AC 25.671-1
such element should be multiplied by its maximum exposure time to determine the probability that the element will be failed when the jam occurs. The probability of all such failure conditions, when combined, must total 1/1000 or less.
8.2.6.2 The probability criterion of 1/1000 is not a failure rate but a time-based probabilistic parameter intended to ensure a minimum residual airplane level of safety following a jam regulated by § 25.671(c)(3). This analysis should help determine intervals for scheduled maintenance activity or operational checks that ensure the availability of alleviation or compensation means.
8.2.7 An applicant may use alleviation to show compliance with § 25.671(c)(3). For this purpose, alleviation may include automatic or manual system reconfigurations, or any other features that eliminate or reduce the consequences of a jam or permit continued safe flight and landing. A local structural failure (for example, via a mechanical fuse or shear-out) that could lead to a surface departure from the airplane should not be used as a means of jam alleviation.
8.2.8 Only the airplane rigid body modes need to be considered when evaluating the airplane response to maneuvers and continued safe flight and landing. All approved airplane gross weights and center-of-gravity locations should be considered when complying with § 25.671(c). However, only critical combinations of gross weight and center of gravity need be evaluated.
8.2.9 Section 25.683 requires testing of the control system to show the system is free from jamming. The referenced CATA paper provides guidance on compliance with this regulation.
8.2.10 Section 25.685 requires each control system be designed and installed to prevent jamming. As part of compliance with this requirement, the applicant should perform a qualitative assessment of the flight control system to identify and address potential jamming scenarios and eliminate any potential sources of jamming.
8.3 Determination of Normally Encountered Flight Control Surface or Pilot Control Positions.
8.3.1 General.
8.3.1.1 Section 25.671(c)(3) requires the evaluation of a jammed flight control surface or pilot control at any normally encountered position of that flight control surface or pilot control. The control positions specified in paragraphs 8.3.2 through 8.3.8 below pertain to both the pilot control and the control surface deflections that are associated with those control positions, and they may be considered normally encountered positions for compliance with § 25.671(c)(3). Lateral (roll), longitudinal (pitch), and directional (yaw) control positions are provided for the takeoff and in- 10
AC 25.671-1
flight phases, but not for the landing phase. The applicant may propose control positions for the landing phase if such jams are being evaluated.
See paragraph 8.2.4. Takeoff is the time between brake release and 35 feet above ground level (AGL). The in-flight phase is from 35 feet after takeoff to 50 feet AGL prior to touchdown and includes climb, cruise, normal maneuvering, descent, and approach. Landing is the time from 50 ft AGL prior to touchdown up to the complete stop of the airplane on the runway.
8.3.1.2 Although 1 in 1000 operational takeoffs is expected to include crosswinds on the order of 25 knots, the short exposure time associated with a jam occurring between V1 and VLOF allows usage of a less conservative crosswind magnitude when determining normally encountered lateral and directional control positions. Given that lateral and directional controls are continuously used to maintain runway centerline in a crosswind takeoff, and control inputs greater than that necessary at V1 will occur at speeds below V1, any jam in these control axes during a crosswind takeoff will normally be detected prior to V1. Considering the control jam failure rate of approximately 10-6 to 10-7 per flight hour combined with the short exposure time between V1 and VLOF, a reasonable crosswind level for determination of jammed lateral or directional control positions during takeoff is 15 knots.
8.3.1.3 The jam positions to be considered in showing compliance include any position up to the maximum position determined by the following maneuvers. The maneuvers and conditions described in this section are only to provide the control surface and pilot control deflections to evaluate continued safe flight and landing capability; they are not to represent flight test maneuvers for such an evaluation. For airplanes with stability augmentation or other systems that move control surfaces independent of pilot controller positions, the control surface deflections used to evaluate continued safe flight and landing capability should be established by the normal airplane control system response to the maneuvers and conditions described in this section.
8.3.2 Jammed Lateral Control Positions.
8.3.2.1 Takeoff.
The lateral control position for wings-level at V1 in a steady crosswind of 15 knots at a height of 10 meters above the takeoff surface. Variations in wind speed from a height of 10 meters can be obtained using the following relationship:
Valt = V10 meters * (Hdesired/10.0)1/7 Where V10 meters = Wind speed in knots at 10 meters above ground level (AGL) 11
AC 25.671-1
Valt = Wind speed at desired altitude (knots) Hdesired = Desired altitude for which wind speed is sought, but not lower than 1.5 meters.
