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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. |
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: |
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. |
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 (Appendix E). The second scenario consisted of applying and holding full rudder pedal from 1953:19, Note 40 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). 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. |
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. |
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. |
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. |
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. |
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