CEN11LA657

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.”

On September 18, 2011, at 2057 central daylight time, N4867K, a Cessna P210N, sustained substantial damage when it collided with trees while on a visual approach to runway 18 at East Texas Regional Airport (GGG), Longview, Texas. The private pilot was not injured. The airplane was registered to and operated by the pilot. No flight plan was filed for the flight that departed Rusk County Airport (RFI), Henderson, Texas, about 2030. Night visual meteorological conditions prevailed for the personal flight conducted under 14 Code of Federal Regulations Part 91. The pilot said he was cleared to land on Runway 18 and entered a right downwind at an approximate altitude of 1,200 feet mean sea level (msl) and an airspeed of 95 knots. He lowered the landing gear and extended the flaps to 10 degrees. The pilot said he lost visual reference to the horizon when he turned onto the base leg due to a “frontal air mass” about 20 miles northwest of the airport. Plus, he was flying over an unpopulated river bottom, which also reduced his visual reference to the ground. As the pilot turned onto the base leg, the airspeed was 90 knots and his altitude was descending through 1,100 feet msl. As he leveled out, the pilot noticed a tree top to the left of the airplane. He immediately added full power, pulled back on the control column and started a climb, but was unable to avoid striking trees. The pilot was able to maintain control of the airplane and landed safely despite substantial damage to both wings and horizontal stabilizers. The pilot said he did not feel any wind gusts, but felt that he encountered some sort of "downdraft" that resulted in him losing approximately 800 feet in less than a minute with no gain in airspeed. A weather study conducted by a National Transportation Safety Board (NTSB) meteorologist revealed the conditions were favorable across central and northeastern Texas for the formation of a weather phenomenon known as a mesoscale gravity wave. This wave most likely originated from a strong downdraft or microburst from a large thunderstorm complex in northern Texas. The wave formed when the downdraft/microburst descended into an existing low-level inversion and a favorable gravity wave duct environment. The wave then propagated eastward toward the accident site. According to the weather study, a mesoscale gravity wave is formed whenever air or a fluid intrudes into another type of air or fluid. In most cases gravity waves dampen out very quickly and rapidly dissipate their energy. However, when the correct environment comes together, mesoscale gravity waves can propagate quite far from their original source, as they did in this case. These waves can result in rapid wind changes and areas of increased wind shear and turbulence. In this accident, the wave dramatically reduced the southerly wind component above the low-level inversion at the time and place of the accident. The surface wind at the airport did not change at the time of the accident dueto the stable low-level inversion. According to a 1997 study conducted by Steve Koch and Christopher O'Handley titled "Operational Forecasting and Detection of Mesoscale Gravity Waves", these waves are rare and difficult to forecast. According to the study, it said, "Current poor understanding of gravity waves and 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. In addition forecasters have often been misled by the existence of large scale gravity waves in 2 noteworthy events." Weather at GGG at 2053 was reported as wind from 170 degrees at 15 knots gusting to 21 knots, visibility 10 miles, clear skies, temperature 29 degrees Celsius, dewpoint 19 degrees Celsius, and a barometric pressure setting of 29.89 inches of Hg. The field elevation at GGG is 365 feet msl.