A History of “Significant Wear” In MD –80 and MD-11 Aircraft.
Dec 14. As the Alaska Flight 261 NTSB Hearing unfolds the FAA has made a statement that the stabilizer problem seen at Flight 261 has not been seen at other carriers. (See Reuters story below for one press account).
Contrary to that, there are seven priors filed with the FAA including five referenced in a July 31, 1997 Douglas Service Bulletin and two others this year alone in two Service Difficulty Reports (SDRs) filed on March 31,2000 and on June 6, 2000. The FAA has also claimed the same jackscrew is used on the MD-11.
From that July 1997 filing with the FAA, that Douglas Service Bulletin said;
“Five operators reported instances of the horizontal stabilizer actuator nut assembly prematurely wearing out. The premature wear of actuator nut assembly and resultant excessive free play on the jackscrew assembly are caused by a jackscrew surface finish which is out of design specification tolerance. Excessive wear of the actuator nut assembly could result in a jammed actuator nut assembly and subsequent loss of horizontal stabilizer function.” Recommended compliance date for a “initial inspection for free play” was “first convenient maintenance period, but not to exceed 400 cycles (landings) from the issue date…”. ( Douglas service Bulletin MD-11-27-067).
An FAA AD (AD-98-16-9) followed nine months later in 1998.
In spite of that 1998 AD the two Service Difficulty Reports this year reported the following;
Occurrence Date, 03/31/2000. “During inspection of the horizontal stabilizer jackscrew assy, metal shavings were found within the grease on and around the jackscrew. Further, the ACME screw and nut wear check showed the end play was beyond limits. The established limits are .030 - .040 inch IAW MM-27-40-1 and .050 was obtained. Non-routine work card 4000277-0055 has been written documenting these discrepancies. Removed, replaced, and ops checked horizontal stabilizer assy IAW MM 27-40-1. (X).” (SDR 2000050900161).
Occurrence Date, 06/14/2000. “The operator is of the opinion that the jackscrew assy has never been removed. However, documentation is unavailable at this time to verify the status of this jackscrew assy. A visual inspection of the horizontal jackscrew assy revealed metal particles in the lubricant on the jackscrew. The operator decided to remove the jackscrew assy for repair/overhaul. (X).” (SDR # 2000080400209).
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Reuters; Tuesday December 12, 9:14 pm Eastern Time
FAA sees no redesign from Alaska Airlines crash
Source;
WASHINGTON, Dec 12 (Reuters) - The Federal Aviation Administration said on Tuesday it saw no need to order a redesign of a tail mechanism at the center of inquiries into the January crash of an Alaska Airlines plane that killed 88 people.
A top FAA official, speaking ahead of hearings into the crash that start Wednesday, said the carrier had experienced problems with the jackscrew device that moves the MD-80's horizontal stabilizer that were not seen at other airlines.
``At this point we don't see design changes as a result of this accident,'' FAA associate administrator Tom McSweeny told reporters, citing the generous safety margin of strength built into the parts.
``There was some fairly significant wear occurring there,'' McSweeny said of Alaska Airlines experience with the jackscrew mechanism.
Everyone on Alaska Airlines Flight 261 was killed Jan. 31 when its pilots lost a battle to control the plane after radioing that they were having trouble controlling the stabilizer mounted on the top of the MD-80's tail.
The National Transportation Safety Board has said its hearing will examine issues including Alaska's maintenance practices, the design of the jackscrew, and FAA's oversight of both the airline and the aircraft design.
The 2-foot (60 cm) jackscrew is normally powered by electric motors and threaded vertically through another part, a gimbal nut, to move the stabilizer which maintains level flight and is also used for gradual climbing or descent.
The jackscrew was retrieved from the seabed off Point Mugu, north of Los Angeles, with threads from the gimbal nut wrapped around it. Investigators, in safety board reports, have questioned whether the jackscrew had been lubricated properly.
McSweeny said the MD-80 jackscrew mechanism had conformed to FAA design rules in 1977 and would still meet today's certification requirements. He said other systems with two jackscrews are used on the larger MD-11 and DC-10 aircraft but they were not necessarily more dependable.
He said Alaska Airlines had been in the process of trying to standardize its lubricants and incompatible greases may have been mixed on the jackscrews.
Traveling from Puerto Vallarta, Mexico, to San Francisco, the pilots of Flight 261 had requested an emergency landing in Los Angeles and were trying to stabilize the plane over the Pacific Ocean when the crash occurred.
Investigators know from the plane's flight data and cockpit voice recorder that Flight 261's stabilizer went into a full nose-down position about 12 minutes before the crash.
The crew regained control after a steep dive by using the elevator panel on the trailing edge of the stabilizer. But they lost control of the jet for good shortly after extending, for a second time, the wing slats and flaps they would need for landing.
Alaska Airlines is the nation's 10th largest carrier and a subsidiary of Seattle-based Alaska Air Group (NYSE:ALK - news).
Boeing Co. (NYSE:BA - news) in 1997 bought McDonnell Douglas which designed the MD-80 and related aircraft that include the DC-9, MD-90 and Boeing 717.
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