Saturday, August 1, 2009
Rotorcraft Report: Cougar Accident Raises Questions Over "30 Minute Rule"
Cougar Helicopters Flight 491 lost tail rotor drive about a minute before it crashed into the ocean off Newfoundland on March 12, killing 17 of the 18 people on board. The aircraft’s flight data recorder had stopped working but investigators were able to piece together information from the health and usage monitoring system (HUMS) and flight control computer. Canada’s Transportation Safety Board (TSB) says that, at the time of failure, the pilots were making a controlled descent to land in the water after the loss of MGB oil pressure, and were at about 500 feet above sea level. Three seconds afterward, the pilot cut off the engines to attempt an engines-off landing.
"It’s showing us that there was control remaining and the guys were doing their best to get the helicopter down on the surface," said Mike Cunningham, TSB investigator-in-charge. The main rotor was working throughout the descent. Fellow investigator Allan Chaulk explained to Rotor & Wing that following the gearbox oil loss, heat was transferred to the tail rotor take-off pinion gear. "The gear rotates at six to seven times the speed of other gears in the assembly, and it wasn’t able to sustain that without lubrication."
"It’s at the top of the gearbox and the first thing to fail," he continued. Photos on the TSB website (http://tsb.gc.ca) show the teeth of the pinion gear completely stripped.
The interval between the loss of transmission oil and contact with the surface was "approximately 10 minutes." The investigation determined the helicopter struck the water at a "moderate" speed. The pilot may have also flared before meeting the surface. Despite this, the helicopter hit the water with a significant impact — reportedly 20G — nose upward and banked slightly to the right.
All flotation devices failed as the helicopter hit the three-meter seas. "The damage done... during the impact was so significant that... it might be unreasonable to expect that this (flotation) system could have even operated," said Cunningham. "And would it have made any difference? If it did, is pretty hard to say."
"One of the bags was ripped right out of its housing," he continued. This failure is still under investigation.
Under Part 29 certification rules for rotorcraft, failures that result in the loss of lubrication to the drive train must allow for 30 minutes of flight time after the crew is presented with the cockpit warning. However, a clause notes that, if a failure mode is determined to be "extremely remote," the 30 minute rule no longer applies.
In an accident update communiqué, the TSB stated: "the investigation has revealed that, even though the Sikorsky S-92A MGB was certificated to meet requirements of (FAR 29), there is a perception in some areas of the aviation community that the MGB can be run in a dry state — 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.’
"Based on the applicable guidance material at the time of certification, the lubrication failure modes of interest were limited to the failure of external lines, fittings, valves, and coolers. This practice was consistent with industry experience, which had found that loss of lubrication tended to be associated with external devices. Therefore, the possibility of a failure at the oil filter was considered to be extremely remote."
During its examination of the helicopter wreckage in March, the board discovered two of the three mounting studs that attach the oil filter assembly to the main gearbox had failed. An FAA directive then ordered the titanium mounting studs be replaced with steel studs. That directive grounded the 91-strong fleet of Sikorsky S-92As, including Cougar Helicopters aircraft, until the studs were replaced. This process is now complete.
The TSB says a metallurgical examination of the titanium mounting studs revealed fatigue cracking and thread damage. The board is still trying to determine the origin of the fatigue cracks.
Sikorsky was made aware of the potential problem last August, when an S-92A was forced to make an emergency landing in Australia after losing MGB oil pressure. The company issued an Alert Service Bulletin in January, asking that operators replace the studs within a year, or by 1,250 flight hours.
The families of 15 passengers who died in the crash and the one survivor, Robert Decker, are suing Sikorsky, its maintenance subsidiary Keystone Helicopters and the parent company, United Technologies Corporation. Decker (27) managed to escape through a window to be rescued by the crew of another Cougar helicopter. On reaching hospital in St. John’s, Newfoundland, he was put on life support and a ventilator: however, he is now said to be recovering at home.
Cougar rescue technicians Ian Wheeler and Stephen LeMessurier were later commended for their actions during the rescue mission. Wheeler is credited with reaching and sending Decker up to the rescue helicopter while waiting in the ocean. LeMessurier was recognized for his efforts to save one of the other people aboard the helicopter, though she did not survive.
At press time Sikorsky commented, "our prayers continue to go out to the families of all those lost in this accident. It is our policy to not comment on pending litigation except to state that we will appropriately defend against these claims." — By Andrew Healey