IFR season is with us and being able to hear ATC under all weather conditions is paramount. In the past we’ve talked about P-Stat build up and how to get rid of it by properly installing static wicks and bonding straps. In most cases wicks and bonding straps do cure P-Stat build up but there is another area on piston Cessna aircraft that is prone to P-Stat build up, especially the 210 and 177 series aircraft. Just what will the pilot notice if the aircraft picks up P-Stat you ask? Well, normally you’ll start hearing a noise in the headset that sounds like frying eggs and the noise will grow in amplitude to the point that you no longer can hear ATC over the aircraft radios. Usually the situation only last for 5-10 minutes but rest assured that’s a long time when you are in IMC and can’t talk to anyone. Chances are you can still transmit but will not be able to receive due to P-Stat build up; this makes for a bad day.

Here’s the problem and how we found it. Jerry Blank, a friend of mine, often flew his T210N in heavy rain and during those flights he would hear a noise in the headset that sounded like frying eggs. The noise would become so great that he no longer could hear ATC until he flew out of the heavy moisture. Avionics West installed static wicks, bonding straps and other bits of magic trying to cure the P-Stat build up in heavy rain but every time Jerry would enter the clouds, the frying egg noise would again return to the headset. One day, while flying in the rain the P-Stat noise again took over the audio in headsets, thus Jerry couldn’t hear anything but the loud noise. For some reason he touched the OAT which is located in the plastic windscreen. At that moment, he received a good shock but the noise in the headset went away. A minute or so later the noise started building in the headset; Jerry touched the OAT gage and sure enough, the noise disappeared. Jerry discovered that anytime the static noise was present in the headset all he had to do was touch the OAT and presto, things got quiet again! Only problem was Jerry’s eyes would roll back in his head every time he touched the OAT due to the high voltage discharge. Being the smart pilot he is, Jerry discovered if he ran a wire from the OAT to a screw on the panel, the noise never occurred and he didn’t have to put up with the electrical shock.

As an aircraft flies through the clouds the plastic windscreen picks up a charge in the cloud. This charge could be as high as 200K volts. While the voltage is high, the current available is low, thus the shock (should you touch the OAT) only hurts for a short period of time. The windshield "floats" in the channel that it lays in and is susceptible to picking up a heavy charge as the aircraft flies through clouds due to the fact the windscreen isn’t grounded to the airframe and the windscreen is plastic. When the static charge is large enough, you’ll start hearing a noise in your headset. Actually windscreen static build up has gone on for decades and Cessna has a fix for it. Service Bulletin SE 77-7 addresses the problem. The service kit basically contains a wire that attaches to the OAT, runs along the OAT light tube and the other end attaches to the top, inside of the cabin. Any P-Stat build up on the windscreen will puddle around the metal OAT and is bled off to ground via the wire that is attached to airframe ground.

We installed kit SE 77-7 in Jerry’s T210 and for a decade he flew through all kinds of weather without a problem. Recently Jerry stated picking up static in the headset while flying in the rain. We checked the static wicks and bonding straps and they were in excellent condition. Even though the wire was still attached from the OAT to the airframe ground, Tom Knoll decided to replace the wire and discovered the terminal attached to the cabin end of the wire had corrosion underneath the terminal lug. Tom replaced the wire and again, the static noise problem disappeared.

Note the pictures; you’ll see the corrosion on the terminal lug. Often during the wet season our shop receives several calls about static build up. We recommend checking the wicks and bonding straps first but if those items check fine, we recommend installing the kit mentioned above. If you can’t find the SE 77-7, your A&P/avionics shop can simply run a wire from the OAT to the airframe and enter the event in the aircraft records. The fix for P-Stat induced via the windscreen is simple and cheap to fix. I’ve never found it to be the case but if an aircraft had just a wet compass attached to the windscreen there would be the possibility of the same problem occurring while flying in and around clouds.