Flt. Lt. Sean Cunningham inquest
Chug,
I concur with Downsizer. Parachute Headbox replacement was in the Topic 1J (all Ejection Seat work covered in this AP). If memory serves me correctly, the Hawk Headbox was replaced every 6 months (circa 1992), but that may well have been extended out by now.
I concur with Downsizer. Parachute Headbox replacement was in the Topic 1J (all Ejection Seat work covered in this AP). If memory serves me correctly, the Hawk Headbox was replaced every 6 months (circa 1992), but that may well have been extended out by now.
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If memory serves me correctly, the Hawk Headbox was replaced every 6 months (circa 1992), but that may well have been extended out by now.
DV
The SI report says the armourers did not have the requisite training.
If maintenance/removal of the seat has been extended from 6 months to, someone said 3 years, then what guarantee is there anyone would gain experience and correct training on a normal tour?
If maintenance/removal of the seat has been extended from 6 months to, someone said 3 years, then what guarantee is there anyone would gain experience and correct training on a normal tour?
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Folks,
I've purposely stayed clear of this thread until now - there have been some very well informed posts here, and I felt I didn't have much to add of value, until now. I hope this contribution may help, or help the exchange of views.
The SI report is not an easy or consistent read. I think (but don't know for certain) there may two reasons for this:
1. Redaction or moderation of comments which reviewers felt may have not agreed with. Motives unknown, but could range from correction of fact to desire to preserve reputations of individuals or organisations. I'm sorry, but I suspect the latter. I wish I didn't.
2. Less speculative - the change of role of the accident investigators. When the AAIB was still the RN AIU, its involvement in SIs was ordered by the RN's Advisor of Aircraft Accidents based on the initial accident report. It's remit was to carry out a technical investigation, and it delivered a separate and complete report to the BoI's president. BoIs were normally adjourned while the AIU did that. The BoI then reconvened, and considered the AIU report as evidence.
Now, the AAIB has been integrated within the SI process, and its investigation is mandated to support the SI as directed. Other posts have commented on the potential problems with this, and I very much agree that having the Accident investigators under the command of the regulatory authority is not the right arrangement to help improve Air Safety. It also leads to reports which combine the technical narrative with the wider organisational aspects, and like others, I found some of these latter paragraphs short on fact and long on opinion.
Looking at the technical issues, I am struck by the role played by RTI/Hawk/059D. In my opinion (and that's all it is) the reason a fatality occurred was the failure of the shackle to release, and that was down to the malpractice that took place during execution of that RTI. The report covers the background to the RTI, but I am surprised by the statement that the AOA did not want to operate aircraft with a cracked cross beam, and probably insisted on the frequent inspection, 'as the visual affect (sic) may undermine aircrew confidence in the system'.
No, check that - i'm astonished. The way the aircrew have confidence in the system they are given to fly is by inspecting and signing the F700. Engineers decide what is acceptable and what isn't up to that point. The day you start managing aircraft maintenance to bolster 'aircrew confidence' you are hazarding safety. And I honestly think that's what happened here.
My years with and around seats and escape systems were relatively uneventful. One reason for that was that the RN operated a system where maintenance on seats on aircraft was intentionally restricted to seat removal and installation - the RN operated a system of 30 week bay servicing, where experienced (civilian mostly) fitters did the work. At sea, a very limited bay servicing facility was provided, but the ship usually carried a couple of spare seats. There was a deliberate policy not to fettle seats while they were in the aircraft.
I'd like to know who came to the conclusion that taking the drogue and scissor shackles apart every 50 flying hours on aircraft wasn't a safety risk. And why they carried on doing intrusive NDT inspections when 1710 had told them they weren't required. Oh, and why they didn't stick to a non-intrusive visual inspection to begin with. OK, I don't have the detail to hand, I could well be off beam here. But neither did the SI, due to lack of records. Bad. The old ES(Air) 'Annex A' and 'Annex B' systems would have provided those records.
