AC 25.671-1
13.1.1 Conditions that Modify Command Signal or System Response.
13.1.1.1 Perturbations, as referred to in this section, are erroneous signals resulting from either internal or external causes, such that the intended command or control signals are altered from their intended characteristics.
13.1.1.2 Internal causes include, but are not limited to, the following:
• Loss of data bits.
• Unwanted transients.
• Computer capacity saturation.
• Processing of signals by asynchronous microprocessors.
• Adverse effects caused by transport lag.
• Poor resolution of digital signals.
• Sensor noise.
• Corrupted sensor signals.
• Aliasing effects.
• Inappropriate sensor monitoring thresholds.
• Frozen or erroneous values
• Structural interactions (such as control surface compliance or coupling of structural modes with control modes) that may adversely affect the system operation.
13.1.1.3 External causes include, but are not limited to, the following:
• Lightning.
• Electromagnetic interference effects (e.g., motor interference, ship’s electrical power and power switching transients, smaller signals if they can affect flight control, transients due to electrical failures, etc.).
• High intensity radiated fields (HIRF).
13.1.2 Compliance.
13.1.2.1 Erroneous signals resulting from the internal and external causes listed in paragraphs 13.1.1.2 and 13.1.1.3 may lead to malfunctions that produce unacceptable system responses, such as limit cycle or oscillatory failures, control surface runaway conditions, loss or reduction of control authority, unresponsive control surface, erroneous control commands, false indication/alert, and command offsets etc. Since any of these responses could present a flight hazard, it is imperative that the command signal remain continuous and free from perturbations and common cause failures. Accordingly, special design measures should be employed to 22
AC 25.671-1
maintain system integrity to ensure capability for continued safe flight and landing. These special design measures can be evaluated through the systems safety analysis process, provided specific care is directed to development methods and quantitative/qualitative showings of compliance. System safety analysis process and probability guidelines associated with hazard assessments is established in AC 25.1309-1B, or later revision.
13.1.2.2 Applicants should consider the following when evaluating compliance with the requirements of §§ 25.671, 25.1301, and 25.1309:
• The flight control system should continue to perform its intended function regardless of any malfunction in the integrated systems environment of the airplane.
• Any system in the aerodynamic loop that has a malfunction should not produce hazardous loads or hazardous deviations in the flight path and must automatically recover its ability to perform critical functions upon removal of the effects of that malfunction.
• Systems in the aerodynamic loop should not be adversely affected during or after exposure to any sources of a malfunction.
• Any disruption to an individual unit or component that occurs as a consequence of a malfunction, and that requires annunciation and flightcrew action, should be evaluated to ensure that the failure can be recognized by the flightcrew, and that the recommended flightcrew action can be expected to result in continued safe flight and landing.
• An automatic change from a normal to a degraded mode that is caused by spurious signal(s) or malfunction(s) should meet the probability guidelines associated with the hazard assessment established in AC 25.1309-1B, or later revision.
• The flight control system should operate appropriately considering other systems’ behaviors. The applicant should assure the compatibility of automatic functions that may dynamically interact or affect flight control in both normal and anticipated abnormal operating conditions and ensure that such interactions (either by airplane response, or by data transfer or data saturation) do not result in inappropriate flight control responses. This should include any potential for adverse coupling of the dynamics of one automated flight function with another (e.g., coupling between automated thrust and flight control functions).
AC 25.671-1
13.1.3 Additional Testing.
The complexity and criticality of the FBW flight control system typically necessitates additional laboratory testing beyond that required as part of individual equipment qualification, validation, and software verification. It should be shown that either the FBW flight control system signals cannot be altered unintentionally, or that altered signal characteristics meet the following criteria:
• Stable gain and phase margins are maintained for all control surface closed-loop systems. Pilot control inputs (pilot in the loop) are excluded from this evaluation.
• Sufficient pitch, roll, and yaw control power is available to provide control for continued safe flight and landing considering all FBW flight control system signal malfunctions that are not extremely improbable.
• The effect of spurious signals on the systems that are included in the aerodynamic loop do not result in unacceptable transients or degradation of the airplane’s performance. Specifically, signals that would cause a significant un-commanded motion of a control surface actuator are readily detected and deactivated, or the surface motion is arrested by other means in a satisfactory manner. Small amplitude residual system motions and oscillations may be acceptable if they are easily controllable.
• It should be shown that the output from the control surface closed-loop system does not result in un-commanded, sustained oscillations of flight control surfaces that can hinder safe flight and landing. The effects of minor instabilities may be acceptable, provided they are thoroughly investigated, documented, and understood. An example of an acceptable condition would be one where a computer input is perturbed by spurious signals, but the output signal remains within the design tolerances, and the system is able to continue in its selected mode of operation unaffected by that perturbation.
13.1.4 Demonstrating System Characteristics. In the context of showing and demonstrating these system characteristics, an acceptable means of compliance with § 25.671 should include:
• Systematic airplane or laboratory validation that includes a realistic representation of all relevant interfacing systems and associated software, including the control system components that are part of pitch, roll, and yaw control. Closed-loop airplane simulation/testing should be included in this laboratory validation.
• Laboratory or airplane testing to demonstrate unwanted coupling of electronic command signals and their effects on the mechanical actuators and interfacing structure over the spectrum of operating frequencies.
• Analysis or inspection to substantiate that physical or mechanical separation and segregation of equipment or components are used to minimize any potential hazards.
AC 25.671-1
13.1.5 Demonstrating Signal Integrity. A successful demonstration of signal integrity should include all elements that contribute command and control signals to the “aerodynamic closed-loop,” which actuates the aerodynamic control surfaces (e.g., ailerons, elevator, rudder, stabilizer, flaps, spoilers, etc.). The “aerodynamic closed-loop” should be evaluated for the normal and degraded modes. Elements of the integrated aerodynamic closed-loop may include, for example, digital or analog flight control computers, power control units, control feedback, major data busses, and sensor signals for items such as air data, acceleration, rate sensors, commands to the surface position, respective power supply sources, etc. Autopilot systems (including feedback functions) should be included in this demonstration if they are integrated with the FBW flight control system.
13.2 Compliance Strategy for Fly-By-Wire Flight Control Systems
13.2.1 A strategy for showing compliance for electronic or fly-by-wire flight control systems should be developed and contain the following:
• Determination of the flight control characteristics that require detailed and specific test strategies; and
• Substantiation of the proposed strategy (flight tests, simulator tests, analyses, etc.)
covering the characteristics and features determined above.
13.2.2 In particular, the following characteristics of flight control laws should be covered:
• Discontinuities,
• Robustness versus piloted maneuvers and/or adverse weather conditions,
• Protection priorities,
• Control law mode changes with and without failures, and
• Determination of critical scenarios for multiple failures.
13.2.3 The strategy should include, but should not be limited to, operational scenarios. The determination that an adequate strategy has been achieved should be based on engineering judgment.
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