Drone / Helicopter Mid Air Collision Over Los Angeles City Hall ‘Probable’ Said NTSB (Airbus AS30B2 N71HD)

July 6, 2020

Stablilizer
Horizontal stabaliser damage on Helinet Aviation Services, ABC7 News AS350B2 ENG Helicopter, over LA City Hall.
Credit: NTSB
On Dec. 4 2019 Airbus AS350B2 N71HD was being operated by Helinet Aviation Services as an electronic news-gathering flight for ABC7 News in Los Angeles under Part 91 and visual flight rules, within Class G uncontrolled airspace.

The U.S. National Transportation Safety Board (NTSB) explain in their safety investigation report (issued June 29 2020) that the pilot, “… reported colliding with an unknown object at 1,100 feet above mean sea level (msl) in the vicinity of Los Angeles City Hall. The pilot conducted a precautionary landing at a nearby helipad. Post-flight examination revealed minor damage to the right horizontal stabilizer and tail rotor blade.”

The helicopter was equipped with a SpiderTrax [sic] flight following system, which indicated the helicopter passing northeasterly in the vicinity of LA City Hall at 7:15 p.m. at 1,104 feet msl (approximately 828 above ground level (agl)).

TailRotor
Horizontal stabaliser damage on Helinet Aviation Services, ABC7 News AS350B2 ENG Helicopter, over LA City Hall.
Credit: NTSB
The pilot and operator reported that no visible evidence of a bird strike was apparent, and believed that the aircraft collided with a small unmanned aircraft (sUAS, or drone).

The three occupants were uninjured.

There was a small round dent which partially punctured the leading edge of the horizontal stabilizer. Further dents and scuff marks were approximately 6 to 8 inches outboard of the round dent on the leading edge and along the upper surface. One of the tail rotor blades exhibited a small gouge in the composite surface approximately midchord. The horizontal stabilizer and tail rotor were sent to the NTSB Materials Laboratory for examination. Visual examination was consistent with an inflight hard-body impact. The dent and scuff marks were compared to the dimensions of common small drones and were consistent with the distance and orientation between the motor and center fuselage/battery compartment.

comparison
Angled view comparing common drone to impact marks. The drone in the photo is an exemplar of a popular drone used to illustrate size comparison. Numerous manufacturers and models of drone fall into this size class. Does not necessarily depict the actual collision object.
Credit: NTSB
Ultraviolet light inspection revealed no evidence of biological material. The dented region exhibiting material transfer was examined using Fourier-transform infrared spectroscopy (FTIR). The white and gray areas were probed, and the data were contrasted with an area of the painted stabilizer skin surface away from the impact zone. In subtracting out the spectrum peaks from the control area away from the impact zone, several peaks consistent with a different or foreign material were noted.

These peaks were consistent with a polycarbonate polymer. Examination of an exemplar popular drone indicated the primary construction material is polycarbonate. (Polycarbonate plastics are used in many other common items, such as safety glasses or light lenses). Under magnification, inside the round dent, a small circular indentation was noted of approximately 0.125 inches (~3 mm) in diameter, exhibiting tears and cracking around its circumference.

Detail
Close-up of damage to Helinet Aviation Services / ABC7 News AS350B2 ENG Helicopter over LA City Hall.
Credit: NTSB
These features were consistent with fore-to-aft impact with a hard, cylindrical object. The propeller shafts on many common small drones are approximately 3 mm in diameter. The damaged portion of the horizontal stabilizer was sent to the National Institute for Aeronautical Research (NIAR) impact dynamics lab (part of the Federal Aviation Administration UAS Center for Excellence). NIAR scanned the damaged component and created a finite element model (FEM) of the stabilizer. The FEM was entered into their impact kinematics simulation with a representative model of a common small drone. The resultant damage was similar to that of the incident, although the calculated relative speeds were higher than those reported by the helicopter pilot. NIAR researchers noted that the collision model they used was of one specific small drone – different construction materials or mass of the colliding object could account for the difference. A small contribution from rotor wash could also contribute.

No evidence was found of a missing drone however, the operator reported that the onboard cameras were stowed and did not record video at the time of the reported collision. The operator and NTSB investigator conducted a search of social media for any possible indications of a lost drone in the area, nothing definitive was found. Two drone detection companies were operating in the area but did not have any targets in the area at the time of the report. Both companies reported less than complete coverage of the area.