AC 27-1B, Chg 7 2/4/2016 responsibility of the operator to demonstrate, and the operations inspector to confirm, that any external load operation, including en route IFR, can be safely conducted.
(16) Rotorcraft Flight Below Instrument Flight Minimum Speed.
(i) The advent of lateral precision with vertical guidance (LPV) precision GPS approach procedures may necessitate flying at airspeeds below the previously published instrument flight minimum speed (VINI) established for most rotorcraft under Appendix B to part 27, paragraph IIc).
(ii) Additionally, some modern automatic flight control systems (AFCS) have been certificated to fly fully-coupled below previously published VMINI in order to improve operational safety while conducting certain operations such as Search and Rescue, steep-approaches, and special rig approaches.
(ili) Certification for flight below previously published VMINI will require the applicant to either reestablish VMINI at a lower airspeed per Appendix B to part 27 or be granted special conditions that specify the AFCS reliability and functionality requirements that allow for coupled flight below the previously established VMINI- (A) If VMINI is to be reestablished, the certification requirements in Appendix B to part 27 for VMINI must be followed. Among the part 27 requirements, below are two items that are often overlooked:
(1) Static and dynamic stability requirements of Appendix B to part 27 must be complied with at all approved descent angles.
(2) Pilot workload needs to be evaluated at the proposed lower VMINI. Legacy AFCS may not be able to cope with higher angles of descent, thus increasing pilot workload beyond an acceptable level.
(B) The FAA has previously approved special conditions that permit rotorcraft to fly below airspeeds at which the stability requirements of Appendix B to part 27 cannot be demonstrated. The bases of these special conditions are that the AFCS has specific modes that permit coupled flight from VMINI to a minimum airspeed (or groundspeed specified by the applicant), stabilized flight at the minimum airspeed (or groundspeed), and flight from the minimum airspeed back to VINI. The following items should be considered if the applicant chooses to utilize a robust AFCS to permit coupled flight below the previously established VMINI. These topics are not all-inclusive, but are general considerations. The actual requirements specified within an individual project's special conditions will depend on the specific functionalities provided by the design.
(1) The rotorcraft must be certificated for IFR flight and meet the part 29 requirements for category A engine isolation.
(2) For system safety calculations for functional failure modes with operations less than VMINI, an exposure time of no less than the entire flight duration should be used.
(3) The AFCS should provide coupled flight modes that permit safe and controlled flight in the three axes at all airspeeds (lateral position and speed, longitudinal position and speed, and height and vertical speed) from the previous MINI to a hover (within the maximum demonstrated wind envelope). The AFCS should also provide automatic transition to the helicopter instrument flight (Appendix B to part 27) envelope as part of normal sequencing. Additional considerations include:
(i) A pilot-selectable "Go-Around" mode to safely interrupt any other coupled mode and automatically transition to the helicopter instrument flight (part 27, Appendix B) envelope.
(ii) A means to prevent unintended flight below a safe minimum height.
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(4) The AFCS sub-mode system architecture should support the following functionalities:
(i) A system for limiting the engine power demanded by the AFCS when any of the automatic piloting modes are engaged, so that FADEC power limitations such as torque and temperature are not exceeded.
(i) If applicable, a system providing the aircraft height above the surface and final pilot-selected height at a location on the instrument panel in a position acceptable to the FAA that will make it plainly visible to and usable by any pilot at their station.
(iii) A system providing the aircraft heading and the pilot-selected heading at a location on the instrument panel in a position acceptable to the FAA that will make it plainly visible to and usable by any pilot at their station.
(iv) If applicable, a system providing the aircraft longitudinal and lateral ground speeds and the pilot-selected longitudinal and lateral ground speeds when used by the AFCS in the flight envelope where airspeed indications become unreliable. This information must be presented at a location on the instrument panel in a position acceptable to the FAA that is plainly visible to and usable by any pilot at their station.
(v) A system providing wind speed and wind direction when automatic piloting modes are engaged or transitioning from one mode to another.
(vi) A system that monitors for flight guidance deviations and failures with an appropriate alerting function that enables the flight crew to take appropriate corrective action.
