Page 18 - October 2022

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to a post I saw today in an online forum by a King Air owner. He was looking for recommendations on where to send his pressurization controller for repair or overhaul. My good friend and colleague, Tom Clements (who also writes for this publication), gently suggested that the controller might not be the source of his pressurization problem.
So, let’s review.
The Ins and Outs of Pressurization
Imagine this scenario: You are nearing your destination airport so you adjust the pressurization controller for the field elevation where you will be landing; you begin your descent and pull the engines back; you notice the cabin rate of climb going up instead of down. This scenario is the most common symptom of a weak pressurization system. The reduction in engine RPM on descent puts less air into the cabin via the flow packs. If you have a weak flow pack, a high cabin leak rate or both, you will experience this scenario. Further, as the cabin altitude continues to rise during descent, the cabin will dump (depressurize) as soon it converges with the aircraft altitude.
Most pilots suspect that the pressurization controller or the outflow valve went bad. But in all my years of working on King Airs I found the most common culprits to be a weak or bad flow pack, an excessive cabin leak rate or both. It is a waste of time and money to change controllers or valves without thoroughly troubleshooting the pressurization system.
Inflow and Outflow
Properly functioning pressurization achieves a balance between the inflow of air pumped into the cabin by the flow packs and the outflow of air through the outflow valve. A weak or dead flow pack can’t pump enough air into the cabin. An excessive leak rate allows too much air out at one time. The trick is figuring out if your problem is inflow (packs), or outflow (leaks) or both.
Flow Packs
Most King Airs have flow packs, except for the straight 90, the A90 and the B90 which have a supercharger for pressurization. All others, from the C90 going forward, have flow packs for pressurization. Pneumatic flow packs are found on older King Airs and electronic packs are found on newer models. Pneumatic packs have filters and orifices that become clogged, thus weakening performance. Pneumatic packs can be cleaned and recalibrated or completely overhauled to restore functionality. I have a great vendor for this. I always get excellent results with pneumatic packs after overhaul.
16 • KING AIR MAGAZINE
OCTOBER 2022