Last summer I supervised a gear overhaul (inspection) on a King Air F90 belonging to a client. We were coming down the home stretch. The motor, gear box and torque tubes were ready for reinstallation. The cabin had been gutted for ease of access and we were about to reinstall all the floorboards that had been removed for the inspection. On a whim, we decided to look around before closing up.
To my dismay, we found a mess of melted ducting. The under-floor heat duct going to the aft cabin and baggage compartment was totally melted and deformed (photos above). Worse yet, the tail deice supply line, which runs right on top of the heat ducting, was melted flat which would point to no air getting to the boots on the tail. That’s a problem.
Seen It Before
Believe it or not, 10 years ago I saw this exact scenario on another F90. It flew to my shop for same-day service on a couple squawks. I’d never seen this King Air before. For one of the squawks, I needed access under the floorboards and there they were – melted heat ducts. The deice line was also melted flat. I checked the duct temperature sensor and it ohm’d out correctly. I needed to troubleshoot this further, but this guy was in a huge hurry. He was hell bent on leaving that same day until I told him that a collapsed deice line was a no-go item. It had to be replaced. I got it ordered for the next day and he left the F90 in my shop overnight. The next morning, I received and installed the new deice line, put the ducts back in and off he went.
That wasn’t how I liked to do things, but he had Phase Inspections coming due very soon, and he scheduled to bring the F90 back to me in a couple of weeks. I knew I could replace the ducts at that time and become more familiar with this King Air. I looked forward to unraveling the mystery of those melted ducts. Unfortunately, I never got the chance. A week later the pilot called to tell me the gear collapsed on landing.
Allow me to digress briefly, because this is a perfect example of what happens when you skimp on a pre-buy inspection. That F90 was recently purchased. The pilot and owner had worked together for a while, both of them new to King Airs. The owner deferred to the pilot in all things aviation-related, and the pilot was extremely confident in his ability to assess and fly any aircraft. The pilot seemed eager to please the owner by saving him money wherever possible. From what I could see, the gear was due for a six-year inspection at the time of purchase. The shop doing the work did not work on King Airs regularly. After the gear collapse, I was engaged to investigate the cause and I found clear evidence the gear was not rigged properly. This would have been found during a proper pre-buy inspection. Enough said?
Fast forward to the F90 with the gear collapsing – here was my opportunity to solve the mystery of the melted heat ducts. We ohm’d out the duct temp sensor and it checked out good. We went a step further and applied heat to it, as called out in the maintenance manual. There are two circuits in this thermostat. The AB circuit gave a proper reading, but the CD circuit wasn’t reading what it should have been.
My client happened to have an E90 in the hangar, so we decided to test that temp sensor. We ohm’d it out and it checked perfectly, but when we applied heat, the CD circuit was out of whack. What were the chances of two bad temp sensors? That wasn’t what we hoped for.
This little sensor lists around $6,100 (that’s correct – six-thousand, one hundred dollars for a thermostat). He ordered it and got it the next day. We put the thermostat through its paces and it passed all tests with flying colors. Even when heat was applied, it met all the prescribed parameters. We installed it along with new heat ducts and a new tail deice line. To date, the new ducts and deice line appear to be holding up. He uses his borescope to check periodically. The jury is still out, though. He needs to fly more and keep monitoring it.
The big question is: What happens when the CD circuit goes high on resistance after 300°F is applied? The Maintenance Manual does not elaborate.
Tail Deice
My overriding concern is the deice line going to the tail. The only way to verify if the tail boots are working is to test them during a ground run and have someone stand outside to watch them inflate. If air is not getting to the tail boots, the gauge for deice boot pressure will still be reading the proper pressure. The pressure gauge cannot differentiate if the line going to the tail boots is blocked.
As we go forward, my client is monitoring his situation and if he finds anything I will let everyone know. Further, I would love to hear if anyone out there has seen melted heat ducts below the floorboards on any model of King Air. Please forward any and all details to me by phone or email. I’m very keen to know if this “mystery melt” is something we all should be digging into or if it’s an oddball occurrence.
Dean Benedict is a certified A&P, AI with over 45 years of maintaining King Airs. He’s the founder and former owner of Honest Air Inc., a maintenance shop that specialized in Beech aircraft with an emphasis on King Airs. Currently, with BeechMedic LLC, Dean consults with King Air owners, operators and maintenance shops on all things pertaining to King Air maintenance. This includes troubleshooting, pre-buys and maintenance management. He can be reached at dr.dean@beechmedic.com or (702) 524-4378.