Avimo Mk 8D pitot/static heads, Bulldog
 

Wonder if any of you can help me with a little query. Tried to carry out a leak check of the pitot line on a Bulldog. Everything is perfect from the head pipes to the ASI. Problem is when I try and carry out a leak check of the system with the head connected. Massive leak from the pitot to the static in the head. I have been told that there is supposed to be a calibrated leak between the two (five knots in three minutes). However, this is just dumping the pressure. I can't even get a reading on either of my pitot/static instruments as the air just escapes too fast.

?????

camlobe

p.s. seem to remember that five knots in three minutes was the allowable leak rate of an aircraft system on test.

Pitot heads
 

The 'calibrated leak' is common on lots of old British aircraft. Same effect on Scout/Wasp helicopters, Chipmunks etc. You can carry out a system leak check if you have the correct long head adaptor that also covers the static slots in the side of the head so you apply static to both pitot and static simultaneously. Don't apply pitot or you'll drive the altimeter backwards against it's stops and risk damage. The ASI will be fine because it will see static inside the capsule as well as outside so it will be balanced. There will be no reading on the ASI, just on the altimeter.

Hope this helps.

This load of old rubbish is also used on the Islander where it causes no end of problems. The leak is an allowance not a right and 5 knots in three minutes would have no significant effect on instrument accuracy. The manufacturers stated life for the item is 900 hours. Few acheive even 100 hours as the heating/cooling cracks the internals. A recent new design is no better.