N332SMPiper PA-42 · 1981

On July 8, 2012, at 2114 eastern daylight time, a Piper PA-42, N332SM, sustained substantial damage during a gravity wave encounter near Charleston, West Virginia. The airline transport pilot and two passengers were not injured. Instrument meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the flight, which departed Yeager Airport (CRW), Charleston, West Virginia at 2052 and was destined for Middle Georgia Regional Airport (MCN), Macon, Georgia. The personal flight was operated under the provisions of Title 14 Code of Federal Regulations Part 91.

The pilot obtained a preflight weather briefing, which depicted an area of convective activity west of CRW that was moving east. He stated that conditions at CRW included calm winds, light rain, and good visibility under a layer of overcast clouds at 11,000 feet. The flight was cleared for takeoff by the CRW air traffic control tower and asked to expedite the takeoff for another airplane on final approach. The pilot complied, but stated that the expedited takeoff precluded him from making a final "sweep" of the area ahead on the airplane's on-board radar, as was his custom, and further stated that the airplane's satellite weather receiver could not obtain a signal prior to takeoff.

The pilot contacted the Indianapolis Air Route Traffic Control Center at 2105 while climbing from an altitude of 5,000 feet after departure from CRW. Noting several lightning strikes on the airplane's Stormscope, the pilot requested and was issued a deviation to the left of his planned course.

At 2110, after clearing the airplane to climb and maintain 23,000 feet, air traffic control queried the pilot regarding the conditions encountered during the departure and climb. The pilot responded that the airplane was experiencing "light bump" at its current altitude and stated that they entered instrument meteorological conditions around 15,000 feet.

Shortly after this transmission, the pilot saw a flash of lightning that "appeared close." The airplane subsequently entered a rapid ascent, with the vertical speed indicator reflecting a climb of more than 4,000 feet per minute (fpm). The pilot reported a "strong kinesthetic sense of upward acceleration," as he disconnected the autopilot and neutralized the flight controls. He stated that "both" of the airplane's gyroscopic instruments tumbled, leaving him with no reference of the airplane's attitude. The pilot made a "mayday" transmission to air traffic control before hearing the airplane's stall warning horn sound, followed by a "falling and twisting" sensation. The pilot suspected that the airplane was in a steep spiraling descent or spin, and responded by reducing the engine power. The pilot then perceived ground lighting as the airplane exited the clouds, and initiated a recovery to a level flight attitude at an altitude of approximately 8,000 feet.

After regaining control of the airplane, the pilot requested and was provided radar vectors back to CRW, and subsequently landed without incident. Upon disembarking, the pilot observed substantial damage to both the left and right elevators.

The pilot held an airline transport certificate with a rating for airplane multiengine land, a commercial pilot certificate with ratings for airplane single engine land and single and multiengine sea, and a flight instructor certificate with ratings for airplane single and multiengine and instrument airplane. He reported 4,080 total hours of flight experience, of which 389 hours were in the accident airplane make and model. His most recent Federal Aviation Administration (FAA) first class medical certificate was issued on January 3, 2012.

According to FAA records, the airplane was manufactured in 1981, and was equipped with two Pratt and Whitney PT6A turboprop engines. The airplane's most recent 100-hour inspection was completed on June 6, 2012 at a total aircraft time of 6,091 hours.

The National Weather Service (NWS) Surface Analysis Chart for 2000 depicted a stationary front across northern West Virginia, stretched west to east from Indiana into Maryland. A surface low pressure center was located in southern Virginia, and a high pressure system was located in eastern Kentucky. Station models around the accident site depicted air temperatures between 70-80 degrees Fahrenheit (F), with temperature-dew point spreads of 4 F or more, variable winds less than 15 knots, partly clouds skies, and light rain.

At 1345, the NWS Storm Prediction Center (SPC) issued a severe thunderstorm watch for West Virginia and Virginia, valid through 2200. The watch discussed the favorable environment for severe thunderstorms with 2-inch diameter hail possible, extreme turbulence, and cumulonimbus tops to 55,000 feet mean sea level (msl).

The CRW surface weather observation at 2054 included wind from 150° at 3 knots, 10 miles visibility, light rain, few clouds at 1,500 feet, a broken ceiling at 4,800 feet, overcast skies at 10,000 feet, temperature 23° C, dew point 21° C, and an altimeter setting of 29.97 inches of mercury. Remarks indicated that rain began at 2012 and ended at 2021, rain began again at 2039, and cumulonimbus clouds distant in all quadrants.

