low IAS

[John Kerr(Guest)]

First flight yesterday on RV9 was made a little more interesting by a low Indicated Air Speed. Upon landing, a pressure (low) showed that the pitot system will not hold pressure. A check of the connections did not reveal any leaks. Where else in the system could ram pressure be escaping? :-/

 

[dynonsupport]

John,
It is possible that there is a leak inside the EFIS. Not something we have ever really seen since it will not pass calibration if it is there, but it is theoretically possible. Neither our static or pitot inputs (or AOA for that matter) should flow any air. If you put 3 PSI on them, they should hold it indefinatley. If we can hold the pressure at the A/S/P ports, then the leak is not in the EFIS.

One thing to check- are you sure that you have your ports connected in the right places? If you hooked Static to your AOA port, the altitude and airspeed would still change, but they wouldn't be accurate since the static would be derived from the cockpit.

 

[John Kerr(Guest)]

Checked everything from the instuments back to the pitot and the leak is from the holes drilled in the bottom of the pitot tube back by the mounting screw. What now? :-/

 

[John Kerr(Guest)]

Just to be sure, tell me the orientation of each of the ports so that I can verify correct connections without pulling my panel. ???

 

[dynonsupport]

Looking from behind, left to right:

AOA PITOT STATIC

The "leaks" in the pitot are drain holes and almost all pitots have these. They are calibrated into the pitot's response, and are not a cause for concern. The only real way to check for leaks is to disconnect the pitot and pressurize the actual line going from the pitot into the aircraft.

 

[John Kerr(Guest)]

It appears in talking with Mike at Tech Support that my pitot might be mis-calibrated. What would be the effect of a piece of "200 mph" duct tape over one of the holes. Indicated Air Speed would rise, yes? What if the tape had a hole smaller than the one in the pitot tube? IAS would rise but not as much, True? Does information exist showing the resulting rate of change with calibration of this type?

John Kerr

 

[dynonsupport]

There is a good amount of engineering that ensures that the pitot probe measures airspeed accurately. If there is something wrong with your pitot, as it sounds like there may be, there is not much you can do in the field to fix it.

If you are thinking of taping the "leak holes" on the back of the pitot, know that they both feed to a common cavity and are not directly connected to the pitot and AOA tubes. Plugging those holes will likely not fix your airspeed problem.

Similarly , partially taping the pitot and AOA themselves would cause all kinds of strange effects. We would not reccomend this.

The replacement pitot that we've previously arranged to send your way should arrive today (Monday, Dec 19). Let us know how it works.