ATSB vs Boeing re Virgin cartridge crack
ATSB vs Boeing re Virgin cartridge crack
Interesting......
An Australian Transport Safety Bureau investigation found multiple occurrences involving fatigue cracks and failures on 737 wing flaps in a location not included in the detailed flap actuation system inspection.
The investigation stemmed from an incident involving a passenger flight from Queensland’s Gold Coast Airport to Sydney, NSW, operated on 27 April 2022 by a Virgin Australia 737-800, registered VH-YFZ.
“Immediately after take-off the pilot noticed the aircraft tended to roll to the right, and so trimmed the rudder to keep wings level,” ATSB Chief Commissioner Angus Mitchell said.
The aircraft no longer required trim when the flaps were retracted for cruise, but the issue returned when the flaps were extended for landing into Sydney.
“A walk-around inspection after the flight found the outboard aft flap on the left wing had not completely retracted, and a subsequent inspection found several components in the aft flap actuation system had failed,” Mr Mitchell said.
The ATSB determined that a pre-existing fatigue crack progressed through the aft flap’s inboard programming roller cartridge, resulting in component failure.
“The last general visual inspection had been carried out on VH-YFZ’s left outboard flap, according to Boeing’s specifications, in October 2020, and no defects were found,” Mr Mitchell said.
“While it could not be determined whether the fatigue crack was present at that inspection, 10 other instances of cracking and/or failure of the programming roller were reported to Boeing between 2017 and 2022, and at least six of these were old enough to have been inspected several times prior to failure.
“Significantly, the area in which the fatigue cracks developed was not included in the detailed inspection that Boeing specified for the flap actuation system.”
Boeing has advised the ATSB that it does not agree that this issue warrants safety action – noting that a review of prior failures showed that aeroplane-level effects were correctly mitigated by flight crews, and the affected aircraft landed without further incident.
“While the ATSB acknowledges that Boeing’s risk management program does not classify this as a safety issue, the ATSB believes the reduction in safety margins involving a passenger-carrying aeroplane, and the frequency of occurrence – particularly in the past five years – warrants safety improvement in the detection of fatigue cracking prior to failure,” Mr Mitchell said.
“A detailed inspection of the flap actuation system already exists, and while it includes the aft flap rollers, it does not include the cartridges that house them. Inclusion of the cartridges in the detailed inspection would provide the greatest opportunity for fatigue cracks to be identified prior to failure.”
Read the report: AO-2020-029: Flight control systems occurrence involving Boeing 737-800, VH-YFZ Gold Coast Airport, Queensland, on 27 April 2022
The investigation stemmed from an incident involving a passenger flight from Queensland’s Gold Coast Airport to Sydney, NSW, operated on 27 April 2022 by a Virgin Australia 737-800, registered VH-YFZ.
“Immediately after take-off the pilot noticed the aircraft tended to roll to the right, and so trimmed the rudder to keep wings level,” ATSB Chief Commissioner Angus Mitchell said.
The aircraft no longer required trim when the flaps were retracted for cruise, but the issue returned when the flaps were extended for landing into Sydney.
“A walk-around inspection after the flight found the outboard aft flap on the left wing had not completely retracted, and a subsequent inspection found several components in the aft flap actuation system had failed,” Mr Mitchell said.
The ATSB determined that a pre-existing fatigue crack progressed through the aft flap’s inboard programming roller cartridge, resulting in component failure.
“The last general visual inspection had been carried out on VH-YFZ’s left outboard flap, according to Boeing’s specifications, in October 2020, and no defects were found,” Mr Mitchell said.
“While it could not be determined whether the fatigue crack was present at that inspection, 10 other instances of cracking and/or failure of the programming roller were reported to Boeing between 2017 and 2022, and at least six of these were old enough to have been inspected several times prior to failure.
“Significantly, the area in which the fatigue cracks developed was not included in the detailed inspection that Boeing specified for the flap actuation system.”
Boeing has advised the ATSB that it does not agree that this issue warrants safety action – noting that a review of prior failures showed that aeroplane-level effects were correctly mitigated by flight crews, and the affected aircraft landed without further incident.
“While the ATSB acknowledges that Boeing’s risk management program does not classify this as a safety issue, the ATSB believes the reduction in safety margins involving a passenger-carrying aeroplane, and the frequency of occurrence – particularly in the past five years – warrants safety improvement in the detection of fatigue cracking prior to failure,” Mr Mitchell said.
“A detailed inspection of the flap actuation system already exists, and while it includes the aft flap rollers, it does not include the cartridges that house them. Inclusion of the cartridges in the detailed inspection would provide the greatest opportunity for fatigue cracks to be identified prior to failure.”
Read the report: AO-2020-029: Flight control systems occurrence involving Boeing 737-800, VH-YFZ Gold Coast Airport, Queensland, on 27 April 2022
Also just read a summary of the case as reported by Australian Aviation in their news email. There was one line in that article that made me wonder if it was an accurate quote:
"The ATSB said the aerospace giant “does not agree” that the particular issue with fatigue cracks requires safety action because it has never led to an accident."
