We are concentrating on more and more esoteric issues and, consequently, may be missing one of the (more important) links in the causal chain.
(Extracts are selective)
In Paragraph 1.18.3.8 of the accident report it is reported:
1.18.3.8 TSB Examination of CHI91 Filter Bowl Mounting Studs and Nuts
Fatigue initiation was at the minor diameter of the first fully engaged thread on one stud and at the inboard radius of the serrations of the other stud. Fatigue cracking in the first engaged thread of a stud is consistent with insufficient preload causing an excessive vibratory loading to be transmitted to the stud.
Galling was observed on the threads of the occurrence helicopter’s studs, as well as on some of the studs removed from other Cougar helicopters. The galling noted on these studs would have been detectable using 10X magnification, and on some studs the damage would have been visible without the aid of magnification (see circled area – Photo 15). The TSB examination suggested that the occurrence nuts and studs had accumulated sufficient galling damage to prevent the correct preload from being applied during installation. The reduced preload led to an increase of the cyclic load experienced by the studs during operation and to initiation and propagation of fatigue cracks. The TSB’s examination of new studs found that even though the studs were manufactured with a coating to prevent galling, galling damage developed after the first installation of a nut, and the damage became more severe the more frequently the nut was removed and re-installed. The occurrence helicopter, as well as at least three other Cougar helicopters, had MGB oil filter bowl attachment nuts with a grey paint residue that had been applied when the MGB was manufactured.
In observation of the intent of the Cougar implementation of the Sikorsky measures, the report states:
1.18.3.9 TSB Examination of Cougar Helicopters’ MGB Filter Replacement Practices
On 07 November 2008, Cougar Helicopters inserted AMM Revision 13 into its maintenance computers and acknowledged it by signing off on the revision. Company procedures require that maintenance personnel become aware of AMM revised procedures. Cougar Helicopters maintenance personnel are required to read each new instruction and acknowledge having done so, by signing off on the mandatory “Must Read” board.
...
Between the time the occurrence helicopter was manufactured and the accident the helicopter underwent 11 MGB oil filter replacements. During the last two MGB oil filter replacements on the occurrence helicopter, AMM Revision 13 was in effect. At the time of the accident, there was no record of the 10X magnification inspection being performed, nor was there a record of a torque wrench being used to measure the run-off torques on any of Cougar Helicopters’ S-92As, even though required by AMM Revision 13. AMM Revision 13 also required the oil filter mounting nuts to be changed at each removal; however, the nuts installed on the occurrence helicopter were original.
...
The report then repeats the Sikorsky messages to the operators:
1.18.3.10 Sikorsky Actions
About 2 months after the filter bowl stud problem was discussed during the Sikorsky webcast, Sikorsky issued SSA-S92-08-007 on 08 October 2008 that stated:
Sikorsky has been advised that an operator experienced the loss of MGB system oil due to a leak at the filter bowl. The investigation revealed that two of the three MGB filter bowl assembly titanium studs had sheared allowing the filter bowl to displace. As a result, Sikorsky is enhancing the current Aircraft Maintenance Manual (AMM) procedures to aid in identifying potentially damaged studs during the removal or installation of the filter bowl. It is recommended that particular attention and care be taken during the removal and installation of the MGB filter bowl assembly to minimize any potential damage to the threaded portions of the mounting studs.
Approximately 3.5 months after the SSA, Sikorsky released ASB 92-63-014 on 28 January 2009 to its customers, which stated:
Undetected damage to an oil filter stud can lead to failure of the stud. Enhanced procedures are being added to the maintenance manual to help identify potentially damaged studs. To further enhance reliability of this connection, the titanium studs are being replaced with steel.
Finally, the report comments on used studs received from the operators (in respect of expected actions by the operators):
Between 05 November 2008 and 23 March 2009, none of the S-92A operators reported to Sikorsky they had found any damaged studs while performing the enhanced inspection, nor had they contacted Sikorsky to
comment on the steps involved with the enhanced procedures.
On 23 March 2009, the FAA issued Emergency AD 2009-07-53 for Sikorsky S-92A helicopters, which required, before further flight, removing all titanium studs that attach the MGB filter bowl assembly to the MGB and replacing them with steel studs. Sikorsky did not receive any reports of damaged studs between issuance of AMM Revision 13 in November 2008 and when AD 2009-07-53 was issued in March 2009. However, it did receive 59 studs from various operators after they had complied with the AD. Sikorsky examined these studs and found that they had varying degrees of galling of the threads, indicating multiple nut removals. Some of the thread damage was visible without the use of magnification. Photo 16 shows a sample of studs returned to Sikorsky, with varying degrees of galling, ranging from coating loss and minor damage to the threads (stud 1), coating loss and moderate damage to the threads (stud 2), to extensive coating loss and severe damage to the threads (stud 3). The thread damage in stud 3 of Photo 16 was visible to the naked eye. Sikorsky could not provide the time-in-service for the returned studs; however, considering the timing of the AMM revision on 05 November 2008 and the issuing of AD 2009-07-53 on 23 March 2009, and the average S-92A utilization times, they would have come from helicopters that had their filter bowls removed at least three times.
If there is a lesson to be learnt here, it is that expectations resulting from 'soft' measures have to be tempered in the light of human involvement. Additional procedures are not always the best way to address issues and sometimes more direct action is called for. Having had the first occurrence (in Australia), it might have been prudent (in hindsight) for Sikorsky to sample 'galling' damage to other studs in the field. This could have been achieved by requesting photographic evidence from the operators whenever filter bowl intervention was necessary (extremely simple in this age of digital cameras). Comparisons could then have been shown in the weekly webcast.
No-one comes well out of this accident but it is now clear that complacency was a real issue - perhaps because of the perceived 'safety' of the latest certification standards.
Jim