AWB 31-3 Issue 1, 10 February 2005 Beech 200/300 Super King Air - Passenger Door Warning Switches
AWB 31-3 Issue 1, 10 February 2005
Beech 200/300 Super King Air - Passenger Door Warning Switches
All Beech King Air and Super King Air aircraft.
The objective of this Airworthiness Bulletin is to alert operators and maintainers to a potential defect within the passenger door warning sense system, which may result in the door warning system being rendered inactive and unable to provide a warning when the passenger door is unlocked or not fully closed.
On 7 September 2004, Beech Super King Air B300 S/No FL-194, was descending through 17,000 ft when the rear passenger door separated from the aircraft.
An inspection of the recovered passenger door was carried out to determine if there was a fault with the door locking mechanism or its warning system. No defect in the door locking mechanism was observed, however one switch in the door warning sense system was found to be intermittently earthing to the door structure, causing deactivation of the door warning system.
The door warning sense system comprises four switches connected in series to provide an earth path for the detector circuit located in the warning annunciator panel. One switch is located in the lower forward fuselage door opening, two are in the upper forward and aft section of the door and the fourth is located adjacent to the door locking mechanism. These switches are actuated by the physical movement of the door locking mechanism and are intended to confirm the correct locked position of the door.
With all latches engaged the four switches will be closed, providing an earth path allowing the warning light "DOOR UNLOCKED" to be extinguished. The annunciator panel test function confirms the integrity of the annunciator flash circuit and serviceability of the warning globes. It does not confirm the integrity of the earth path through the four switches.
The switches in the door are mounted via two screw/nut combinations on a mounting plate with a single adjusting slot machined into it. A screw/nut/washer combination through the adjustment slot and another screw/nut/washer combination as a pivot point secure the adjustable plate to the fixed mount. Figure 1 below shows the layout.
The design of this assembly allows the possibility that a terminal on the switch will foul the adjustment screw retaining nut and, in doing so, provide the earth path necessary to extinguish the door unlocked warning light. This effectively removes the door handle position switch from the sense circuit, i.e. the handle could be in any position (locked or unlocked) and it would not be detected. With the sense circuit partially disabled, the position of the door lock handle could not be detected.
The maintenance procedure for adjustment of the switches requires that "two attaching screws" are loosened and the switch is positioned to extinguish the warning light. The maintenance manual does not contain cautions regarding the possibility of contact between the switch terminal and the adjustment screw retaining nut.
Figure 1: Showing the location of the aft latch hook switch.
It is recommended that all operators examine all internal switches in the door for possible earthing of a switch terminal to the retaining nut. Inspection of the switches may be accomplished by removing the door trim in accordance with the procedure in the maintenance manual. Using a bright light and mirror if necessary, inspect all internal door switches for the interference condition described. Any interference or potential interference, which could cause earthing, should be reported to Raytheon or Beech before attempting adjustment. The manufacturer's advice must be sought to correct the interference or potential interference condition. Adjusting the gap incorrectly may result in the switches not being in the correct position to work correctly when the door is closed.
Report any defect found to CASA. Service Difficulty Reporting is available online.
Enquiries with regard to the content of Airworthiness Bulletins should be made via the direct link e-mail address included on the Airworthiness Bulletin web site, AirworthinessBulletin@casa.gov.au or in writing to Airworthiness Standards Branch, GPO Box 2005, Canberra, ACT, 2601