Monday, October 20, 2003
FAA Regulators Order Maintenance Changes on Beech 1900 Aircraft
The Federal Aviation Administration (FAA) has ordered maintenance changes for the twin-engine Beech 1900 turboprop, a popular regional aircraft that has been involved in two crashes this year that killed 23 people.
Operators of the Beech 1900, 1900C and 1900D, manufactured by Raytheon Aircraft Co. [NYSE: RTN], need to correct maintenance manual information on the plane's elevator system, located on the horizontal section of the tail to control aircraft pitch, according to the FAA's airworthiness directive (AD 2003-20-10).
An illustration accompanying the written instructions on elevator system repairs shows a key part installed backward, the FAA said. If the text alone were followed, the part likely would be installed properly. But following the illustration alone likely would result in a maintenance error. Regulators also are requiring an additional visual inspection each time certain elevator work is performed.
The case involves the Aug. 26 crash of a Beech 1900D twin turboprop near Yarmouth, Mass. (CRAN, Sept. 8,). Shortly before plummeting steeply into the water, the pilots radioed in a pitch trim problem. The Colgan Air aircraft had just come out of maintenance and was on a re-positioning flight from Barnstable Municipal Airport, Hyannis, Mass., to New York's Albany International Airport for return to revenue service. The crash was the second of a Beech 1900D in eight months and the second where compromised pitch control was involved.
An Air Midwest Beech 1900D crashed Jan. 8 at Charlotte, N.C., having just undergone maintenance Jan. 6 (CRAN, July 28). While the investigation into that crash continues, the case has focused on the tensioning of the elevator control cables performed during the airplane's Detail Six (D6) check, a routine maintenance interval check, and the likelihood that the pilots of the overloaded and tail-heavy accident flight could not arrest the airplane's nose-up pitch. Their inability to overcome the airplane's pitch-up after liftoff was due to limited elevator control movement -- estimated at some 30 percent less than if the elevator had been properly rigged. It is a different situation than the Colgan Air crash, which involved the pitch trim control, but in both cases the Beech 1900D maintenance manual has come under intense scrutiny.
"An incorrectly installed elevator trim system component, if not detected and corrected, could result in difficulties in controlling the airplane or a total loss of pitch control," FAA regulators said in their directive, which took effect Oct. 15. While the FAA order could be an indicator of what caused the Colgan Air crash, National Transportation Safety Board (NTSB) investigators will not formally determine that for months.
The pitch trim control system on the Beech 1900 is located in the cockpit's center pedestal, and cables run through the airplane to the tab control in the elevator. The pedestal from Colgan Air Flight 9446 was recovered from the water and shipped to NTSB headquarters in Washington, D.C. The inspectors traced the pitch trim cables coming from the pedestal, the drum on which they were wound, and their route through various pulleys. The drum, about four inches in diameter, fits easily in the palm of one's hand.
The two sides of the drum are different. One side is flat. The other is machined out, and is described as the "keyed" side. When properly installed, the flat side of the drum should be shown.
But the illustration in the maintenance manual is reversed, showing the direction of the cables' winding around the take-up drum. As one Beech 1900D maintenance expert said, "I guess you could wind the cables on the wrong way, but it might take some doing. You would have to reverse the direction of cable winding on the drum and swap the cable attach points over."
If this error occurred, the trim would be set opposite to what it should be for takeoff -- say 10 degrees nose-down -- and it would look correct on the trim wheel indicator in the cockpit. But once airborne, with a tendency to pitch-up, the situation would be compounded by the pilot's reaction, to oppose the limited out-of-trim situation. The result would be a confused worsening of the situation. If the pilots were trimming electronically, they would be thinking most about a trim runaway (as the Colgan accident pilots said on the radio) and opposing it on the trim thumbwheels. Unfortunately, their trained and instinctive input would run the trim even further in the wrong direction -- all the way to the stops. In the brief time available, neither pilot would have perceived the real cause.
Whether elevator authority alone would be sufficient to counter a worsening out-of-trim condition is under review by investigators.
The instructions in the Beech 1900D maintenance manual for elevator trim tab cable installation, which the Colgan Air technicians would have followed, contains a series of written steps and an accompanying illustration. The written instructions do not mention the orientation of the drum and do not caution that the flat side should face the technician. The illustration shows the keyed side facing out. One question sure to arise in the investigation is whether the drum should be designed for installation only in the proper orientation, in the manner of a key that can only be inserted one way into a lock.
The potential for incorrect installation is obvious, particularly since there is a natural human tendency to focus on an illustration for mental reinforcement that the work is being done correctly. In this case, the illustration leads down the path of error.
Of course, reversed pitch trim controls would be noted during a post-maintenance check of elevator trim operation. It is not known if this check was done prior to the accident flight.
The discovery that the drum could be installed backwards prompted Raytheon to issue a Safety Communiqu� to all its operators and customers, warning that the maintenance manual shows the drum "180 degrees from the installed position. The flat side of the drum should be shown instead of the open, keyed side," the communiqu� advised.
The message also urged operators to conduct an elevator trim operational check at the next maintenance opportunity, should any other reversed pitch trim controls be lurking in the fleet.
There are nearly 370 Beech 1900-series planes in the U.S. fleet, with fewer than half flown by regional airlines.