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What is the orgin for the blocked static port rule-of-thumb

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Instrument Rating

Static instruments - when the normal static source is blocked and you use the alternate inside the cabin I have been told to use the 7/7 rule. The Airspeed indicator will read 7kts faster and the altimeter will read 70ft higher. Do you know how these numbers came about or any more information on it?

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1 Answers



  1. Matthew Waugh on Jul 30, 2010

    I’m going to go with you as the origin – although apparently you can trace it back at least one more generation.

    I have never heard that. I have always been told that when on alternate static the instruments will not be accurate. I seem to recall from my ground school training on one plane that the manual had specific values published, but since I wasn’t required to memorise them for any test I never knew them, as opposed to a lot of information that I have retained for 24 hours and then forgotten.

    I think your information is a lot more useful than “not accurate”, but if your information is not accurate then perhaps it is no more useful. If I had a blocked static port and my only option was to shoot an ILS to minimums where 70 ft. was going to make the difference then my goal would have shifted to having my accident on the airport, as close to the fire station as possible and consider any improvement on that a bonus.

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