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Alternate Static Preflight Check

Asked by: 2998 views Aircraft Systems, Commercial Pilot, General Aviation

G'day,

Why is it that when checking the alternate static source on an aircraft like a C172, a momentary change in indication is noticed in the altimeter and VSI but not the Airspeed indicator? I have so far assumed it's down to the sensitivity threshold of the instruments.

Additionally if this check is done when stationary on the ground then the static pressure shoud be the same both inside and outside the cockpit correct? why then is a momentary change noticed at all on any instrument when testing the alt static source?

 Cheers

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1 Answers



  1. KDS on Mar 05, 2020

    I cannot give you a good answer, but since your question has sat for so long without any answer, I will give you my mediocre answer.

    It has to do with the design of the valve in the Cessna. The opening and closing of the valve is what make the pressure change. If you do the same thing in a Piper, you should not see a change.

    Incidentally, one of my more memorable aviation experience is when I was flying Part 135 and one of my peers came all too close to killing himself. He took off in a Cherokee 6 into what he thought would be an ice-free flight that turned out to be otherwise. As the aircraft iced up, he tried not to frighten the passengers, but things kept getting worse and finally he decided to break the glass on the VSI because the static port was iced over. One of the lessons we all learned from that event was that breaking the VSI glass sounds simple, but it is not. He never did get the glass to break. Later, when he was telling me the story of his survival he said he had to do it because there was no alternate air in the Piper. I took him to the airplane and showed him how it was there, but just tucked away under the panel. Since that day I have found it is something that a lot of people who fly Pipers have never noticed.

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