[[email protected]]

The Southco autopsy photos. View at your discretion. Yes......the guts are hanging out.

So, as a refresher, This latch failed when I opened the fuel door and the handle remained "pulled out". I ran to shut off the bay door power, but didn't get there fast enough. I then spent time spraying that door handle until it worked freely.
Subsequently, the mechanical release mechanism failed on this latch. The mechanical release on the other latch still worked.

Image #1 shows the latch with its cover removed. The latch has 3 main areas. The left portion is the mechanical latch with release capability and microswitch (more on this later) The center portion is the linear gear mechanism. The right portion is the motor portion. In this image, the release arm is clearly disconnected from its linkage to the latch. The latch linkage is a "U" shaped slot in the mechanical release lever of the latch. The microswitch on the latch is maintained  active when the latch is open. It has NO and NC wires available external to the whole assembly. The center linear gear mechanism is stuck in the half activated position. When the latch assembly is not energized, the spring would normally push the gear mechanism such that it would move all the way right, spinning the motor in reverse, in the process. The motor is coupled to the linear gear by a "bow tie" shaped coupling. The motor itself appears unremarkable in this photo, except that the rightmost margin area appears sooty and the adjacent nylon appears burned. 
Normally at rest while latched, the release arm is located in the slot of the latch release lever, the linear gear body is pushed all the way to the right, and the motor is at rest with no impressed voltage. When the latch is activated into the electrical release case,  a short 12VDC pulse is placed on the latch. This causes the motor to spin, moving the gear to the left, impacting the release lever, thereby causing the lever to rotate open and force the strike from its jaw. When the latch is activated into the mechanical release case, the gear and motor are irrelevant because the release arm pulls directly on the release lever, thereby directly causing the latch to open. Upon opening the latch assembly, the release arm was decoupled from the release lever, eliminating that method of release. The motor had also failed, eliminating any hope of that function.

Image #2 shows a closer view of the release arm and the slot in the release lever. It appears that the release arm is quite a bit thinner than the width of the release arm slot. It also appears that there is sufficient space between the top of the release lever slot and the adjacent wall of the latch assembly. This was probably the way that the release arm became uncoupled.

Image #3 shows the seized motor and the gear/spring not being able to push back.

Image #4 shows the melted right side of the motor. The rotor shaft uses the plastic housing as a bearing. Once gone, the motor does not move.

Image #5 shows the paddle/ "bowtie" used to couple the motor to the linear gear.

Image #6 shows the rating plate. Clearly, this device needs some sort of management of the voltage applied to it. It cannot be just connected to the bay handle switch. A quick flick of the handle will probably cause at least a quarter second of voltage to be applied. That is 5 times rating. This is why I say some sort of "one shot" circuit needs to be used. A "one shot" is used to control the length of the pulse applied, as well as to control the time to re-trigger. Additionally, the striker should be able to be removed directly from the bottom of a wedged-open bay door. In this way the latch would not be over stressed, electrically, and a way to overcome complete failure would be achieved. The microswitch is not rated, which means its not going to withstand use at a 5 amp rating, as the motor draws. If it was capable, the microswitch could have been used to cut off power while the latch was open.

Bob appreciate you tearing into this and analyzing the failure. This is how I learned how stuff works over the years, by just taking things apart. So if I understand what you are saying, the microswitch is not being used? I would take it that the microswitch is depressed when the latch is opened or closed? You could use the micro switch, would need to just run it to a relay to handle the load. I would assume that is why Southco put that microswitch in there, so that it could be used to stop the motor. Would be curious to see if one could find the installation information for those latches, I bet they have a suggested wiring diagram on how they should be used. 

As Richard said, great that you are contributing this information. I am sure it will help someone else some day.