FAA warns of unintended release of rotary seatbelt buckles

 - September 20, 2006, 5:20 AM

Pilots flying almost every type of business jet and airliner and some turboprops face a risk of inadvertent seatbelt release, according to a bulletin the FAA has released. Special Airworthiness Information Bulletin (SAIB) NM-06-29 warns that unguarded rotary seatbelt buckles can be activated unintentionally, possibly releasing the seatbelt at an inopportune moment.

The SAIB, which is not mandatory, follows an NTSB recommendation (A-03-57) that summarized investigators’ conclusion that the captain’s rotary seatbelt buckle released during the Jan. 8, 2003, crash of an Air Midwest Beech 1900D in Charlotte, N.C. “The buckle released,” according to the NTSB, “when the pilot pulled the control yoke aft and contacted the vanes of the rotary buckle.”

The unguarded rotary buckles, made by Pacific Scientific, have four vanes on the top of the buckle that make it easy for pilots to release the lap and shoulder belts. The vanes need to turn just 30 to 40 degrees to release, according to Pacific Scientific’s component maintenance manual.

NTSB investigators conducted tests on a Beech 1900D using a person of approximately the same height as the captain of the Air Midwest flight. “The tests showed that it was possible for the yoke to contact the rotary seatbelt buckle vane and rotate the seatbelt buckle,” the NTSB recommendation stated.

The FAA SAIB recommends that pilots test rotary seatbelt buckles installed in airplanes that they fly, using a procedure developed by a test pilot who found that he could unlatch his rotary buckle in flight. The procedure involves placing a five-foot-tall person in a pilot’s seat, moving the seat full forward, and rotating the yoke left and right while pulling it fully aft to see if it interferes with the vanes on the rotary buckle. (To view the SAIB and the list of affected aircraft, see www.faa.gov/aircraft/safety/alerts/saib/media/NM-06-29.pdf.)

The Air Line Pilots Association (ALPA) agrees with the FAA about the potential problem of inadvertent seatbelt release and addressed the subject in an April 2003 letter to the NTSB regarding the Air Midwest crash.

According to the letter, “The evidence suggests that the captain’s seatbelt released either during the impact sequence or during the efforts to fly the airplane while in the upset. If the seatbelt latch released prematurely while the captain was attempting to gain control of the airplane, it would have exacerbated an already serious situation.”

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According to ALPA, this is not a new issue; the letter noted that Boeing had
issued Service Letters in 1992 (737-SL-25-54, 757-SL-25-40) to tell operators that an improved buckle was available. Test pilots flying a 737-400 found that a clipboard attached to the yoke unlatched the rotary buckle. In 1992 Pacific Scientific published Service Bulletin 1117032-25-02 recommending installation of guarded seatbelt buckles in Boeing airplanes equipped with the company’s unguarded buckles.

Not all rotary buckles are susceptible to the inadvertent release problem. Schroth Safety Products consulted with the FAA, according to a company spokesman, and was told that the Schroth buckles are not cause for concern. The Schroth buckles are streamlined and have a lower profile and a different type of internal release mechanism.

The SAIB notes that the unguarded rotary buckle problem doesn’t affect airplanes with sidesticks or joysticks. Helicopters aren’t immune to this problem, however. Don Eisentraut, vice president of safety engineering for forensic engineering consulting firm ARCCA, during work designing crash-resistant seating for military helicopters, found that “unguarded buckles could be struck by the cyclic and released.”

The solution to the problem was the guarded buckle, he said. “There has to be a tradeoff between a buckle that is easily releasable for egress but not releasable inadvertently.”

The SAIB suggests replacing the rotary buckles with guarded buckles if the test shows that the control yoke contacts the vanes. “It isn’t necessary for the buckle to disengage,” the SAIB noted, “for the test to be considered a failure.”

Despite the issuance of the SAIB, the NTSB’s 2003 recommendation remains classified as open and awaiting an acceptable response from the FAA.