Helicopter EC225 MGB - Rotor&Wing

Monday, October 10, 2016

EASA Orders Tougher Super Puma Gear Rules

EASA’s end to a flight ban on some Super Pumas is contingent on those helicopters’ manufacturer and operators replacing transmission gears with tougher versions and closely monitoring the shortened service life of the new components.

 

The European agency’s Oct. 7 decision was not matched by the civil aviation authorities of the U.K. or Norway, which said they would leave flight bans in place for Airbus Helicopters AS332L2s and EC225LPs. Those bans went into effect in early June after Norwegian investigators concluded a main transmission gear fracture was the most likely cause of an April 29 EC225LP crash that killed 13 on a flight returning from an offshore oil rig. The Norwegian probe is ongoing.

 

The U.S. FAA also imposed a flight ban in early June.

 

In lifting the ban, EASA said that it had been persuaded by additional Airbus testing on second-stage planetary gears in the main gearbox of the AS332L2 and EC225LP transmission.

 

The April 29 crash occurred when a CHC Helikopter EC225LP’s main rotor broke free of the aircraft as the aircraft cruised at 2,000 feet on approach to Flesland Airport about 13 nautical miles to the southeast, near Bergen, Norway. The crash killed the 11 passengers and two pilots on board.

 

Initial efforts investigated the role of possible failures in the main rotor head suspension bar assembly, the main gearbox’s conical housing or that gearbox’s epicyclic module. After recovering and examining more debris, investigators found evidence that a second-stage planetary gear (which is contained in the epicyclic module) had fractured.

 

Norway’s accident investigation board said that not only was gear fracture the accident’s most likely cause but that it appeared to have developed in a way that certification processes may not have foreseen and maintenance procedures may have been unable to detect. These possibilities contributed to the international groundings.

 

After its own work on possible suspension bar assembly failures, EASA said,

Airbus also “determined that the likely cause relates to the rupture of the second stage planet gear.”

 

The cause of the gear fracture “is still not fully understood,” EASA said, but “it involved cracking of the planet gear bearing outer race, some spalling and propagation of a crack into the rim of the gear, finally resulting in its rupture.”

 

Airbus identified two planet gear configurations of its current design and found one configuration “has higher operating stress levels that result in more frequent events of spalling, associated with rolling contact fatigue, while the other exhibits better reliability behavior,” EASA said in its Oct. 7 airworthiness directive.

 

That directive specifies that AS332L2s and EC225Ls cannot be returned to flight until they are fitted with the more reliable configuration of the second-stage planet gears (those with part numbers P/N 332A32-3335-04 or -06). Furthermore, to keep flying, those helicopters must have the more reliable planet gears removed before they reached half of their previously prescribed service life.

 

In addition, to keep flying their AS332L2s and EC225Ls, operators must inspect the aircraft’s main gearbox particle detectors and oil filters every 10 flight hours.

 

“By limiting the type design to the gear configuration with lower stress levels and better reliability and specifying a reduced life limit, combined with more effective oil debris monitoring procedures and other operational controls, an acceptable level of safety can be restored,” EASA said.

 

The U.K. CAA disagreed. Noting that “the safety of those who travel on offshore helicopter flights is a key priority,’ it said Oct. 7 that the accident “is still under investigation by the Norwegian authorities and we remain in close contact with all offshore helicopter operators to continue to assess the situation.” It added that it is united in its approach with Norway’s CAA and “both agencies now await further information from the accident investigation before considering any future action.”