Greetings All,

For some time I have noticed during my takeoff roll my VSI and Altimeter begin to rise even before I break ground. Once airborne I do not notice any significant differences between indicated airspeed, altitude or rate of climb/descent vs my GPS indicated data. I just did the 24 month IFR check and there were no leaks. My fuselage mounted ports (2) are not the ones identified in the parts manual, but rather they are about a 1/2" - 5/8" dia rounded top (looks like a small mushroom head) that protrude about 1/8" at the highest point in the center of the port.

Can the geometry of the mushroom top create a low pressure over the port causing a slight vacuum creating the movement on my VSI and Altimeter? the technician performing the static test suggested getting either a rear mounted flush port or the flat 1 1/2" dia style port (available through Spruce) and try it to see if it clears up the situation.

Any thoughts, suggestions or has anyone seen this before? Does anyone know of a fix or is this expected on my Comanche?

Thanks in advance.

Paul

Paul,

Yes, I have seen this twice before on a Diamond and on a GlasAir III. Not qualified to give advice but did want to let you now it is not new news - at least to me.

Paul,
I know you said you had your static check done, but this is indicative of a leak all the way. Reading a climb while accelerating shows that the static system is being vented to the cabin. Also showing higher than actual airspeeds are indicative of a lowered static pressure. Do you have an alternate static valve installed on your system? Is it turned on? That is where I would start. Also, many times, no mater how good the shop, something can be knocked loose, or just not reattached after a static check, and that could also be your issue. It sounds as if you have the later model static ports (Button tops as opposed to the skin hole and shield) which is good. The skin holes always fill up with water and cause erratic indications to say the least! But no, your style static ports are not the culprit, venting to the cabin is.
-Zach

Paul,

I like Zach's answer better.

FYI. Even though the Twin has been under our family's care via licensed mechanics for all 44 years, I was surprised to see the hodge-podge, out of spec hoses and just flat old/decrepit hoses behind the panel between the static system and the instruments. I had them all replaced. This was done just prior to doing the same thing for the vacuum system hoses.

Hindsight is 20/20 - should have made this part of the panel renovation in 2007. While I was very pleased with the avionics shop I used at the time, they could have had more of my money if they'd suggested this infrastructure improvement.