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NTSB Final Report: Manassas, VA

 

Location:

Manassas, VA

Accident Number:

DCA19LA154

Date & Time:

06/04/2019, 0719 EDT

Registration:

N83AU

Aircraft:

AURORA PAV

Aircraft Damage:

Substantial

Defining Event:

Flight control sys malf/fail

Injuries:

N/A

Flight Conducted Under:

Part 91: General Aviation - Flight Test

Analysis

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.

Probable Cause and Findings
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. 

Findings

Aircraft

Auto flight system - Malfunction (Cause)

Factual Information

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

None.

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.

PERSONNEL INFORMATION

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.

AIRCRAFT INFORMATION

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.

METEOROLOGICAL INFORMATION

Weather was clear with calm wind.

COMMUNICATIONS

There were no communications difficulties with the accident flight.

AERODROME INFORMATION

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.

RECORDED DATA

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

Uncontrolled descent

Flight control sys malf/fail (Defining event)


 

Aircraft and Owner/Operator Information

Aircraft Make:

AURORA

Registration:

N83AU

Model/Series:

PAV

Aircraft Category:

Helicopter

Year of Manufacture:

 

Amateur Built:

No

Airworthiness Certificate:

Experimental

Serial Number:

1

Landing Gear Type:

Skid

Seats:

0

Date/Type of Last Inspection:

 

Certified Max Gross Wt.:

900 lbs

Time Since Last Inspection:

 

Engines:

 Electric

Airframe Total Time:

 

Engine Manufacturer:

Magicall

ELT:

 

Engine Model/Series:

 

Registered Owner:

Aurora Flight Sciences

Rated Power:

 

Operator:

Aurora Flight Sciences

Operating Certificate(s) Held:

Certificate of Authorization or Waiver (COA)

 

Meteorological Information and Flight Plan

Conditions at Accident Site:

Visual Conditions

Condition of Light:

Day

Observation Facility, Elevation:

KHEF, 200 ft msl

Distance from Accident Site:

0 Nautical Miles

Observation Time:

1056 UTC

Direction from Accident Site:

Lowest Cloud Condition:

Clear

Visibility

10 Miles

Lowest Ceiling:

 

Visibility (RVR):

 

Wind Speed/Gusts:

Calm /

Turbulence Type Forecast/Actual:

/

Wind Direction:

 

Turbulence Severity Forecast/Actual:

/

Altimeter Setting:

30.19 inches Hg

Temperature/Dew Point:

8°C / 7°C

Precipitation and Obscuration:

 

Departure Point:

Manassas, VA (KHEF)

Type of Flight Plan Filed:

None

Destination:

Manassas, VA (KHEF)

Type of Clearance:

None

Departure Time:

0701 EDT

Type of Airspace:

Class D

 

Airport Information

Airport:

Manassas (KHEF)

Runway Surface Type:

Asphalt

Airport Elevation:

200 ft

Runway Surface Condition:

Dry

Runway Used:

34L

IFR Approach:

None

Runway Length/Width:

3500 ft / 100 ft

VFR Approach/Landing:

Precautionary Landing

 

Wreckage and Impact Information

Crew Injuries:

N/A

Aircraft Damage:

Substantial

Passenger Injuries:

N/A

Aircraft Fire:

None

Ground Injuries:

N/A

Aircraft Explosion:

None

Total Injuries:

N/A

Latitude, Longitude:

38.721944, -77.517778


Administrative Information

Investigator In Charge (IIC):

William R English

Report Date:

03/13/2020

Additional Participating Persons:

 

Publish Date:

03/13/2020

Note:

The NTSB traveled to the scene of this accident.

Investigation Docket:

http://dms.ntsb.gov/pubdms/search/dockList.cfm?mKey=99537





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