Safety News

Fallout From Alaska Crash: Overhaul facilities and operators of Boeing DC-9s, MD-80s, MD-90s, and 717s could face new maintenance restrictions and requirements if that aircraft maker and the FAA accept federal safety recommendations issued as a result of an Alaska Airlines crash three years ago.

The U.S. National Transportation Safety Board at a December 10, 2002 meeting ruled that Alaska Airlines Flight 261 crashed on January 31, 2000 when the pitch control system broke apart in flight because the airline had failed to sufficiently lubricate the jackscrew assembly that drove it.

The NTSB cited several factors that helped cause the MD-83’s crash into the Pacific Ocean off of southern California, which killed all 88 people on board. It found, for instance, that:

• Alaska extended the interval for lubricating the jackscrew assembly, and the FAA approved that extension. That change "increased the likelihood that an unperformed or inadequate lubrication would result in excessive wear" of that critical component;
• The airline extended, and the FAA approved the extension of, the primary check for excessive wear with no data to justify the change. Even though NTSB investigators determined that this check was unreliable, the safety board found, its extension allowed wear on the accident aircraft’s assembly "to progress to failure without the opportunity for detection."
• Even though the jackscrew assembly is an integral and essential part of the aircraft’s horizontal pitch trim system, its design "does not provide redundancy with regard to wear." The accident demonstrated, the board said, that that critical flight-control system is vulnerable to a single-point failure

The safety board said that, at the time of the accident, Alaska’s maintenance program "had widespread systemic deficiencies," that the FAA "did not fulfill its responsibility to properly oversee" that maintenance, and that FAA surveillance of Alaska "had been deficient for at least several years."

In approving the staff’s findings on the accident, the four presidential appointees currently serving on the NTSB stopped short of calling for a redesign or recertification of the MD-80 or the DC-9, MD-90, and 717, which use similar horizontal pitch trim designs.

Redesign or recertification was the subject of extended, heated debate within the safety board. Investigators found that these aircraft depend on maintenance and inspection instead of built-in safeguards to minimize critical failures. Observations of maintenance and inspection practices and results at Alaska and other airlines found that errors in work on jackscrew assemblies were commonplace. For instance:

• Effective lubrication may not be achieved. (In fact, investigators found virtually no usable grease on the accident aircraft’s jackscrew; they believe its Zerk fitting had been clogged for quite some time before the accident, and that mechanics that had signed off lubrication tasks had simply gone through the motions of hooking a grease gun to the fitting without verifying that grease entered the assembly.
• Maintenance practices vary widely. Alaska lubricated jackscrew assemblies every 2,000 flight hours at the time of the accident. American Airlines did so every 940 hours.
• So-called "end play" checks on the acme nut in the assembly (the primary means of assessing wear, the effectiveness of lubrication, and the residual safety of the part) are unreliable. In one case, investigators said, end-play checks indicated that the threads on the nut and jackscrew not only had not worn, but apparently had grown thicker with use. In another case, an assembly that tested as being well within wear limits was found to be worn to nearly twice those limits. (Keep in mind that worn part would have be cleared to fly for hundreds of hours under standards and practices in place prior to the Alaska accident.)

As a rule of thumb, the NTSB’s head of aviation investigations, John Clark, told the meeting, maintenance and inspection efforts are considered to be 80 percent reliable. He said it is not sufficient to rely on them to prevent failure of critical systems.

Nonetheless, the safety board opted not to call on the FAA and Boeing to redesign what appears to be an insufficiently safe or redundant unit. When confronted with a rare, difficult-to-replicate problem with the rudder control system of 737s five years ago, the NTSB called for the system’s redesign, which Boeing has since completed. When faced with problems common and widespread in aircraft akin to the DC-9, the safety board called on the FAA and Boeing "to conduct a systematic engineering review" to identify means of preventing catastrophic failures of the jackscrew assembly and to make design changes "if practicable."

In the meantime, the safety board wants to clamp down on maintenance and inspection practices.

It said jackscrew lubrication should be made a required inspection item and operators of DC-9s, MD-80s, MD-90s, and Boeing 717s should have to clean old grease from the assembly before applying fresh grease.

The NTSB called for development of a more reliable end-play check that ensure the jackscrew assembly won’t fail even if excessive wear is missed on two consecutive inspections.

It said operators should have to permanently track end-play measurements by aircraft tail number and jackscrew assembly serial number. They also should be required, it said, to record average wear rates for each airplane and implement a program to analyze causes of excessive or unexpected wear. The NTSB wants that data reported to the FAA.

The NTSB also would have maintenance shops required to record and report to customers end-play measurements on overhauled jackscrew assemblies. It said the FAA should require those facilities to "obtain specific authorization to perform such overhauls" and to demonstrate they "possess the necessary capability, documentation, and equipment for the task."

The safety board has no power to enforce these recommendations, and the FAA can take years to decide how specifically it will address NTSB findings.–By Jim McKenna