Waypoints: Something's not right

After another 30 minutes the rain slacked off as the thunderstorm moved to the northeast. To the southwest, where I intended to go, I could see layers of clouds and improving visibility, even in the darkness. The radar app confirmed no more rain of significance between my location at Lancaster, Pennsylvania, and home at Frederick, Maryland.

I splashed across the ramp to my Bonanza A36 sitting next to Lancaster Avionics. The technicians earlier in the day had installed a Garmin Flight Stream 510 and updated software in various boxes, allowing me to update the GPS’s navigation database wirelessly and to share flight plans between the panel and my tablet computer—as well as display ADS-B traffic and weather from the panel on my tablet.

The line of thunderstorms had shown up just as technician Andrew Knowlton handed me the logbook entry. It was closing time for the shop as the thunder boomed outside. I dashed to the FBO to wait out the storm.

Afterward, I carefully preflighted the airplane, always especially suspicious after any work has been done. Lancaster Ground Control fed me the IFR clearance and I taxied out. A flash of red caught my eye. The airspeed indicator on the Garmin G500 had a red line next to the “airspeed” of 54 knots. Strangely, I glanced outside to make sure I wasn’t actually taxiing that fast. The L3 Trilogy backup display showed the same speed, within a knot or two. At the run-up area I tried running up the engine, somehow hoping the system would clear itself. No such luck.

Ground control cleared me back to the FBO, where I shut down and restarted, with no change in the displays. Knowing the avionics shop was closed I called the owner on his cellphone, wondering whether he could have someone come out the next day—a Saturday—to troubleshoot. It rang and rang, but finally someone answered. It was Knowlton, still in the office doing some paperwork. The owner’s cellphone had rolled over to the office number.

Back on the avionics shop’s ramp, Knowlton looked puzzled, assuring me that he had not touched the pitot-static system. He applied gentle air pressure through the pitot probe, sending the airspeed indications up, as expected. But when he stopped, the airspeeds settled back down to between 45 and 55 knots.

Fifteen minutes later he had removed a dozen or more screws from the cabin’s aft bulkhead to expose the empennage. Black flexible tubes feed in from the static ports on each side to a tee fitting that leads to the static port drain. I opened the drain, hoping to see some water emerge. Dry. Having opened the static system, I applied power to the panel again, hoping it had cleared the lines. No such luck.

Knowlton had me stand at the left static port with a tissue next to the port. He did the same on the right side while blowing high-pressure air out through the ports. Sure enough, a little mist of water showed up on the tissue. He tried again. No more water.

He closed off the line and I once again applied power to the panel. Success! Both airspeed indicators read zero. He applied gentle air pressure to the pitot probe, and both rose and descended as expected.

A few minutes later he had buttoned up the back and I was a taxiing out into the night, relieved but thoroughly suspicious of the airspeed indicators as I rolled down the dark runway. All was fine and I climbed into the night sky headed for home.

I’ve owned this 1972 Bonanza since 1999 and have flown behind the G500 and Trilogy for four years. Never in all those years with mechanical gauges, and now digital displays, have I seen any errant indications related to the pitot-static instruments. The airplane has flown through many heavy rain showers and sat outside for days on end during intense storms at EAA AirVenture, Sun ’n Fun, and on ramps all around the country. But apparently this particular day the airplane was angled in just such a way that the driving rain entered the static ports, hanging out just inside the ports and not making it to the drain. The blockage was just enough to fool the airspeed indicators into thinking they were going faster than they were.

One of the rewards of aviation is knowing that you never know it all.

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