8.3.2.2 In-flight.
8.3.2.2.1 The lateral control position to sustain a 12 degree-per-second steady roll rate from 1.23 VSR1 to VMO/MMO or VFE, as appropriate, but not greater than 50 percent of the control input.
8.3.2.2.2 Directional control inputs may only be used to the extent necessary to minimize sideslip.
8.3.3 Jammed Longitudinal Control Positions.
8.3.3.1 Takeoff.
Three longitudinal control positions should be evaluated:
8.3.3.1.1 Any control position from the position the controls naturally assume without pilot input at the start of the takeoff roll to the position that occurs at V1 using the manufacturer’s recommended procedures.
Note: It may not be necessary to consider this case if it can be shown that the pilot is aware of the jam before reaching V1 (for example, through a manufacturer’s recommended AFM procedure).
8.3.3.1.2 The longitudinal control position at V1 based on the manufacturer’s recommended procedures including consideration for any runway condition for which the airplane is approved to operate.
8.3.3.1.3 Using the manufacturer’s recommended procedures, the peak longitudinal control position to achieve a steady airplane pitch rate of the lesser of 5 degrees per second or the pitch rate necessary to achieve the speed used for all-engines-operating initial climb procedures (V2+XX) at 35 feet.
8.3.3.2 In-flight.
The maximum longitudinal control position is the greater of the following:
8.3.3.2.1 The longitudinal control position required to achieve steady state normal accelerations from 0.8g to 1.3g at speeds from 1.23 VSR1 to VMO/MMO or VFE, as appropriate.
8.3.3.2.2 The peak longitudinal control position commanded by the stability augmentation or other automatic system in response to atmospheric discrete vertical gust defined by 15 feet per second (fps) from sea level to 20,000 feet.
AC 25.671-1
8.3.4 Jammed Directional Control Positions.Takeoff.
The directional control position for takeoff at V1 in a steady crosswind of 15 knots (at a height of 10 meters above the takeoff surface). Variations in wind speed from a height of 10 meters can be obtained as shown in paragraph 8.3.2.1.
8.3.4.2 In-flight.
The directional control position is the greater of the following:
8.3.4.2.1 The peak directional control position commanded by the stability augmentation or other automatic system in response to atmospheric discrete lateral gust defined by 15 fps from sea level to 20,000 feet.
8.3.4.2.2 Directional control position required for lateral/directional trim from
1.23 VSR1 to the maximum all engines operating airspeed in level flight with climb power, but not to exceed VMO/MMO or VFE as appropriate.
While more commonly a characteristic of propeller aircraft, this addresses any lateral or directional asymmetry that can occur in flight with symmetric power.
8.3.5 Control Tabs, Trim Tabs, and Trimming Stabilizers.
8.3.5.1 Any tabs installed on control surfaces are assumed jammed in the position associated with the normal deflection (as defined in this section) of the control surface on which they are installed.
8.3.5.2 Trim tabs and trimming stabilizers are assumed jammed in the positions associated with the manufacturer’s recommended procedures for takeoff, and normally used throughout the flight to trim the airplane from
1.23 VSR1 to VMO/MMO or VFE, as appropriate.
8.3.6 Speed Brakes.
The applicant should assume that speed brakes are jammed in any position for which the speed brakes are approved to operate during flight at any speed from 1.23 VSR1 to VMO/MMO or VFE, as appropriate. Asymmetric extension and retraction of the speed brakes should be considered. Roll spoiler jamming (asymmetric spoiler panel) is addressed under paragraph 8.3.2 of this AC.
8.3.7 High Lift Devices.
Leading edge and trailing edge high lift devices should be assumed to jam in any position for takeoff, climb, cruise, approach, and landing. Applicants should analyze the potential for skew, and asymmetric extension and retraction, of high lift devices.
Section 25.701 contains a requirement for flap and slat mechanical interconnection unless the airplane has safe flight characteristics with asymmetric flap and slat positions.
