On occasion flying across the mid west in the summer, where from 4000ft the view across the prairie in any direction is such, you could easily be convinced the world is flat, I have encountered a similar phenomena in my 172. Flying straight and level, you find yourself descending, the only way to maintain altitude is to enter a climb, sometimes as much as 500ft
pr min. In fact there have been times where my climb has been so steep my IAS has been as low as 65kts, and only just maintaining altitude. A few minutes later, no problem maintaining altitude, and airspeed back to normal. This can happen 2 or 3 times during a 2 hour flight. I put it down to flying through sinking cooler air. Any thoughts?
Regards,
W.B.