Passengers and pilots in a series of survivable Boeing 737 crashes may have been injured by approved cabin overhead systems or crew seat belt restraints designed or tested to inadequate levels for an actual crash.
A series of new recommendations from the US National Transportation Safety Board (NTSB) call on the US Federal Aviation Administration to modify the design and test requirements related to passenger service units (PSUs) in Next Generation Boeing 737 family aircraft, and to analyse aircraft from other manufacturers for similar problems.
PSUs, which are mounted above each seat on the underside of the overhead bins, include supplemental oxygen generators, oxygen masks and ventilation air vents, and typically weigh about 5.7kg (12.5lb).
Fallen PSU in Denver runway excursion
Other recommendations call on the FAA to develop test criteria and performance measures for the negative-g straps that are part of flight deck seats, and to switch out Ipeco-built strap attachment brackets with more robust replacements.
"This letter discusses the circumstances of several survivable accidents that have occurred in the last three years in which overhead bins and passenger service units (PSU) on Boeing 737 airplanes became separated from their attachments during the accident sequences, likely increasing the number of reported occupant injuries, particularly injuries to the head and face," the NTSB said in the 24 February letter.
"In addition to this occupant safety hazard, the negative-g strap attachment bracket (used as part of the flight crew five-point restraint assembly) failed in two cases, possibly contributing to back injuries to the flight crewmembers."
The recommendations are linked to several completed and on-going accident investigations, including the 16 August 2010 crash of an Aires Airlines 737-700 short of the runway at San Andres Island, Colombia; the 22 December 2009 runway excursion by an American Airlines 737-800 in Kingston, Jamaica; the 25 February 2009 crash of a Turkish Airlines 737-800 in Amsterdam, and the 20 December 2008 runway excursion by a Continental Airlines 737-500 in Denver.
Common themes in the post-accident cabin investigations of the crashed aircraft were PSUs that were either hanging from the overhead bins or had fallen into the seats or into the aisle. In addition to the potential for passenger head injuries, skull fractures and lacerations as the PSUs fell, the devices could also restrict access to emergency escape routes.
A key failure mode the NTSB has identified is movement of the overhead bin structure during the crash, allowing for higher than expected loads to be transferred to the PSUs, fracturing the outboard clamps holding the devices in place.
In addition to asking the FAA to analyse the structural attachments, the NTSB also asked for backup safety devices that would "capture" the PSU if it does break lose.
For the crew seats, the NTSB said that negative-g seat belt brackets made of 1.6mm-thick (0.063in) sheet aluminium provided by Ipeco had failed in several of the accidents. In one case, the Ipeco bracket on one seat failed while a different seat with a 2.4mm-thick bracket built by AmSafe Aviation did not break.
In addition to new test criteria and performance measures for the negative-g strap assemblies, the NTSB has asked the FAA to require airlines to retrofit the Ipeco brackets "with stronger brackets".