A Door Unlock annunciator that illuminates in flight is a discomforting sight for any pilot, especially in a pressurized aircraft. Unless a second crew member or passenger can check to make sure the main cabin door is secure and locked, there is not much that can be done to remedy the problem except reduce speed, reduce cabin pressure by raising the cabin altitude setting on the pressurization controller, and land and investigate.
This scenario has happened to some pilots of JetProp Twin Commanders, who eventually discover that the problem is not an unsafe door, but rather an unreliable switch on the aft fuselage baggage door. In flight, air pressure across the fuselage tends to suck the baggage door away from the fuselage just enough to trip the switch that triggers the Door Unlock annunciator.
Once the reason for the annunciator has been identified and the pilot knows it is not due to a true unsafe condition, the tendency is to fly on. Of course, that introduces another, potentially serious problem: if the Door Unlock annunciator is continuously illuminated because of a faulty baggage door sensor, how will the pilot know if an actual unsafe condition arises with the main cabin door or baggage door?
Twin Commander Aircraft has developed a solution to the baggage door switch problem in the form of Custom Kit (CK) 194. The kit calls for removal of the sensor switch on the forward lower baggage door hinge, and replacement of the stop latch that secures the door in the closed position with a new stop latch and Door Unlock sensor switch.
Moving the sensor switch to the rear latch mechanism provides a better, more secure location with improved circuitry, according to Twin Commander. CK194 can be installed on 690D serial numbers 1500-1531, and 695As from serial number 96000 through 96061. The kit should take about six hours to install.
For more information about CK194 contact Twin Commander Aircraft or your authorized Twin Commander Service Center.