Page 17 - October 2022

Pressurization Problems?
by Dean Benedict
Yesterday I got a call from a mechanic working on a pressurization problem with a King Air C90. The pilot’s squawk was that the altitude warning light came on while at cruise; the pilot descended. The mechanic did not know if the cabin dumped all at once, but the way he told it, it didn’t sound that way. (I know this mechanic; it probably didn’t occur to him to ask the pilot.)
By the time he called me, this mechanic had already changed the pressurization controller, the safety valve and the solenoid valve. In other words, he was just throwing parts at the problem – very expensive parts, I might add. This guy had squandered $10,000 to $15,000 of the customer’s money with no fix in sight. This infuriates me and it’s not my airplane!
Confusion
I am confused as to why so many pilots and mechanics go straight to the controller, and then the outflow and safety valves when the pressurization system goes awry. Why is this?
In 45-plus years working with King Airs, I have seen maybe six pressurization controllers that needed to be changed. Most of them had suffered external damage such as a lens getting broken after something was dropped on it or coffee got spilled all over. Pressurization controllers in King Airs rarely fail. The same holds true with the outflow and safety valves. These are at the bottom of my pressurization troubleshooting list.
Flow Packs & Cabin Leaks
The vast majority of pressurization problems (95% or more) stem from one or both flow packs becoming weak
and/or leaks in the cabin. Usually, it is a combination of both. You can carve that in stone.
The C90 this guy called me about is owned by an air ambulance operation and has over 9,000 hours. In my experience these medevac birds are run hard and put away wet – that’s horse talk for not properly cared for. The first place I would check is the ducting in the nose.
Crossover Duct
All King Air 90 models have a weak link: the pressurization air goes from the cabin to the nose; goes through the A/C evaporator; then goes through a crossover duct to the heater on the other side; and finally back into the cabin. After 9,000 hours these ducts get old and brittle, they split and leaks develop.
Other King Air models have a version of this crossover duct but it is plumbed differently. The 90 has the most problems with this duct, so if you’ve got a 90, put it at the top of the list when troubleshooting pressurization.
Pressurization Review
Twelve years ago, my “Maintenance Tip” for this magazine was on pressurization. It appeared in the September/October issue in 2010. Not only does it apply to the mechanic that phoned me yesterday, it also applies
OCTOBER 2022
KING AIR MAGAZINE • 15