N602RNRaytheon COBRA · 2006

Note: This case was reclassified from an accident to an incident as a result of applicable revisions to 49 Code of Federal Regulations Part 830.2, as amended at 75 FR 51955, Aug. 24, 2010. The case was previously identified under accident number DEN08FA160.

On September 24, 2008, at 0841 Pacific daylight time, an experimental Raytheon Cobra, unmanned aircraft system (UAS), N602RN, was destroyed when it lost engine power and subsequently impacted in desert terrain near Whetstone, Arizona. The aircraft was registered to and operated by Raytheon Missile Systems Advanced Programs of Tucson, Arizona. There were no injuries to persons on the ground. Visual meteorological conditions prevailed for the Title 14 Code of Federal Regulations Part 91 test flight, and no flight plan was filed. The aircraft had departed from a paved airstrip at the Unmanned Vehicles International (UVI), Incorporated facility near Whetstone, Arizona, at approximately 0809.

The purpose of the flight was to test a photo-mapping package for the U. S. Army Corp of Engineers. The Cobra UAS was to serve as an aerial platform for the device. The test consisted of the aircraft performing an automatic takeoff and climb to 1,000 feet above ground level (agl), fly a programming leg on which the internal mapping system would align, and then enter a grid series of 17 parallel, 5 mile long north-to-south, south-to-north legs that progressed west to east along which the camera would photograph and record the terrain over which it was being flown. Following the mapping legs. The aircraft then was to descend to pattern altitude and enter a non-standard traffic pattern from which it would execute an automatic landing.

The aircraft was computer controlled through software that established altitudes, airspeeds and GPS waypoints for the aircraft to fly. Commands to the vehicle were sent via omni radio and executed by an internal pilot that sat at a control console located in the back of a company half-ton truck. For this flight, a Raytheon internal pilot was instructing a U. S. Army student pilot, who manned the computer console, monitored the wing camera display, and held the remote control box in case it was needed. Other persons associated with the vehicle operation were a Test Director, a company observer, a company crew chief and a U. S. Army crew chief in training. All of these persons were in radio communication with the internal pilots in the truck.

The aircraft underwent preflight and systems tests. The aircraft's engine was started at 0805 and the aircraft's left wing-mounted camera was activated. Following automatic and manual engine run ups, the aircraft was taxied to a position on the UVI runway at which time the student pilot entered the command for an automatic takeoff. The aircraft took off to the south and climbed to 5,200 feet msl (1,000 feet agl), and began its programmed mission.

Following the final pass, the aircraft was to enter a single figure-eight pattern that was to set it up for landing. During the turn from north to east, the aircraft engine lost all power. The autopilot then commanded the aircraft to descend to hold airspeed and maintain it's ground track profile. An observer watching the aircraft from the ground queried the internal pilot at the command station regarding the aircraft's continuing descent. The internal pilot then noted the red-highlighted warning on the console indicating the engine RPMs were at zero. The autopilot executed a turn to the south, which aligned the aircraft with the runway at an altitude approximately 800 feet above the ground. The internal pilot told the student pilot at the console to input the "Land Now" command. The student pilot switched the remote control box to "manual" instead and the aircraft pitched down approximately 45 degrees and descended rapidly. The Test Director, who was also watching from the ground, radioed the command to "pull up." In the wing camera monitor, the runway was visible and coming up fast. The internal pilot switched the remote control box back to "automatic." The aircraft leveled off and corrected back over the runway at an altitude of approximately 60 to 70 feet. At approximately 1,400 feet down the runway, the aircraft entered a steep right diving turn and impacted the terrain.

The aircraft was automatic piloted on a pre-programmed schedule. A company-trained internal pilot and a student pilot monitored the aircraft's progress through laptop software programmed into the aircraft's autopilot.

Internal Pilot

The internal pilot was a Raytheon employee and held a commercial pilot's certificate with single-engine land, instrument airplane, and glider ratings. The internal pilot also held a flight instructor's certificate for single-engine land airplanes and gliders.

The internal pilot held a second class medical certificate with no restrictions, dated May 15, 2008.