8.3.8 Load Alleviation Systems.
AC 25.671-1
8.3.8.1 Gust Load Alleviation Systems.
At any airspeed between 1.23 VSR1 to VMO/MMO or VFE, as appropriate, the control surfaces are assumed to jam in the maximum position commanded by the gust load alleviation system in response to a discrete atmospheric gust with the following reference velocities:
• 15 fps equivalent airspeed (EAS) from sea level to 20,000 feet (vertical gust),
• 15 fps EAS from sea level to 20,000 feet (lateral gust).
8.3.8.2 Maneuver Load Alleviation Systems.
At any airspeed between 1.23 VSR1 to VMO/MMO or VFE, as appropriate, the control surfaces are assumed to jam in the maximum position commanded by the maneuver load alleviation system during a pull-up maneuver to
1.3g or a pushover maneuver to 0.8g.
8.4 Assessment of Continued Safe Flight and Landing.
To show, as required by § 25.671, that the airplane remains capable of safe flight and landing after a flight control system failure, the applicant should consider the following maneuverability and structural strength criteria. A pilot assessment of the airplane handling qualities should be performed, although this does not supersede the maneuverability criteria provided below.
8.4.1 Flight Characteristics.
8.4.1.1 General.
8.4.1.1.1 Following a control system failure, appropriate operating procedures may be used, including system reconfiguration, operating limitations, and flightcrew resource management. The procedures necessary for safe flight and landing should not require exceptional piloting skill or strength.
8.4.1.1.2 Additional means of control, such as a trim system, may be used if the applicant can show that the systems are available and effective. Credit should not be given for use of differential engine thrust to maneuver the airplane. However, differential thrust may be used following the recovery to maintain lateral or directional trim following the flight control system failure.
8.4.1.1.3 For the longitudinal pilot control or control surface jam during takeoff prior to rotation, the applicant should show that the airplane can be safely rotated for liftoff without consideration of field length available.
8.4.1.2 Transient Response.
8.4.1.2.1 There should be no unsafe conditions during the transient condition following a flight control system failure. The evaluation of jam conditions 14
AC 25.671-1
per § 25.671(c)(3) may be assumed to begin at 1g wings-level flight. For failure conditions addressed by § 25.671(c)(1) and (c)(2), operating and environmental conditions should be considered as described in paragraph 8.1.4.3. During the transient response, continued safe flight and landing is generally defined as not exceeding any one of the following:
• A load on any part of the primary structure sufficient to cause a catastrophic structural failure.
• Catastrophic loss of flight path control.
• Exceedance of VDF/MDF.
• Catastrophic flutter.
• Excessive vibration or buffeting.
• Bank angle in excess of 90°.
8.4.1.2.2 In connection with the transient response, applicants must show compliance with § 25.302 for applicable failure conditions. While VF is normally an appropriate airspeed limit to be considered regarding continued safe flight and landing, temporary exceedance of VF may be acceptable as long as the requirements of § 25.302 are met.
8.4.1.2.3 Section 8.3 of this AC provides a means of determining control surface deflections for the evaluation of flight control jams. In some cases, airplane roll or pitch rate, or normal acceleration are used as a basis to determine these deflections. The roll or pitch rate and/or normal acceleration used to determine the control surface deflection need not be included in the evaluation of the transient condition. For example, the in-flight lateral control position determined in paragraph 8.3.2.2 of this AC is based on a steady roll rate of 12 degrees per second. When evaluating this condition, whether by analysis, simulation, or in-flight demonstration, the resulting control surface deflection is simply input while the airplane is in wings-level flight, at the appropriate speed, altitude, and so forth. During this evaluation, the airplane’s actual roll or pitch rate may or may not be the same as the roll or pitch rate used to determine the jammed control surface position.
8.4.1.3 Delay Times.
8.4.1.3.1 Applicants should account for potential delays in pilot recognition, reaction, and operation of any disconnect systems, if applicable:
Delay = Recognition + Reaction + Operation of Disconnect 15
AC 25.671-1
8.4.1.3.2 Recognition is the time from the failure condition to the point at which a pilot in service operation may be expected to recognize the need to take action. Recognition of the malfunction may be through the behavior of the airplane or a reliable failure alerting system, and the recognition point should be identified but should not normally be less than 1 second. For flight control system failures, except the type of jams addressed in § 25.671(c)(3), control column or wheel movements alone should not be used for recognition.
