Sitting still for long periods isn’t good for machinery, and it’s particularly tough on aircraft. Moisture collects in places where it would normally be evaporated by engine heat or blown away by airflow. Things that should move freely start to stick; corrosion begins and progresses undetected. Fuel gets contaminated, rubber hardens and cracks, and living things find homes in places where they’re not welcome.
One should exercise caution in returning an aircraft to service after a long inactive spell. At the very least, a careful pilot would want to do an exceptionally thorough preflight, followed by a long enough ground run to turn up any obvious problems with the engine or brakes. A few high-speed taxi tests might not be a bad idea. Better would be a complete annual inspection or the equivalent. Either way, it goes without saying that repairs shouldn’t be attempted by anyone who is not familiar with the appropriate techniques.
According to the pilot’s family, an amateur-built RV-6 that crashed in August 2009 had sat on the ramp “for about a year” without being flown. About a week before the accident, the pilot got the engine started and flew one circuit around the traffic pattern. On the morning of the accident, he removed the spark plugs and sandblasted them, ran a compression check, and cleaned “goop” from the air filter.
A witness on the field saw the RV take off and climb to no more than 200 agl before the engine began making “popping” noises. The airplane turned crosswind and then quickly turned downwind; the pitch attitude gradually increased, but the airplane didn’t climb. Abeam the numbers, it began a steep left turn as if to return for landing. The nose dropped and the airplane hit the ground, killing the pilot. Fire consumed the wreckage.
The NTSB blamed the accident on “[t]he pilot's failure to maintain adequate airspeed that led to a stall/spin,” but also noted that examination of the engine revealed one discrepancy. The valve train and cylinders appeared normal and only traces of metal were found in the oil filter, but all the spark-plug gaps were between 0.028 and 0.030 inches. This was far outside the range of 0.016 to 0.021 recommended by the manufacturer, and all but one plug failed bench tests when subjected to pressures of more than 80 psi. Whether or not the sandblasting widened the gaps, the pilot either didn’t check them before reinstalling the plugs or didn’t know what the correct range was. Given the successful flight around the pattern the week before, it seems as though his attempt at preventive maintenance may instead have prevented the engine from making full power.
The pilot was not the airplane’s builder. He had a private pilot certificate and 302 hours of total flight time. There is no report of what experience, if any, he had in aircraft maintenance. Of course, no mechanic’s certificate is required to work on a homebuilt, and cleaning and gapping spark plugs is one of the preventive-maintenance chores all owners who hold at least a private pilot certificate are specifically authorized to do—but that doesn’t make an engine any more forgiving if the work isn’t done correctly, and it doesn’t make the consequences less severe. Nor does it diminish the critical importance of maintaining airspeed when faced with the loss of engine power. If a misfiring engine created this emergency, attempting a tight turn at low airspeed, low altitude, and a high angle of attack was what made it unrecoverable.