Given all that, I am surprised (very) that the introduction of the RTI, and the way it was carried out, weren't cited as at least an 'aggravating' factor. I'm also frankly staggered, given my experience of the RAF's very thorough and complex authorisation systems, of the findings into RAFAT engineering personnel's lack of training and qualification to service seats. I don't like saying this, but I hope the SEnGO involved is now doing something a long way away from aircraft maintenance management.
As Tuc and others so rightly say, all that is really required (on the technical side) is for everyone at the sharp end, and in the EA areas, to do their damned job in accordance with basic engineering principles. Don't look to the regulator to make it all better - they just write regulations. Don't look to VSOs to sort it out - most of them aren't engineers.
Final point. The saddest part of this report is the long list of recommendations, most of which boil down to 'do what you are supposed to do'. That hasn't happened, and there needs to be some serious heart searching as to why. Building a huge and complex regulatory set is OK, as long as the system being regulated has the means of compliance. I don't think, in this case, it had.
Best regards as ever to all those affected by this,
Engines
I've purposely stayed clear of this thread until now - there have been some very well informed posts here, and I felt I didn't have much to add of value, until now. I hope this contribution may help, or help the exchange of views.
The SI report is not an easy or consistent read. I think (but don't know for certain) there may two reasons for this:
1. Redaction or moderation of comments which reviewers felt may have not agreed with. Motives unknown, but could range from correction of fact to desire to preserve reputations of individuals or organisations. I'm sorry, but I suspect the latter. I wish I didn't.
2. Less speculative - the change of role of the accident investigators. When the AAIB was still the RN AIU, its involvement in SIs was ordered by the RN's Advisor of Aircraft Accidents based on the initial accident report. It's remit was to carry out a technical investigation, and it delivered a separate and complete report to the BoI's president. BoIs were normally adjourned while the AIU did that. The BoI then reconvened, and considered the AIU report as evidence.
Now, the AAIB has been integrated within the SI process, and its investigation is mandated to support the SI as directed. Other posts have commented on the potential problems with this, and I very much agree that having the Accident investigators under the command of the regulatory authority is not the right arrangement to help improve Air Safety. It also leads to reports which combine the technical narrative with the wider organisational aspects, and like others, I found some of these latter paragraphs short on fact and long on opinion.
Looking at the technical issues, I am struck by the role played by RTI/Hawk/059D. In my opinion (and that's all it is) the reason a fatality occurred was the failure of the shackle to release, and that was down to the malpractice that took place during execution of that RTI. The report covers the background to the RTI, but I am surprised by the statement that the AOA did not want to operate aircraft with a cracked cross beam, and probably insisted on the frequent inspection, 'as the visual affect (sic) may undermine aircrew confidence in the system'.
No, check that - i'm astonished. The way the aircrew have confidence in the system they are given to fly is by inspecting and signing the F700. Engineers decide what is acceptable and what isn't up to that point. The day you start managing aircraft maintenance to bolster 'aircrew confidence' you are hazarding safety. And I honestly think that's what happened here.
My years with and around seats and escape systems were relatively uneventful. One reason for that was that the RN operated a system where maintenance on seats on aircraft was intentionally restricted to seat removal and installation - the RN operated a system of 30 week bay servicing, where experienced (civilian mostly) fitters did the work. At sea, a very limited bay servicing facility was provided, but the ship usually carried a couple of spare seats. There was a deliberate policy not to fettle seats while they were in the aircraft.
I'd like to know who came to the conclusion that taking the drogue and scissor shackles apart every 50 flying hours on aircraft wasn't a safety risk. And why they carried on doing intrusive NDT inspections when 1710 had told them they weren't required. Oh, and why they didn't stick to a non-intrusive visual inspection to begin with. OK, I don't have the detail to hand, I could well be off beam here. But neither did the SI, due to lack of records. Bad. The old ES(Air) 'Annex A' and 'Annex B' systems would have provided those records.