8.4.1.3.3 Applicants should use the reaction times in the following table:
Table 1. Reaction Times for Flight Conditions Flight Condition Reaction Time On ground 1 second* In air (<1,000 feet AGL) 1 second* Manual flight (>1,000 feet AGL) 1 second* Automatic flight (>1,000 feet AGL) 3 seconds * 3 seconds if control must be transferred between pilots.
8.4.1.3.4 The time required to operate any disconnect system should be measured either through ground tests or during flight testing. This value should be used during all analysis efforts. However, flight testing or manned simulation that requires the pilot to operate the disconnect includes this extra time; therefore, no additional delay time would be needed for these demonstrations.
8.4.1.4 Maneuver Capability for Continued Safe Flight and Landing.
The applicant should show that each of the following maneuvers can be performed, using the procedures specified in the AFM, following the failure:
8.4.1.4.1 A steady 30° banked turn to the left and right.
8.4.1.4.2 A roll from a steady 30° banked turn through an angle of 60° so as to reverse the direction of the turn in not more than 11 seconds. (In this maneuver, the rudder may be used to the extent necessary to minimize sideslip, and the maneuver may be unchecked.)
8.4.1.4.3 A pushover maneuver to 0.8g, and a pull-up maneuver to 1.3g.
AC 25.671-1
8.4.1.4.4 A wings-level landing flare in a 90° crosswind of up to 10 knots (measured at 10 meters above the ground).
Note: For the case of a lateral or directional control system jam during takeoff that is described in paragraph 8.3.2.1 or 8.3.4.1, respectively, the applicant should show that the airplane can safely land on a suitable runway with any crosswind from 0 knots to the crosswind level and direction at which the jam was established.
8.4.1.4.5 The airplane remains on the paved runway surface during the landing roll, until reaching a complete stop.
8.4.1.5 Control Forces.
8.4.1.5.1 The short- and long-term control forces should not be greater than 1.5 times the short- and long-term control forces allowed by § 25.143(d).
8.4.1.5.2 The FAA has typically considered “short-term,” as used in § 25.143, to mean the time required to accomplish a configuration or trim change.
However, taking into account the capability of the flightcrew to share the workload, the short-term forces of § 25.143 may be appropriate for a longer duration, such as the evaluation of a jam on takeoff and return to landing. Additionally, the flightcrew may be able to control the airplane by other means, such as applying alternate control in lieu of the jammed control. In such case, procedures on load sharing or control by other means should be provided in the AFM and should be evaluated.
8.4.1.5.3 During the recovery following the failure, transient control forces may exceed these criteria to a limited extent.
8.4.2 Structural Strength for Flight Control System Failures.
8.4.2.1 Failure Conditions Specified in § 25.671(c)(1) and (c)(2).
The applicant should show that the airplane maintains structural integrity during the transient response and for continued safe flight and landing.
This should be accomplished by showing compliance with § 25.302, where applicable. For systems not covered by § 25.302, the criteria in that regulation may be used. In lieu of those criteria, the airplane should be shown capable of withstanding the loads occurring during the transient response with a factor of safety of 1.25. For the continuation of flight, the following loads should be applied: For failures that would be detected, the loads resulting from the conditions specified in paragraphs 8.4.2.2.1 and
8.4.2.2.2, considered as ultimate; for failures that would not be detected, the loads specified in § 25.302(c)(2)(i)(A)-(E), considered as ultimate. See also § 25.1329(g).
AC 25.671-1
8.4.2.2 Jam Conditions Specified in § 25.671(c)(3).
Following a jam, the applicant should ensure that the flight control system can withstand the loads occurring as a result of the jam. This would include the mitigating effects of any shear-out or override functions used to alleviate the jam. The applicant should also show that the aircraft maintains structural integrity for continued safe flight and landing. For these conditions, the loads, considered as ultimate, should be derived from the following conditions, considered separately, at speeds up to the maximum speed allowed for the jammed position or for the failure condition.
8.4.2.2.1 Balanced maneuver of the airplane between 0.25g and 1.75g with high lift devices fully retracted and in en route configurations, and between 0.6g and 1.4g with high lift devices extended.
