High density altitude impairs 172’s takeoff performance

The pilot completed a normal landing to the south at a grass airstrip near a lake that was surrounded by 75′ pine trees. He added that he and his passengers ate lunch at the airstrip in Coolin, Idaho, and during that time, he noticed “mostly calm” wind with an “occasional gust from the south.”

Due to the runway gradient, he decided to take off downhill to the north because the wind sock was indicating a calm wind.

During the soft field takeoff, the takeoff roll was normal, but about 100′ above ground level, he noticed that the “climb had slowed” and the “airspeed was dropping.” He lowered the nose, the Cessna 172 “descended quickly,” and then touched down on the runway with about 30′ remaining.

The airplane overran the runway, crossed a road, and hit a dumpster and trees.

The fuselage and both wings sustained substantial damage.

The pilot reported that the airplane was 25 pounds under maximum gross weight.

An automated weather observation station (AWOS) 13 nautical miles from the accident site reported about the time of the accident wind from 240° at 6 knots. A review of four hourly AWOS recordings, south and east of the accident site, around the time of the accident, revealed that the wind was variable from the southwest to west at 5 to 10 knots, gusting 15 to 18 knots.

The takeoff was on runway 33. The calculated density altitude was 3,700′.

According to the FAA density altitude Koch Chart, the airplane would have likely experienced a 32% decrease in the normal climb rate and a 50% increase in the normal takeoff distance.

The FAA’s Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B, contained a section titled, “Effect of Obstructions on Wind,” which stated: “Another atmospheric hazard exists that can create problems for pilots. Obstructions on the ground affect the flow of wind and can be an unseen danger. Ground topography and large buildings can break up the flow of the wind and create wind gusts that change rapidly in direction and speed. These obstructions range from man-made structures, like hangars, to large natural obstructions, such as mountains, bluffs, or canyons. It is especially important to be vigilant when flying in or out of airports that have large buildings or natural obstructions located near the runway. The intensity of the turbulence associated with ground obstructions depends on the size of the obstacle and the primary velocity of the wind. This can affect the takeoff and landing performance of any aircraft and can present a very serious hazard.”

It is likely that, during the initial climb in high-density altitude conditions, the airplane encountered a quartering tailwind gust as the airplane climbed above the trees, which resulted in a loss of lift and an inability to gain altitude during the initial climb.

Probable cause: The pilot’s decision to take off in high-density altitude and gusting quartering tailwind conditions, which resulted in a loss of lift and an inability to gain altitude during the initial climb.

NTSB Identification: GAA17CA526

This September 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.