Policy Statement Subject: Low-Speed Alerting and/or Protection Date: Proposed Policy No: PS-ANM-25-16 Initiated By: ANM-111 Summary This policy statement clarifies and supplements the advisory material related to the means of compliance with Title 14, Code of Federal Regulations (14 CFR) 25.1329(h). It is intended to clarify the existing advisory material and to ensure that applicants understand all appropriate design considerations that are related to low-speed alerting and protection. The low-speed alerting and protection must comply with related regulations that ensure that the flightcrew is aware of the low airspeed condition and can determine the appropriate actions, or that flight guidance systems (FGSs) prevent significant unwanted airspeed excursions outside the range of the normal flight envelope. Compliance with § 25.1329(h) and related regulations (e.g., §§ 25.1322, Flightcrew alerting, and 25.1309, Equipment, systems, and installations) should preclude a majority of situations that have contributed to low-speed incidents and accidents. This policy statement is necessary to address a safety vulnerability, as evidenced in the following four recent incidents and accidents that involved the failure to maintain proper airspeed: 1) Thomsonfly Ltd, Boeing Model 737-300, near Bournemouth, United Kingdom on September 23, 2007—This incident involved deceleration after the flightcrew did not notice the autothrottle failure or the autothrottle disengagement. The flightcrew allowed the airspeed to decrease 20 knots below Vref before they initiated recovery. After multiple stick shaker activations and an aerodynamic stall, the flightcrew eventually regained control, and the airplane landed safely. 2) Empire Airlines, Model ATR-42-300, in Lubbock, Texas on January 27, 2009—This accident involved a deceleration to stick shaker and autopilot disconnect following a flap anomaly. The probable cause was the flightcrew’s failure to monitor and maintain a minimum safe airspeed while executing an instrument approach in icing conditions, which resulted in an aerodynamic stall at low altitude. 3) Colgan Air, de Havilland Model DHC-8-400, in Clarence Center, New York on February 12, 2009—This accident involved decelerating to a low speed, which activated 2 the stall warning while on approach. The probable cause of this accident was the captain’s inappropriate response to the activation of the stick shaker, which led to an aerodynamic stall from which the airplane did not recover. The flightcrew’s failure to monitor airspeed also contributed to this accident. 4) Turkish Air, Boeing Model 737-800, in Amsterdam, Netherlands on February 25, 2009— This accident involved a radio altimeter anomaly that led to an inappropriate autothrottle thrust reduction. The flightcrew failed to recognize the airspeed decay and the pitch increase until the moment the stick shaker was activated. Subsequently, the approach-to-stall recovery procedure was not executed properly, causing the airplane to stall and crash. Some of the airplanes in the cited incidents and accidents may have conformed to the guidance for low-speed awareness cues in paragraph 2.3 of Appendix 1 of Advisory Circular (AC) 25-11A, Electronic Flight Deck Displays, dated June 21, 2007. However, these accidents and incidents indicate that the current means for flightcrew speed awareness have been ineffective, especially when accompanied with multiple flightcrew distractions. In each of the cases, the flightcrew did not respond to the low-speed cues until stall warning. The flight guidance system did not provide adequate protection in any of these cases. Analysis of the factors leading up to the incidents have demonstrated the need for more robust protections or low-speed alerting beyond those that have been historically provided. The addition of §§ 25.1329(h) and 25.1322 to the regulations in recent years in conjunction with applying the guidance in this policy should improve the effectiveness of low-speed alerting. Definition of Key Terms In the text below, the terms “must,” “should,” and “recommend” have a specific meaning that is explained in the Attachment to this policy. Current Regulatory and Advisory Material The regulations applicable to low-speed protection and alerting are §§ 25.1329(h), 25.1322, and 25.1309(b)(2). These sections lay out the regulatory basis for the installation and approval of low-speed protection and/or alerting systems within the FGS. Section 25.1309(b)(2) states: “The airplane systems and associated components, considered separately and in relation to other systems, must be designed so that the occurrence of any other failure conditions which would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions is improbable.” Section 25.1329(h) states: “When the flight guidance system is in use, a means must be provided to avoid excursions beyond an acceptable margin from the speed range of the normal flight envelope. If the airplane experiences an excursion outside this range, a means must be provided to prevent the flight guidance system from providing guidance or control to an unsafe speed.” 