ERA13FA026

The pilot’s decision to depart under visual flight rules in dark night instrument meteorological conditions, which resulted in subsequent spatial disorientation, uncontrolled descent, and impact with trees and terrain.

According to the operator, the airline transport-rated pilot sent a text indicating that he was intending to depart on a scheduled repositioning flight from a helipad located behind the pilot's home to an airport located about 25 nautical miles southwest. Data recovered from a handheld GPS device showed that the helicopter lifted off from the helipad and began accelerating forward while turning right and maintaining a relatively constant altitude. During the 27 seconds of recorded flight that followed, the helicopter's right turn rate increased, shortly decreased slightly, and then significantly increased again as the helicopter began to descend. The helicopter subsequently impacted trees and terrain. Although the weather reporting stations closest to the accident site and at the destination airport generally reported that visual meteorological conditions prevailed, the presence of calm wind, near-coincident temperatures and dew points, and the clear night sky favored the formation of patch radiation fog and/or dew on the surface. Visible satellite imagery captured about 1 hour after the accident depicted a band of low stratiform clouds or fog/mist over the accident site and along the adjacent river valley. Additionally, several witnesses near the helipad reported that the lighting and weather conditions about the time of the accident were "dark" and "foggy." Postaccident examination of the wreckage revealed no evidence of any preimpact mechanical malfunctions or failures of the airframe or engine that would have precluded normal operation. The helicopter was not equipped to operate in instrument meteorological conditions (IMC). Although the airline transport-rated pilot possessed airplane and helicopter instrument ratings, his most recent instrument proficiency check was completed about 8 months before the accident in an airplane, not a helicopter, and no evidence was found indicating that he was current or proficient in operating a helicopter in IMC. Regardless, the fog should have been an indication to the pilot that IMC existed, and he should not have decided to operate the helicopter in such conditions. The flight profile and the presence of radiation fog during dark night conditions, which would have obscured visual references such as the trees, are consistent with the pilot having experienced spatial disorientation, specifically a vestibular illusion known as the "graveyard spiral."

