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 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 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� 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.
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. Since the Beech 1900D is in service in other nations, as well as in the United States, it may be useful to note that duplicate inspection of controls typically is required before an aircraft can be returned to service.
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. The Federal Aviation Administration (FAA) followed with an Oct. 10 airworthiness directive (AD 2003-20-10), effective Oct. 15, calling for revisions to the maintenance manual and an enhanced operational check “every time” maintenance is done on the elevator trim system.
The case is a classic illustration of human factors in maintenance, and how unclear or insufficiently specific instructions, combined with a revered but reversed illustration, could lead to a flawed installation and the complete opposite airplane reaction to what pilots would expect from their trim control inputs. Murphy’s Law (“If something can go wrong, it will”) had deadly consequences. Another maxim applies, too. It is not only the primary flight controls that need to be checked as “full and free and moving in the correct direction,” but secondary controls (like trim) as well. Winding the trim all the way to the stops may have been the pilots’ natural response, but it may also prove to have been the curtain-closer.
Elevator Trim Tab Cable Installation
From the maintenance manual (extracts):
a. Position the cable in the slot in the cable drum … and install the cable lock pin. Refer to Figure 201.
b. The cable must be installed as shown in the illustration. From the pin wrap each cable 21/4 turns around the drum and tape the two cables together to prevent backlash from the drum (step ‘p’ calls for removal of the temporary tape).
c. Set the tab indicator at 0.
d. Assemble the cable guard over the drum, install the sprocket on the shaft and carefully position the shaft assembly in the pedestal, installing the lower end of the chain over the sprocket.
Source: Beech 1900D maintenance manual, Chapter 27-30-04
Flat Side Out – Not Keyed Side
Raytheon Aircraft Safety Communiqu� No. 234 (extracts)
To: All Beech Model 1900 airliner owners and operators,
Raytheon aviation centers, chief pilots, directors of operations, directors of maintenance, all Raytheon authorized service centers, and international distributors and dealers
Models: Raytheon Aircraft Beech Model q900, 1900C … and
1900D series
Subject: Elevator Trim Cable Installation
1900D Aircraft
In the 1900D AMM [aircraft maintenance manual], Chapter 27-30-04, Figure 201, the elevator trim tab control cable drum is depicted 180 degrees from the installed position. The flat side of the drum should be shown instead of the open, keyed side.
In order to assure correct operation of the elevator trim system, RAC [Raytheon Aircraft Corp.] recommends that the ELEVATOR TRIM OPERATIONAL CHECK be accomplished at the aircraft’s next maintenance opportunity and any subsequent time that maintenance is performed on the elevator trim system.
1900/1900C Aircraft
A similar illustration is shown in the 1900/1900C AMM, Chapter 27-10-00, Figure 9, with the same orientation issue on the elevator trim tab control system.
Both Model 1900D and 1900/1900C airliner maintenance manuals are being revised to clarify these illustrations.
Source: RAC
Duplicate Inspection of Controls
The requirement in New Zealand, where the Beech 1900D is operated (extracts):
A person shall not certify an aircraft … for release to service after the … disturbance, or adjustment of any part of an aircraft control system … unless a duplicate safety inspection has been performed that includes:
An inspection after maintenance by a person authorized … to certify the release to service of the control system, and
A second inspection by another person who … has adequate training, knowledge and experience to carry out the second inspection. Source: US dollars 43.113, Civil Aviation Rules, Part 43, Civil Aviation Authority of New Zealand, July 28, 2003