8.4.2.2.2 Vertical and lateral discrete gusts corresponding to 40 percent of the limit gust velocity specified at VC in § 25.341(a) with high lift devices fully retracted, and a 17 fps vertical and 17 fps head-on gust with high lift devices extended.
9 EVALUATION OF ALL-ENGINES-FAILED CONDITION—§ 25.671(d).
9.1 Explanation.
Section 25.671(d) states: “If all engines fail at any point in the flight, the airplane must be controllable, and an approach and flare to a landing and controlled stop, and flare to a ditching, must be possible without requiring exceptional piloting skill or strength.”
9.1.1 The intent of § 25.671(d) is to assure that in the event of failure of all engines, and given the availability of a suitable runway, the airplane will be controllable and an approach and flare to a landing and controlled stop is possible. Although the rule assumes that a suitable runway would be available, the FAA recognizes that, with all engines inoperative, it may not be possible to reach a suitable runway or landing surface; in this case, the airplane must still be able to make a flare to landing attitude.
The rule also requires that a flare to a ditching be possible in the event all engines fail.
9.1.2 Compliance with § 25.671(d) generally necessitates that fully powered flight control systems lacking manual reversion capability must have a source for emergency power, such as an air-driven generator, windmilling engines, batteries, or other power source capable of providing adequate power to the flight control system through landing.
9.1.3 An applicant may use analysis, simulation, or any combination thereof to show compliance in lieu of flight test, if the applicant shows the method to be reliable.
AC 25.671-1
9.2 Procedures.
9.2.1 Section 25.671(d) requires applicants to show that it is possible, without requiring exceptional piloting skill or strength, to maintain control following the failure of all engines. This showing should include the time it takes for activating any backup systems. The airplane should also remain controllable during restart of the most critical engine, while following the engine restart procedures recommended in the AFM.
9.2.2 The most critical flight phases, especially for airplanes with emergency power systems dependent on airspeed, are likely to be takeoff and landing. Credit may be taken for hydraulic pressure or electrical power produced while the engines are spinning down and any residual hydraulic pressure remaining in the system. Sufficient power must be available to complete a wings-level approach and flare to a landing and controlled stop.
Analyses or tests may be used to demonstrate the capability of the control systems to maintain adequate hydraulic pressure and electrical power during the time between the failure of the engines and the activation of any backup systems. If any of the backup systems rely on aerodynamic means to generate power, then a flight test demonstration should be performed to demonstrate that the backup system could supply adequate electrical and hydraulic power to the flight control systems. The flight test should be conducted at the minimum practical airspeed – in accordance with AFM recommendations – that would be required to perform an approach and flare to a safe landing and ditching attitude.
9.2.3 The maneuver capability following the failure of all engines should be sufficient to complete an approach and flare to a landing, and flare to a ditching. Note that the airplane weight could be extremely low (for example, the engine failures could be due to fuel exhaustion). The maximum speeds for approach and landing may be limited by other requirements (for example, tire speeds, flap, or landing gear speeds, and so forth), or by an evaluation of the average pilot’s ability to conduct a safe landing or ditching.
At an operational weight determined for this case and for any other critical weights and centers of gravity identified by the applicant, and at speeds down to the approach speeds appropriate to the airplane configuration, the airplane should be capable of the following:
9.2.3.1 A steady 30° banked turn to the left and right.
9.2.3.2 A roll from a steady 30° banked turn through an angle of 60° so as to reverse the direction of the turn in not more than 11 seconds. (In this maneuver, the rudder may be used to the extent necessary to minimize sideslip, and the maneuver may be unchecked.)
9.2.3.3 A pushover maneuver to 0.8g, and a pull-up maneuver to 1.3g.
9.2.3.4 A wings-level landing flare in a 90° crosswind of up to 10 knots (measured at 10 meters above the ground).
AC 25.671-1
Note: If the loss of all engines has no effect on the control authority of the aircraft (for example, manual controls) then the results of the basic handling qualities flight tests with all engines operating may be used to demonstrate the satisfactory handling qualities of the airplane with all engines failed.
