Maintenance Tip: The King Air Cabin Door

I have a customer that had a cabin door problem for two-and-a-half years. He kept receiving the cabin door warning light on the annunciator panel when he was in flight, but never on the ground. When the light came on, it was never at the same altitude and happened at various times throughout the flight – sometimes early, other times later. It was also intermittent, and didn’t show up on every flight. This issue was as variable and unpredictable as they come; hence, it was extremely difficult to troubleshoot. But, whenever I see a King Air squawking the cabin door light, nine times out of 10 it is a bad switch, so that’s where I start.

The Three-Switch System

First of all, I must specify that all King Airs have the same door – from the oldest A90 to the newest 350 – it is the same door and same door frame. The only change made throughout the years was beefing it up to accommodate greater pressurization demands as the King Air product line was improved and expanded.

Older King Airs have three switches pertaining to the cabin door. An actuated switch gives continuity and shuts off the cabin door light on the annunciator panel. When a switch is not properly actuated, it stays “open” (no continuity) and the warning light stays on. There are two switches on the door frame located at the lower forward latch bolt. Both are activated by a spring steel actuator. When the door is closed and latched, the bolts extend into the door frame and hold the door shut. When the lower forward latch bolt extends, it activates the two switches – one extinguishes the cabin door light on the annunciator panel and the other turns off the cabin door step lighting. The third switch is in the cabin door handle inspection light hole. Prior to departure, when you pull the door shut and put the handle in the locked position, you have to push the light button so you can see inside the inspection hole to get a visual that the handle mechanism is in the locked position. You can’t get a clear view of the switch in that hole, but it is there. If that switch does not activate, you’ll have a warning light on the panel. So, if you can see that the door is properly locked, but you have a cabin door warning light on, maybe the switch is bad and not activating when it should.

Typical Problems

The inspection hole switch is the first place I look when the cabin door light fails to turn off. The actuator on that switch can become bent due to over-travel, and a bent actuator prevents continuity, so the warning light stays on. Being over-zealous with the cabin door handle will cause over-travel and this will bend the actuator. Eventually the actuator will fail and the switch will need replacement.

Whenever a switch is changed, the pins must be checked for correct rigging in accordance with the maintenance manual. The switches in the door frame have a spring steel actuator tab and this tab has a relief hole where cracks can develop. Obviously, cracks will weaken the tab. If the tab breaks, it’s a major ordeal to replace it due to the location of the tab in the cabin door frame. The switches in the door frame can fail in other ways, but a broken tab is the bigger bummer.

The Five-Switch System

New King Airs and many 300 and 350 models have five switches in the cabin door warning system. They have the three switches already discussed, plus a switch at each upper door hook. The King Air with that maddening cabin door light squawk is a 300, and it has the five-switch system.

The first time the aircraft was brought in with this squawk, I went straight to the switch in the door handle inspection hole. Sure enough, that switch would not “ohm-out” properly – it gave really odd readings on the ohmmeter, and it never gave the same reading twice. When the cabin door is locked, a good switch should read zero, meaning it is activated, there is continuity, and no warning light showing up on the panel. But this switch, when the door was locked, was reading 50 or 60, then 120, then 300 ohms – anything but zero.

The inconsistency in the switch seemed to support the intermittent nature of the squawk, so we changed the switch and rigged it per the maintenance manual. It was good on ground running and I thought that was the end of it, but I was wrong!

The Boomerang Squawk – It Keeps Coming Back!

The next time I saw the particular 300, we checked the switches again and they were good. While checking the door frame switches, we looked at the cabin door locking bolts since we were in the neighborhood. There are four of them, two on the forward edge of the door and two on the aft edge. The door frame switches, you will recall, are at the lower forward door bolt. We found that bolt right on the ragged edge of being out of rig and not activating the door frame switches, so we rigged it per the maintenance manual. Again, this was one of those things that could create intermittent failure, so I thought we had it licked this time. Everything was good on the ground, but the squawk eventually came back with the same intermittent quirks as before. It didn’t happen every flight and it didn’t happen at the same altitude. Sometimes it would come on, go off, then come on again. Sometimes it flickered.

