that freezes in flight. This then restricts the NW Steering cables on landing, gradually thaws on the ground resulting in it all working again. Happens several times - access panels removed, puddle found, much head scratching before theory worked out.

[framer]

If you want to read a report about a runway excursion resulting from not stabilising at 40% on the NG there is a very good one on the CAANZ website where a PacBlue NG was asked to expedite while lining up at intersection Golf on 16R in Sydney. It drives home the importance of allowing the thrust to settle on the gauges, not just having the levers in the usual place.
If you are an examiner, give your next RTO scenario a freshen up by failing an engine at 20kts on the take- off roll, it is a great learning experience for the crew and will be talked about in the cruise thus spreading the message through the pilot group .
I like the fuselage drain theory below, sounds reasonable.

[BARKINGMAD]

We seem to have drifted off (pun intended!) the original posting which describes a LANDING incident, with all that that implies.

Why so many replies referring to swing on TAKE-OFF?

If we want a discussion on failure of engines to stabilise or accelerate symmetrically, may I suggest a new thread, otherwise may we return to the flight mode as per the OP?
:confused:

[framer]

Fair call Barkingmad.
I have just read the report the OP mentions. It is definitely something that is worthy of a Bulletin in my mind but technically Boeing don't have to bring it to the attention of the airlines because of the low rate of occurrence as mentioned earlier.
There seem to be two possibilities for causing the rate jam,
1/ an internal foreign object
2/ an external foreign object
Is the external case something that could be addressed during walk-around or would an external foreign object be so minuscule that there is no benefit in checking?
This from the report:The second cause of a rate jam is debris, such as ice or stones, becoming lodged in the external linkage that operates the steering valve. A jam of the external summing linkage that operates the input lever to the steering valve will have the same effect (rate jam) as a jam of the valve’s internal slide/sleeve. The steering assembly is shielded by a plastic cover, 35 but is not sealed and is exposed to environmental conditions while the aircraft is on the ground, when the gear is extended on approach for landing, and until the gear is retracted after take-off. No testing or research has been conducted by Boeing to attempt to replicate a rate jam scenario due to an external linkage jam.

[Machinbird]

From the TSB Canada report
The TSB asked Boeing if, when using the engineering simulation, it would have been possible for the PF to keep the aircraft on the runway using only the rudder pedals.
Additional simulations completed by Boeing consisted of modifying the rudder pedal inputs to attempt to keep the aircraft on the runway. Boeing ran 2 scenarios with the engineering simulations.
The first scenario consisted of an increase of right rudder pedal input (up to full rudder deflection) starting from a neutral position at 1953:13 (i.e., 3 seconds after the start of the uncommanded left veer). This time was chosen as it was the time at which right control-wheel input was applied instead of right rudder pedal. During the simulation, from 1953:13, an increase of right rudder pedal input was applied, reaching full rudder deflection at 1953:17; therefore a gradual application of rudder pedal was involved, with maximum pedal reached after 4 seconds. Rudder pedal was not released until the simulation was terminated. The jam is assumed to have released at 1953:18 Note 39 (http://tsb.gc.ca/eng/rapports-reports/aviation/2010/a10q0213/a10q0213.asp#fnb39) (Appendix E (http://tsb.gc.ca/eng/rapports-reports/aviation/2010/a10q0213/a10q0213.asp#appen-e)).
The second scenario consisted of applying and holding full rudder pedal from 1953:19, Note 40 (http://tsb.gc.ca/eng/rapports-reports/aviation/2010/a10q0213/a10q0213.asp#fnb40) exceeding the amount of rudder pedal input that was recorded on the FDR at that time, when the aircraft heading started to return to the runway heading. This rudder pedal input was held for approximately 3 seconds, until the aircraft was recovering back toward the runway centerline (Appendix E (http://tsb.gc.ca/eng/rapports-reports/aviation/2010/a10q0213/a10q0213.asp#appen-e)).
These simulations showed, in both scenarios, that there would have been sufficient control available to prevent significant deviation from centerline. Rudder pedal only was used during the simulations. Additional directional control would have been available from differential braking or differential reverse thrust. Boeing simulations suggest that there was enough control power from the rudder pedals to keep the aircraft on the runway, assuming that the rate jam cleared at 1953:18, as explained above.
Maybe a little taildragger time would be a good training experience:E:}.

