Page 25 - August 2015 Volume 9, Number 8

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back, and turned to remain in the pattern. Now I had time to select alternate air – reaching over to the lever on the right side panel – and no change took place! You see, the open drains still permitted cabin air pressure to enter the instruments since the drains were downstream of the source. I figured that depressurizing the cabin would be my only solution and since I was only at pattern altitude I used the Dump switch instead of the more gentle method of turning off both left and right bleed air switches. Immediately, as the cabin ascended to aircraft altitude, all instruments returned to normal operation, allowing for an uneventful approach and landing. (Followed by a rather eventful meeting with the shop supervisor!)
Here is the part of this story that absolutely “blew my mind.” Before I dumped, the airspeed had dropped so much that it was actually reading below zero! A quick glance at it, without observing its previous slow demise, would have left an observer believing we were overspeeding, past VMO. I am quite happy that this whole event transpired in clear weather conditions and that I had not departed into low clouds.
It had never occurred to me previously just how little the actual difference is between pitot and static pressure that leads to our normal IAS indications. Merely having the static pressure about one thousand feet lower than it should have been led to more than a 150 KIAS difference!
Imagine what would happen if those drains were opened in normal cruise flight, while fully pressurized. Instead of simply sensing an incorrect static pressure from the beginning – as happened to me – now the system would instantaneously experience a huge pressure increase, with the “felt” altitude going from, say, 24,000 feet to 8,000 feet in a heartbeat. A colleague said this happened while he was flying a Convair 580 and not only were six instruments ruined, but it included having the glass faces of the VVIs ejected from the panel!
In conclusion then, two things: First, add one more pitot-static malfunction into your storage bin of facts: The effect of introducing pressurized cabin air into the static lines. Second, know your magic numbers. If and when your airspeed indication is malfunctioning, it certainly is comforting to have solid torque and airframe configuration combinations in mind that you know will yield safe and appropriate speeds. KA
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.
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.
24 • KING AIR MAGAZINE
SEPTEMBER 2015