Bell's fourth flight test vehicle. Photo courtesy of Bell
The FAA is proposing two additional special conditions for certification of the Bell 525 Relentess related to the prototype's unique fly-by-wire controls. Wednesday's proposed rules follow previous rules issued in May regarding putting in a suitable mode annunciation to flight crews for changes in flight control modes on the prototype.
Bell told R&WI that both conditions address gaps in current federal regulations. "The release of the special conditions is the culmination of several years of collaboration between the FAA and Bell," Bell told R&WI. "As such, the Bell 525 is well positioned to comply with these special conditions."
The new rules would require Bell to include in the 525's design measures addressing two features: control margin awareness and flight envelope protection. Both rules are directly related to the 525's fly-by-wire flight control system.
"The system design must ensure that the flight crew is made suitably aware whenever the means of primary flight control approaches the limits of control authority," the notice states of the control margin awareness condition. "For the context of this special condition, the term 'suitable' indicates an appropriate balance between nuisance and necessary operation."
Under flight envelope protection, the FAA details several requirements.
"Onset characteristics of each envelope protection feature must be smooth, appropriate to the phase of flight and type of maneuver, and not in conflict with the ability of the pilot to satisfactorily change rotorcraft flight path, speed, or attitude within the approved flight envelope," the document states, among other requirements.
The FAA is accepting comments for both notices until July 23.
Bell is targeting 2019 for achieving certification of the aircraft. A fourth 525 prototype flew for the first time last week, with the fifth and final one to join the fleet near year's end. Once certified, the Relentess will be the first commercial helicopter to be flown with fly-by-wire flight controls.
Update: This article has been updated to reflect comments from Bell.