Cirrus Design Corp SR22 N122ES and Robinson Helicopter Company R44 II N7518Q accident near Frederick, Maryland

Probable cause

The failure of the helicopter pilots and the airplane pilot to maintain an adequate visual lookout for known traffic in the traffic pattern, which resulted in a midair collision. Contributing to the accident were the airplane pilot's descent below the published airplane traffic pattern altitude (TPA) and the helicopter pilot's climb above the proper helicopter TPA as prescribed in the Federal Aviation Administration's Aeronautical Information Manual for airports without published helicopter TPAs. Also contributing to the accident were the lack of a published helicopter TPA, the absence of radar equipment in the tower, and the controller's inadequate task prioritization.

NTSB analysis

The private airplane pilot was conducting a personal cross-country flight, and the commercial helicopter pilot and flight instructor were conducting a local instructional flight. A review of radar and voice communications revealed that the accident airplane pilot first contacted the nonradar-equipped tower when the airplane was 10 miles from the airport and that the local controller (LC) then acknowledged the pilot's transmission and instructed him to contact the tower when he was 3 miles from the airport. At this time, the LC was also handling two helicopters in the traffic pattern, one airplane conducting practice instrument approaches to a runway that intersected the runway assigned to the accident airplane, another airplane inbound from the southeast, and a business jet with its instrument flight rules (IFR) clearance on request. About 1 minute after the accident airplane pilot first contacted the LC, the LC began handling the accident helicopter and cleared it for takeoff. One minute later, the controller issued the business jet pilot an IFR clearance. When the accident airplane was 3 miles from the airport, the pilot reported the airplane's position to the controller, but the controller missed the call because she was preoccupied with the clearance read-back from the business jet pilot. About 1 minute later, the controller instructed the accident airplane pilot to enter the left downwind leg of the traffic pattern on a 45-degree angle and issued a landing clearance. She advised that there were three helicopters "below" the airplane in the traffic pattern, and the pilot replied that he had two of the helicopters in sight.

Data downloaded from the airplane and witnesses on the ground and in the air indicated that, as the airplane entered the downwind leg of the traffic pattern, it flew through the accident helicopter's rotor system at the approximate point where the helicopter would have turned left from the crosswind to the downwind leg. Because of a specific advisory transmitted on the tower radio frequency advising of traffic on the downwind, the pilot of each accident aircraft was or should have been aware of the other. A witness in the helicopter directly behind the accident helicopter had a similar field of view as the accident helicopter, and he reported that he acquired both accident aircraft in his scan before the collision. Given this statement and that the accident helicopter had two commercial pilots in the cockpit, the pilots should have had the situational awareness to understand the conflict potential based on the airplane's position reports. Although the airplane was equipped with a traffic advisory system, its capabilities could have been limited by antenna/airframe obstruction or an inhibition of the audio alert by the airplane's flap position.

The airplane's data indicated that the collision occurred at an altitude of about 1,100 ft mean sea level (msl). The published traffic pattern altitude (TPA) for light airplanes was 1,300 ft msl. Although several different helicopter TPAs were depicted in locally produced pamphlets and posters and reportedly discussed at various airport meetings, there was no published TPA for helicopters in the airport/facility directory or in the tower's standard operating procedures. According to the Federal Aviation Administration's Aeronautical Information Manual, in the absence of a published TPA, the TPA for helicopters was 500 ft above ground level; therefore, the appropriate TPA for helicopters at the accident airport was about 800 ft msl. The lack of an official helicopter TPA, which was published after the accident, significantly reduced the potential for positive traffic conflict resolution.

Review of the airport procedures, tower capabilities, and the controller's actions revealed no specific departure from proper procedures. Because the tower was not equipped with radar equipment, all of the sequencing and obtaining of traffic information had to be done visually. This would have been especially difficult at the accident airport due to the local terrain and tree lines that extend above the pattern altitudes from the tower controllers' view, which can cause aircraft to easily blend in with the background. Further, the controller spent a lengthy amount of time on the task of issuing the IFR clearance to the business jet while handling multiple aircraft in the traffic pattern. It is likely that the lack of radar equipment in the tower and the controller's inadequate task management also significantly reduced the potential for positive traffic conflict resolution.

Text from the NTSB. Open the NTSB docket for ERA15FA025 for the full record.