The , responding to a formal recommendation, is telling the U.K. Aviation Accidents Investigation Branch (AAIB) that it has no plans to require that landing gear designs factor in the effects of soft ground on gear breakaway sequences, an AAIB report reveals.
“The FAA reviewed this issue with[the European Aviation Safety Agency] and concluded that a change to the regulatory requirements for landing gear failures to account for the effects of landing on different types of surfaces is not warranted at this time,” the AAIB says in its recently released annual safety report.
“Such a change would require significant industry support and data that is currently unavailable,” the report adds.
The issue came to light following the 2008 non-fatal crash-landing of a(BA) just short of its intended runway at . During the accident sequence, the 777’s left main gear attachments sheared away as designed, but the right main gear attachments did not. The resulting damage included a center fuel tank rupture.
The 777’s main landing gear is designed to break away when overloaded via strength-controlled fuse pins. The pins break in a specific sequence, primarily to minimize the risk of fuel tank ruptures.
As part of the AAIB’s investigation, Cranfield Impact Centre (CIC) researchers studied the BA 777 accident sequence. They concluded that the 777 gear breakaway sequence is different depending on the surface where the accident occurs.
“In the soft ground accident simulation, the results showed that only one of the fuse pins failed,” the AAIB says in its final report on the accident. “A delayed build-up of shear forces in the pins [when compared to impact with hard ground] prevented most of them from reaching their failure loads,” it adds,
This can transfer loads to other areas, including the rear spars, causing damage to fuel tanks. This is what investigators believed happened to the BA 777.
CIC notes that its simulation used even softer ground than the soil present at the accident site, likely distorting the results. “However, the analysis did indicate that landing gear interaction with soft ground can substantially modify the breakaway sequence,” the AAIB says.
The findings led the AAIB to recommend that the FAA and EASA re-examine certification requirements. The FAA says that while it has no immediate plans to make changes, it would consider the AAIB’s recommendation the next time applicable U.S. certification regulations are revised.
The FAA rejected another recommendation from the same report urging improvement of 777 interior lighting. During the probe, the AAIB learned that some passengers reported a “fog” in the cabin soon after the plane went down. Investigators determined the haze was mercury vapor released when some of the plane’s indirecting lighting system bulbs broke in the crash.
The 777’s indirect lighting, which sits above outboard stowage bins, uses tubular florescent bulbs. The AAIB says the bulbs contain mercury vapor that “may present a hazard when broken.”
In addition, broken glass from the bulbs presented a possible hazard to evacuating passengers.
The AAIB’s findings led it to recommend that the FAA orderto modify 777 indirect lighting design. The U.S. regulator looked at the issue, but determined that the risk of broken bulbs impeding an evacuation “would be extremely low,” the AAIB says.
The FAA notes that despite evidence of an unsafe condition, Boeing made changes to the lighting systems a decade ago, starting with line no. 454. The BA 777 was line no. 342.