Tue, Nov 17, 2009 — David Evans
|23 October 2009 European Aviation Safety Agency (EASA)AD No. 2009-0230-E
Subject: Emergency Airworthiness Directive for Sikorsky S-92 helicopters
Applies to all Sikorsky S-92A helicopters and requires inspection within 10 flight hours and continued inspections every 10 flight hours thereafter of the main gear box (MGB). Effective date 27 October 2009.
According to this AD:
“Cracks have reportedly been found in the MGB assembly mounting feet, pad and foot ribs during regular inspections of the MGB feet and mounting bolts. In one case, the mounting foot was completely severed from the MGB. Other cases include cracks at the fore and aft mounting bold location on the right hand side mounting foot.
“This condition, if not detected and corrected, could lead to failure of the MGB attachment, possibly resulting in MGB detachment and consequent loss of control of the helicopter.
“For the reasons described above, this EASA Emergency AD requires repetitive inspections of the MGB assembly mounting feet pad and foot ribs for cracks and, if any cracks are found, the replacement of the MGB assembly with a serviceable unit. Replacement of the MGB assembly does not constitute a terminating action for the repetitive inspections.”
These inspections relate to the fatal crash 12 March 2009 of an S-92A near St. John’s, Newfoundland. The case is now under investigation by the Transportation Safety Board (TSB) of Canada. The TSB issued a communiqué 18 June 2009 indicating the MGB is a focus of the investigation, indicating:
“The investigation has revealed that, even though the Sikorsky S-92A MGB was certificated to meet requirements of Part 29 of the Federal Aviation Regulatios (FAR 29) of the United States FAA, there is a perception in some areas of the aviation community that the MGB can be run in a dry states – that is, without lubricating oil – for 30 minutes. FAR 29 does not require run-dry operation of a gearbox to meet the 30-minute ‘continued safe operation’ … As a result of the fracture of the filter bowl mounting studs, resulting in the loss of a large quantity of oil, the certification guidance material is being reviewed. Additionally, the FAA and Sikorsky Aircraft are working to identify all the modes of failure that might lead to Sikorsky S-92A MGB oil loss, determining their probability of occurrence, and developing appropriate mitigation strategies.”
The S-92A was certified after Sikorsky demonstrated that the chance of complete oil loss was “extremely remote,” only to suffer a series of gearbox oil-loss incidents after the helicopter entered service. The MGB, which transmits power from the engines to the main rotor blades, is one of the most critical components in a helicopter. Pilots can recover from an engine failure, but MGB seizure is considered catastrophic.
Given the previous incidents and the fatal crash, one must ask: how many cracks must be found before immediate grounding is appropriate?