[MacDaddy]

Boeing is well aware of the issue.

The slang term for it is "Beached Whale" coined after an occurrence several years ago in OAK involving the Southwest Shamu 737 on takeoff.

The official description is a "rate jam of the nose wheel steering metering valve".

In the 3 event I am aware of recently, two of which left the runway surface, the crews applied full opposite direction corrective measures with no effect. The key in all of these events is that the crew continued to employ stopping effort during the attempted directional corrections. So in the cases of the aircraft departing the runway surface they did so at slow speed.

The analysis of one of the aircraft found debris in the NLG steering filter but the findings were not conclusive. The other two teardowns resulted in no findings.

As the TSB report states the rate of occurrence is within Boeing Safety acceptable risk of less than 10 to -9. Until there is a hull loss they are simply going to accept this problem.

[awblain]

MacDaddy,

10 to the minus 9 fatal accident risk per hour seems a bit ambitious.

Say NGs fly 5000 hours a year? There are 4000 of them (now, not through their lifetime). They've been around for 15 years. There's been three reported incidents ("recently" you say, not over the lifetime). To be generous, 15x5000x4000/3 = 10 to the 8. So, you have to assume that the least sophisticated crew anywhere won't steer off a cliff or into a colleague if this happens to them in more than about 1 in 10-100 incidents?

The 10^9 is a case that can be argued, but it doesn't seem open and shut.

I guess if the typical flight time is 3 hours, then it makes the odds per takeoff a bit better.

[No Fly Zone]

I sure NOT a landing gear engineer or even a Boeing driver. That said, there is clearly a problem. It may appear to be limited to the 73 series, but that is probably a function of their great numbers. Common sense suggests an issue with hydraulic valve control, perhaps similar to the rudder control problem the was found - and fixed a few years ago.
While aggressive use of the rudder may help at the instant of landing, rudder authority obviously decreases with declining speed. I don't know what other options those crews may have. Boeing's suggestion that the number of incidents suggest 'acceptable risk' is not satisfactory and the problem should be identified - and corrected - IMO. This must be a horrific experience for flight crews; after making a stable approach and a good landing, to discover that they cannot steer their airplane once rudder authority is gone.
Most earlier Boeing airplanes provided nose wheel steering control only at the left seat. Does anyone know of this issue has ever appeared on an aircraft that provides a steering tiller for both seats? It may be a place to begin the inquiry. And finally, the fine engineers at Boeing are not idiots! I suspect that under the convenient cover of proprietary engineering details, the already know the reason for this performance defect. Sadly, their corporate masters won't allow them to publish the details or to provide a physical correction. "It does not happen often and retrofitting the entire fleet is too expensive," is probably the line from the executive suite. Sad, but true. This nose wheel steering issue is not the first instance of such cheap behavior from Boeing; they resisted (paying for) the 73's rudder control issues for as long as possible, using PR fluff to cover them while the engineers fixed the issue. I do not know, yet suspect, that Boeing already has a fix for this steering issue, but does not want to pay the hefty cost to retrofit the suspect fleet - or admit to a defect. Good company, great products and a typical, deny everything until proven otherwise corporate culture. Sad, but probably true.:ugh: At the end of the day, I'd guess that Boeing drivers are still better pilots than are AB drivers; Some degree of operation remains in their hands, not those silly computers. Blast away: I've always made it perfectly clear that I am NOT a transport class airplane driver and yes, I have very thick skin. Blast as will, but please do not loose sight of the original post's objective: Low speed steering.

[PEI_3721]

The safety statistics might indicate that there are many more events on a range of aircraft due to NWS problems than would be expected; IIRC a CAST group investigated the maintenance aspects re runway excursion (off the side).
Many years ago the BAe146/Avro RJ suffered a spate of incidents, but diligent safety investigation by both the gear manufacturer and BAe identified contributory issues, many as above.
The 146 problems generally occurred during landing, but with a temporary procedure of checking the NWS tiller position after lowering the gear the hazard could be contained – centre the tiller.
Even so these manufacturers did not write the problems off to statistics, they took action.
IIRC, set-up / maintenance changes cured the problem.