Given all that, I am surprised (very) that the introduction of the RTI, and the way it was carried out, weren't cited as at least an 'aggravating' factor. I'm also frankly staggered, given my experience of the RAF's very thorough and complex authorisation systems, of the findings into RAFAT engineering personnel's lack of training and qualification to service seats. I don't like saying this, but I hope the SEnGO involved is now doing something a long way away from aircraft maintenance management.
As Tuc and others so rightly say, all that is really required (on the technical side) is for everyone at the sharp end, and in the EA areas, to do their damned job in accordance with basic engineering principles. Don't look to the regulator to make it all better - they just write regulations. Don't look to VSOs to sort it out - most of them aren't engineers.
Final point. The saddest part of this report is the long list of recommendations, most of which boil down to 'do what you are supposed to do'. That hasn't happened, and there needs to be some serious heart searching as to why. Building a huge and complex regulatory set is OK, as long as the system being regulated has the means of compliance. I don't think, in this case, it had.
Best regards as ever to all those affected by this,
Engines
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One thing that has come to light during the course of the last couple of posts is the need for the head box (cute container) to be changed, periodically, in situ. How was the shackle and scissor unit disconnected in this standard servicing procedure? Never memtioned in the SI or Inquest. Someone out there must know. PM me if you wish.
DV
DV
-re the almost certainly erroneous claim that Martin Baker didn’t send servicing information to MoD, I’ve just remembered that MoD has form here which was the subject of a question to a Minister (Ingram) on Mull of Kintyre.
Despite the AAIB report detailing a series of faults in an avionics system, MoD claimed it was perfectly serviceable. (Part of their lie that, just because the SuperTANS held data in battery backed-up memory, that meant the entire Nav System was both serviceable and accurate – completely disregarding the inconvenient fact it was not even cleared to be fitted to the aircraft, never mind used). Related to this, MoD claimed it had no servicing information on how to set up the system’s Automatic Gain Control. I wrote to my MP – he asked Ingram, who replied that MoD stuck by their statement. I wrote again, giving him the figure (in dB) that MoD claimed they weren’t aware of – something I could only have obtained from the APs. MoD declined to respond. Not a million miles from what they're claiming here.
Lessons – MoD usually have the information if you know where to look. MoD don't look. They also lie. And Ministers are content.
Despite the AAIB report detailing a series of faults in an avionics system, MoD claimed it was perfectly serviceable. (Part of their lie that, just because the SuperTANS held data in battery backed-up memory, that meant the entire Nav System was both serviceable and accurate – completely disregarding the inconvenient fact it was not even cleared to be fitted to the aircraft, never mind used). Related to this, MoD claimed it had no servicing information on how to set up the system’s Automatic Gain Control. I wrote to my MP – he asked Ingram, who replied that MoD stuck by their statement. I wrote again, giving him the figure (in dB) that MoD claimed they weren’t aware of – something I could only have obtained from the APs. MoD declined to respond. Not a million miles from what they're claiming here.
Lessons – MoD usually have the information if you know where to look. MoD don't look. They also lie. And Ministers are content.
Engines, excellent post, Sir. Thank you! I don't think that it is any longer appropriate to apologise for revealing or commenting upon glaring airworthiness shortcomings, RAF or otherwise. The emperor's apparel is notable by its absence, and the sooner we all accept that the more pressure can be applied to make himself decent again.
Just to avoid confusion, I take it by your references to the AAIB you mean the MAAIB, or as they seem to prefer, the MilAAIB? As far as I know, the civilian AAIB was not involved in this SI, though I may be wrong. If not, is this another worrying departure from past BoI practice. AAIB Inspectors were often ignored or over ruled, but at least they were witness to evidence available to the BoI's and could make their concerns apparent at a later date. Oh, wait, have I just answered my own query?
Mortmeister, thanks for the update re box changes. As Distant Voice asks, what was the procedure required to do that in situ? Presumably much the same as with the NDT check for beam cracks, ie undo and remove the shackle bolt from the scissor jaws and replace when done? So was the tightening of the bolt not described in anyway other than the generic default of leaving 1.5 threads showing?
