Welcome Guest. Sign in or Signup

1 Answers

Alternate Static Preflight Check

Asked by: 4306 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

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes


Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.