Photo courtesy of Bell Helicopter
The No. 1 525 prototype’s main rotor blades appear to have struck its tail and nose during a high-speed, engine-out test that ended with the July 6 crash that killed its two pilots, Rotor & Wing International has learned.
“We saw signs consistent with a blade strike on the tailboom,” the National Transportation Safety Board’s investigator-in-charge for the probe, John Lovell, told R&WI, adding that investigators also believe “the nose was struck.”
The fly-by-wire Bell Helicopter super-medium, twin-engine helicopter broke up in midair during a flight to expand its operating envelope that included a test of the 525’s performance in one-engine-inoperative conditions as the aircraft approached its never-exceed speed (Vne), Lovell said. Bell early last March said it had flown the 525 above 200 kt in a shallow dive. A flight tracking service, Flightradar24, had reported July 6 that its last data set on the 525 flight put the aircraft at 199 kt at 1,975 ft.
Prior to the breakup, Lovell said, “data indicates that main rotor rpm dropped significantly.” He also said some of the aircraft’s main rotor blades “appeared to have dropped from their normal plane” of rotation.
Test pilots in a Bell 429 chase aircraft reported that some of the main rotor blades were moving out of plane before the aircraft broke up over Italy, Texas, about 30 nm south-southwest of the flight’s launch point, Bell’s Xworx research center at Arlington Municipal Airport.
The data also contains indications of significant vibration in the main rotor during the accident sequence, Lovell said. Investigators are working to identify when that vibration began and its cause.
Lovell leads a team of six investigators from NTSB’s Washington headquarters. Bell is a party to the NTSB’s investigation as is General Electric, maker of 525’s CT7-2F1 engines. The FAA by law is a party to NTSB probes.
Those investigators have an abundance of data to examine in seeking clues to the causes of the accident.
“This is one of the few accidents where we have so much data,” Lovell said. That includes telemetry that the 525 was transmitting back to the Xworx, telemetry from previous 525 flights and the observations of the test pilots in the chase 429. NTSB specialists have been able to extract data from a flight test recorder (or telemetry module) retrieved from the heavily burnt wreckage, Lovell said, adding that the recorder data aligns with the flight’s telemetry. He said there don’t appear to have been any significant breaks in the telemetry stream.
“We’re going through all that data to understand what happened and in what sequence,” Lovell said. “We’re still looking at the plots of telemetry data.”
Bell also has a full systems simulator for the 525 that includes main rotor mast, swashplate and actuators and other flight control and systems hardware as well as the current version of the flight control’s software. The simulator can be used to re-create actual flights using telemetry data and to test refinements of flight control software and systems.
The manufacturer has described the 525 as the first commercial helicopter using all fly-by-wire flight controls. Its development will continue, the head of Bell parent Textron said.
“We do remain committed to the Bell 525 program,” Textron Chairman, President and CEO Scott Donnelly told financial analysts July 22. “We'll work to ensure the aircraft will be a safe, reliable and high-performance helicopter.”
Bell had aimed to achieve FAA certification and begin customer deliveries of the 525 next year, but Donnelly said Bell has suspended flight testing until the investigation determines the crash’s causes. Non-flight certification and 525 program activities are proceeding, he said, but “we do not have an estimate as to when flight testing might resume or the length of delay in certification or first deliveries.”