Well if accurate, that would be an interesting risk management practice... only dealing with a problem once an accident (or two) have been attributed to it?
"The ATSB said the aerospace giant “does not agree” that the particular issue with fatigue cracks requires safety action because it has never led to an accident."
Well if accurate, that would be an interesting risk management practice... only dealing with a problem once an accident (or two) have been attributed to it?
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"The ATSB said the aerospace giant “does not agree” that the particular issue with fatigue cracks requires safety action because it has never led to an accident."
Boeing are run by accountants not engineers anymore.
I expected that Boeing would be very sensitive to regaining a reputation for prudence after the Max criminal conspiracy. This even fails the cynical “Making you believe that safety is our first priority is our first priority “ test.
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I heard a Virgin 737 crew at Melbourne recently who asked ATC if they had recorded any unusual wind gusts. They had landed on 16 and as they rolled through the intersection with 27 they stated that the aircraft suddenly yawed to the right. So is this another case of uncommanded yaw that Boeing will just consider to be a sudden wind gust? The wind on the day was a constant 160 at 15. Boeing is just going to stonewall any suggestion that their product requires any extra inspections.
I'm astonished. If true, Boeing has truly lost the plot.
What does Boeing have to lose in adding the cartridges to the detailed flap actuation system inspection?
(Perhaps Boeing is working on a modification such that the flap actuation systems on both sides have the same failure, simultaneously, so that the pilots aren't distracted by unexpected roll... /sarcasm off.)
What does Boeing have to lose in adding the cartridges to the detailed flap actuation system inspection?
(Perhaps Boeing is working on a modification such that the flap actuation systems on both sides have the same failure, simultaneously, so that the pilots aren't distracted by unexpected roll... /sarcasm off.)
Boeing’s risk management program does not classify this as a safety issue
As a result of these reports, Boeing performed a safety analysis of this condition to determine whether the event presented a potential safety issue to the affected 737 fleet. As part of the study, Boeing performed an aerodynamic review of worst-case-scenario events involving the loss of different sections of trailing edge flaps, and the effect those losses would have on the pilot’s ability to control the aircraft in the roll axis. For each of the studied events involving skews or losses of the outboard aft flap, Boeing found that the aircraft remained well within the bounds of controllability by pilots, and the issue was evaluated to result in a slight increase in workload for the pilot in the worst possible case. Based on prior occurrences, Boeing determined that fleet experience aligned with that severity study, and that the rate of occurrence was less than that required by the United States Federal Aviation Administration for the level of hazard presented.
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Do operators get to send Boeing a bill for conducting the inspection? Do operators get to send Boeing a bill for the repair of defects and damage discovered in the course of inspections, whether or not the inspections are included in the maintenance manual? I would have thought the answers are 'no' and 'no', but I'd be happy to stand corrected.
Are owner/operators prohibited from making additional inspections?
Is there some reason to not extend the flaps for inspection prior to the walk-around?
I'd fault the ATSB for not showing a comparison to an undamaged version of the part and installation to more clearly indicate the likely origin of the stress concentration that produced the fatigue crack and how visible the crack might be. The reason this is a problem is that if inspection is to take place it's important to know exactly where to look.
From past experience this sort of failure is from a transition from a section that is very stiff to a section that is not. In some failures I call it the "can opener" where a vertical rib (with high stiffness) ends in a flat area. A crack will eventually initiate at that point, just like the way a can-opener that cuts a triangular opening creates a high stress concentration to pierce the can lid. Unlike the designs I have seen before, this case looks to be severely limited for space to ease that transition. It looks like there is a place where two ribs terminate on either side of the part at the same spot - which is where the crack started.
I also expected a diagram of the operation of the lower flap in regards to the upper flap - this would show how/what position the parts would have been in to cause the pushrod to fail.
In regards to the pushrod - if it pushes the flap to extend then if the flap jammed in position and did not easily/fully extend it would cause the pushrod to buckle. Then, when the retraction took place forces applied would try to straighten the pushrod, I cannot tell if only a single cycle of buckling/straightening would fracture it completely, but it would take only a couple of cycles at most.
The ATSB should have been able to identify the amount of deflection the failed pushrod was subject to and seen any beach marks it there was low-cycle fatigue failure. This would tell what the range of extension at the time the pushrod buckled.
It is possible the load required to break the pushrod was in concert with breaking of the programming roller.
On the money side - the pressure is likely that owner/operators don't want to absorb the cost of replacing these parts and either have contracts that would transfer that cost to Boeing or are simply lobbying Boeing to let them take care of it on their own schedule. If Boeing adds this, then not only is Boeing admitting there is a problem, it allows the airlines to skip paying for it, but it also forces the airlines to schedule the inspection which means down-time. Perhaps it also requires a new part, some amount of qualification, a forced replacement program that might also do other damage carrying it out, changing a known problem into an unknown number of problems.
The owners and operators ultimately pay for the effort - it's just a matter of when.
