Longview Airport
Aircraft Accident/Incident Report

Longview, Texas 75603
Sunday, September 18, 2011 20:57 CDT

The commercial pilot was returning to his home airport at night and entered the traffic pattern on a right downwind leg for runway 18 about 835 feet above the runway elevation. While turning to the base leg from about 735 feet above the runway elevation, the pilot lost visual reference to the horizon due to a frontal air mass about 20 miles northwest of the airport, and he had no visual reference to the ground because he was flying over an unpopulated river bottom. After leveling the airplane on the base leg, the pilot suddenly saw a treetop to the left of the airplane and immediately added full power and initiated a climb, but he was unable to avoid striking trees. The pilot was able to maintain control of the airplane and land safely. The pilot said that he did not feel any wind gusts but thought that he encountered some sort of “downdraft” that resulted in a rapid loss of altitude with no gain in airspeed. A weather study conducted by an NTSB meteorologist revealed that conditions were favorable for the formation of a weather phenomenon known as a mesoscale gravity wave. These waves can result in rapid wind changes and areas of increased windshear and turbulence. The study determined that the wave likely originated from a strong downdraft or microburst from a large thunderstorm complex in central and northeastern Texas then propagated eastward toward the accident site. At the time and place of the accident, the wave dramatically reduced the southerly wind component above the low-level inversion; therefore, it is likely that, as the pilot descended, he inadvertently encountered a sudden wind shift or rapid decrease in the speed of the southerly wind component that resulted in a rapid loss of altitude. A review of available weather information revealed that the mesoscale gravity wave was not forecasted; however, such waves are rare and difficult to forecast. A research study suggests that there is a “poor understanding of gravity waves” and that “acknowledged skepticism of their existence stems from the fact that it is both difficult and tedious to conduct a study of even a single event.” The study further notes that “forecasters have often been misled by the existence of large scale gravity waves.”

The flight's inadvertent encounter with a rare and unforecasted mesoscale gravity wave while on base leg, which resulted in a sudden loss of altitude and subsequent collision with trees.