The internal pilot reported having approximately 4,000 total flying hours, 3,000 of which he stated were in single-engine airplanes. The internal pilot reported having about 1,000 hours in gliders and approximately 60 hours piloting Cobra aircraft.

Student Pilot

The student pilot was an employee of the U. S. Army Corps of Engineers. The student pilot reported no civilian flight certificates or ratings.

The FAA special program letter covering Raytheon's operation of the Cobra UAS at UVI required that the internal pilot hold at least a private pilot certificate, a third class medical certificate with no limitations, and have a minimum of three years experience operating radio-controlled aircraft.

The aircraft was serial number 002. It received an experimental airworthiness certificate from the FAA on January 12, 2007. The aircraft was being operated under a special program letter authorizing Raytheon Missile Systems to fly the Cobra aircraft within the confines of the UVI facility.

A Certificate of Authorization (COA) was not required for the flight. The flight was approved through the experimental certificate program letter. Although carrying a military program package, the flight was considered civilian and not military.

The aircraft was 9 feet long and had a wingspan of 10 feet. The aircraft's empty weight was 65 pounds. The maximum gross weight of the aircraft was 110.3 pounds. The gross weight of the aircraft on the day of the accident was approximately 106 pounds at takeoff. The center of gravity was at 29.09 inches with a mean aerodynamic chord of 26.3 percent, placing the vehicle within its operating envelope.

A single Desert Aircraft DA-150 2-cycle, horizontally opposed, carbureted, 16.5 horsepower engine powered the aircraft. The aircraft operated within an airspeed regime of 45 to 55 knots for climbs, cruise, descent, and landing. The stall speed for the aircraft was approximately 38 knots. The engine, serial number DA2178, had a total operating time of 35.9 hours at the time of the accident. The company's established time between overhauls for the engine was 250 hours.

The aircraft was controlled by a ground-based command center. This consisted of Piccolo II software run by a laptop computer, into which commands were programmed and then sent to the autopilot through an omni radio transmitter. The signals were then received and integrated with the aircraft's on-board Global Positioning system (GPS) receiver, which in turn provided commands to the autopilot. A remote control box with manual power and steering controls was also available and served as a backup control system to the command center.

The aircraft underwent a condition inspection on January 31, 2008. At that time the aircraft had 16.1 hours total flying time. Prior to the accident, the aircraft had flown 57 times. At the time of the accident, the aircraft had logged a total flying time of 25.7 hours.

At 0755, weather conditions at the Sierra Vista Municipal Airport (KFHU), Sierra Vista Arizona, 10 miles south of UVI were clear skies, 70 statute miles visibility, variable winds at 1 knot, temperature 70 degrees Fahrenheit (F), dew point 41 degrees F, and altimeter 30.21 inches.

At 0855, weather conditions were clear skies, 70 statute miles visibility, winds 100 degrees at 8 knots, temperature 75 degrees F, dew point 45 degrees F, and altimeter 30.22 inches.

The aircraft came to rest 1,417 feet from the approach end of the runway and 118 feet, 8 inches right of the runway centerline in sage brush covered desert terrain. The wreckage and debris trail fell along a 290-degree magnetic heading.

The primary impact point was a 4-foot long, 6-inch wide and 1-inch deep ground scar that ran along a 281-degree magnetic heading. Small pieces of fiberglass consistent with the wing structure were found along the ground scar. The ground scar ended approximately 21 feet prior to the aircraft main wreckage. Small pieces of fiberglass and foam filler material rested between the ground scar and the main wreckage along a 290-degree path.

The aircraft main wreckage consisted of the payload compartment that contained the photo-mapping package, fuel tanks, avionics and the engine, the left wing, the tail boom and empennage, the landing gear, and propeller.

The forward portion of the payload compartment was crushed inward. The nose gear was broken aft. All three propeller blades were fractured. Two blades were broken rearward at the hub. One propeller blade rested just in front of the main wreckage. The left main landing gear was broken aft. The right main landing gear was broken aft and separated. The vehicle's right wing was broken aft from the carry through spar and separated. The spar was bent aft. The left wing was broken aft and downward, but remained attached to the payload compartment. The left wing aileron was broken at the inboard hinge. The outer two-thirds of the aileron remained attached at the outboard hinge. The inner portion was missing. The tail boom was twisted to the right approximately 70 degrees. The empennage was intact.

