I mentioned in another thread (or two) that I was seeing about 8 knots more True Airspeed than I usually do on my recent trip to Minneapolis, given the same power settings that I usually use. I knew it wasn't right, but it really didn't bother me on th trip - the airplane flies fine, regardless of the TAS shown on the gauges! I did a little troubleshooting today, and tracked down the culprit - only took three hops to have it figured out and fixed. Dang, forced to go flying to fix it....
Call it a "Successful Test" of the alternate static air source!
The night before I left for the North, I decided to install a new Aux battery for my EFIS. The old one had seen a couple of deep discharge cycles during pre-flight testing, and was getting tired. I located this ahead of my EFIS MDU's, and right on top of the AHRS, so getting to it is a matter of four screws and two minutes. However, to make the job easy, I disconnected the pitot and static lines into the AHRS for access.
Of course, when I noticed the higher that normal airspeed, my mind went first to those two disconnects - did I get them on tight? They were the first thing I checked, and they looked fine. So I traced both lines to the other instruments, and voil?! I found an open end that had pulled out of it's ferrule at a "T" on the backup ASI. This line had gotten caught in a bind by a cable, and must have been a poor connection in the first place. It took nothing at all to make a new section of line, test it for a solid connection, and do a quick leak-down test on the whole system. Problem solved!
The net affect was that the static system was open to the cabin. (Don't worry guys - as soon as I noticed the problem, I knew I wasn't filing IFR for the trip - that's why I was waiting for good weather!) The affect on the airspeed was identical on the EFIS and backup ASI - they read about 8 knots higher than normal, and the stall came about 10 knots high as well (that's how I confirmed I really had a problem - did a little stall work to see what it was indicating). The affect on the altitude was an error of about 200 feet - the GRT EFIS shows the delta between GPS and pressure-generated altitude when you are dialing in an altimeter setting, and I noticed on the trip home that every time I adjusted the baro, I was seeing about a "+200" indication between GPS altitude and barometric. I didn't, however, pay enough attention to see if that means I was indicating high or low, and I really wish I knew that!
So that's the story. All fixed. Don't be surprised if you use an alternate static source to see higher than normal airspeeds!
Paul
Call it a "Successful Test" of the alternate static air source!
The night before I left for the North, I decided to install a new Aux battery for my EFIS. The old one had seen a couple of deep discharge cycles during pre-flight testing, and was getting tired. I located this ahead of my EFIS MDU's, and right on top of the AHRS, so getting to it is a matter of four screws and two minutes. However, to make the job easy, I disconnected the pitot and static lines into the AHRS for access.
Of course, when I noticed the higher that normal airspeed, my mind went first to those two disconnects - did I get them on tight? They were the first thing I checked, and they looked fine. So I traced both lines to the other instruments, and voil?! I found an open end that had pulled out of it's ferrule at a "T" on the backup ASI. This line had gotten caught in a bind by a cable, and must have been a poor connection in the first place. It took nothing at all to make a new section of line, test it for a solid connection, and do a quick leak-down test on the whole system. Problem solved!
The net affect was that the static system was open to the cabin. (Don't worry guys - as soon as I noticed the problem, I knew I wasn't filing IFR for the trip - that's why I was waiting for good weather!) The affect on the airspeed was identical on the EFIS and backup ASI - they read about 8 knots higher than normal, and the stall came about 10 knots high as well (that's how I confirmed I really had a problem - did a little stall work to see what it was indicating). The affect on the altitude was an error of about 200 feet - the GRT EFIS shows the delta between GPS and pressure-generated altitude when you are dialing in an altimeter setting, and I noticed on the trip home that every time I adjusted the baro, I was seeing about a "+200" indication between GPS altitude and barometric. I didn't, however, pay enough attention to see if that means I was indicating high or low, and I really wish I knew that!
So that's the story. All fixed. Don't be surprised if you use an alternate static source to see higher than normal airspeeds!
Paul