My wonder is the possibility the flap coming loose could impact the tail. They say outboard flap, but perhaps it could pivot and be swept off in an unexpected direction.
Is it any concern the pilots attributed the noise to previously identified fan blade damage?
"The flight crew discussed potential sources for the noise, and attributed it to some slight damage on one of the engine fan blades that had been previously identified and logged for maintenance"
Why would the operator tolerate any damage to a fan blade?
Is there some reason to not extend the flaps for inspection prior to the walk-around?
I'd fault the ATSB for not showing a comparison to an undamaged version of the part and installation to more clearly indicate the likely origin of the stress concentration that produced the fatigue crack and how visible the crack might be. The reason this is a problem is that if inspection is to take place it's important to know exactly where to look.
From past experience this sort of failure is from a transition from a section that is very stiff to a section that is not. In some failures I call it the "can opener" where a vertical rib (with high stiffness) ends in a flat area. A crack will eventually initiate at that point, just like the way a can-opener that cuts a triangular opening creates a high stress concentration to pierce the can lid. Unlike the designs I have seen before, this case looks to be severely limited for space to ease that transition. It looks like there is a place where two ribs terminate on either side of the part at the same spot - which is where the crack started.
I also expected a diagram of the operation of the lower flap in regards to the upper flap - this would show how/what position the parts would have been in to cause the pushrod to fail.
In regards to the pushrod - if it pushes the flap to extend then if the flap jammed in position and did not easily/fully extend it would cause the pushrod to buckle. Then, when the retraction took place forces applied would try to straighten the pushrod, I cannot tell if only a single cycle of buckling/straightening would fracture it completely, but it would take only a couple of cycles at most.
The ATSB should have been able to identify the amount of deflection the failed pushrod was subject to and seen any beach marks it there was low-cycle fatigue failure. This would tell what the range of extension at the time the pushrod buckled.
It is possible the load required to break the pushrod was in concert with breaking of the programming roller.
On the money side - the pressure is likely that owner/operators don't want to absorb the cost of replacing these parts and either have contracts that would transfer that cost to Boeing or are simply lobbying Boeing to let them take care of it on their own schedule. If Boeing adds this, then not only is Boeing admitting there is a problem, it allows the airlines to skip paying for it, but it also forces the airlines to schedule the inspection which means down-time. Perhaps it also requires a new part, some amount of qualification, a forced replacement program that might also do other damage carrying it out, changing a known problem into an unknown number of problems.
The owners and operators ultimately pay for the effort - it's just a matter of when.
My wonder is the possibility the flap coming loose could impact the tail. They say outboard flap, but perhaps it could pivot and be swept off in an unexpected direction.
Is it any concern the pilots attributed the noise to previously identified fan blade damage?
"The flight crew discussed potential sources for the noise, and attributed it to some slight damage on one of the engine fan blades that had been previously identified and logged for maintenance"
Why would the operator tolerate any damage to a fan blade?
So the solution is simple: Deliberately avoid inspecting the area and leave it to crews to “correctly mitigate” the “aircraft-level effects” when there are further failures. Job done!
And yet there are still those who‘ll argue there’s no such thing as “affordable safety” …
(I’m always struck by the blissful ignorance of punters. When I explain to them that they’re flying in a cocoon of slowly propagating cracks and there’s no such thing as a perfectly serviceable aircraft, they simply refuse to believe me. Just like when I say they’re in airspace that’s not under air traffic control….)
It seems that the parcel in this game of pass-the-risk-parcel is now in Ms Spence’s lap (as well as all the crews’ and their passengers’). Does Ms Spence issue an AD mandating the inspection?
And yet there are still those who‘ll argue there’s no such thing as “affordable safety” …
(I’m always struck by the blissful ignorance of punters. When I explain to them that they’re flying in a cocoon of slowly propagating cracks and there’s no such thing as a perfectly serviceable aircraft, they simply refuse to believe me. Just like when I say they’re in airspace that’s not under air traffic control….)
It seems that the parcel in this game of pass-the-risk-parcel is now in Ms Spence’s lap (as well as all the crews’ and their passengers’). Does Ms Spence issue an AD mandating the inspection?
There's absolutely nothing stopping an operator from adding additional tasks to an inspection regime. Getting Boeing to cover the costs would be another matter.
Last edited by 717tech; 21st Dec 2022 at 02:25. Reason: slight improvement of spelling
Indeed. And there’s absolutely nothing stopping crews from extending the flaps and doing an inspection prior to each flight. Nothing, that is, apart from commercial imperatives.
According to the ATSB:
We know where Boeing stands. One wonders what the ATSB will do if everyone else stares into the middle distance.
According to the ATSB:
The ATSB expects relevant organisations will address all safety issues an investigation identifies.
Indeed. And there’s absolutely nothing stopping crews from extending the flaps and doing an inspection prior to each flight. Nothing, that is, apart from commercial imperatives.
One wonders what the ATSB will do if everyone else stares into the middle distance.
