The US FAA has denied a request by Embraer to withdraw a proposed airworthiness directive (AD) requiring operators of as many as 77 US-registered E-170 aircraft to make electrical systems changes aimed at preventing possible loss of aileron, spoiler and rudder control if the aircraft were to experience a rotor disk failure in the turbofan engine.

The agency on 14 January will issue a final version of the directive, giving operators of certain aircraft 6,000 flight hours to make changes that may include modifying electrical wiring in overhead panels in the cockpit, changing power sources to air data probes and modular avionics units and modifying wiring to slat and flap actuator control electronics. The alterations, which the FAA says could cost as much as $5,628 per aircraft, were defined in three earlier Embraer service bulletins.

Spawning the bulletins and AD was Embraer's "re-assessment" of the E-170, which revealed that a worst case rotor disk failure could possibly cause the "loss of electrical power supply to the following aircraft systems: air data systems, ailerons and multi-function spoilers and rudder, which results in the loss of aircraft pitch and yaw control", according to the FAA.

During the comment phase after FAA proposed the rule in July, Embraer asked for its withdrawal based on "service experience".

The Brazilian airframer believes the "probability of a rotor burst combined with the probability of a disk trajectory that hits the specific wiring bundle is extremely rare". Instead, Embraer suggested issuing a special airworthiness information bulletin (SAIB) to alleviate concerns.

After consulting with Brazilian airworthiness authorities FAA determined that even if the risk is low for a catastrophic event stemming from a broken disk, aircraft certification requirements "do not permit the use of probability as a risk reduction", according to the AD.

Source: Air Transport Intelligence news