It was a bright, hot day in the central Everglades. A. was driving Slytherin, my favorite airboat, and we were returning from a wading bird colony. As we cruised down the airboat trail going the usual speed (20 mph), the boat began accelerating at an extremely rapid pace. I turned to look at A. and she looked scared. She was trying to communicate with us, but with our ear protection on (not to mention the deafening sound of a Lycoming O-540 aircraft engine reaching perhaps 70-80 mph when a 40 lb 2-blade propeller is 4 feet behind you) we of course couldn't hear what she was yelling, although she kept shaking her head.
I quickly gathered that the boat was accelerating of its own accord, and I was so glad that we were on a straight-away, instead of having to weave around narrow curves a little further up the trail and potentially face the disastrous consequences of suddenly meeting another airboat head-on.
A moment later, the engine stopped. A. had her wits about her, and had done the only thing she really could do, even if the engine didn't appreciate it: she cut off the magnetos and the fuel line. We sat tensely as the airboat planed to a stop (yeah, that's always an interesting problem--airboats don't have brakes), wondering if the wake would wash over the low transom and flood the boat as it came off plane. No, it didn't, and we ripped off our ear protection to talk to A. She spoke breathlessly, still shaken - and explained that pants tied to the cage had come loose and flown through the prop, and suddenly the gas pedal had gone limp and the boat had surged forward, full speed ahead.
The gas pedal is connected to the throttle return spring via a long, thin cable. The throttle return spring lends the necessary tension to control the throttle (and therefore the speed) via a butterfly valve. As the pedal is pushed down, the throttle return spring contracts and the throttle is opened. Thus, it had happened that the pants had completely ripped off the throttle return spring, and suddenly the throttle valve was wide open, causing the boat to reach maximum speed. We turned off the batteries ("batteries off!" we always repeated to each other, for safety) and I went to the back of the boat to investigate - which is when I discovered the first issue -- the throttle return spring dangling at the end of the disconnected cable. That was easy to reattach, which I did right off.
I also found that the timing belt had come off the flywheel, and we were a bit stumped by that. We didn't want to have to loosen the alternator to get it back on. I then did something that you generally should never do - although the magnetos, fuel line, and batteries were off, and the engine had been cooling down a little bit. I fed the timing belt partly back on the groove in the flywheel, and since the belt was tensioned tightly, I slightly turned the propeller to feed the belt completely back into place. Just a disclaimer - don't ever get near the prop if the engine is hot and the magnetos aren't grounded. Aircraft engines can start when you manually turn the prop and the magnetos are on or improperly grounded - you can never be too careful. We were all a bit shaken and probably not thinking the clearest. I think right after that, I called my adviser, who helpfully answered, as he always did, and I explained what had happened. Turns out we could have driven the boat back to the ramp without putting the timing belt back on, but anyways...
A couple lessons learned or relearned, and a good chance to practice fixing an airboat on the fly... I remember we found some remnants of P's pants partly hooked on the cage - they were completely shredded.
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