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On February 20, 1997, at 1027 eastern standard time (est), a Dehavilland DHC-8-102, N828MA, operated as Mesaba Flight 3098, by an airline transport pilot, experienced an uncommanded left rolling bank while in cruise flight. The pilot corrected the situation and the airplane was landed at Detroit Metropolitan Airport, Michigan, without further incident. Post-flight examination of the left wing showed the left outboard spoiler segment partially extended. Visual meteorological conditions prevailed at the time of the incident. The flight was being conducted as domestic commuter air carrier service under 14 CFR Part 121. An IFR flight plan was on file. There were no reported injuries by the two flight crew members, one flight attendant or 23 passengers on board the airplane. The flight originated at Romulus, Michigan, at 1007 est, and was en route to Knoxville, Tennessee.
In his written statement, the captain said that after departing Detroit Metropolitan Airport, "the first officer needed excessive right wing down trim to maintain a wings level attitude. The left outboard roll spoiler indicated approximately one-half deployment on the PFCS (Powered Flight Controlled Surfaces)." The crew maintained control of the airplane with trim and control wheel input. "As per maintenance's request, we tried to depressurize the system with the spoiler one and two push-off switches on the center console. This had no effect on the problem." The crew declared an emergency and returned to Detroit Metropolitan Airport. The airplane was landed without further incident.
The airplane, a Dehavilland DHC-8-102, serial number 333, was manufactured in July, 1992. The airplane had undergone a line check inspection on February 18, 1997. At the time of the incident, the airplane had 10,466 total flight hours.
A post-incident inspection on the airplane was conducted by Mesaba Airlines Maintenance at Detroit, Michigan, on February 20, 1997. The examination revealed that the left outboard roll spoiler was "deployed approximately half-way." The roll spoiler servoactuator (part number A44700-009) was removed and retained for further testing.
TESTS AND RESEARCH
An examination and disassembly of the roll spoiler servoactuator (serial number 1713) was conducted at Dowty Aerospace, Duarte, California on April 29, 1997. The roll spoiler servoactuator was manufactured by Dowty Aerospace in the first quarter of 1992.
Disassembly of the servoactuator revealed that the plug (part number A50993-1), which is fusion-welded into the base of the piston (part number A50992-1), had separated and lodged itself between the base of the piston and the shims, restricting the piston's movement.
Examination of the airplane's previous maintenance history revealed that the airplane had experienced a similar incident on April 4, 1995, near Moline, Illinois. During that incident, the airplane's left inboard roll spoiler servoactuator had jammed in an intermediate position. A post-incident examination and disassembly of the left inboard roll spoiler servoactuator revealed that the plug had separated from the piston assembly and restricted the piston from full travel (see CHI95IA116).
Dowty Aerospace had recorded seven previous failures of fusion welds in pistons on DHC-8 roll spoiler servoactuators prior to this incident. The first failure was reported by Horizon Air, Incorporated, in August, 1994, when a roll spoiler servoactuator malfunctioned during a check of the airplane's flight controls on the ground. The second failure occurred on N828MA on April 4, 1995. A third operational failure of a fusion weld occurred on a DHC-8 airplane operating out of Charleston, West Virginia, on January 22, 1997 (see CHI97IA075).
Between June 5, 1995, and January 20, 1997, eight additional roll spoiler servoactuators with the fusion welded piston design were returned to Dowty Aerospace by DHC-8 operators. Four of the eight units had failed welds around the piston plugs.
Dowty Aerospace sent the piston from the Horizon Air servoactuator to M. M. A. Laboratories, Huntington Beach, California, for failure analysis on September 1, 1994. Results of the analysis confirmed failure in the fusion weld.
Dowty Aerospace then conducted impulse testing on three randomly selected pistons from their stock. The pistons were subjected to 50,000 cycles at 0 to 3,000 pounds per square inch (psi) at rates of 0.5 seconds at 0 psi, to 2 seconds at 3,000 psi. A magnetic particle inspection following the testing revealed no cracks in the welds.
Following the second operational failure of a piston plug fusion weld, Dowty Aerospace issued a service letter to all DHC-8 operators informing them of the weld failures and requested that these operators monitor their fleets for similar problems. On March 31, 1995, Dowty Aerospace converted its manufacturing of their roll spoiler servoactuators to incorporate a new piston which did not require a welded plug. On May 31, 1995, Dowty Aerospace scrapped its stock of 41 pistons which had the fusion welded plug.
Parties to the investigation were the Federal Aviation Administration, the Transportation Safety Board of Canada, Dowty Aerospace Los Angeles, and Mesaba Airlines.