One incident that occurred while I was flying remains a mystery to me until today. It was during a flight home from Tioman Island to KL (Tioman being the island I fly to most often for reasons the reader will ultimately find out as he or she continues to read my blogs).
The weather was simply beautiful at that time which was around six in the evening and we were flying at 2,500 feet. When we were roughly abeam Genting Highlands on our right I saw the RPM Indicator suddenly reading 2,450 rpm (which for the C172 means that the propeller was turning at that speed). Normal cruise setting is usually 2,200 to 2,300 rpm during level flight.
I automatically assumed that the plane was diving because only when a plane is diving will the RPM increase by itself ( a result of gravity pulling the plane down and increasing its speed). So I pulled the control column back to maintain level flight and reduced the power by pulling the throttle lever back towards me. I managed to hold the nose level by pulling back hard on the control column and holding it there but the throttle lever was forcefully pulling itself back into the instrument panel. I pulled the throttle lever back again with all my strength but the engine responded by again pulling it back into the panel. The altimeter was showing that we were descending at about 100 feet per minute with my nose level which is not too alarming.
This tug of war between myself and the engine continued a few times and I was fearful that the throttle cable behind the panel would snap because the force that was pulling it back in was really great. If it had snapped, the engine would turn at maximum speed and, if prolonged, damage itself. Worse still I would then lose control of the delinquent aircraft. I looked at the Airspeed Indicator to see if the speed of the aircraft had increased correspondingly and it was up to about 100 knots, which is on the fast side but not yet in the danger zone. Probably by pulling hard on the control column I had managed to keep the plane's speed within limits (high speed can weaken the aircraft's structure).
There was nothing else I could do other than pray that the situation would not worsen. I had three passengers with me who had never flown in a small plane before this and I could not let them see that I was in trouble. I decided to descend a bit lower as we would soon be required by Air Traffic Control (ATC) to maintain 1,500 feet as we enter the KL control zone. This enabled me to reduce the pressure I was putting on the control column and I also would be able to test if the engine RPM would increase further as we descended. Just as suddenly as it started, the engine RPM normalised and became controllable again. I could pull the throttle lever back and forth without forcing it. Pressure on the control column was also relieved.
After changing the radio frequency from Lumpur Information's to Sempang Tower's, I was mighty glad to be able to speak to my home airport controllers again. What a satisfying feeling it was to land the aircraft as normal and without my passengers noticing any difference. It could have been a different story though.
I asked around but none of the pilots I spoke to had experienced it the way I did. I asked the engineer to check the plane thoroughly after the flight and he found nothing out of the normal. My guess is that I had entered an area of clear air turbulence (CAT) where there was a strong downdraft coming from the front of the aircraft. This would have increased both my propeller RPM and the airspeed while causing the plane to descend. However I still cannot believe the really strong force that was pulling the throttle lever back into the panel.