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Nine months after a fatal engine-failure accident in Hawaii, the FAA released three special airworthiness information bulletins (SAIBs) warning operators and maintainers that there might be a problem with the Rolls-Royce 250 engine's compressor splined adapter coupling. The FAA is considering "mandatory action" on this issue and has issued a proposed airworthiness directive.

The part that is under suspicion of failing and causing the helicopter's engine failure is a compressor splined adapter coupling that wasn't a Rolls-Royce part. The part was an FAA-approved unit made under FAA PMA (parts manufacturer approval) regulations, which permit companies to manufacture parts even if they aren't the original type certificate holder.

In this case, two companies, EXTEX and Alcor (and Superior Air Parts's turbine parts division, which was sold to Extex), made PMA adapter couplings for the Rolls-Royce 250 engine by reverse-engineering the original part and testing it to make sure it met all applicable requirements. The FAA approved the EXTEX and Alcor parts, but it appears that there could be a problem with the original Rolls-Royce adapter coupling, because a number of operators reported problems with the Rolls-Royce coupling to the FAA and to an industry-maintained maintenance-issue database. There were also two reports on the EXTEX coupling.

The National Transportation Safety Board preliminary report on the accident does not mention engine failure. But the NTSB investigator-in-charge on that accident, Tealeye Cornejo, confirmed that "there was an engine failure" and that the failure is "definitely part of our investigation." Four people were killed in the McDonnell Douglas 369D helicopter tour accident near Volcano, Hawaii on June 15, 2003. The accident helicopter's engine was equipped with the EXTEX adapter coupling.

The FAA SAIBs are a bit vague, but what is interesting is that two FAA Aircraft Certification Offices issued three SAIBs simultaneously on the same part. Actually, each of the three SAIBs is for a version of the part. One SAIB (NE-04-56) covers the original Rolls-Royce compressor splined adaptor coupling part number 23039791-1, -2, and -3. The other two SAIBs are for the PMA versions of the parts. There is one SAIB (NE-04-57) covering Alcor Engine Company's coupling part number 23039791AL-1, -2, and -3, and another (NE-04-58) for EXTEX's part number E23039791 and EH23039791 with the same dash numbers.

The information in the SAIBs notes that in the EXTEX part's case, "we have received two reports of the failure of two EXTEX compressor splined adapter couplings. The most recent failure of the EXTEX coupling was due to fatigue originating from fretting on the outer diameter of the coupling where it mates with the compressor impeller. Failure of this part results in immediate and total loss of power from the engine."

The Rolls-Royce SAIB notes that "the FAA has received reports of failures of Rolls-Royce compressor adapter couplings." But it doesn't say that there are many more reports on Rolls-Royce coupling problems than there are EXTEX reports.

Alcor's PMA coupling seems to be included simply because Alcor obtained PMA for the part and other companies' versions of the part seem to be having problems. But nothing in the Alcor version of the SAIB mentions problems with the Alcor part. In fact, the SAIB notes, "There have been no reported failures of the Alcor compressor splined adapter coupling. However, we are issuing this SAIB on the Alcor coupling due to its identicality to the RRC [Rolls-Royce] coupling."

A search of the FAA's service difficulty reporting database reveals seven reports of problems with Rolls-Royce adapter couplings. The narrative on these seven reports refers to 13 couplings with problems. There are two additional Rolls-Royce adapter coupling problem reports in the Helicopter Association International's MMIR database; these reports didn't make it into the FAA SDR database for some unknown reason. There were two reports about two EXTEX couplings; neither were in the FAA SDR database, but both were in the HAI MMIR database.

It seems likely that FAA interest in the coupling problem stems from the June 2003 accident, but other failures of adapter couplings probably generated FAA attention, too. "There have been nine failures of the adapter coupling that have resulted in total engine power loss," the FAA stated in response to questions submitted by Aviation Maintenance. "Seven [were] manufactured by Rolls-Royce and two manufactured by EXTEX. There have not been any failures of Alcor or Superior [which sold its turbine parts business to EXTEX] parts."

Although since the accident EXTEX has stopped manufacturing its compressor splined adapter coupling, the company did not want to comment for this article. EXTEX issued a service bulletin (T-081) on March 15, 2004 recommending that couplings be removed by March 1, 2005. Couplings with less than 600 hours total time should be removed within the next 150 hours of engine operation, and couplings with more than 600 hours should be removed within 100 hours, the company stated.

The reason for the removal, according to the bulletin: "There have been many reports of compressor adapter couplings developing fatigue cracks. Two of those adapters were manufactured by EXTEX. The fatigue is initiated by fretting on the outer diameter that mates with the impeller. The exact cause of the fretting is still under investigation."

Rolls-Royce also issued information to 250 operators, an Alert Commercial Engine Bulletin dated September 9, 2003. In the bulletin, Rolls-Royce recommends replacement of the 23039791 adapter coupling with a new coupling part number 23076559 "the next time the affected module or component is at an approved repair/overhaul facility and the compressor rotor is disassembled for any reason." Rolls-Royce declined to comment on this issue.

On April 6, the FAA met with personnel from Alcor, EXTEX, and the Modification and Replacement Parts Association at the FAA's Engine Directorate in Burlington, Massachusetts to discuss the Rolls-Royce 250 adapter coupling issue. MARPA is recommending issuance of a single airworthiness directive to cover all three parts.

On July 1, the FAA released a notice of proposed rulemaking for an airworthiness directive targeting the Rolls-Royce, EXTEX/Superior, and Alcor couplings. Comments on the proposed AD are due by August 30. For more information, see www.airweb.faa.gov, then click on the Regulations link and then FAR NPRMs. Interestingly, the FAA's investigation has thus far determined that the adapter coupling problem is not a manufacturing problem, which essentially means that there is no flaw in the EXTEX and Alcor PMA coupling adapters. "The problem may be a combination of design and assembly," the FAA said. "If a new adapter coupling is installed in an impeller with fretting damage, the new coupling is susceptible to rapid fretting damage and may develop fatigue cracks. Also, if a coupling adapter of improper diameter is installed, then there would be a higher risk of fretting damage and part failure." Proper assembly procedures are a key to making sure the coupling adapter doesn't fail, according to the FAA.

The FAA held a meeting in May to discuss an issue related to this accident and the adapter couplings, a gathering of the Joint Management Team, composed of personnel from the FAA, Aerospace Industries Association, General Aviation Manufac-turers Association, and Air Transport Association. The FAA's presentation focused on the issue of how PMA manufacturers ensure that their parts remain safe once they are operating in the field. This issue is known as continued operational safety. The FAA is concerned that PMA manufacturers are trying to shift the burden of operational safety to the OEM, if the PMA part is identical to the OEM part. And if there is a problem with an OEM part and the same PMA part, should the FAA issue a blanket airworthiness directive to cover all the parts together or single ADs against each part? These questions weren't answered at the meeting, but the FAA presentation there did outline possible actions to ensure public safety and this may result in some new requirements for PMA manufacturers. One idea may be to assign more stringent requirements for critical PMA parts to ensure continued operational safety. Meanwhile, NTSB investigator Cornejo said that NTSB investigators are working on the final report on the Volcano, Hawaii accident and its cause. "It's still under investigation," she said. —By Matt Thurber