Incident Details
On September 18, 2011, at 2057 central daylight time, a Cessna P210N, registration N4867K, sustained substantial damage when it collided with trees during a visual approach to runway 18 at East Texas Regional Airport (GGG) in Longview, Texas. The private pilot, who was also the registered owner and operator, was not injured. The flight departed Rusk County Airport (RFI) in Henderson, Texas, around 2030, and no flight plan was filed. Night visual meteorological conditions prevailed for the personal flight, conducted under 14 Code of Federal Regulations Part 91.
Pilot Account
The pilot reported being cleared to land on Runway 18 and entering a right downwind at approximately 1,200 feet mean sea level (msl) with an airspeed of 95 knots. He lowered the landing gear and extended the flaps to 10 degrees. While turning onto the base leg, the pilot lost visual reference to the horizon due to a “frontal air mass” located about 20 miles northwest of the airport. Additionally, flying over an unpopulated river bottom reduced his visual reference to the ground. At the turn onto base leg, airspeed was 90 knots and altitude was descending through 1,100 feet msl. As he leveled out, the pilot observed a tree top to the left of the airplane. He immediately applied full power, pulled back on the control column, and initiated a climb but was unable to avoid striking trees. Despite substantial damage to both wings and the horizontal stabilizers, the pilot maintained control and landed safely. The pilot stated he did not feel wind gusts but believed he encountered a “downdraft” that caused a loss of approximately 800 feet in less than a minute with no gain in airspeed.
Weather Study
A weather study conducted by a National Transportation Safety Board (NTSB) meteorologist determined that conditions across central and northeastern Texas were favorable for the formation of a mesoscale gravity wave. This wave most likely originated from a strong downdraft or microburst associated with a large thunderstorm complex in northern Texas. The wave formed when the downdraft or microburst descended into an existing low-level inversion and a favorable gravity wave duct environment, then propagated eastward toward the accident site.
According to the study, a mesoscale gravity wave occurs when air or a fluid intrudes into another type of air or fluid. While gravity waves typically dampen quickly and dissipate energy, the correct environmental conditions can allow them to propagate far from their source. Such waves can produce 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 remained unchanged due to the stable low-level inversion.
A 1997 study by Steve Koch and Christopher O'Handley, titled "Operational Forecasting and Detection of Mesoscale Gravity Waves," noted that these waves are rare and difficult to forecast. The study stated, "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 Conditions at GGG
At 2053, weather at East Texas Regional Airport 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 mercury. The field elevation is 365 feet msl.