3 Section 25.1322 requires that flightcrew alerting provide information about non-normal operation and appropriate reactions. It further provides requirements for the various types (e.g., warnings, cautions, and advisories) and characteristics of alerts. Section 25.1329(h) was introduced with Amendment 25-119. It pertains to the FGS, which includes the functions of autopilots, flight directors (FDs), and automatic thrust control. Section 25.1329 also addresses both fully automatic and combined automatic-manual flight using any portion of the FGS (i.e., autopilot, FD, or automatic thrust control). The remaining manual flight scenarios (those not covered by § 25.1329) are generally covered by the use of stick force gradients that deter further speed decreases, or by the use of flight envelope protection features of the primary flight control system. Section 25.1322 provides the criteria that alerts must meet. It was revised with Amendment 25-131. This amendment is a substantial update of previous alerting requirements to include requirements for alerts that are readily and easily detectable and intelligible by the flightcrew under all foreseeable operating conditions and timely attention-getting cues through at least two different senses. AC 25.1329-1B, Change 1, Approval of Flight Guidance Systems, dated October 16, 2012; AC 25.1322-1, Flightcrew Alerting, dated December 13, 2010; and AC 25-11A; provide guidance on this subject. Relevant Past Practice Before Amendment 25-119 (effective May 11, 2006) was added to the regulations, low-speed protection and/or alerting other than stall warning was not specifically required during automatic flight, and stall warnings were considered to provide adequate protection. However, with the adoption of Amendment 25-119, protection against low airspeed excursions is now required whenever the FGS (which includes the autopilot, FD, and autothrust functions) is in use. When the FGS is in use, a means must be provided to avoid excursions beyond an acceptable margin from the speed range of the normal flight envelope. If the airplane experiences an excursion outside this range, the FGS must not provide guidance or control to an unsafe speed. The phrase “to an unsafe speed” is intended to mean that the FGS should not control or provide guidance that would eventually lead to an aerodynamic stall (or even to a stall warning) or a speed that is in excess of the maximum operating speed, regardless of the maneuver being conducted at the time. Section 25.1329(h) is intended to eliminate unwanted airspeed excursions, through automatic FGS control or through alerting the flightcrew. The preferred implementation is for the FGS to automatically provide control and/or guidance to avoid these excursions. However, an acceptable means of compliance with § 25.1329(h) may also be to implement an alert to increase the flightcrew’s awareness, recognition, and ability to intervene when a potential airspeed excursion occurs. AC 25.1329-1B provides guidance for several methods of implementing such an alert. Previous accidents have shown that visual alerts alone with no accompanying aural alert have been ineffective in ensuring flightcrew awareness of low airspeed excursions. Therefore, an aural 4 alert is considered an essential aspect of any alerting intended to show compliance with § 25.1329(h). In addition to § 25.1329(h), other changes to part 25 also relate to the issue of low-speed alerting: Amendment 25-121 (effective October 9, 2007) revised the majority of subpart B to mandate more stringent requirements for performance and handling qualities in icing conditions. Stall warning, as part of the Amendment 25-121 package, must meet the new regulatory margins specified in § 25.207, Stall warning, for flight in icing conditions. Although these stall warning requirements are not “low-speed alerting,” they are functionally similar since they increase the warning margin for flight in icing conditions. Amendment 25-131 (effective January 3, 2011) introduced new and updated regulatory requirements for flightcrew alerting (§ 25.1322), as well as new flightcrew alerting guidance material and definitions (AC 25.1322-1). The combination of these new regulatory requirements affects the low-speed protection and/or alerting requirement of § 25.1329(h) because they provide additional, more specific requirements. This policy statement clarifies and supplements existing guidance on the subject. In addition to the policy considerations described above, the reliability requirements of § 25.1309(b)(2) apply to low-speed alerting and protection systems. According to that regulation, failure conditions that would reduce the ability of the crew to cope with adverse operating conditions must be improbable. A low-speed excursion is an example of this type of adverse operating condition. This may not be clearly understood from previous guidance. The FAA’s analysis shows that adherence to the contents of the regulatory and guidance material outlined in this policy statement likely could have prevented the situations that led to the incidents and accidents previously described. In 2011, the Aviation Rulemaking Advisory Committee working group was tasked to provide information to use to develop standards and guidance material for low speed alerting. The FAA initiated rulemaking activity. However, after further review, the FAA determined that the combination of current existing regulations and guidance could provide adequate protection. 