On August 4, 1995, an Air Tractor AT-401 (registration N4543F) was involved in an aviation accident near Aurora, NE. Investigators recorded the probable cause as: A cracked exhaust tower on the number 4 cylinder prevented the exhaust valve from opening, causing a blow back condition and subsequent engine power loss. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot noticed white smoke from the engine, discontinued a swath run, and turned back. Shortly after leveling out, the engine lost most of its power. The airplane nosed over after contacting corn. Examination found a cracked exhaust tower on the number 4 cylinder.
Incident Overview
At the beginning of a swath run, the pilot observed white smoke emanating from the right side of the engine. He discontinued the run and turned the airplane back toward his home airport. Just as the airplane leveled out from the turn, the pilot reported that the engine lost most of its power. He then headed the airplane for a pasture but was unable to glide to it. Shortly after its wheels contacted the corn, the airplane nosed over and came to rest on the ground.
Engine Examination
Examination of the engine revealed that the number 4 cylinder's exhaust tower was cracked where the rocker arm shaft passed through the cylinder head. The owner/mechanic of the airplane stated that this damage prevented the cylinder's exhaust valve from opening. As a result, a blow back condition was created that affected the other cylinders. The owner indicated that the ultimate consequence of this condition is that the engine loses most of its power. This explanation was confirmed by the FAA Principal Maintenance Inspector who conducted the engine examination.
Probable cause
A cracked exhaust tower on the number 4 cylinder prevented the exhaust valve from opening, causing a blow back condition and subsequent engine power loss.