Sunday, August 7, 2016

Testing Of Static/Pitot System & Transponder Completed

One of the checks that was performed earlier in the week was the testing of the Dynon 261 Mode S transponder and the static/pitot systems. The FAA requires transponder testing to take place every two years. Unfortunately, I forgot to bring the camera to the hangar and didn’t think about the camera in my phone, DUH … so don’t have any photos of the setup to share.

The testing is conducted by connecting a very sensitive “test box” to the static ports and pitot tube. The "test box" can simulate flying at various altitudes and airspeeds by varying the pressures on the static ports and pitot tube. The test box also has the capability of decoding the transponder's transmission by covering up the aircraft's transponder antenna with a cup that receives the transmitted signal while at the same time preventing the signal from radiating beyond the aircraft during testing. The goal is to make sure the transponder can transmit “squawk codes” along with reporting the aircraft’s altitude correctly to air traffic controllers.

Fellow RV-12 builders may want to take note of the following: The Dynon manual makes it very clear that when performing the static system/transponder testing, it is CRITICAL  that the SkyView be powered off when making any pressure changes at the “test box” …. Not doing so can damage the sensors in the ADAHRS unit. Because Primary Flight Displays like the Dynon SkyView D1000 are not commonplace yet, test shops are not familiar with this because they are mostly working with “steam gauges” in the typical Piper or Cessna … so don’t take it on blind faith that your test shop is up to speed. I asked the gentleman performing the testing if he had ever tested a Dynon SkyView D1000 system before and aware of the required testing procedure and was told “no”.... at that point I showed him the Dynon manual and had him read the paragraph that spelled out how the testing needs to be performed and why. I was told following the Dynon testing procedure would not be an issue for him, so we proceeded with the testing. Every time the pressure needed to be changed, we powered off the SkyView and when the pressure change was stabilized at “the box” we powered the SkyView back up.

When the testing first began, I was told there was a very large leak in the static system. The test connections at the static ports were checked, but they were OK, meaning the leak was someplace in the static system. Followers of the DOG Aviation Blog know I have run an additional static line from the tail cone up to the instrument panel for the backup instruments … so I thought that would be a good spot to begin troubleshooting the leak. I was praying the problem would not be in the tail cone so opened up the static line coming from the tail cone where it first appears in the instrument panel and capped off the line. This solved the leak issue … it was a HUGE relief knowing the problem was at the instrument panel where things are much easier to get to as opposed to crawling inside the tail cone for a repair.

At this point we added the backup instruments one at a time until the problem was identified to be coming from the vertical speed indicator (VSI). We hooked up the box directly to the VSI and it still failed so the issue was at the VSI itself. After a little inspection with a mirror I discovered the root of the problem. Me! Apparently, I over tightened the fitting on the VSI’s static port and cracked the pot metal casting. This was hard to spot but I noticed a small line on the casting and after scraping away some of the paint, one could see there was clearly a crack. We completed the testing with the static tube going to the backup VSI capped off.

We both learned something interesting regarding the SkyView 261 transponder that also may be of use to fellow RV-12 builders. The Dynon SkyView 261 transponder is a remote unit, so there are no switches or buttons on the unit … as such, all control is done through the SkyView D1000. When we began the actual testing with “the box”, I was asked to place the transponder in the Alt mode so it reports the altitude. Normally, there is a button for this on the transponder, but the Skyview uses function buttons to turn the transponder on and off. But there was no indication appearing for Alt … I was asked if there were any pressure switches installed which verify flight and I said “No”.  So we both came to the conclusion perhaps the SkyView needs to be fooled into thinking it is flying. So “the box” was set up so an airspeed of 90 knots was simulated and the display for the transponder now had a button for Alt. When Alt was selected, it allowed the transponder to begin transmitting the altitude along with the “squawk” code. Learn something every day when building an airplane.

Once the static leak was resolved and the idiosyncrasies of the Dynon Skyview sorted out, the DOG Aviation RV-12 passed with flying colors. I was told the transponder is reporting the altitude within 10 feet which is great! Also, the transponder was able to correctly transmit the various combinations of “squawk” codes that were required for the testing.

When the testing was completed, I immediately picked up the phone and called UMI and fortunately they had a new VSI in stock which they put it in the mail that day for a two day priority shipment. I received the new unit and installed it yesterday … this time I was much more careful when tightening the fitting for the static port. Below is a photo of the cracked casting for the static port on the VSI after some of the paint was removed in the area.
The source of the leak in the static system was traced to a crack in the static port on the UMI vertical speed indicator after flaking some of the paint away. Obviously, the fitting was over tightened by me and the pot metal split where the fitting screws in.