Primary finding
Probable cause
The inaccurate obstacle height entered into mission planning that resulted in the aircraft wing’s impact with a tree during descent, resulting resulted in a loss of control.
Investigator assessment
Analysis narrative
A Pyka Pelican Cargo unmanned aircraft system (UAS), N136ZP, was substantially damaged after it impacted a tree and subsequently the ground during its descent to land at Summit Point Training Facility in Summit Point, West Virginia. The flight was operated by Pyka Inc. as a developmental flight test and was conducted under the provisions of Title 14 Code of Federal Regulations Part 91 and a Federal Aviation Administration (FAA) Certificate of Authorization (COA). There were no injuries. The Pelican Cargo is a fixed-wing UAS with four electrically-driven, wing-mounted motors (figure 1). Each motor has two propellers and are powered by a battery system. The airframe has a fixed tricycle landing gear. The accident aircraft held an experimental airworthiness certificate. Figure 1. Pyka Pelican Cargo. (source: Pyka Inc.) According to the operator, the aircraft departed from the same runway as the intended landing with no issues. The flight was conducted autonomously with the option for manual control. There was a pilot in command, a remote pilot, two visual observers (one positioned at the beginning of the final approach path, and one positioned abeam the tallest obstacle along the approach course), and a ground control station operator as part of the operation. During the flight, multiple passes were conducted to both ensure a stable approach and terrain clearance for landing. The pilot elected to proceed with landing, which the aircraft accomplished autonomously. During descent, at about 60 ft above ground level, the aircraft wing contacted a tree and subsequently struck a powerline before impacting the ground and fragmenting. No postcrash fire was reported and there were no injuries. During preflight planning, the operator stated that personnel measured nearby trees as being about 46 ft (14 meters) tall. However, these measurements were taken relative to the elevation of the runway, but the elevation of the trees was lower than that of the runway. The measured height of the trees was input as obstacles into the aircraft’s mission planning software. As a result, the aircraft interpreted these obstacles as extending 46 ft above the ground from where the trees were located. Furthermore, the operator reported finding, after the accident occurred, that one tree that was about 15 ft taller than the surrounding trees. This taller tree was missed during preflight planning and was not input into the mission planning software. The Pelican Cargo was not equipped with light detection and ranging (LIDAR) equipment or analogous remote sensing technology for obstacle detection and collision avoidance. Such equipment could have detected the impending conflict with the tree and aborted the landing autonomously.