With credits to Cloyd VanHook, BAC founder and first President:
My airspeed indicator suddenly seems to be reading much too low. What should I check?
With credits to Cloyd VanHook, BAC founder and first President:
I posted a few weeks ago asking if I had water in the Pitot-static system, or something more serious. I think I now have the problem solved, but for everyone’s info here’s the background, symptoms, etc.
About three or four weeks ago on the takeoff roll it seemed like I wasn’t gaining speed. Before I got up to indicated rotation speed, she flew herself off the ground. I was showing maybe 60 MPH or so, so I kept pushing the nose down, but she kept wanting to climb, at low indicated airspeed. I don’t remember the exact numbers but they were well below what I was used to seeing. I stayed in the pattern, intending to just go back and land. On downwind she seemed fine, engine-wise; just showing low airspeed. I decided the plane was OK, but that something was wrong with the airspeed indicator. I was bringing the plane home; it had been at another field 30 miles away, while they were working on the runways as home. Figured I may as well go ahead and fly home. When I got on the ground, 7′ MSL, the altimeter was showing about 200′.
I took the plane to the avionics shop to have the airspeed and a few other things worked on. The avionics shop said there were bugs (literally) in the Pitot system. He said he blew out the Pitot, and bug eggs came out of the Pitot tube. I do NOT have a heated Pitot. I have the old metal tubing, maybe 1/4 diameter, with a flapper on the end that flaps back and opens the air to the Pitot when you get moving. No removable cover on it. Brought the plane back and things seemed to be better.
About 3 weeks ago I took her up again, and saw the same symptoms. Showed low airspeed on the take off roll, but she wanted to fly. On climb-out the ASI was showing below the white arc, and dropping down to almost 0. On downwind it seemed to be reading OK at least closer to what it should. Seemed to be OK, perhaps even better, on final. Landed, decided to do it again; same thing. Went around the pattern, same indications; landed and parked her. Altimeter was showing about 100′.this time.
We pulled the belly panel, and broke the union at the low point on both the Pitot and static system lines. The A&P said that only one single drop of water came out of the static system. But at the union, the fitting has a fairly small inner diameter; small enough that a drop of water could plug it. The Pitot line was dry and clear.
There’s an AD back around 1965 (AD65-11-01) that required the tubing from both static ports (you know where they are, just ahead of the tail) to be run up to a tee near the top of the fuselage. This was to prevent water from getting into the static system. This AD has been complied with on my plane. The tubing must run uphill a foot or more, before it hits the tee and comes back down to the low point. I don’t know how water got in there, other than condensation.
SOOOOOOOOOO if you ever have a similar problem, take a look at the low point drain.