Incorrect Installation Contributed to Beech 1900D Crash

Air Safety Week, Oct 13, 2003

Reversed installation of pitch trim may have resulted in the deaths of two pilots, prompting a note to operators warning that the maintenance manual was incorrect and highlighting once again the potential for deadly error on the maintenance floor.

The case involves the Aug. 26 crash of a Raytheon Aircraft Corp. Beech 1900D twin turboprop near Yarmouth, Mass. Shortly before plummeting steeply into the water, the pilots reportedly radioed a pitch trim problem. The Colgan Air aircraft had just come out of maintenance and was on a 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, NC, having jusy come out of maintenance Jan. 6. While the investigation into that tragedy continues, the case has focused on the tensioning of the elevator control cables done during the airplane's D6 check, and the likelihood that the pilots of the overloaded and tail-heavy accident flight could not arrest the airplane's noseup 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 for nose-down than if the elevator had been properly rigged. That's 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 for perceived shortcomings in specificity, clarity and comprehensiveness. This was evidenced in the Air Midwest hearings by the metaphysical debate over cable "rigging" and cable "tensioning." (See ASW, May 26)

The two cases also raise questions about the rigor of required functional tests performed of flight control and trim systems following maintenance - and of flight test. The Colgan Air aircraft could have had a full load of 19 passengers on its accident flight.

Both pilots of Colgan Air Flight 9446 were killed when the airplane impacted the water about 3:30 p.m. local time. The pitch trim control system is located in the cockpit's center pedestal, and cables run through the airplane to the tab control in the elevator. The smashed-up pedestal was recovered from the water and shipped to National Transportation Safety Board (NTSB) headquarters in Washington, DC. The examining group 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.

Given that the illustration in the maintenance manual is reversed, the direction of cables' winding around the take-up drum could be wrong. 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."

Assume this concatenation of error occurred. Trim would be set opposite to what it should be for takeoff - say 10[bar] 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 electrically, they would be thinking uppermost about a trim runaway (as the 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, a-x, 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.