Were the box changes arranged to coincide with the crack checks or done completely independently? It seems these bolts were having to be regularly removed and replaced on a frequent basis, or were the old ones ever re-used? I'm sorry to ask so many questions, but it is because the SI didn't.
Just to avoid confusion, I take it by your references to the AAIB you mean the MAAIB, or as they seem to prefer, the MilAAIB? As far as I know, the civilian AAIB was not involved in this SI, though I may be wrong. If not, is this another worrying departure from past BoI practice. AAIB Inspectors were often ignored or over ruled, but at least they were witness to evidence available to the BoI's and could make their concerns apparent at a later date. Oh, wait, have I just answered my own query?
Mortmeister, thanks for the update re box changes. As Distant Voice asks, what was the procedure required to do that in situ? Presumably much the same as with the NDT check for beam cracks, ie undo and remove the shackle bolt from the scissor jaws and replace when done? So was the tightening of the bolt not described in anyway other than the generic default of leaving 1.5 threads showing?
Were the box changes arranged to coincide with the crack checks or done completely independently? It seems these bolts were having to be regularly removed and replaced on a frequent basis, or were the old ones ever re-used? I'm sorry to ask so many questions, but it is because the SI didn't.
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Chug,
Thanks. No, my bad, I meant the MAAIB, or MilAAIB as they are now called. Sorry for any confusion.
I noted that the Convening Authority's (I guess DGMAA) final remarks partially echo my points over the way the RTI was handled, and the safety hazard it presented. I have learned over the years that rushing into issuing Tis is usually a bad move. Making them complex is another bad move. NDT techniques should be a last resort. You should get rid of Tis as soon as practicable.
Good rule of thumb - do as little as possible to the aircraft to make it safe. Don't do more than you need, as it usually causes problems. All this applies in spades to explosive and armament systems. Sadly, experience levels are dropping and so is the level of understanding of the issues behind these rules of thumb.
Stay safe, all,
Best regards as ever
Engines
Thanks. No, my bad, I meant the MAAIB, or MilAAIB as they are now called. Sorry for any confusion.
I noted that the Convening Authority's (I guess DGMAA) final remarks partially echo my points over the way the RTI was handled, and the safety hazard it presented. I have learned over the years that rushing into issuing Tis is usually a bad move. Making them complex is another bad move. NDT techniques should be a last resort. You should get rid of Tis as soon as practicable.
Good rule of thumb - do as little as possible to the aircraft to make it safe. Don't do more than you need, as it usually causes problems. All this applies in spades to explosive and armament systems. Sadly, experience levels are dropping and so is the level of understanding of the issues behind these rules of thumb.
Stay safe, all,
Best regards as ever
Engines
I think some of the contributors here will be interested in the SI into the Tucano engine failure / gear-up landing in Jan 13 (see separate thread). Firstly, a familiar tale of airworthiness failings at the EA. Secondly, it appears that the MilAAIB's human factors specialists (who I believe are civvies at RAFCAM) were looking for failings that were not there, came up with some hare-brained stuff about cockpit gradient, and had their theory politely dismissed by the panel. Good too see some forthright comment from the Convening Authority, too.
Chug, DV
The RTI was not around when I worked on Mk10 seats so I would not like to comment on that process.
Parachute Headbox changes were a fairly common event, either when called up for bay maintenance, or certainly when I worked on Hawks, contaminated with vomit!
To replace the Headbox Assembly it was a matter of disconnecting the Harness top and bottom locks, sticker straps, scissor shackle bolt and then 2 bolts to take the Headbox from the Seat structure.
Fitting was the reverse, but the new Headbox always came from the Para Bay with a brand new bolt and stiff nut. This is critical because if you re-use the stiff nut the plastic insert becomes less effective. If the RTI has been calling for that connection to be disturbed every 50 flight hrs, were they replacing the nut, or just reusing the old one?
