Narrative:

On an IFR flight as I was approaching a line of clouds with some early buildups; I asked for an amended clearance with a 90 degree turn to the left to try to circumvent the buildups. The controller instead told me that there was a gap up ahead that should allow safe passage to my destination and cleared me to a VOR and then to my destination. I checked the course and it actually looked like there was a large gap; so I accepted the clearance. I had to get cleared higher several times in order to stay above the clouds. As I was about to enter the gap between the clouds; the controller suddenly called and said that there was heavy precipitation straight ahead as well as on both sides. I told him that the gap was clear; and I could see clear blue sky all the way through. At that moment; the clouds below the gap seemed to rise up and engulf me. For about a minute or so; I was subjected to severe turbulence and occasional ice pellet showers. No ice was accumulated on the airframe; and the engine did not quit. I was able to maintain wings level and did not lose much altitude. During that time; I obtained a low manifold pressure warning and saw 9 inches indication on the manifold pressure gauge. As I came out of the cloud; I told the controller that I needed to descend as soon as possible because of the loss of manifold pressure and was cleared to do so. After descending to around 7000 ft; the manifold pressure returned to normal as did all other engine parameters. I proceeded to land at a local airport without incident. I have noticed no further engine issues in subsequent flights. In this situation; one of my errors was to trust the controller's radar more than my eyes. I was in VMC observing the weather build up ahead; and I asked for a routing away from it that made sense with what I was observing out the window. Even though I was actually given an unsolicited clearance to a temptingly quicker way around the buildups; I should have stuck to my guns. Another error was to misjudge the tops of the clouds ahead of me. In retrospect; the air was rising; moist; and unstable; and even though I was heading for what looked like an easy pass between the building clouds; I was unable to recognize the rapidity with which the clouds were rising directly in front of me.

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Original NASA ASRS Text

Title: M-20 Pilot flew into buildups at FL240 while trying to out-climb and avoid convective weather.

Narrative: On an IFR flight as I was approaching a line of clouds with some early buildups; I asked for an amended clearance with a 90 degree turn to the left to try to circumvent the buildups. The Controller instead told me that there was a gap up ahead that should allow safe passage to my destination and cleared me to a VOR and then to my destination. I checked the course and it actually looked like there was a large gap; so I accepted the clearance. I had to get cleared higher several times in order to stay above the clouds. As I was about to enter the gap between the clouds; the controller suddenly called and said that there was heavy precipitation straight ahead as well as on both sides. I told him that the gap was clear; and I could see clear blue sky all the way through. At that moment; the clouds below the gap seemed to rise up and engulf me. For about a minute or so; I was subjected to severe turbulence and occasional ice pellet showers. No ice was accumulated on the airframe; and the engine did not quit. I was able to maintain wings level and did not lose much altitude. During that time; I obtained a low manifold pressure warning and saw 9 inches indication on the manifold pressure gauge. As I came out of the cloud; I told the controller that I needed to descend as soon as possible because of the loss of manifold pressure and was cleared to do so. After descending to around 7000 FT; the manifold pressure returned to normal as did all other engine parameters. I proceeded to land at a local airport without incident. I have noticed no further engine issues in subsequent flights. In this situation; one of my errors was to trust the controller's radar more than my eyes. I was in VMC observing the weather build up ahead; and I asked for a routing away from it that made sense with what I was observing out the window. Even though I was actually given an unsolicited clearance to a temptingly quicker way around the buildups; I should have stuck to my guns. Another error was to misjudge the tops of the clouds ahead of me. In retrospect; the air was rising; moist; and unstable; and even though I was heading for what looked like an easy pass between the building clouds; I was unable to recognize the rapidity with which the clouds were rising directly in front of me.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.