On the next shop visit we checked the switches again and found a bad one in the door frame! Here’s a switch that checked good the last time, but now it’s bad? Grrr! Airplanes can bedevil the best of us, and this one was giving me a run for my money! We put in a new door switch, it was good on the ground, but (you guessed it!) the squawk came back.

Peeling the Onion, Layer by Layer!

Troubleshooting is like peeling an onion – you can’t just dive in and start replacing stuff willy-nilly. You need thorough knowledge of the system you’re working on. You observe what is working correctly and what is not, and you go from there.

The next time the King Air 300 came in to the shop, we zeroed in on the upper door hook switches and found the aft door hook switch failing to make proper connection on an intermittent basis. Being as this squawk was so squirrelly, finding another intermittent switch gave me hope. We changed it, rigged it, and it worked perfectly on the ground. On the flight home, 20 minutes after reaching 28,000 feet, the darn light showed up again. This time it stayed on until landing, which had not happened before. I’ll be honest, I was crushed.

Meanwhile, the owner was furious and I didn’t blame him one bit. By this time we had replaced three switches; each one being verified bad at the time, but the squawk kept coming back. What to do?

An Atypical Problem

At that point, I was thinking it must be a mechanical problem, not a switch problem. The next time the owner was in our area with the aircraft, he stopped by and we tore that door apart – molding and panels were removed. It was disassembled to where I could see all five switches in operation. We closed and latched the door, fired the aircraft up, taxied out for ground running and pressurized it on the ground.

I deliberately did not use a huffer for this test. I wanted the vibration from the running engines in case that was a factor in the problem. I hoped to simulate flight as much as possible. After some time, the forward upper hook began to move ever-so-slightly away from its proper over-center position. Eventually the switch lost continuity and the light came on. Eureka! For the first time, I had a hard fail on the ground and I saw it with my own eyes!

We taxied back and shut down. I went straight to the rig holes for the upper hooks thinking they would be out of rig, but they were good. They were properly adjusted per the maintenance manual … hmmm. I saw some index markings on the mechanism for the upper hooks which got me curious. This aircraft had less than 5,000 cycles, so the side bolts hadn’t been replaced; the upper hook is to be replaced at 10,000 cycles, so it hadn’t been changed either. So why were there index markings at the upper hooks? It made no sense.

I called the pilot who had been flying this King Air for seven to eight years, but before he came along, the aircraft was in a hurricane down in Florida. Was it possible that this wasn’t the original door to the aircraft? Unfortunately, the logbooks had little to say about the cabin door. Regardless, although the rig holes checked out good, I delved into the upper door hook rigging. Back in the hangar, the cabin door was open and lying on a table for support. I locked the door handle and inserted my rig pin into the upper forward rig hole; it went right in. This was normal and exactly what I was hoping for. Next, I began to adjust the rod that holds the upper forward hook in its over-center position. I gradually lengthened the rod, which in turn increased the pressure on the hook. With each slight adjustment, I checked the rig pin to ensure I wasn’t throwing the rigging off. The pin still went in. So far, so good!

I had to make sure that these adjustments did not affect the door handle operation, so we closed the door and locked it several times. The handle operated normally and was not too stiff. Finally, we ran the aircraft on the ground exactly as before – a full power run-up with pressurization. The upper forward hook stayed put. (Thank God!) Even better, the boomerang squawk had not reared its ugly head since that time.

Epilogue

This was a very unusual problem. The pilot flew this 300 for at least five years before this squawk cropped up, seemingly out of the blue. Was it the hurricane from years ago? Did the fuselage get tweaked a little bit? I doubt we’ll ever know for sure. All I can say is that I found a combination of bad switches and a couple of rigging issues that came together to create the ultimate boomerang squawk.

I do know from experience that a King Air door can become slightly tweaked over time. This is more likely with single cable doors that have been subjected to heavy use (Note: adding a second door cable is easily done to any door with a kit). There is a good reason for that placard on the door that states only one person at a time be allowed on it, and I encourage you to be vigilant in making sure all passengers observe this. Additionally, don’t be over-forceful in locking the cabin door.

I wish you many squawk-free hours in your King Air.