The right wing rested in a sage bush approximately 14 feet north-northeast of the main wreckage on a magnetic heading of 324-degrees. The inboard portion of the left wing aileron was located approximately 17 feet north-northeast of the main wreckage on a magnetic heading of 335-degrees.

Flight control continuity was confirmed at the accident site.

The aircraft wreckage was recovered and transported to the Raytheon facility in Tucson, Arizona, for further examination.

The aircraft had flown on the morning of September 23, 2008, over the UVI facility, with the Army package on board. The aircraft was flown for approximately 38 minutes. On landing, immediately after the airplane touched down on the airstrip, the engine stopped. The aircraft rolled to a stop. The test team retrieved the aircraft and the Army team downloaded the data from their mapping package. The download took a couple of hours to perform.

Raytheon and the Army Corps of Engineers had planned for a second flight that day. The aircraft was fueled, all preflight checks were performed and the ground team positioned the aircraft next to the runway in preparation for flight. The engine was started. During the power checks as the power was advanced to the higher rpms, the engine stopped. The team re-examined the preflight actions, checked fuel to the engine, and attempted to restart the engine. The engine failed to operate. Several more attempts were made to start the engine, without success. The flight test director then determined to suspend flying for the day, return the aircraft to the Raytheon facility in Tucson, and trouble shoot the engine there.

At the Raytheon facility, the carburetor was replaced and the engine was run for approximately 5 minutes. The aircraft was then loaded on to a truck for the next day's flight.

On September 24, 2008, the aircraft was operated for 39.3 minutes. During the flight, the engine ran at a near-constant power setting between 5,200 to 5,400 rpms for approximately 30 minutes.

Following the accident on September 24, 2008, the Piccolo software file was downloaded from the command center and sent to Cloudcap Technologies, the autopilot, command center and software manufacturer, for examination. The data set showed that when the command to "land now" was selected, the autopilot selected a waypoint that was collocated over the end of the airstrip. While enroute to that waypoint, on an approximate heading of 170-degrees, the command center sent a command to the Cobra that read "Heading command 244-degrees."

On September 25, 2008, the aircraft's engine was taken to Desert Aircraft (DA), Incorporated for disassembly and examination.

The ignition module was removed and examined and showed no damage. It was then installed on a good engine and ran the engine. The DA technician conducting the test noted that one of the two coils was bad, but not to the point that it prevented the engine from running.

The engine's carburetor was examined. The base plate was removed and the rubber diaphragm that covered the fuel ducting was removed. A piece of white plastic material approximately 1/64 inch was observed in the elbow of the fuel inlet. Fuel was present in the carburetor. The high-end and low-end needles were examined. The high-end needle adjustment was at a setting that provided a very lean mixture.

The engine body was examined. The engine was made of aluminum and finished in its natural color. The external examination showed that the right cylinder head had a pinkish tint to it. The crankshaft was turned a quarter revolution by hand. It moved freely with no resistance noted.

The two spark plugs were removed. The diodes were round and light brown in color.

The left cylinder was removed exposing the piston, piston rings, piston rod and pins. The piston was unremarkable. The top of the piston showed even, light browning from normal firing. The piston also showed light lubrication.

The right cylinder was removed exposing the piston, rings, rod and pins. The top of the piston was dark brown and a heavier sludge of lubricant was observed around the piston ring. The piston ring showed heavy wear. The piston wall was blue in color and showed rubs and scoring marks running in the direction of the piston.

A DA technician stated that there was clear evidence of "blowby," on the right piston, a condition where hot gases from the cylinder firing are getting beyond the piston ring and down the cylinder walls. Over time, this condition degrades the piston and engine performance, and can lead to engine failure.

