FAA to require flight mode annunciation in Bell 525

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The U.S. Federal Aviation Administration (FAA) plans to introduce a special condition to the certification of the Bell 525 Relentless, requiring the aircraft to give flight crews a “suitable mode annunciation” for events that significantly change the aircraft’s operating mode.

The 20,000-pound gross weight 525 offers a standard seating configuration for 16 passengers and two pilots, with a typical cruise speed of 155 knots for distances of over 500 nautical miles.
Bell hopes to certify the Bell 525 by the end of 2018. The 20,000-pound gross weight aircraft offers a standard seating configuration for 16 passengers and two pilots, with a typical cruise speed of 155 knots for distances of over 500 nautical miles. Bell Helicopter Photo

The FAA said the requirement, detailed in a notice of proposed special conditions published on Dec. 7, is necessary due to the 525’s fly-by-wire flight control system — the first such system in a civilian helicopter.

In the 525’s flight control system, pilot control inputs through the mechanically linked cockpit controls are transmitted electrically to each of the aircraft’s three flight control computers (FCCs). These control input signals are then processed and transmitted to the hydraulic control actuators that control the main and tail rotors. The FCCs process the pilot control input signals depending on the flight control mode in effect.

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“The applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature,” the proposal states. “These proposed special conditions contain the additional safety standards that the Administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards.”

In the absence of adequate standards under part 29 of the Federal Aviation Regulations to alert the pilot of the flight control mode in operation, the special condition requires a suitable mode annunciation for events that significantly change the operating mode of the system, but do not merit the traditional warnings, cautions, and advisories.

Bell is aiming for certification of the 525 in 2018. The company resumed the aircraft’s flight test program in July 2017, almost exactly a year after the program had been grounded following the fatal crash of the first prototype during flight testing.

Comments on the proposal close Jan. 22, 2018.

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