ERA11FA101 · Cessna 172H

The pilot and both crewmembers of the helicopter recalled routine radio communication as the helicopter approached the destination airport. They established visual contact with two airplanes that had announced their positions in the traffic pattern; one on the downwind leg and one on short final. The airplanes were also identified by the traffic avoidance system onboard the helicopter. The pilot followed behind and north of the second airplane and continued to the west side of the airport to complete a landing at the helipad. During the descent, about 500 feet above ground level (agl), the pilot "saw about 2 feet of white wing right outside." He "pulled power" and then felt contact with an airplane. The airplane's right wing separated before it departed controlled flight and descended to the ground, fatally injuring both occupants. The helicopter subsequently landed with minor damage and no injuries to the 3 occupants.

Interpolation of radar data revealed that the accident airplane departed from the same airport about 21 minutes prior to the accident and completed a right downwind departure, contrary to the established left traffic pattern. The airplane’s transponder appeared to be off for about 3 minutes after takeoff before transmitting the visual flight rules transponder code (1200) for the remainder of the observed flight; the transponder appeared to be on and functioning at the time of the collision. The airplane proceeded north of the airport before reversing course and returning to approach the airport from the northeast. The last target was observed about 1.2 nautical miles north of the airport on a track leading toward the west side of the landing runway at an altitude of 500 feet agl. About 25 seconds later, the helicopter passed northeast of the airport on a modified left base, about 500 feet above traffic pattern altitude (1,500 feet agl), crossed the final approach course, and turned parallel to and on the west side of the runway. Although only the helicopter was observed by radar at the time of the collision, extrapolation of the accident airplane’s previously observed targets and flight path placed the airplane at the accident site about the same time the helicopter was observed there. An analysis of the relative positions of the airplane and helicopter based on radar data indicated that the airplane remained below the helicopter pilot's field of view as the helicopter overtook the airplane from behind and descended upon it from above. Although the data indicated that the airplane would likely have been visible to the pilot of the helicopter, it is important to note that the onboard traffic avoidance system (TAS) did not provide the pilot with any alert of its presence because the system operated on line-of-sight principles. If an intruder aircraft’s antenna was shielded from the TAS antenna, the ability of the TAS to track the target would be affected. If a TAS equipped aircraft was located directly above an intruder, the airframe of one or both of the aircraft could cause the TAS’s interrogations to be shielded, depending on antenna location (either bottom or top-mounted).

All other airplanes in the traffic pattern were acquired visually by the pilot and crew as their positions were confirmed by the helicopter's onboard traffic avoidance system and the position reports provided by the pilots of each airplane. Because of the high-wing structure of the airplane, and its relative position and altitude, the helicopter's image was either blocked from the airplane pilot's view by the left wing, or was above and behind the airplane in the seconds before collision. Further, no radio position reports from the accident airplane were confirmed. The helicopter pilot’s unalerted detection of the airplane against a complex background of ground objects would have been difficult because of both the lack of apparent contrast between the airplane and the ground, its size in the windscreen, its relative lack of movement within the pilot’s field of view, and the position and angle of the sun. In addition, the helicopter pilot’s familiarity with the customary routes used by fixed-wing pilots to fly into and out of the airport also made detection of the airplane less likely, because the airplane was not in a location that normally contained conflicting traffic. Finally, before the helicopter turned and overtook the airplane, the helicopter pilot’s visual attention would have likely been directed toward the landing area, which would also have limited opportunities for detection of the airplane. The airplane's departure and arrival were contrary to published Federal Aviation Administration guidance, the airplane owner's guidance, and the airplane pilot's guidance to his own students with regard to pattern entry at the destination airport.

Verbatim NTSB analysis and probable cause from the NTSB dataset