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leaks. So how do we complete this test and determine how badly your airplane leaks?
We start by turning back on the right pack and giving plenty of time for things to return to normal, at the maximum attainable ∆P. Now we turn both bleed air switches off simultaneously. (If you have three-position switches – as all of the 200- and 300-series do – make sure you go only to the center, not bottom position. You don’t want to lose the inflation pressure for the door seal.) Observe the peak on the cabin VVI.
If it is less than 2,500 fpm, then you have an airplane that meets Beech’s specifications. Congratulations! Sadly, a leak rate this low is exceedingly rare to find. You have a one-in-a-thousand, exceedingly tight, airplane. More typically, you will see a leak rate of 3,500 to 5,000 fpm. Realize this, too: If the combination of weak inflow and excessive outflow prevents your airplane from attaining the proper maximum ∆P of 6.4-6.6 psid, then this check will not be valid since you have not attained the “push” that would exist if you could get to the proper maximum ∆P. To better explain: If you can only get 5.0 psid maximum and the peak leak rate at that ∆P is 4,000 fpm, perhaps it would be 5,500 fpm at 6.5 psid.
My personal criteria for deciding that a King Air’s pressurization system is satisfactory looks at two things:
First, can either side’s flow pack alone maintain full ∆P when at cruise power? Second, can I pull both power levers back to the gear horn’s setting, with both flow packs operating, and not have the cabin start to climb? If both of these are true, then I see no reason to spend money and time on overhauling flow packs and/or finding and sealing cabin leaks.
I hope this presentation of basic rules of pressurization will help you better understand your system and troubleshoot problems when they arise.
Watch a complete tutorial on how to perform flow pack, leak rates and altitude checks in your King Air at https://www.youtube.com/watch?v=WAzgeLnfa1I KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 50 years and is the author of “The King Air Book” and “The King Air Book II.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his books, contact Tom direct at twcaz@msn.com. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at editor@blonigen.net.
MARCH 2024
KING AIR MAGAZINE • 23