US regulators are proposing a number of special conditions relating to the certification of the Bell 525 Relentless super-medium-twin and its fly-by-wire controls.

The changes come in the wake of a July 2016 fatal accident involving the programme's second flight-test vehicle.

Two pilots were killed in the incident which was blamed on flight-control system design flaws that contributed to a feedback loop causing an uncontrollable vibration during high-speed, one-engine-inoperative testing.

In part, the vibrations were aggravated by a particular envelope protection feature which was not designed to cope with severe oscillations up to six times per second, instead amplifing the effect.

The US Federal Aviation Administration says that the 525's flight envelope protection (FEP) system must have limit values that are compatible with a number of requirements, including: structural limits; safe and controllable manoeuvring; rotor rotational speed limits; blade stall limits; and engine transmission torque limits.

In addition, the certification requirements appear to specifically address the causes of the 2016 crash: "The FEP system must not create unusual or adverse flight characteristics when atmospheric conditions or unintentional pilot action causes the approved flight action to be exceeded," says the FAA document.

If there is a single FEP failure, the helicopter should also continue to be controllable, says the FAA "without requiring exceptional pilot strength or skill". That also appears to allude to the fatal accident, in which the vibrations were so severe as to render pilot intervention highly difficult.

The Bell 525 and the Leonardo AW609 are the first two commercial rotorcraft to seek an airworthiness certificate with a fly-by-wire control system. Although new for the commercial market, Bell introduced an earlier version of the 525's fly-by-wire system on the V-22 Osprey tiltrotor, which it produced for the military with Boeing as a partner.

A separate special conditions document addresses the fact that current certification regulations "do not contain adequate or appropriate safety standards" to allow for the FBW system, says the agency's proposal.

It notes that as FBW controls do not have the "stops" associated with a hydro-mechanical system, pilots will be unaware that they approaching the limits of the control margins.

"Without a constant correlation between cockpit control and main or tail rotor actuator positions, the [flight control system] may not provide tactile control margin feedback to the pilot through cockpit control position relative to the control position physical stop or limit, for all flight conditions," it says.

That issue must be addressed to ensure certification, says the FAA.

Bell intends to achieve FAA approval for the Relentless in 2019, with service entry following shortly afterwards.

The manufacturer recently performed the maiden sortie of the programme's fourth test aircraft from its Amarillo, Texas site. Since returning to flight testing in July 2017, the 525 fleet has accumulated some 400h, says Bell.