The FAA published notice May 27 that it is seeking special conditions for certification of Bell’s 525 super-medium twin to address “a novel or unusual design feature associated with fly-by-wire flight control system (FBW FCS) functions that affect structural integrity of the rotorcraft.” The agency noted that “the applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature” and that the “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.”
Specifically, the FAA noted that current regulations are inadequate to determine the effect of systems and system failures from the FBW FCS and related systems (stability augmentation, load alleviation, flutter control and fuel management) on the structural integrity of the aircraft and structural performance. The problem is that existing standards, according to the FAA, do not account for the fact that active flight control systems provide automatic responses to inputs from sources other than the pilots and that “certification of these systems requires issuing special conditions.”
The FAA observed, “In the past, traditional rotorcraft flight control system designs have incorporated power-operated systems, stability or control augmentation with limited control authority, and autopilots...These systems are integrated into the primary flight controls and are given sufficient control authority to maneuver the rotorcraft up to its structural design limits...The FBW FCS advanced technology with its full authority necessitates additional requirements to account for the interaction of control systems and structures. The regulations defining the loads in 14 CFR Part 29 do not fully account for the effects of systems on structural performance. Automatic systems may be inoperative or they may operate in degraded mode with less than full system authority and associated built-in protection features. Therefore it is necessary to determine the structural factors of safety and operating margins such that the probability of structural failures due to the application of loads during FBW FCS malfunctions is not greater than that found with rotorcraft equipped with traditional flight control systems. To achieve this objective and to ensure an acceptable level of safety, it is necessary to define the failure conditions and their associated frequency of occurrence.”
The FAA further points out, “Traditional flight control systems provide two states, either fully functioning or completely inoperative. These conditions are readily apparent to the flight crew. Newer active flight control systems have failure modes that allow the system to function in a degraded mode without full authority and associated built-in protection features. As these degraded modes are not readily apparent to the flight crew, monitoring systems are required to provide an annunciation of degraded system capability.”
The FAA went on to publish a detailed list of special conditions for the 525. Significant among them: With the FBW FCS system, fully operative strength requirements of Part 29 need not be investigated beyond limit conditions “when it can be shown that the rotorcraft has design features that will not allow it to exceed those limit conditions.” In evaluating failure conditions shown not to be extremely improbable, loads assume failures begin in 1g level flight and include pilot corrective actions. The special conditions also provide that special periodic inspections, daily checks and pre-flight inspections may be used in lieu of failure-detection and -indication systems, but “must be limited to components that are not readily detectable by normal detection and indication systems and where service history shows that inspections will provide an adequate level of safety.”