Page 23 - August 2015 Volume 9, Number 8

I am sure you have noticed also that to achieve a 500 fpm cabin climb and a 500 fpm cabin descent, it is the rare Controller in which the rate knob can remain in the same position! Invariably, a higher setting – maybe 1:00 o’clock – will be needed to get the climb but a lower setting – maybe 11:00 o’clock – will be needed for the descent. Sorry to increase your workload, but it is a fact of King Air life.
Have you noticed the little access panel in the upholstery on the right side of the baggage compartment down just above the floor? Know what’s inside? Well, looky there, it’s a petcock drain of some sort, just like the static line drains behind the right side upholstery in the cockpit.
But this is not a static line drain. Instead, it is a drain at the low spot of the line between the Controller in the cockpit and the Outflow Valve on the back wall of the baggage compartment. If it is inadvertently left open, then all of the carefully-regulated vacuum that the Controller is using to modulate the Outflow Valve gets overwhelmed by cabin pressure leaking into the line. The controller loses its ability to work the Outflow valve correctly. Although the most typical result is runaway ∆P to maximum – turn off the Flow Packs when you are ready to depressurize for landing – we have also heard of cases in which cabin altitude merely “stuck” at one value and could not be raised or lowered.
I will close with this reminder: Both the Outflow and the Safety valves have self-contained Maximum Differential Pressure and Negative Differential Pressure relief functions. It is almost impossible for these functions to fail so long as the valve is installed in the aft pressure bulkhead correctly. That is the reason this discussion of abnormalities has not mentioned what to do if you exceed maximum ∆P...it just won’t happen.
To repeat what I wrote at the start: The malfunctions I have reviewed here, although possible, are quite rare. But being unable to reach maximum ∆P because of lack of sufficient inflow and too much outflow – leaks – that’s where the bulk of pressurization problems lie. KA
About the Author: King Air expert Tom Clements has been flying and instructing in King Airs for over 43 years, and is the author of “The King Air Book.” 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 book, go to www.flightreview.net. 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 kblonigen@cox.net.
AUGUST 2015
KING AIR MAGAZINE • 21