A British agency concluded last month that grease may have played a role in four recent incidents in which landing gear on commercial planes operated by a Pakistani airline caught fire during landings.
The agency did not venture an opinion, however, about whether the fires were more likely caused by application of too much grease or by use of an unapproved product. The grease supplier maintained that its product was not a primary cause.
According to a September bulletin posted by the Air Accidents Investigation Branch of the United Kingdoms Department for Transport, all four planes were Boeing 777s operated by Pakistan International Airlines. The fires occurred in May and June, three during landings in Manchester, U.K., the fourth in Toronto. None caused any injuries.
The wheel assemblies in all four cases were lubricated with Nyco Grease GN 22, a product supplied by the French specialty lubricant maker, Nyco S.A. The investigations branch noted that Boeings maintenance manual for the 777 recommends wheel assemblies be lubricated with general purpose wide-temperature greases meeting the MIL-PRF-81322 specification, specifically mentioning three particular products. Nyco 22 meets the mil-spec but is not currently among the products mentioned by the 777 manual.
The investigations branch also cited a 2001 Boeing maintenance tip advising that wheels and brakes occasionally catch fire after routine landings, and that such fires are usually triggered by ignition of excessive grease that has accumulated in the brake assembly or on the axle. It added that fires may also be caused by residual cleaning fluids and leaking hydraulic fluid.
The AAIBs bulletin did not explicitly suggest a cause of the PIA fires, but it did note that the airline has stopped using Nyco 22 on wheel assemblies of its 777s and switched to one of the products mentioned in the 777 maintenance manual. The bulletin added that PIA, which began flying 777s earlier this year, is now using less grease in accordance with Boeings recommendations.
A spokesman for the investigations branch said the agency would have mentioned a primary cause of the fires if it had been able to identify one. PIA did not respond to requests for comment about the fires and investigation.
A Nyco official told Lube Report that its grease may not be ideally suited for 777 wheel assemblies but that the use of Nyco 22 was not a primary cause of the fires. Aviation Business Manager Eric Piveteau said Nyco 22 has been safely used for a wide variety of applications on numerous types of aircraft for nearly 20 years. He added that it is specifically recommended for wheel assemblies of some aircraft and was formerly one of the products specifically mentioned for 777 assemblies.
He added that Nyco learned after AAIB started its investigation that Nyco 22 and another product had been removed from Boeings recommendation. The products had been removed some time before the fires, but the plane manufacturer did not notify Nyco, he said.
We are still looking into this, Piveteau said. We have spoken with Boeing, but the rationale is not clear. If you look at the data submitted to the [U.S.] Navy [which formerly administered MIL-PRF-81322], there is nothing to suggest a gap in performance of the products that were recommended.
Piveteau noted, however, that wheel assemblies on 777s are exposed to more extreme conditions during landings than other aircraft – including temperatures that reach 500 to 600 degrees C.
We know that this is a demanding application, he said. It is conceivable that part of the explanation [for the PIA fires] is that Nyco 22 is not best-suited for this application, even though it meets MIL-81322. But the grease itself is not the primary cause.
Piveteau noted that Nyco is participating in an inter-industry effort being managed by SAE International to develop better standards for aviation greases. The organizations AMS-M Committee recently finished drafting a specification for a general purpose aviation grease and is now tackling a specification for wheel bearing greases.