The helicopter was not equipped with any flight data recording devices, nor was it required to be; however, a hand-held global positioning system (GPS) receiver was recovered from the wreckage, and found to contain data pertaining to the accident flight. The initial data point was recorded at 0634, at the 4PN5 helipad. The helicopter's position began tracking northeast at 0635:59, at a GPS altitude of 442 feet. Over the next 27 seconds, the helicopter began to accelerate to about 85 knots ground speed, while turning to the right and maintaining a relatively constant altitude, within about 50 feet of the helicopter's initial recorded altitude as it began to accelerate. The helicopter maintained a consistent right-turning track for the entirety of the flight. Between 0636:09 and 0636:17, the calculated turn rate of the helicopter increased from about 3 degrees per second to about 18 degrees per second. Over the next 7 seconds, the turn rate decreased to about 14 degrees per second. The calculated turn rate between the final two recorded GPS positions increased again to about 31 degrees per second. Over the final 5 seconds recorded for the flight, the helicopter descended from a GPS altitude of 503 feet to 455 feet, which corresponded to an approximate 575-foot per minute average descent rate for that portion of the flight. The initial impact point (IIP) was located about 300 feet northwest of the helicopter's final GPS recorded position. The departure heliport, 4PN5, was located about 1,000 feet southwest of the accident site. The heliport was comprised of a 65-foot square turf and gravel helipad, which was located in the backyard of the pilot's home at an estimated elevation of 400 feet. A rural neighborhood surrounded the heliport to the north, east, and south. The west bank of the Delaware River was located about 1/2-mile east of the heliport, at an elevation of 120 feet. Spatial Disorientation According to the FAA Airplane Flying Handbook (FAA-H-8083-3), "Night flying is very different from day flying and demands more attention of the pilot. The most noticeable difference is the limited availability of outside visual references. Therefore, flight instruments should be used to a greater degree.… Generally, at night it is difficult to see clouds and restrictions to visibility, particularly on dark nights or under overcast. The pilot flying under VFR must exercise caution to avoid flying into clouds or a layer of fog." The handbook described some hazards associated with flying in airplanes under VFR when visual references, such as the ground or horizon, are obscured. "The vestibular sense (motion sensing by the inner ear) in particular tends to confuse the pilot. Because of inertia, the sensory areas of the inner ear cannot detect slight changes in the attitude of the airplane, nor can they accurately sense attitude changes that occur at a uniform rate over a period of time. On the other hand, false sensations are often generated; leading the pilot to believe the attitude of the airplane has changed when in fact, it has not. These false sensations result in the pilot experiencing spatial disorientation." According to the FAA Instrument Flying Handbook (FAA-H-8083-15), a rapid acceleration "...stimulates the otolith organs in the same way as tilting the head backwards. This action creates the somatogravic illusion of being in a nose-up attitude, especially in situations without good visual references. The disoriented pilot may push the aircraft into a nose-low or dive attitude." The FAA publication Medical Facts for Pilots (AM-400-03/1), described several vestibular illusions associated with the operation of aircraft in low visibility conditions. Somatogyral illusions, those involving the semicircular canals of the vestibular system, were generally placed into one of four categories, one of which was the "graveyard spiral." According to the text, the graveyard spiral, "…is associated with a return to level flight following an intentional or unintentional prolonged bank turn. For example, a pilot who enters a banking turn to the left will initially have a sensation of a turn in the same direction. If the left turn continues (~20 seconds or more), the pilot will experience the sensation that the airplane is no longer turning to the left. At this point, if the pilot attempts to level the wings this action will produce a sensation that the airplane is turning and banking in the opposite direction (to the right). If the pilot believes the illusion of a right turn (which can be very compelling), he/she will reenter the original left turn in an attempt to counteract the sensation of a right turn. Unfortunately, while this is happening, the airplane is still turning to the left and losing altitude. Pulling the control yoke/stick and applying power while turning would not be a good idea–because it would only make the left turn tighter. If the pilot fails to recognize the illusion and does not level the wings, the airplane will continue turning left and losing altitude until it impacts the ground." An autopsy was performed on the pilot by the Office of the Coroner, Bucks County, Pennsylvania. The listed cause of death was "multiple injuries." The FAA's Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma, performed toxicological testing on the pilot. No cyanide, ethanol, or drugs were detected in the samples submitted for testing. The samples submitted for carbon monoxide testing were deemed "unsuitable for analysis." The National Weather Service (NWS) Area Forecast applicable to the area surrounding the accident site, which was issued at 0445, predicted broken to scattered ceiling with visual meteorological conditions generally prevailing around the region. No AIRMETS or SIGMETS were current for the area of the accident flight at the time of the accident. Geostationary Operational Environmental Satellite -14 infrared satellite image for 0645 depicted a band of mid-level stratiform-type clouds over the accident site with a radiative cloud top temperature that corresponded to cloud tops near 16,000 feet. The first available visible satellite image at 0732 showed a band of low stratiform clouds or fog/mist over the accident site, and along the Delaware River Valley. The weather conditions at Doylestown Airport (DYL), Doylestown, Pennsylvania, located about 11 nautical miles south of the accident site, at 0654, included calm winds, clear skies below 12,000 feet, 10 statute miles visibility, a temperature and dew point of 1 degree C, and an altimeter setting of 29.99 inches of mercury. The weather conditions at LOM, located about 25 nautical miles southwest of the accident site, at 0635, included calm winds, clear skies below 12,000 feet, 5 statute miles visibility, a temperature and dew point of 3 degrees C, and an altimeter setting of 29.97 inches of mercury. According to the U.S. Naval Observatory, on October 9, 2012, the beginning of civil twilight occurred at 0648 and sunrise occurred at 0716. The moon set at 1902 on the preceding evening, and did not rise again until 0949 on the morning of the accident. Several witnesses who lived in the vicinity of the accident site were interviewed separately shortly after the accident. Each was asked to describe the weather and lighting conditions that prevailed at the time. The witnesses consistently described the weather as "foggy," with some stating that it was "very" or "extremely" foggy. One witness described the fog as being very dense, like "pea soup," while another estimated the visibility to be about 1/8th-mile. The witnesses also consistently described the lighting conditions as "dark" or "very dark". Another witness who lived about 1 mile north of the accident site described the weather conditions about 1 hour after the accident as cloudy with no fog. According to the NWS, "Radiation fog forms at night under clear skies with calm winds when heat absorbed b