Last week I helped one of our customers import an RV-7A. The airplane is a very nice example and sports an EFIS system and autopilot. My job was to conduct a two-year re-certification of the pitot and static systems.

On the surface, this appeared to be a simple enough task, so the altimeter, transponder and encoder were removed and sent to us for bench checking. When we were done on the bench, I talked to the owner to schedule the system leak check and discovered that there was also an EFIS system.

Since the system included a standby altimeter and airspeed indicator, two calibrations had to be carried out: one on the EFIS and one on the standby altimeter. CAR 625 Appendix C requires calibration of all altimetry devices if the aircraft is to be operated in Mode C airspace.

We started in on the leak check and it quickly became apparent that we had some major leaks. We started troubleshooting the system by clamping off the static line in various spots to try and isolate the problem. Eventually, we discover the problem in the TruTrak autopilot control head. It has a pitot and static port in the form of a plastic block on the back of the instrument. The block had cracked at both the pitot AND static ports giving us a massive leak.

We also found some homemade tees in the system made from copper pipe soldered together. The tees were connected to the Nylaflow lines with ¼” rubber hose that simply slide over the tees and Nylaflow lines. AN fittings were used on the back of the instruments and on the ADAHRS box, but the beads at the end of the fittings had been cut off to make the lines fit on easier. None of the fittings nor the tees provided a tight fit to the rubber hose. I also suspected that the homemade tees were leaking on the butt type solder joints. We decided that all the fittings and tees had to be replaced.

All of this started me thinking about the design of the static and pilot systems. The first real issue with this installation was the sheer complexity of the system. Starting at the dual static ports in the back, the system moves forward through a tee using three rubber hoses as connectors. The line travels forward to a proper tee mounted in the back of the autopilot with two rubber hose connectors. Next it goes to the airspeed indicator and altimeter; each with a homemade tee, cut-off AN fitting and three rubber hose connectors. From there it goes to one final homemade tee and three more hose connectors finishing up at the ADAHRS box and encoder through cut-off AN fittings.

Every one of those tees, rubber hose connectors and fittings has the potential for leakage. In addition, the devices themselves can leak. If you’re designing a new static system for your plane, try and keep the system as simple as possible.

In this system, the encoder is somewhat redundant as the EFIS system will talk directly with the transponder. It also makes the re-certification easier as the correlation between encoder and altimeter is moot as the data is coming from the same device. Using the EFIS to drive the autopilot would also reduce the number of devices. Finally, having a redundant altimeter and airspeed may be a bit of overkill for a VFR aircraft. Of course, this is up to the builder and his comfort level in the reliability of the EFIS.

Next, and somewhat obvious, is to use the right lines and fittings in your system. If you plan on using nylon lines, use the fittings available for them. Make your tee fittings right at the devices rather than in the lines. Most devices have a 1/8” pipe thread hole in the static and pitot ports. The nylon tees are available with one branch having a 1/8” pipe thread. This eliminates more connections reducing the potential leaks.

As for lines, Nylaflow lines are OK, but we find the best results using aluminum hard lines with AN fittings. The weight difference is negligible and the potential for leakage is far less. From a servicing point of view, it does require access to the back of the panel to release and cap lines while you’re troubleshooting issues. It also becomes a bit more difficult to remove instruments with hard lines attached. Also, not all of the encoders have the ability to use hard lines.

My final point is to add a service port to your static system. A simple tee with one branch capped off that’s easily accessible makes the job of static leak checks so much easier.

In summary:

• Keep the system simple.
• Eliminate standby instruments if practical.
• Use as few joints as possible
• Use the correct fittings and hoses
• If possible, use aluminum lines and fittings
• Add a service port.

With two-year re-certifications costing from $500- $1000, it makes a lot of sense to design your system to reduce this as much as possible.