Flight History
On July 6, 2016, at approximately 1148 central daylight time, an experimental Bell 525 helicopter (N525TA) broke up in flight and impacted terrain near Italy, Texas. The two test pilots on board received fatal injuries, and the helicopter was destroyed. The helicopter was owned by Bell Helicopter Textron, Inc. and operated under 14 Code of Federal Regulations Part 91 as a developmental flight test. Visual meteorological conditions prevailed at the time. The flight originated from Arlington Municipal Airport, Arlington, Texas.
Earlier that morning, at about 0630, the pilots, flight test engineers, and a chase helicopter crew briefed the planned flight. The accident helicopter, accompanied by a chase helicopter, was to proceed to the Arlington Initial Experimental Test Area, about 30 miles south of Arlington, to perform in-flight tests. The test objectives included evaluating engine loads at maximum continuous power, simulated two-to-one-engine failures, longitudinal roll oscillations, and run-on landings in a heavy, forward center-of-gravity configuration.
During the tests, the pilots used one-engine-inoperative (OEI) special training mode software, which reduced power output of both engines to simulate single-engine power. The test card for the simulated engine failure directed the pilot to delay response by about 1 second before lowering the collective to increase rotor rotation speed (Nr). The lowest allowable Nr was 86%; below that, the test would halt. Flight test engineers monitored real-time telemetry with oversight from the flight test director, who communicated with both helicopter crews.
About 0959, weather was deemed acceptable, and at 1038 the helicopter departed for the test area, followed by the chase helicopter. The pilots established maximum level flight airspeed (Vh) at 148 knots calibrated airspeed at 4,000 ft density altitude. After performing steady-heading sideslips and level turns, they began the simulated engine failure tests.
At about 1108, the pilots set the OEI training mode shaft horsepower as predetermined. The first three tests were in level flight at 102, 131, and 145 KCAS. Subsequent tests were at 155, 160, 165, and 175 KTAS, requiring a shallow descent. These tests resulted in rotor speed decay of 5 to 13% Nr, and recovery required lowering collective to near or below 50%. Build-up tests and recovery times were recorded. Prior to the final test, flight test engineers received warnings and alerts related to main rotor, tail rotor, pylon, and tail boom loads, which were expected as airspeed increased.
At about 1148, the final test was performed at 185 KTAS, the helicopter's never-to-exceed speed. OEI was engaged, and Nr dropped to about 91% within 1.5 seconds. The pilot reduced collective, stopping the decay, and Nr leveled around 92% with collective at 58%. About 7 seconds after arresting the decay, the structural dynamics engineer observed increased engine vibrations and called "knock-it-off." The test director radioed the call, and other engineers repeated it. The chase helicopter crew, positioned about 100 ft above and to the right of the accident helicopter, heard the call and observed the rotor blades flying high and the rotor appearing wobbly and slow. They radioed, "Hey, you're flapping pretty good," but received no response. About 21 seconds into the test, the main rotor severed the tail boom, and telemetry was lost. The chase crew saw the tail and fuselage jack-knife and debris separate. They radioed a major accident and landed near the wreckage to assist.
Personnel Information
The pilot held a letter of authorization (LOA) from the Federal Aviation Administration dated December 2, 2015, authorizing him as pilot-in-command of the Bell 525 experimental helicopter. He completed crew resource management training on January 12, 2015. He graduated from the United States Naval Test Pilot School in 2010 and worked on flight test projects involving Bell AH-1W and UH-1Y helicopters. He was hired by Bell Helicopter on September 23, 2013, as a pilot for the 525 program.
The copilot also held an LOA dated December 2, 2015, authorizing him as pilot-in-command of the Bell 525. He completed CRM training on January 12, 2015. He completed U.S. Navy flight training in 2000 and graduated from the USNTPS in 2006, later working on AH-1W and UH-1Y test programs. He was hired by Bell on August 2, 2010, as a pilot for the 525 program.
