Ivan Kristensen
Well Known Member
On a recent trip back to Canada from Florida (Dec. 16, 2013) I experienced some pretty serious A/S (airspeed) indication fluctuations. In fact, when I landed at my home field CYKF Kitchener Ontario my A/S was reading zero and had done so through 4000' and below.
A little background. A couple of months ago before I left for Florida I had a similar thing happen to me when landing here at my home field just not as severe, some fluctuations below normal air speeds on short final. This worried me some but the next time I flew the airplane the A/S seemed to function normally so I ignored it.
While in Florida a couple of friends of mine and I flew a trip into the Islands. My buddy flew a couple of the legs and noted that he thought the A/S indication acted strange but only below about 80kts. showing about 10kts low and fluctuating some. This confirmed what I had seen also on recent legs.
Now fast forward to our trip home. At 12000Ft. over Western Pennsylvania I was right in the top of a cloud deck and picking up a trace of ice. I asked for and was approved by Cleveland center for a climb to 13thousand and spun the Alt selector on the A/P to this altitude.
This is where it get really interesting. The airplane immediately commenced the climb, as it should. The trouble is that it steepened the climb progressively to the point where it would have stalled had I not caught it and disconnected the A/P and manually lowered the nose. Keep in mind that in my GRT EFIS I set the climb to an A/S of 120kts for an enroute climb (100kts for a missed approach)
My flying buddy mentioned earlier was sitting in the right seat for this event. He and I debated this scenario for some time and concluded that it had to be a completely blocked Pitot tube or line somewhere between the wing root and the Pitot mast. Reason for this conclusion of the location of the blockage was that all three of the EFIS screens (two GRT and one Dynon all with independent AHRS) all read the same thing.
Now we all know that with a blocked Pitot the A/S indicator works as an altimeter. Right!!!! Now if that is true then that would explain the steep A/P climb from 12000 to 13000ft. In the climb the airspeed would increase, the A/P would continue to steepen the climb in search for 120kts. until stall occurred.
Again, if this theory is accurate then on the descent the airspeed would decrease and that is exactly what happen. At 4000ft the A/S read zero and stayed there until landing. I was very cold on that day, 15F aloft and 20F on landing.
A working heated Pitot mast was confirmed and switched on during the flight. What we found later after closer investigation was water in the Aluminum Pitot line between the Pitot mast and the root of the wing. some low volume compressed air applied at the wing root produced a fair bit of water at the Pitot inlet. This water had obviously frozen solid and trapped air-pressure inboard giving erroneous airspeed indications.
I have since test flown the airplane and all is working perfectly again. Has anyone experienced a similar problem?
A little background. A couple of months ago before I left for Florida I had a similar thing happen to me when landing here at my home field just not as severe, some fluctuations below normal air speeds on short final. This worried me some but the next time I flew the airplane the A/S seemed to function normally so I ignored it.
While in Florida a couple of friends of mine and I flew a trip into the Islands. My buddy flew a couple of the legs and noted that he thought the A/S indication acted strange but only below about 80kts. showing about 10kts low and fluctuating some. This confirmed what I had seen also on recent legs.
Now fast forward to our trip home. At 12000Ft. over Western Pennsylvania I was right in the top of a cloud deck and picking up a trace of ice. I asked for and was approved by Cleveland center for a climb to 13thousand and spun the Alt selector on the A/P to this altitude.
This is where it get really interesting. The airplane immediately commenced the climb, as it should. The trouble is that it steepened the climb progressively to the point where it would have stalled had I not caught it and disconnected the A/P and manually lowered the nose. Keep in mind that in my GRT EFIS I set the climb to an A/S of 120kts for an enroute climb (100kts for a missed approach)
My flying buddy mentioned earlier was sitting in the right seat for this event. He and I debated this scenario for some time and concluded that it had to be a completely blocked Pitot tube or line somewhere between the wing root and the Pitot mast. Reason for this conclusion of the location of the blockage was that all three of the EFIS screens (two GRT and one Dynon all with independent AHRS) all read the same thing.
Now we all know that with a blocked Pitot the A/S indicator works as an altimeter. Right!!!! Now if that is true then that would explain the steep A/P climb from 12000 to 13000ft. In the climb the airspeed would increase, the A/P would continue to steepen the climb in search for 120kts. until stall occurred.
Again, if this theory is accurate then on the descent the airspeed would decrease and that is exactly what happen. At 4000ft the A/S read zero and stayed there until landing. I was very cold on that day, 15F aloft and 20F on landing.
A working heated Pitot mast was confirmed and switched on during the flight. What we found later after closer investigation was water in the Aluminum Pitot line between the Pitot mast and the root of the wing. some low volume compressed air applied at the wing root produced a fair bit of water at the Pitot inlet. This water had obviously frozen solid and trapped air-pressure inboard giving erroneous airspeed indications.
I have since test flown the airplane and all is working perfectly again. Has anyone experienced a similar problem?
I was just happy to find an avionics shop on the field and willing to do it.
