I just had a similar experience with pitot system, but this was not because I'd forgotten the cover. I was down having a look at an airplane I might want to buy and we were going to fly it back to where I'd left my own aircraft for repairs. This is a pretty complex twin, but thankfully one of the instructors that checks people out in this type was with me for the whole 4hrs flight. As we pop into clouds on the IFR clearance, I can see that the VSI, Altimeter and speed don't seem to behave normally. We switch to alternate air, and it gets a little better, but still very erratic. Either big jumps, or frozen needles etc and not much in between. Turns out we have some water in the pitot static system and it needs to be drained, but the experienced instructor decides to continue our flight as we were now on top and forecast was good for destination. This is a high speed stalling airplane, so had I been alone and new to the aircraft, it would have been a bit scary.
There is no real way of checking for this on a preflight, so this is another reason one has to be alert and perceptive on the climbout before one gets into cloud. An AOA meter should be standard equipment on all aircraft. Thankfully the FAA have started to realise this and have made it a lot easier to get approval for AOA retrofits. I don't know how it is in EASA land, but I'm assuming they will see the benefits, too.
Last edited by AdamFrisch; 22nd Feb 2013 at 20:05.