National Transportation Safety Board
Aviation Accident Final Report
Date & Time:
06/04/2019, 0719 EDT
Flight control sys malf/fail
Flight Conducted Under:
Part 91: General Aviation - Flight Test
The aircraft experienced a vibrational excitation environment which exceeded the previously measured accelerations used to establish an autopilot ground-sensing threshold. The excessive oscillations activated the ground-sensing system, which transitioned the autopilot from Autoland into "ground" mode and commanded a reduction in rotor speed.
The National Transportation Safety Board determines the probable cause(s) of this accident to be:
a loss of hover lift thrust provided by the eight lift motors due to a false on-ground rotor speed reduction command, resulting from excessive vibration.
Auto flight system - Malfunction (Cause)
HISTORY OF FLIGHT
On June 4, 2019, at 0709 eastern daylight time (EDT), an Aurora Flight Sciences Pegasus Passenger Air Vehicle (PAV) unmanned aircraft system, N83AU, crashed during landing on runway 34L at the Manassas Regional Airport, Manassas, VA. There were no injuries, and the aircraft was substantially damaged. The aircraft was operated as a developmental test flight under the provisions of 14 CFR part 91 and an FAA Certificate of Waiver or Authorization (COA).
The aircraft was positioned on runway 34L for a vertical takeoff and landing, along with low speed maneuvering test. The aircraft took off normally and the pilot, who controlled the aircraft from a ground station in a building on the east side of the airport, entered pre-planned flight test stability check maneuvers, including brief lateral maneuvers followed by forward translations along the runway centerline in accordance with the test plan. The flight test engineers noted some brief data dropouts and abnormal motor speeds, and the team decided to terminate the flight. The pilot entered the autoland command, and the aircraft initially responded. After a small descent, the aircraft motors went to idle and the aircraft descended vertically at a high rate and impacted the runway, breaking the aft booms, the horizontal stabilizer, and other significant structure. The #3 motor exhibited a brief flash of fire or sparking on impact.
The aircraft remained with the planned test area.
INJURIES TO PERSONS
DAMAGE TO AIRCRAFT
There was structural damage to the booms, horizontal tail, vertical tails, and warping of the main aircraft structure. The aft section of the booms, with the two aft-most lift motors, were fractured.
The pilot, age 46, was an FAA certified commercial pilot airplane and helicopter, and reported 3,500 hours total time.
The aircraft was also supported by a visual observer stationed nearby the runway, and a team of flight test engineers in the control station located in a building on the east side of the airport.
The PAV is a developmental unmanned aircraft intended for Urban Air Mobility applications. The aircraft consists of a center fuselage which contains two (simulated) occupants, the main power battery, avionics and flight control computers, and an essential bus flight control battery. At the aft of the fuselage is an electric motor with pusher propeller for fixed-wing mode flight (this motor was disabled during the accident flight).The aircraft has a forward canard, main wing, and an aft mounted horizontal stabilizer with dual vertical stabilizers. A boom runs along each side containing 8 total vertical takeoff/landing (VTOL) electric motors.
The aircraft was controlled from a ground control station located at the Manassas Airport, and linked to the aircraft via a 2.4gHz connection with two radio nodes.
The normal landing maneuver for the PAV was an autoland, which establishes a vertical descent and transitions to on-ground mode using a combination of squat switches and a time derivative of acceleration known as "jerk" logic. When the aircraft contacts the ground and either the squat switches close or an acceleration spike is detected, the logic switches to ground mode and commands the VTOL motors to idle. The aircraft was equipped with a radar altimeter, but in this test configuration it was not used for ground detection in the autoland sequence.
Weather was clear with calm wind.
There were no communications difficulties with the accident flight.
The Manassas Regional Airport/Harry P. Davis Field (HEF), was located about 3 miles southwest of Manassas, Virginia, at an elevation of 192 feet mean sea level (msl). The airport has an FAA Air Traffic Control Tower which operates from 0630-2230 local time daily. The airport has two parallel runways 16L/34R, and 16R/34L (the accident runway). Runway 16R/34L had an asphalt surface 3,175 feet long and 100 feet wide. There were no reported abnormalities with the airport or facilities.
A review of the recorded data was provided by the operator/manufacturer revealed that airframe vibration occurred in a resonant mode and was transmitted through the structure into the flight controller. The accelerations resulting from the vibrations briefly exceeded the jerk logic threshold and the aircraft entered the ground mode, subsequently commanding the motors to shutdown.
History of Flight
Flight control sys malf/fail (Defining event)
Aircraft and Owner/Operator Information
Year of Manufacture:
Landing Gear Type:
Date/Type of Last Inspection:
Certified Max Gross Wt.:
Time Since Last Inspection:
Airframe Total Time:
Aurora Flight Sciences
Aurora Flight Sciences
Operating Certificate(s) Held:
Certificate of Authorization or Waiver (COA)
Meteorological Information and Flight Plan
Conditions at Accident Site:
Condition of Light:
Observation Facility, Elevation:
KHEF, 200 ft msl
Distance from Accident Site:
0 Nautical Miles
Direction from Accident Site:
Lowest Cloud Condition:
Turbulence Type Forecast/Actual:
Turbulence Severity Forecast/Actual:
30.19 inches Hg
8°C / 7°C
Precipitation and Obscuration:
Manassas, VA (KHEF)
Type of Flight Plan Filed:
Manassas, VA (KHEF)
Type of Clearance:
Type of Airspace:
Runway Surface Type:
Runway Surface Condition:
3500 ft / 100 ft
Wreckage and Impact Information
Investigator In Charge (IIC):
William R English
Additional Participating Persons:
The NTSB traveled to the scene of this accident.