With the retirement of our much-loved author of Maintenance Tips, Dean Benedict, I’ve been given the honor of becoming a maintenance columnist for King Air magazine.
I’m humbled to follow in the footsteps of great authorities like Dean and Tom Clements, and I hope you’ll enjoy what I bring to the table.
A brief background
I cut my teeth in general aviation in 1973 as a right-seater on a Beech 18 and a general assistant in my father’s aircraft management/pilot services business. I went to A&P school for the 1976 calendar year and proudly took home my fresh A&P in January 1977, having won the Knox Family Foundation Award along the way.
I spent a four-year stint in the U.S. Air Force as I studied and then taught avionics. That made a nice addition to my skillset and my resume. Afterward, I returned to GA with a King Air E90 to help maintain and manage.
My career since has been one that’s made me a “Jack of all trades, master of none.” I’ve done time in all parts of several shops, including maintenance, avionics and serving as a chief inspector in three of them.
King Airs have always been a love and became a career focus in 2006 when I landed a job teaching them. I’ve taught classes at some of the well-known instructional organizations and conducted many on-site training classes. My personal achievements include the development of type-specific, hands-on engine rigging training that has been well received in the King Air community. I continue to instruct when I can.
With luck, I can provide both entertainment and useful technical information for you, the readers.
Since it just wouldn’t be right to try to walk in Dean’s “Maintenance Tips” shoes, without further nonsense, welcome to “Tech Tidbits.”
The Keith Temperature Control System
The venerable King Air has somehow stayed the course for many years and is still very much the same airframe that was first a hit in the 1960s. That’s not to say the aircraft hasn’t evolved, and what may be one of the most significant changes to the B200s and B300s came in 2006 when the environmental system was finally updated with a complete system mentality by what was then Hawker Beechcraft Corporation. I suspect most of the development was done under Raytheon’s ownership. This development was a joint effort between Beechcraft’s engineers and a team from what was then known as Keith Products. Those of us in the trenches at the time simply dubbed it the Keith
System, as opposed to the Legacy System that preceded it. Keith later became part of Air Comm Corporation, which later became part of Ace Thermal Systems. Maintainers will know Keith Products from Sikorsky, Citations, Bonanzas and many others.
What made the Keith version of the B200/B300 temp control was the approach to intelligently steering the hot/warm engine bleed air to where it could be most used or disposed of. In my opinion, the biggest contributor was that the system would route the bleed air to the back baggage deck anytime we were trying to cool the aircraft, allowing the air conditioning to truly recirculate and cool efficiently. The legacy aircraft always added the hot/warm P3 to the cabin, even when we are trying to cool it down. Yep, we need to pressurize, but where we bring in the bleed air can be an amazing change.
The system also became dual zone, allowing the cockpit and cabin to have separate temperature selections for comfort. No more robbing Peter to pay Paul when it came to warm feet in the winter.
Cooling the incoming bleed air using the bypass valves and the heat exchangers in the wing has always been at the core of temp control, but that’s never been cool enough for humans except when we’re trying to heat the cabin, and even then it’s usually reduced from the engine delivery temperature.
The Max Heating Mode and Max Cooling Mode diagrams will give you a general idea of what’s going on.
Some items of note are the six temperature sensors (including OAT) and four servo motors. The servo that controls the heat bypassing the cabin also controls the heat to the cabin floor outlets. They trade off. In AUTO, all the valves are handled by the system controller. That computer takes the temperature requests from the zones and turns that into servo positions and air conditioning on/off to achieve those temperatures.
When it comes to system troubleshooting, there’s good news and bad news. First, the good news: The system will try to communicate the failure of a temp sensor should it become what the controller considers out of range. If the crew reports the cockpit blower pulsating, that’s a sure sign that a temp sensor has failed or disconnected. The photo captioned “cockpit duct temp sensor on B300” shows a favorite problem area. It’s the temp sensor for the cockpit duct, and it’s just forward of the copilot’s rudder pedals. Apparently, it gets kicked and the connector comes loose.
Another piece of good news is the ability to hook up a PC for diagnostics. If you have doubts over which sensor may be faulty, running a terminal emulator program and plugging into the controller with a USB cable can be a super diagnostic aid. Even parked in the hangar, a notable difference between sensor temp indications can take you right to the troublemaker. In addition, the data screen will show (in Fahrenheit!) the cockpit and cabin temp knob positions as well as the travel positions of the four servos. In the controller data photo, the system is in cooling mode so there’s no Add Heat servos trying to add any warmth.
Of course, with good news, there’s always a bit of payback. If you haven’t yet had the experience of downloading, running and setting up a terminal emulator program on your PC or laptop, then you’re in for a bit of a learning curve. For whatever reason, Textron Aviation still hasn’t put any of the laptop troubleshooting information in the maintenance manual, so you’ll need to consult a couple of Model Communiqués.
Us old-timers may have some experience with com ports and baud rates, but if that’s Greek to you, be sure to check out Model Communiqué ME-TP-0014. That MC has detailed information regarding the necessary downloads and setup process for the PuTTY terminal emulator program. You can find that app directly in the Microsoft Store now (you Apple folks are on your own).
The next communiqué that’s a must-have is ME-TP-0017, which has descriptions for each of the readouts as well as troubleshooting hints.
Locating and changing servos can be challenging, although they are not items that fail very often. If you do have to change a servo, getting there will be the hard part. Once there, four bolts and a connector should have you on your way. One caution here: Be sure to lift gently as you remove the servo. If it’s stuck, you might consider additional access through a duct hole. The servos drive a “D” shaft that has a thin spot. If you break it, you’ll be diving inside the plenum to remove the butterfly valve and change the part.
One last downside to this system, which most air crews are familiar with. To allow the controller maximum flexibility over temp control, Beech incorporated an AUTO position on the ENVIR BLEED AIR – Normal/Low selection. That switch in AUTO allows the controller to decide whether to put the flow control in Normal or Low, but it only does so for heat demand purposes. There is no accounting for any engine ITT concerns. This auto operation has resulted in many reported cases of cabin pressure bumps during cruise flight. The auto controller is selecting back and forth in an effort to increase or reduce the bleed duct temp, and occasionally it can’t make up its mind. Many hours have been wasted when techs try to troubleshoot what seems to be a pressurization problem. I strongly suggest that flight crews revert to manual selection as required for takeoff and ITT considerations and leave AUTO bleed air selection out of your operation.
Until next time, keep them flying!