- Joined
- Sep 25, 2019
- Messages
- 828
- Location
- Lancaster, Pennsylvania
- Display Name
- Rick A
- Home Airfield
- KLNS
At the tail end of an extended trip that added up to around 30 flight hours, whereas the last leg of it included rain while parked outside, then soon flying VFR into very cold temperatures soon afterwards. The altitude hold function began a rhythmic cycle of climbing 60' above the selected altitude, then descending to a point 60' below the selected altitude, in a perfectly symmetrical and predictable wave pattern.
After living with it (and video taping the instruments for the radio shop) I disconnected and hand flew much of the last portion of the trip. But did reengaged at times with normal results. This occurred in relatively smooth air, so I don't believe it was turbulence induced. I disconnected the autopilot and reengaged for normal altitude hold, then it went back to normal ops before I initiated and hand flew the descent and landing.
Upon speaking with the radio shop and showing their experienced technician the video, he asked that I flight test and help point him in the right direction. Which I did, and after 20 minutes of altitude hold the wave action never occurred, so I could only answer what I believe to have noticed during the trip, and the flight conditions when it happened.
Conditions - OAT in the single digits. High barometric pressure (30.60 to 30.70). Trim was definitely stiffer than normal when spinning the wheel.
Their questions were:
Did the autotrim wheel move, and if yes, did it move in the appropriate direction of of altitude excursions? I did feel it move while the wave action was happening, but don't remember if it was moving in the proper directions (but assume it was).
Did I activate Alternate Static source? No. They asked for it to be done on the test flight when the excursions were occurring.
I recall the pressure sensor was located under the panel and hooked up with the static system right after the autopilot was installed in 2001. We had to relocate the sensor (still under the panel) then change the enroute antenna used from top to bottom due (#2 Nav com now on top) to an altitude jump when transmitting, which solved it (also solved the shadowing weak reception / transmitting issue when the theory was the wing was between the antenna and a distant ATC ground comm site).
The radio shop as a used altitude sensor for the 55X available if needed for the trouble shooting and purchase if necessary.
One theory floated by the shop was water may have entered the static system the previous night and morning, and froze during the flight northbound. But summarily discounted due to the VSI and Airspeed indicators displaying normally during the excursions.
At this point I believe there's not much I should do but fly it until it happens most/all of the time, and then work on the cause. Not ready for an hourly rate investigation and parts swapping fishing expedition (yet).
I asked if strong RFI from outside sources could be a possible cause? The excursions occurred in or around military areas, and who knows what the heck may be blasted around in the name of security. The worst examples of the excursions happed within close proximity to Washington DC. The tech discounted the RFI theory as possible, but not likely.
Any opinions on potential causes would be appreciated, and suggestions / recommendations of anything to try next.
After living with it (and video taping the instruments for the radio shop) I disconnected and hand flew much of the last portion of the trip. But did reengaged at times with normal results. This occurred in relatively smooth air, so I don't believe it was turbulence induced. I disconnected the autopilot and reengaged for normal altitude hold, then it went back to normal ops before I initiated and hand flew the descent and landing.
Upon speaking with the radio shop and showing their experienced technician the video, he asked that I flight test and help point him in the right direction. Which I did, and after 20 minutes of altitude hold the wave action never occurred, so I could only answer what I believe to have noticed during the trip, and the flight conditions when it happened.
Conditions - OAT in the single digits. High barometric pressure (30.60 to 30.70). Trim was definitely stiffer than normal when spinning the wheel.
Their questions were:
Did the autotrim wheel move, and if yes, did it move in the appropriate direction of of altitude excursions? I did feel it move while the wave action was happening, but don't remember if it was moving in the proper directions (but assume it was).
Did I activate Alternate Static source? No. They asked for it to be done on the test flight when the excursions were occurring.
I recall the pressure sensor was located under the panel and hooked up with the static system right after the autopilot was installed in 2001. We had to relocate the sensor (still under the panel) then change the enroute antenna used from top to bottom due (#2 Nav com now on top) to an altitude jump when transmitting, which solved it (also solved the shadowing weak reception / transmitting issue when the theory was the wing was between the antenna and a distant ATC ground comm site).
The radio shop as a used altitude sensor for the 55X available if needed for the trouble shooting and purchase if necessary.
One theory floated by the shop was water may have entered the static system the previous night and morning, and froze during the flight northbound. But summarily discounted due to the VSI and Airspeed indicators displaying normally during the excursions.
At this point I believe there's not much I should do but fly it until it happens most/all of the time, and then work on the cause. Not ready for an hourly rate investigation and parts swapping fishing expedition (yet).
I asked if strong RFI from outside sources could be a possible cause? The excursions occurred in or around military areas, and who knows what the heck may be blasted around in the name of security. The worst examples of the excursions happed within close proximity to Washington DC. The tech discounted the RFI theory as possible, but not likely.
Any opinions on potential causes would be appreciated, and suggestions / recommendations of anything to try next.
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