Topic-icon ATA 34-11, Airspeed Reading High

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2 months 2 weeks ago - 2 months 1 week ago #731 by Mark D Francis
Crew report of the Standby Airspeed indication reading high.

As discussed here , cabin pressure leaks into a static system will cause an altimeter to read low in flight. When there is a leak on the pitot side, airspeeds will indicate high.

For some reason..... Boeing runs two alternate static lines and two pitot sense lines from the front of the aircraft to a Elevator Feel Actuator (limits elevator movement at high speeds) behind the aft pressure dome. We isolated our leak to one of the pitot lines running to the back of the aircraft.

We had to do some serious gutting of the aircraft to expose the lines running towards the back. By process of elimination (disconnecting and plugging the line at different points) we found this in the left sidewall about 10 feet aft of the main cargo door.



This is 3/8" OD tubing. That hole was so big, leak detector didn't bubble at all.

Actual isolation involved applying pressure into the system with a pitot/static test box. We used 300 Knts as a test speed. Of the sections we had connected, we knew we still had the leak in our line just by watching how fast the tester was able to reach 300. If it was slow to increase.... we still had our leak. Removing test pressure caused the test box indication to go from 300 to 0 in about two seconds.
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Last edit: 2 months 1 week ago by Mark D Francis.

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