NTSB Factual Report

On June 4, 2006, at 1400 Pacific daylight time, a Cessna 180A, N180LG, sustained substantial damage when the pilot's seat slid aft and the airplane veered from the runway during the takeoff roll at Marina Municipal Airport, Marina, California. The airplane came to rest inverted. The private pilot and one passenger were not injured. The pilot, who was also the registered owner of the airplane, was operating it under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed, and no flight plan had been filed for the local area personal flight. The flight was destined for Frazier Lake Airpark, Hollister, California.

The pilot stated that during the takeoff roll, his seat slipped aft and the throttle control slipped from his grasp so he was unable to pull it to the closed position. The airplane veered left from the runway and nosed over. He had adjusted his seat prior to takeoff and noted no slippage. He believed that he had also secured the after market seat stop called a SAF-T-STOP on the seat track but was not certain. The pilot stated that he normally flew the airplane with the seat in its forward most position. The seat and seat track were last inspected during the annual inspection in September 2005.

The seat track and locking mechanism were examined on June 20, 2005, by the National Transportation Safety Board investigator, and compared with data obtained from airworthiness directive (AD) 87-20-03 R2. AD 87-20-03 R2 defines the airworthiness requirements and minimum acceptable wear on the seat locking mechanism and seat track components. The seat track roller assembly was intact and operational. The seat track holes were measured using an electronic micrometer. The holes were all within specified tolerances. The roller housing had a light coat of dust and grime. The rollers turned freely on their axle bolts. The spring that keeps the locking pin in place provided positive engagement. The tang thickness was greater than 1/2 of the roller housing thickness.

Two bolts were installed through the seat track; one through the most forward hole and one through the most aft hole. The purpose of the bolts was to serve as the fore and aft seat stops. The bolts fit perpendicular to the seat track and were secured to the track using nuts. The forward bolt shank that was visible through the seat track hole had noticeable rub marks on its upper surface. When the bolt was removed and examined, there were no rub marks on the bottom portion of the bolt.

The seat lock pin was engaged in each of the seat track holes and did not disengage with an applied force. The investigator noted that when the seat pin was engaged in the most forward hole with the bolt through it, specified depth engagement of the seat pin was not possible. When the locking pin passed through the hole, it would hit the shank of the bolt prior to reaching its full locking depth. There was no installation record of the bolt seat stop in the airplane maintenance records.

The airplane inside seat track was equipped with a SAF-T-STOP. The stop was not tightened on the track and was free to move fore and aft. When the investigator tightened the stop on the seat track, the seat did not move past the stop. The purpose of the SAF-T-STOP is to prevent the seat from slipping aft if the primary seat locking mechanism fails.

According to Cessna Aircraft Company, the approved seat stop uses a U-shaped piece of metal that fits over the seat track and is held using a pin through the side hole. This is secured using a cotter pin. The design does not allow the seat lock pin to engage in the hole through which it is secured.

NTSB Probable Cause

the installation of the unapproved forward seat stop by unknown maintenance personnel, which did not allow for full depth engagement of the seat lock pin, resulting in the seat slipping aft during the takeoff roll and subsequent loss of directional control. A contributing factor to the accident was the pilot's failure to secure the backup seat stop.