The carburetor that was on the engine during the previous day's flight was examined. The position of the high-end needle was noted to be the same as the position of the screws on the crashed carburetor. The carburetor was installed as received on a good engine and an attempt to run that engine was made. The engine started and operated in the lower rpm regime, approximately 2,000 to 2,500 rpm. When the power was advanced to 4,500 to 5,000 rpm, the engine would surge and sputter, and then stop. The DA technician then set the carburetor's high-end needle to the factory settings. The engine was restarted and ran according to specifications throughout all power settings selected.

Raytheon's flight profiles for the Cobra consisted of the aircraft doing an automatic takeoff and a full power climb at 5,000 to 5,400 rpm until level off. At level off, the power was reduced and some maneuvers were executed to confirm the integrity of the GPS guidance system. Then the power was advanced to a setting of 5,000 to 5,400 rpm and the aircraft would cruise at this power setting for the duration of its mission. This period of high power cruise was approximately 25 minutes to 30 minutes long. Raytheon has kept the Cobra in the air at these power settings for much longer durations. Depending on the gross weight, the Cobra can remain aloft for up to 3 hours. The weight at which the Cobra was operated on September 24 and 25, 106 pounds, restricted the Cobra to approximately one hour and 30 minutes of total flight time.

According to Desert Aircraft, the engine requires a fuel-oil mixture to lubricate and cool the engine. Raytheon uses a 91-octane fuel with 2-cycle oil mixed in at a ratio of 1 part oil to 1,000 parts of fuel. DA recommends using a lighter mineral oil initially to "break in" the engine. At the time the engine was examined at DA, Raytheon representatives expressed they were not aware as to how long the engine would need to be run with mineral oil before going to the 91-octane mix. Desert Aircraft recommended using the mineral oil for the first 10 hours of engine life before going to synthetic oil. Technicians at Desert Aircraft stated that for constant, high power operation of the engine they recommend a fuel ratio of 1 part synthetic oil to 40 parts of fuel. One technician stated that the DA-160 engine is the most popular engine being used by radio-controlled (RC) airplane hobbyists flying typically one-quarter scale sized model aircraft. When being operated by hobbyists, they are usually changing power settings frequently during flight of their aircraft to do aerobatics, rapid climbs and descents, and so forth. This constant change in power aids in the lubrication and cooling of the engine.

When asked about inspections on the DA-160 engine for potential problems, the DA technicians did not have specific times in mind. They mentioned that most hobbyists contact them if they have a problem, but most times those problems do not manifest themselves until a hobbyist crashes his/her RC aircraft. One technician said that based on the wear he observed on this engine; an inspection at 40 to 50 hours would be prudent.

The engine was retained for further examination.

The engine was examined again in Dallas, Texas, on January 20, 2009. The examination showed that both cylinders exhibited a pint tint to their exterior. The right cylinder interior wall exhibited scoring marks consistent with interference between the piston and the wall of the cylinder.

According to the Airframe and Powerplants mechanic who conducted the examination, engine lubrication and cooling is dependent on the oil to fuel ratio that is supplied. If the engine is operated with less oil to fuel, the engine will have insufficient lubrication and heat will build up. When the heat becomes excessive, the pistons expand to a point that piston-to-cylinder wall interference can occur resulting in engine seizure and complete power loss.

This was the third accident involving a Raytheon Cobra UAS that the National Transportation Safety Board had investigated. On October 24, 2007, N605RN crashed at the UVI facility while preparing to enter the landing pattern under manual control. On July 28, 2008, N601RN collided with a stadium light pole at the United States Air Force Academy, Colorado Springs, Colorado, when the vehicle overshot the final turn of the landing pattern.

The National Transportation Safety Board also conducted an investigation into an accident that occurred on April 25, 2006, involving a General Atomics Predator B UAS that crashed in Nogalas, Arizona. On October 21, 2008, the National Transportation Safety Board issued 22 recommendations on UAS safety to both the operator and the Federal Aviation Administration. Recommendations to the FAA included the requirement for established procedures for handling piloted aircraft emergencies be applied to unmanned aircraft systems, and that all unmanned aircraft systems operators report to the FAA in writing within 30 days of occurrence, all incidents and malfunctions that affect safety; require that operators are analyzing these data in an effort to improve safety; and evaluate these data to determine whether programs and procedures, including those under air traffic control, remain effective in mitigating safety risks.