5 Policy To improve the effectiveness of low-speed alerting, this policy is intended to clarify design considerations related to the low-speed alerts and protection requirements. In addition to applying the guidance provided in AC 25.1329-1B, AC 25.1322-1, and AC 25-11A, an airplane that has Amendments 25-119, 25-121, and 25-131 in its certification basis must satisfactorily address the following regulations for low-speed alerts and/or protection: 1. Alerts for Excursions Outside Acceptable Margins. If a low-speed alert is used as the means of compliance with § 25.1329(h), the alert should ensure that the flightcrew is immediately aware of any excursions beyond an acceptable margin from the speed range of the normal flight envelope. In accordance with § 25.1322(b), this alert must be a caution or a warning. Since automatic flight control presupposes that the flightcrew will not have their hands on the controls, aural and visual indications are presumably the essential attention-getting senses required by § 25.1322(c)(2). In addition, the alert should be time-critical for warnings and for cautions, and be consistent with the elements of the related time-critical warning in accordance with AC 25.1322-1. 2. Reliability of Protection/Alerts. The reliability of the low-speed protection or alert required by § 25.1329(h) must meet the requirement of § 25.1309(b)(2). Failure of the low-speed protection or alert required by § 25.1329(h), which would reduce the capability of the airplane or the ability of the crew to cope with adverse operational conditions, must be improbable. 3. Icing Conditions. If certification is for flight in icing conditions, the low-speed protection or alert should also meet § 25.1329(h) in 14 CFR appendix C icing conditions. In addition, the low-speed protection or alert should meet the criteria stated in items 1 and 2 in this policy for appendix C icing conditions. Effect of Policy The general policy stated in this document does not constitute a new regulation. Agency employees and their designees and delegations must not depart from this policy statement without appropriate justification and concurrence from the FAA management that issued this policy statement. The authority to deviate from this policy statement is delegated to the Transport Standards Staff Manager. Whenever a proposed method of compliance is outside this established policy, the project aircraft certification office has to coordinate it with the policy issuing office using an issue paper. Similarly, if the project aircraft certification office becomes aware of reasons that an applicant’s project should not be approved even if its proposal meets this policy, the office must coordinate its response with the office that issued this policy. Applicants should expect that certificating officials would consider this information when making findings of compliance relevant to new 6 certificate actions. In addition, as with all guidance material, this policy statement identifies one means, but not the only means, of compliance. Implementation This policy discusses compliance methods that should be applied to new type certificate, amended type certificate, supplemental type certificate, and amended supplemental type certification programs. This policy applies to new designs and to significant changes to existing designs, unless an exception can be shown in accordance with § 21.101(b)(2) or § 21.101(b)(3). The compliance methods apply to those programs with an application date that is on or after the effective date of the final policy. If the date of application precedes the effective date of the final policy, and the methods of compliance have already been coordinated with and approved by the FAA or its designee, the applicant may choose to either follow the previously acceptable methods of compliance or follow the guidance contained in this policy. Conclusion The FAA has concluded that it is necessary to clarify the policy on low-speed alerting and/or protection items. This policy statement provides new guidance on the recommended steps to address low-speed alerting and/or protection. If other data were to be presented that demonstrated otherwise, the FAA might reconsider the intent and content of this policy. END Attachment Attachment 7 Terms Table A-1 defines the use of key terms in this policy statement. The table describes the intended functional impact. Table A-1 Definition of Key Terms Regulatory Requirements Acceptable Methods of Compliance (MOC) Recommendations Recommend Language Must Should Meaning Refers to a regulatory requirement that is mandatory for design approval Refers to instructions for Refers to a a particular MOC recommended practice that is optional Functional Impact No Design Approval if not met Alternative MOC has to be approved by issue paper None because it is optional