With regard to tightening the Nut, as I have stated previously on this thread, I was taught 'max 1.5 threads showing above the nut,' but critically to ensure that there was freedom of movement of the Drogue Withdrawal Line in the Scissor Shackle. As for exactly how that was stipulated in the 1J I don't recall, but we were certainly aware of over tightening said bolt at Chivenor back in the early 90s.
For me, a major factor in this incident was leaving the seat on the actuator down limit because the same pilot generally flew the same aircraft. This was against the procedure for af, bf and tr and would seriously restrict the view of the Seat Pan Pin for anyone entering the cockpit. Had the Actuator failed (as they sometimes do) it makes it a much more difficult task to remove the seat for rectification.
In 6 years on a TWU on a Sqn of around 20 aircraft, student pilots, fast turnarounds, high pressure, I never saw a Seat Pan Firing Handle in this condition, although I was aware that it could exist.
The RTI was not around when I worked on Mk10 seats so I would not like to comment on that process.
Parachute Headbox changes were a fairly common event, either when called up for bay maintenance, or certainly when I worked on Hawks, contaminated with vomit!
To replace the Headbox Assembly it was a matter of disconnecting the Harness top and bottom locks, sticker straps, scissor shackle bolt and then 2 bolts to take the Headbox from the Seat structure.
Fitting was the reverse, but the new Headbox always came from the Para Bay with a brand new bolt and stiff nut. This is critical because if you re-use the stiff nut the plastic insert becomes less effective. If the RTI has been calling for that connection to be disturbed every 50 flight hrs, were they replacing the nut, or just reusing the old one?
With regard to tightening the Nut, as I have stated previously on this thread, I was taught 'max 1.5 threads showing above the nut,' but critically to ensure that there was freedom of movement of the Drogue Withdrawal Line in the Scissor Shackle. As for exactly how that was stipulated in the 1J I don't recall, but we were certainly aware of over tightening said bolt at Chivenor back in the early 90s.
For me, a major factor in this incident was leaving the seat on the actuator down limit because the same pilot generally flew the same aircraft. This was against the procedure for af, bf and tr and would seriously restrict the view of the Seat Pan Pin for anyone entering the cockpit. Had the Actuator failed (as they sometimes do) it makes it a much more difficult task to remove the seat for rectification.
In 6 years on a TWU on a Sqn of around 20 aircraft, student pilots, fast turnarounds, high pressure, I never saw a Seat Pan Firing Handle in this condition, although I was aware that it could exist.
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A number of people have said that it used to be generally known that it was possible for the pin to be in but the SPH to be unsafe. Was this just a theoretical possibility or something actually experienced? If so, how did it arise? We know that one route to the unsafe Position 2 was by having the crotch strap through the SPH. Were there other practical ways that Position 2 could be achieved? Was the importance of correct strap in emphasised in seat training?
EG
EG
Mortmeister, thanks again for an informative post, and a detailed one at that! So removing the bolt in situ predated the RTI, which however greatly increased the frequency of it.
It would be interesting to know if that was always the case, ie routine and non routine removal and replacement of the Headbox being done in situ, rather than in the then seat bays, dating all the way back to the 70's?
Your reminder of the reasons for non routine removal remind us of the real world of life on the line and rings particular bells with me, having had to clean out a JP cockpit after myself more times than most people have...well let's just not go there!
Again the reminder of how corporate knowledge; checking for free movement of the drogue line, leaving the seat actuator set to fully down, even perhaps the vital importance of only using an entirely new nut and bolt to re-secure the drogue shackle, and the critical importance of the correct position of the SPH and Pin, can be lost with cutbacks. More holes in more cheese slices?
Can anyone predate Mortmeister with their recollections of the when and where of headbox changes?
It would be interesting to know if that was always the case, ie routine and non routine removal and replacement of the Headbox being done in situ, rather than in the then seat bays, dating all the way back to the 70's?
