A major step has been taken to ensure that a primary flight control system will not wear beyond acceptable limits in service. The Federal Aviation Administration (FAA) recently ordered that end-play measurements of jackscrews be recorded during overhaul.
The directive is a direct outgrowth of the fatal January 2000 crash of an Alaska Airlines [ALK] MD-83 twinjet. All 88 passengers and crew aboard were killed when the jackscrew controlling the pitch trim system for the horizontal stabilizer failed, causing the stabilizer to flip up and separate from the airplane. The airplane careened end-over-end into the waters off Los Angeles, Calif. (See ASW, Jan. 20, 2003)
When the wreckage was pulled from the water, National Transportation Safety Board (NTSB) investigators were dismayed to discover that the threads were completely stripped from the acme screw, causing it to break free of the acme nut, with resulting loss of the stabilizer.
Upon further investigation, investigators found that during the accident airplane’s last overhaul, two years before the crash, the jackscrew end-play was right at the limit where replacement would be required. A second measure of the freeplay between the threads of the acme screw and the acme nut was conducted, which measured right under the maximum tolerance of 0.040 inch. The jackscrew was not replaced, was likely not lubricated, and the threads wore to a nubbin, causing the crash. (See ASW, Jan. 1, 2001)
By a flight standards information bulletin for airworthiness (FSAW 04-03) published March 3, 2004,
maintainers are now required to record the end-play measurement and, if the work is performed by a contract maintenance operation, to provide the measurement to the operator. The FSAW affects all Douglas DC-9, MD-80, MD-90 and Boeing [BA] B717 twinjets. The B717 is a derivative of the Douglas designs and features the same distinctive T-tail and associated jackscrew assembly for pitch trim control.
When the Alaska accident airplane was returned to service, the FSAW notes that the end-play measurement had not been recorded at the component overhaul site. By requiring the end-play measurement to be recorded, the FSAW maintains that the wear rate can be monitored more effectively.
The FSAW declares that the NTSB’s documented “design, operational and maintenance faults involving the airplane’s horizontal stabilizer trim system … may have contributed to the loss” of the Alaska jet. The use of the word “may” evades a mountain of direct evidence compiled during the course of the NTSB’s investigation indicting the design and maintenance of the system. The investigation determined that the pitch trim system lacked redundancy (for “fail safe” design), lubrication intervals were extended with no substantiation that thread wear would remain within safe limits, and end-play checks were performed with non-standard tools, giving rise to false readings of thread wear during maintenance.
In its Jan. 8, 2003, letter to the FAA, the NTSB stressed that an excessively worn jackscrew is an invitation to catastrophe:
“Because the MD-80 jackscrew assembly’s structural function is critical to the safety of flight, and that structural function cannot be maintained without proper acme nut and screw thread engagement, it is essential that acme nut thread wear be regularly monitored. The failure to adequately monitor acme nut threat wear may result in continued operation of an airplane with excessive nut thread wear. As demonstrated by the Alaska Airlines Flight 261 accident, because no other structure performs the function of the jackscrew assembly, the loss of acme nut and screw engagement as a result of excessive wear will most likely have catastrophic results.”
Among its 16 recommendations, the NTSB said maintenance facilities that overhaul jackscrews should be required to record the end-play measurement, and to inform their airline customers.
The FSAW responds to that recommendation, calling for a detailed recording of the work done. Furthermore, the FSAW declares that FAA principal maintenance inspectors (PMIs) should ensure that the new record-keeping requirements are included in the overhaul recording systems of the airlines under their purview.
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Now Required for Jackscrew Overhaul Records
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Requirements in FSAW 04-03
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NTSB Concerns
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| A detailed description of the work performed, including a description of the inspections and repairs performed. | Only one of the maintenance facilities visited by the NTSB used a detailed set of work cards to document each step. |
| A listing of the final fits and clearances. | One maintenance facility’s end-play measurement limit was 0.025 inch; another’s was 0.030 inch. A third maintenance facility had no self-imposed limit at which the acme nut had to be replaced. |
| A detailed description of the testing and testing results. | One facility placed the jackscrew on a wooden cradle and moved the nut; another used a magnet to hold the dial indicator in place. |
| Identification of the standards or technical data used to test the item. | At other facilities, jackscrew overhauls were conducted in accordance with the DC-9 maintenance manual, but the manual contained no requirement to document the end-play measurement after overhaul, or that the receiver of the overhauled unit was provided with the measurement. |
| The identity of the person who performed the work. | The procedure is generally performed and signed off by a single maintenance technician, and that technician’s work is not required to be inspected. |
| Sources: FAA, NTSB: Recommendation letter of Jan. 8, 2003, and final report of Flight 261 crash investigation, adopted Dec. 30, 2002 | |