Altitude and speed differences on alternate static

[Rob Simpson]

Hi All,

Recently back in the air 337G S/N 1807 after some major works over a few years and now doing the shakedown flights. I do have an wing heavy issue that I put in a separate post, but also have a question on pitot static systems and indications.
All new pitot staic hoses installed everywhere during the rebuild including new static bowls and alternate static valve and all tested and no leaks anywhere, but on check flights, the aircraft seems slow, indication 132 knots at 2000 feet with 24/24 set.
Just as a trial, i pulled the alternate static valve and the altitude indication increased by 250 feet and the airspeed went to 141 knots indicated.

I am surprised at the difference in indications and am wondering if others know of differences in indications with alternate static on versus off, or has anyone got any clues as to why I may actually have a higher static pressure with the valve in normal position.

Any clues would be greatly appreciated.

regards

Rob

[mshac]

You clearly have a flow restriction in your regular static system that is relieved when you go to the alternate static system.

Pitot static is one of the simplest systems on an aircraft.

[CenterlineT]

It seems the static system senses more ambient pressure outside vs inside the cabin.
I don't know if you have two static ports or one. If you have one port then the heavy wing maybe the culprit (the static port faces the wind which it will compensate if there are two ports one on each side of the boom or fuselage, but not with one port) . For that you may want to check the rigging and also do a weight and balance to make sure your heavy wing is either weight related vs control surface vs wing or tail boom misalignment related. That may solve both your problems.

[mshac]

If the normal static port has a restriction somewhere in the system, it will reduce airflow. This will cause a lower indicated airspeed, and a lower indicated altitude. VSI would show a lower rate of climb/decent.

[Mich3773]

This was a good question about something I had forgotten, even though it was posted literally right in front of me. See the picture! I tested it in low level cruise. Airspeed dropped by ~10 kts, altitude increased by 150 ft.