Alternate Static Test

jwilbur · #1 09-30-2018, 06:28 PM

Question: If I'm flying at 3000 feet MSL, 120 KIAS, OAT ~ 55 F, fresh air vents in the cabin open, and I switch to my alternate static source (what you would do if the static ports got plugged with ice), what would you expect to see on the Altitude/IAS indicators? I tried this as a test today. I was expecting to see a jump of some sort but I saw nothing. Switched to cabin-air for about 3 seconds before switching back. I tried it twice. Would you assume there's a leak in my static system or is this an inconclusive test?

Bevan · #2 09-30-2018, 06:46 PM

Yes, I would suspect a leak in the static system. If it?s tight, you should see the altimeter jump/drop when activating the alternate static switch.

Bevan

Bavafa · #3 09-30-2018, 07:05 PM

Quote:

Originally Posted by Bevan

Yes, I would suspect a leak in the static system. If it?s tight, you should see the altimeter jump/drop when activating the alternate static switch.

Bevan

Agreed completely. Mine always did it to the point that I was afraid to damage my ASI due to sudden change in the pressure.

jwilbur · #4 09-30-2018, 07:23 PM

When I checked it during phase 1 it jumped as expected. But I didn't record the conditions at that time, just the very useful phrase, "checked alt-static. It works." I was just wondering if maybe there are conditions where there might be a less obvious jump.

I'll be looking for a leak.

Thanks.

Bavafa · #5 09-30-2018, 08:42 PM

For the leak, start at your alternate static port or other connections within the cabin. If there is no jump, good chances are that you are on your cabin pressure the entire time. I would suspect a change even with a leak if it is not within the cabin.

BobTurner · #6 09-30-2018, 10:09 PM

1. Try it again with air vents closed.
2. Watch your indicated airspeed for changes, valve open/closed.
3. In my airplane going to alternate static shows a very small change on the main EFIS - because the air-data computer is in the back, close to the outside static opening and far from the alternate opening. My backup EFIS has its static port close to the alternate valve, and it shows a bigger difference when opening the alternate.

F1R · #7 10-01-2018, 05:42 AM

Quote:

Originally Posted by jwilbur

Switched to cabin-air for about 3 seconds before switching back. I tried it twice.

Are you certain the alternate static valve is actually working? if the alternate valve is not opening, or if the internal line is blocked you would also see no change.

jwilbur · #8 10-01-2018, 09:48 AM

Quote:

Originally Posted by F1R

Are you certain the alternate static valve is actually working? if the alternate valve is not opening, or if the internal line is blocked you would also see no change.

Good point. I've been assuming a working valve. This is the easiest thing I can check and will do so first. Thanks very much for suggesting it.

jwilbur · #9 10-01-2018, 05:06 PM

Well,

I systematically tee-ed off each section of my static system. There are no leaks and the valve works fine. I thought maybe a tube hadn't been seated properly somewhere and I had fixed it in the process of checking it while unplugging and plugging things back in. .... I took it for a flight. Brought it to 6000 feet. I tried the valve with vents open and closed. The only thing I see is a quick little jump in the VSI (both main and backup screens). Definitely not as dramatic as I expected but it does appear to be working.

az_gila · #10 10-01-2018, 05:15 PM

One easy test on the entire system.

Pressurize, (or preferably suck

) your static system on the ground as if you were doing a leak test, say to a few 100 ft on the altimeter, with the alternate static switch closed. Use one of the static ports and seal off the other static port.

Open the switch and the altimeter should quickly return to measure local altitude.