Your reminder of the reasons for non routine removal remind us of the real world of life on the line and rings particular bells with me, having had to clean out a JP cockpit after myself more times than most people have...well let's just not go there!
Again the reminder of how corporate knowledge; checking for free movement of the drogue line, leaving the seat actuator set to fully down, even perhaps the vital importance of only using an entirely new nut and bolt to re-secure the drogue shackle, and the critical importance of the correct position of the SPH and Pin, can be lost with cutbacks. More holes in more cheese slices?
Can anyone predate Mortmeister with their recollections of the when and where of headbox changes?
Chug,
Headbox changes were never done in the Seat Bay on the Mk10 as they were on a different servicing cycle and controlled differently. I would say the whole thing was designed in this way.
The Seat was serviced annually and controlled by the Seat Bay (Eng Tech W) whereas the Parachute Headbox and PSP were on 26 week cycles and controlled by the Survival Equipment Bays (SE Fitt).
We did usually try and keep the SE in line with the Seat where possible to keep things simple. Likewise, Seat Cartridges were I think 5 years and we tried to keep them in line with the Seat if possible (makes life easier!).
I think what you are asking is how often was the Scissor Shackle disturbed during an average year? Well on calendar, normally only twice a year, but every time the Seat was removed for access or loose article checks, it would be disconnected as part of the Seat removal, but never as frequently as every 50 flight hrs!
Headbox changes were never done in the Seat Bay on the Mk10 as they were on a different servicing cycle and controlled differently. I would say the whole thing was designed in this way.
The Seat was serviced annually and controlled by the Seat Bay (Eng Tech W) whereas the Parachute Headbox and PSP were on 26 week cycles and controlled by the Survival Equipment Bays (SE Fitt).
We did usually try and keep the SE in line with the Seat where possible to keep things simple. Likewise, Seat Cartridges were I think 5 years and we tried to keep them in line with the Seat if possible (makes life easier!).
I think what you are asking is how often was the Scissor Shackle disturbed during an average year? Well on calendar, normally only twice a year, but every time the Seat was removed for access or loose article checks, it would be disconnected as part of the Seat removal, but never as frequently as every 50 flight hrs!
Last edited by Mortmeister; 15th Feb 2014 at 11:12.
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Excellent post Mortmeister, it fills a lot of holes.
So MoD was in the know in the early 90s. Fits with the MB statement, that they claim they couldn't find, and the Tornado incident.
Agreed. Although I can not understand why no one thought of pressing down on the base of the firing handle, to make sure it was seated correctly, before inserting the pin.
Probably becaause people were properly trained, applied common sense, and treated an ejection seat with respect.
DV
With regard to tightening the Nut, as I have stated previously on this thread, I was taught 'max 1.5 threads showing above the nut,' but critically to ensure that there was freedom of movement of the Drogue Withdrawal Line in the Scissor Shackle. As for exactly how that was stipulated in the 1J I don't recall, but we were certainly aware of over tightening said bolt at Chivenor back in the early 90s.
For me, a major factor in this incident was leaving the seat on the actuator down limit because the same pilot generally flew the same aircraft. This was against the procedure for af, bf and tr and would seriously restrict the view of the Seat Pan Pin for anyone entering the cockpit. Had the Actuator failed (as they sometimes do) it makes it a much more difficult task to remove the seat for rectification.
In 6 years on a TWU on a Sqn of around 20 aircraft, student pilots, fast turnarounds, high pressure, I never saw a Seat Pan Firing Handle in this condition, although I was aware that it could exist.
DV
Last edited by Distant Voice; 15th Feb 2014 at 10:28.
Quote:
With regard to tightening the Nut, as I have stated previously on this thread, I was taught 'max 1.5 threads showing above the nut,' but critically to ensure that there was freedom of movement of the Drogue Withdrawal Line in the Scissor Shackle. As for exactly how that was stipulated in the 1J I don't recall, but we were certainly aware of over tightening said bolt at Chivenor back in the early 90s.