The 2154 observation at CRW included wind from 210° at 7 knots, 10 miles visibility, light rain, few clouds at 1,000 feet, a broken ceiling of cumulonimbus clouds at 4,800 feet, overcast skies at 10,000 feet, temperature of 23° C, dew point temperature of 21° C, and an altimeter setting of 30.00 inches of mercury. Remarks indicated that rain ended at 2100, rain began at 2139, occasional in-cloud and cloud-to-ground lightning distant southwest through northwest, cumulonimbus clouds all quadrants moving east, and valley fog.

SIGMET 1E was issued at 2055 and valid for the accident site at the accident time. The SIGMET warned of an area of severe thunderstorms moving from 280° at 20 knots, with tops above 45,000 feet, hail to 2 inches in diameter, and wind gusts to 60 knots possible.

The Area Forecast issued at 2045, valid at the accident time, forecasted a broken ceiling at 6,000 feet, overcast skies at 10,000 feet with tops to 22,000 feet. Scattered light rain showers were expected with widely scattered thunderstorms and light rain, with cumulonimbus tops to 45,000 feet.

The NWS Charleston, West Virginia (RLX) Doppler (WSR-88D) base reflectivity images indicated that 30-45 dBZ reflectivity values occurred along the accident airplane's route of flight between 2107 and 2113, which indicated "light to very strong" echoes.

The RLX WSR-88D base velocity image depicted several transverse, southwest-to-northwest-oriented gravity wave bands located perpendicular to the airplane's flight path, which the airplane would have passed through between 2109 and 2113.

The 2000 upper air sounding for Roanoke, Virginia, located approximately 72 nautical miles southeast of the accident site, depicted a conditionally unstable vertical environment supportive of cloud formation, rain showers, and thunderstorms, with moderate potential for downbursts and strong wind gusts. The sounding indicated low-level wind shear, as well as the potential for clear air turbulence in several layers between the surface and 39,000 feet, with the highest potential for turbulence near 20,000 feet. Furthermore, a temperature inversion was located at 20,000 feet, with a high potential for updrafts and downdrafts near the inversion. Along with the convective activity, the area of the accident site was conducive for the formation and propagation of gravity waves and strong turbulence.

The weather briefing obtained by the pilot prior to flight contained the valid METARs, TAFs, Area Forecast, SPC outlook, SIGMETs, AIRMETs, CWAs, NWS watches and warnings, PIREPs, forecast winds aloft, and a text radar summary valid at 2045. There was no record or knowledge of any additional weather information received by the accident pilot.

According to FAA records, the airplane was manufactured in 1981, and was equipped with two Pratt and Whitney PT6A turboprop engines. The airplane's most recent 100-hour inspection was completed on June 6, 2012 at a total aircraft time of 6,091 hours.

GPS Receiver Data

A Garmin GPSMAP 496 was removed from the airplane and downloaded by the NTSB Vehicle Recorders Laboratory. The accident flight recording began at 2052 as the airplane departed CRW to the southwest, then turned south-southeast and climbed to its planned cruise altitude of 23,000 feet at an average recorded groundspeed of 217 mph. Between 2113 and 2115, the airplane's groundspeed began to decrease and the airplane entered a rapid ascent, followed by a rapid descent. During this time, recorded groundspeed fluctuated from 13 mph up to 728 mph, though these values were likely inaccurate due to problems with the GPS satellite lock or with the GPS unit's internal predictive algorithm, often seen when an aircraft experiences unusual attitudes or non-normal motion. After 2115, the airplane's groundspeed and altitude stabilized at approximately 150 mph and between 8,300 and 9,000 feet, as the airplane turned back towards CRW, then subsequently landed, with the recording ending at 2138.

Gravity Waves

A gravity wave is an atmospheric phenomenon that is formed when an area of unstable air rises or descends into a layer of stable air. This initial disturbance is the source of a series of waves that propagate outward like ripples in a pond, resulting in updrafts, downdrafts, areas of increased wind shear, and turbulence. Gravity waves have many sources, including thunderstorms, jet streams, and up- or downdrafts. In most cases, the energy from an initiating source dissipates quickly, along with any associated gravity waves. However, when a favorable environment is present in the form of a low-level temperature inversion or stable layer, with a conditionally unstable layer immediately above, a condition is formed known as a gravity wave duct. The gravity wave duct effectively traps the energy of the gravity wave and prevents its dissipation. These environmental conditions were present near the accident location at the time of the accident and at the accident airplane's altitude just prior to the encounter.

While current weather forecasting technology is able to predict favorable environments for the generation of gravity waves and gravity wave ducts, the technology lacks the ability to predict if, where, and at what intensity these phenomena may form.