9.2.4 The applicant should show that it is possible to perform a flare to a safe landing and ditching attitude, in the most critical configuration, from a stabilized approach using the recommended approach speeds and the appropriate AFM procedures, without requiring exceptional piloting skill or strength. For transient maneuvers, any short or long-term control forces that exceed the short- and long-term control forces permitted by § 25.143 must be evaluated for acceptability.
9.2.5 Finally, assuming that a suitable runway is available, it should be possible to control the airplane until it comes to a complete stop on the runway. A means of positive deceleration should be provided. A suitable runway would have the lateral dimensions, length and load-bearing capability that meets the requirements defined in the emergency procedures of the AFM. It is not necessary to consider adverse environmental conditions (for example, wet or contaminated runway, tailwind) when demonstrating compliance for the on-ground phase.
10 EVALUATION OF CONTROL AUTHORITY AWARENESS—§ 25.671(e).
10.1 Section 25.671(e) requires the airplane be designed to indicate to the flight crew whenever the primary control means is near the limit of control authority. This requirement can be met through natural or artificial control feel forces and/or cockpit control movement if shown to be effective, or by flightcrew alerting in accordance with §§ 25.1309 and 25.1322. Suitability of alerting should take into account that some pilot-demanded maneuvers (for example, rapid roll) are necessarily associated with intended full performance, which may saturate the surface. Therefore, simple alerting systems, which would function in both intended and unexpected control-limiting situations, should be properly balanced between needed flightcrew awareness and nuisance alerting. Nuisance alerting should be minimized.
10.2 Depending on the application, suitable indications may include cockpit control position, annunciator light, or surface position indicators. Furthermore, this requirement applies at the limits of control authority for a given flight condition and configuration, not necessarily at the limits of any individual surface travel.
10.3 When the airplane is equipped with an unpowered manual flight control system, the pilot may be aware of the limit of control authority through other tactile means. In this case, no other means of indication may be required.
AC 25.671-1
11 EVALUATION OF FLIGHT CONTROL SYSTEM SUBMODES—§ 25.671(f).
11.1 Section 25.671(f) requires appropriate flight crew alerting be provided whenever the airplane enters any mode that significantly changes or degrades the normal handling or operational characteristics of the airplane. Flightcrew alerting must meet the requirements of §§ 25.1309 and 25.1322. Some systems, especially electronic flight control systems, have sub-modes of operation not restricted to being either on or off.
The means provided to the flightcrew to indicate the current sub-mode of operation may be different from conventional flight crew alerting, provided it meets the requirements in §§ 25.1309 and 25.1322.
11.2 Mode transitions – When the flight control system transitions from one mode to another, the transient effects should not prevent continued safe flight and landing or the ability to meet the criteria in paragraph 8.4.1.2 of this AC.
11.3 Loss of protection functions due to mode changes – Mode changes can result in loss of protection functions. Appropriate flight crew alerting should be provided if the loss of these protection functions significantly degrades the normal handling or operational characteristics of the airplane.
12 ACCEPTABLE MEANS OF COMPLIANCE.
The FAA recognizes that it may be neither practical nor appropriate to demonstrate compliance by flight test for all the failure conditions noted in this AC. Except as provided elsewhere by regulation or policy, an applicant may show compliance by analysis, background simulation, a piloted simulator, flight test, or combination of these methods as agreed with the FAA. Simulation methods should include an accurate representation of the airplane characteristics and of the pilot response, including time delays as specified in paragraph 8.4.1.3 of this AC. Compliance with § 25.671 may necessitate flight manual abnormal procedures. Verification of the efficacy of these procedures may be accomplished in flight, or by using a validated piloted simulator if the agreement of the FAA is previously obtained. Advisory Circular 25-7D, or later approved revision, provides guidance on simulation and the acceptable use of simulations.
13 EVALUATION OF FLY-BY-WIRE FLIGHT CONTROL SYSTEMS.
13.1 Evaluation of Command Signal Integrity This section provides guidance and identifies issues related to command signal integrity that should be investigated for airplanes using fly-by-wire (FBW) flight controls to comply with the provisions of §§ 25.671, 25.1301, and 25.1309. It is necessary to ensure that any flight control system control signals will not be adversely altered by internal and external interference. This section documents a means of compliance with §§ 25.671, 25.1301, and 25.1309 that ensures control signals are not adversely altered from internal and external interference.