faulty airspeed indication on a C337H

faulty airspeed indication on a C337H

Tech support

Our customer with a Cessna T337H with the horton stol conversion has been experiencing a problem with the indicated airspeed.   During cruise the airspeed indicator does not indicate more than 140 kts.  the aircraft owner has on a number of occasions flown along side another 337 and compared airspeeds to verify the indicated airspeed is faulty.   We have been trouble shooting the pitot and static system trying to pin point a fault.  The pitot and static system do not have leaks, the airspeed indicator works as it should and is accurate within a few kts when testing with a Barfield dps 400 pitot static test set.  We have switched during flight to alternate static source which made no difference.  We installed another airspeed indicator for the purpose of of one test flight and same result, won't go past past the 140kts mark.  The strut has not sustained any visual damage as far as we can tell and there is no obstruction at the two opening facing the airflow.  On the ground it all looks good in the air it is not.  Seems to be related to retracting the wheels (although we did not put out the gear at high speed to see what this would do) or the air flow over the strut is

We are fresh out of ideas and are looking for suggestions from yourselves on  where to go from here.  I have cc'd the aircraft owner/operator on this mail and have said that we will ask he bring the aircraft back to our offices when we have formulated a plan of action to resolve this problem.

Thank you
Regards

Rolfe R Brindeau

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Re: faulty airspeed indication on a C337H

Hi,

I had a similar problem a few years ago.  A little bug had built a partial nest in the pitot line.  As pitot pressure increased, it would block the line ( I can't prove that, but it is the only thing I could come up with at the time)  I did the same things you have done.... even blew the pitot line out with low pressure air... no luck.  Finally, I blasted it, with instruments disconnected of course.  A small chunk came out, and the problem went away. 

While not very scientific, I had run out of ideas.

good luck

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