With regard to tightening the Nut, as I have stated previously on this thread, I was taught 'max 1.5 threads showing above the nut,' but critically to ensure that there was freedom of movement of the Drogue Withdrawal Line in the Scissor Shackle. As for exactly how that was stipulated in the 1J I don't recall, but we were certainly aware of over tightening said bolt at Chivenor back in the early 90s.
BN:-
Mortmeister:-
There seems to be an issue here, doesn't there? Mortmeister reckons the seat was designed by MB and accepted into RAF service to have the headbox changed in situ, ie the shackle bolt was to be routinely removed and replaced by undoing it and retightening it in situ. Yet Background Noise reminds us that the drogue shackle free movement within the scissor shackle after such in situ tightening was not protection against jamming during a zero zero operation of the seat.
Is this not where a Safety Case should have offered that protection? Is this not where the absence of a Safety Case in this tragedy was a major contributory cause?
The drogue shackle can be 'loose' when fitted at rest but jam later when the drogue shackle is pulled through the scissor shackle. See page 38, para c of the report
Headbox changes were never done in the Seat Bay on the Mk10 as they were on a different servicing cycle and controlled differently. I would say the whole thing was designed in this way.
Is this not where a Safety Case should have offered that protection? Is this not where the absence of a Safety Case in this tragedy was a major contributory cause?
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Before you go too far down this route, please remember that a stiff nut must be replaced every time it is removed. Tuc has mentioned it earlier.
Like the 1 & 1/2 threads, it is basic engineering practice and written down somewhere.
It is quite relevant for a few reasons:-
1. Probably not mentioned in RTI's and maybe not in AP's
2. Provision of spares for replacing said nut
3.....A stiff nut used several times over would lose its stiffness and could lead to overtightening and 'cutting' an extention of the thread down the shank of the bolt.
Many PPRuNers have been on here expressing their astonishment at seat pin slacktitude. Not many will post saying they haven't re-used a stiff nut
Like the 1 & 1/2 threads, it is basic engineering practice and written down somewhere.
It is quite relevant for a few reasons:-
1. Probably not mentioned in RTI's and maybe not in AP's
2. Provision of spares for replacing said nut
3.....A stiff nut used several times over would lose its stiffness and could lead to overtightening and 'cutting' an extention of the thread down the shank of the bolt.
Many PPRuNers have been on here expressing their astonishment at seat pin slacktitude. Not many will post saying they haven't re-used a stiff nut
Is this not where a Safety Case should have offered that protection? Is this not where the absence of a Safety Case in this tragedy was a major contributory cause?
I sometimes wonder if I am reading the same report as some, or misinterpreting it. As I read it (but feel free to correct me) the techies followed the AP, leaving 1.5 threads exposed and with the shackle able to move freely. They were not blameworthy in any way.
In reality the AP and the MB tech pubs they were drawn from were wrong and had been for decades. Not only would 1.5 threads be 'grossly overtightened' and the 'shackle check' would not actual ensure a release at low ejection speed. Moreover, the small variance in bolt length would vary the torque from shackle to shackle.
The other thing I am missing is why the forensic tear-down of the seats by MB post-usage did not notice the witness marks on the shackle - they do still return the seats after live-use for such checks and reports don't they? Or have we 'saved' money by not bothering post-ejection?
Then again, without a Safety Case just what effect would these reports actually have….
SPHLC:-
Absolutely correct, but the SI found that new nut or old nut, it was possible to overtighten the bolt such that the scissor shackle would not release it at a force equivalent to that sustained in a zero zero seat firing, and yet still leave the drogue line apparently free movement. The shortcoming seems to be that the generic standard of 1.5 threads showing was not appropriate in this case, and some more precise guidance was required, or indeed the redesign which we are now told is happening.
My point is that requirement should have been flagged up if there had been a Safety Case. There wasn't.
Before you go too far down this route, please remember that a stiff nut must be replaced every time it is removed.
My point is that requirement should have been flagged up if there had been a Safety Case. There wasn't.