(vii) An alerting system that provides visual or aural alerts, or both, to the flight crew when (if applicable) the stored or pilot-selected safe minimum height is reached, when an AFCS malfunction occurs, and when the AFCS changes modes automatically from one AFCS mode to another.
For normal transitions from one AFCS mode to another, a single visual or aural alert may suffice.
For an AFCS mode malfunction or a mode having a time-critical component, the flight crew alerting system must activate early enough to allow the flight crew to take timely and appropriate action. The alerting system means must be designed to alert the flight crew in order to minimize
(5) The reliability of the AFCS must be related to the effects of its failure. The occurrence of any failure condition that would prevent continued safe flight and landing must be extremely improbable. For any failure condition of the AFCS that is not shown to be extremely improbable, the helicopter must be safely controllable and capable of continued safe flight without exceptional piloting skill, alertness, or strength. Additional unrelated probable failures affecting the control system should be evaluated. Additionally, the AFCS must be designed so that it cannot create a hazardous deviation in the flight path or produce hazardous loads on the helicopter during normal operation or in the event of a malfunction or failure, assuming corrective action begins within an appropriate period of time. Where multiple systems are installed, subsequent malfunction conditions should be evaluated in sequence unless their occurrence is shown to be improbable. A functional hazard assessment and a system safety assessment should be provided in accordance with section 27.1309 of this AC to address the failure conditions associated with the AFCS modes. The assessments should consider all systems required for AFCS mode operation, including the AFCS, all associated AFCS sensors (for example, radio altimeter), and primary flight displays. Electrical and electronic systems with catastrophic failure conditions (for example, AFCS) must comply with the § 27.1317(a)(4) High Intensity Radiated Field requirements.
(6) The AFCS modes should be demonstrated in the requested flight envelope.
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AC 27-1B, Chg 7 2/4/2016
(7) For any single failure or any combination of failures of the AFCS that is not shown to be extremely improbable, the recovery should not result in a loss of height greater than half of the minimum use height.
(8) For coupled flight below the previously established VMINI, at the maximum demonstrated winds, the helicopter must be able to maintain any required flight condition and make a smooth transition from any flight condition to any other flight condition without requiring exceptional piloting skill, alertness, or strength, and without exceeding the limit load factor.
(9) For coupled flight below the previously established VMINI, the following stability requirements replace the stability requirements of paragraph IV, V, and VI of of Appendix B to part 27:
(i) Static Longitudinal Stability: The requirements of Appendix B to part 27, paragraph IV are not applicable.
(i) Static Lateral-Directional Stability: The requirements of Appendix B to part 27, paragraph V are not applicable.
(iii) Dynamic Stability:
(A) Any oscillation must be damped and any aperiodic response must not double in amplitude in less than 10 seconds. This requirement must also be met with degraded upper mode(s) of the AFCS.
(B) After any upset, such as a wind gust, the AFCS must return the aircraft to the last commanded position within 10 seconds or less.
(10) With any of the upper mode(s) of the AFCS engaged, the pilot must be able to manually recover the aircraft and transition to the normal (Appendix B to part 27) FR flight profile envelope without exceptional skill, alertness, or strength.
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2/4/2016 FIGURE AC 27.APХ B-1 ROTORCRAFT DYNAMIC STABILITY 35 30
ACCEPTABLE
REQUIREMENTS FOR IFR
UNACCEPTABLE Page Apdx B - 21 -0.2 0.05 Ø.15 AC 27-1B, Chg 7
DAMPING RATIO
AC 27 APPENDIX B (AMENDMENT 27-51) AIRWORTHINESS GUIDANCE FOR ROTORCRAFT INSTRUMENT FLIGHT.
a. Explanation.
(3) Amendment 27-51 revised section VIII (b)(5)(i), which requires isolating the pilot instrument system from any other operating systems, to only apply to pneumatic systems. This change allows other types of pilot instrument systems to be connected to other systems on the rotorcraft and allows the use of modern technology to monitor and display highly integrated information. This guidance remains unchanged except paragraphs (b)(9)(v) and (b)(9)(vi) of this AC section do not apply to nonpneumatic systems.
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