History of Flight
On December 4, 2019, at 7:15 pm Pacific Standard Time (PST), the pilot of an Airbus AS-350-B2 helicopter, registration N71HD, reported a collision with an unknown object at 1,100 feet above mean sea level (msl) near 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 one tail rotor blade. No visible evidence of a bird strike was apparent, and the pilot and operator believed the aircraft collided with a small unmanned aircraft (sUAS, or drone). The helicopter was operated by Helinet Aviation Services as an electronic news gathering flight for ABC7 News under 14 CFR Part 91, with night visual meteorological conditions prevailing.
The pilot reported fully night conditions and difficulty distinguishing lights due to city lights. He was transitioning between story locations at 1,100 ft msl at 100–110 knots. He noted clouds above but not in his immediate area. He did not see anything before hearing a loud noise. The area of the event was east of City Hall, bounded by Los Angeles St, Alameda St, Cesar Chavez Ave, and East 2nd St. The Los Angeles Police Department searched the area but found no drone.
The operator reported onboard cameras were stowed and did not record video. Social media searches for a lost drone yielded nothing. Two drone detection companies operating in the area reported no targets at the time, though coverage was incomplete. The FAA was queried for sUAS airspace authorizations in nearby Class B, but none were active.
Aircraft Information
The Airbus (Eurocopter) AS-350-B2 is a six-seat, single-engine helicopter powered by a Turbomeca Arriel 1D1 turboshaft engine. N71HD was delivered in May 2005 to Helinet. The pilot held an FAA commercial pilot certificate with rotorcraft-helicopter and instrument ratings and had 3,753 hours of flight experience.
Tests and Research
NTSB Materials Laboratory examined the horizontal stabilizer and tail rotor. Visual examination was consistent with an in-flight hard-body impact. Dent and scuff marks were compared to dimensions of common small drones and were consistent with the distance and orientation between a motor and center fuselage/battery compartment. Ultraviolet light inspection revealed no biological material. Fourier-transform infrared spectroscopy (FTIR) on the dented region identified foreign material consistent with a polycarbonate polymer, a material used in small drone construction. A small circular indentation (about 3 mm diameter) inside the dent exhibited features consistent with fore-to-aft impact with a hard, cylindrical object, such as a propeller shaft on many common small drones.
The damaged stabilizer was sent to the National Institute for Aeronautical Research (NIAR) impact dynamics lab for finite element modeling and impact kinematics simulation with a representative small drone model. The resultant damage was similar to the incident damage, though calculated relative speeds were higher than reported by the pilot. NIAR researchers noted differences in construction materials or mass could account for the discrepancy.
Damage Details
A small round dent partially punctured the leading edge of the horizontal stabilizer, with additional dents and scuff marks extending 6–8 inches outboard. One tail rotor blade exhibited a small gouge in the composite surface near mid-chord.
Other Incidents
The NTSB has investigated three positively confirmed drone collisions (DCA17IA202AB, DCA18IA264AB, DCA20IA081AB) and two additional reports with consistent evidence. Other reports were determined to be birds, foreign object damage, or maintenance issues. None resulted in substantial damage or injuries. Similar events include a 2018 EC130-B4 collision over Kauai and a 2018 Cessna 170 collision near Aurora, Oregon.
Regulatory Information
14 CFR 107.29 prohibits small UAS operation at night unless waived. 14 CFR 107.51(b) limits nighttime sUAS altitude to 400 feet above ground level unless within 400 feet of a structure. The reported collision altitude was 1,100 ft msl (about 828 ft agl).