Aircraft Information
The accident helicopter was a conventional main rotor and tail rotor design. On April 25, 2016, it received its latest experimental research and development airworthiness certificate from the FAA. It was a manufacturing prototype being developed for certification under 14 CFR Part 29. The operating limitations specified that pilots must hold a temporary LOA, the helicopter must be maintained under an FAA-approved inspection program, day visual flight rules operations were authorized, and all flights must be within the Arlington Initial Experimental Test Area. The helicopter weight at the time was estimated at 19,975 lbs.
The Bell 525 had a five-bladed main rotor rotating counterclockwise when viewed from above, with a fully articulated system using elastomeric bearings. Fluid-elastic dampers moderated lead-lag motion. The five blades were identified by colored stickers: blue, orange, red, green, and white in order of advancing rotation. The helicopter also had a four-bladed, fully articulated, canted tail rotor with blades in blue, orange, red, and green. It was powered by two General Electric CT7-2F1 turboshaft engines and a Honeywell RE100BR auxiliary power unit. The flight control system was triple-redundant fly-by-wire with a triplex hydraulic system. The landing gear was retractable tricycle type.
The cockpit seated two pilots side-by-side with a center console. Each pilot had a cyclic side-stick, collective side-stick, and pedals. The instrument panel had four primary flight display/multifunction display panels. The center console contained Garmin Touch Control panels, landing gear handle, Nav/Com panel, and flight test switch panel including OEI training mode controls. Above the GTCs were engine control COSIF knobs. Each pilot had an additional display unit for flight test parameters such as density altitude, airspeeds, torque, load factor, rates, slip angle, and rotor flapping angles.
The OEI training mode was a GE software capability that simulated a single-engine failure without shutting down an engine. When engaged, both engines reduced power to mimic single-engine power. If power demand exceeded available power, Nr would droop. The pilot had to reduce collective or apply aft cyclic to recover Nr to 103%. Bell modified the production OEI software to eliminate automatic disengagement at 90% Nr, allowing testing at lower Nr. Manual exit methods included pressing the engine fail button on the GTC, exiting the OEI page, or moving the COSIF switch. The production version includes automatic disengagement at 90% Nr and other conditions.
The Power Situation Indicator on the PFD displayed Nr as a vertical scale above 90% and as an analog arc below 90%, with color bands. The CAS displayed warning messages; "ROTOR RPM LO" appeared when Nr dropped below 90%. An aural tone also sounded, but for the accident helicopter, it was a master warning tone not unique to low Nr. The test team considered this sufficient for development testing.
Meteorological Information
Arlington Municipal Airport, 31 miles north-northwest of the accident site, recorded at 1145: wind from 170° at 15 knots, 10 miles visibility, clear sky, temperature 32°C, dew point 23°C, altimeter 29.95 inHg. Hillsboro Municipal Airport, 15 miles south-southwest, recorded at 1136: wind from 190° at 16 knots gusting to 22 knots, 10 miles visibility, scattered clouds at 3,000 ft, temperature 31°C, dew point 23°C, altimeter 29.98 inHg.
Airport Information
The accident helicopter was a conventional main rotor and tail rotor design. On April 25, 2016, it received its latest experimental research and development airworthiness certificate from the FAA. It was a manufacturing prototype being developed for certification under 14 CFR Part 29. The operating limitations specified that pilots must hold a temporary LOA, the helicopter must be maintained under an FAA-approved inspection program, day visual flight rules operations were authorized, and all flights must be within the Arlington Initial Experimental Test Area. The helicopter weight at the time was estimated at 19,975 lbs.
The Bell 525 had a five-bladed main rotor rotating counterclockwise when viewed from above, with a fully articulated system using elastomeric bearings. Fluid-elastic dampers moderated lead-lag motion. The five blades were identified by colored stickers: blue, orange, red, green, and white in order of advancing rotation. The helicopter also had a four-bladed, fully articulated, canted tail rotor with blades in blue, orange, red, and green. It was powered by two General Electric CT7-2F1 turboshaft engines and a Honeywell RE100BR auxiliary power unit. The flight control system was triple-redundant fly-by-wire with a triplex hydraulic system. The landing gear was retractable tricycle type.