(3) Navigation System - one navigation system, including necessary sensor inputs such as directional gyros.
(4) Transponder.
(5) ICS System - required for two pilot approval.
(G) Instrument Lights (or equivalent).
(B) Avionic and instrument systems that should remain operational, for IF approved rotorcraft, after total failure of the electrical generating system. The rotorcraft must be capable of flight for a minimum of 30 minutes. The suggested minimum equipment is as follows:
(1) Magnetic Compass.
(2) Airspeed-Altitude Attitude Presentation.
(3) Communications One VHF System.
(4) Instrument Lights (or equivalent).
(5) ICS System-For Two Pilot Approval.
(iii) Directional Instruments. A magnetic, gyro stabilized direction indicator is specified because navigation in instrument flight must be precise. In rotorcraft, the nonstabilized magnetic indicator is subject to many errors, particularly in turbulence. Therefore, it is inappropriate as the primary source of directional information, but it is adequate as an emergency source. A nonslaved directional gyro is also inappropriate as the primary source of directional information because of drift and the requirement to set it to some other precise reterence.
(A) As a minimum for single pilot IFR, a nonstabilized magnetic indicator (such as a "whiskey compass") and a magnetic gyroscopically stabilized direction indicator system (slaved) are required.
(B) The minimum for dual pilot certification includes the instruments required for single pilot, and an additional independent gyroscopically stabilized directional indicator system (slaved or
(14) FR Electrical System.
(i) General.
(A) The entire electrical system, both AC and DC portions, should be reviewed with IFR operation in mind. This review is necessary since most of the rotorcraft presently certificated do not include IFR operation as part of their certification. Many aspects of normal operation and results of failure conditions may be entirely acceptable for VER operation but unacceptable for IFR operation.
(B) Provisions should be made for a capability to continue flight to destination and alternate airports in the event of a single failure in the electrical system. Section 27.1351 of this AC contains the definition of a "single failure." The evaluation of the system under failure conditions should consider not only the failure itself, but also the recommended cockpit procedure to respond to any failure.
(C) The fault analyses of the electrical system and the results of the system testing to validate that analysis serves as a good starting place for the electrical system review. Failure of each generator, each battery, and each component, such as switches and relays, should be accounted for first Page Apdx B - 15
AC 27-1B, Chg 7 2/4/2016 (D) System failure such as tripped circuit breakers, blown fuses, loss of busses, loss of feeders, loss of ground terminals, and failure of electrical disconnect plugs should also be considered.
(E) Routing of all wiring from each power source throughout the distribution system should be reviewed. In all instances, feeder wires should be routed separately from small gage control wiring. In addition, wiring for each power system should be separated to the maximum extent practical from the wiring associated with other required power systems.
(F) A single electrical disconnect plug should not contain wiring for more than one generating system. Many systems incorporate automatic feeder fault protection that disables a power source experiencing a short circuit on its feeder, and in some instances, passive protection has been provided for the feeders.
(G) There may be other failures that should be considered that are peculiar to the specific design being evaluated and, if so, an appropriate accounting of these failure should also be made.
(H) Single engine rotorcraft upgrades from VFR to IFR will require careful evaluation of the electrical system. These aircraft normally do not have distribution systems that can tolerate bus or feeder failures, and these failures would result in loss of the entire electrical system. Normally these systems are modified such that distribution system and power supply failures will only result in a partial loss of electrical capability. The power supply problem has been accounted for by the installation of a second generator in some instances or by adding extra battery capacity in others. When an extra battery is added, or a larger battery is substituted, the ampere-hour capacity should be based on one-half the time associated with a worst case maximum flight duration consideration. Additionally, in all instances so far the standby attitude system has been provided a separate power supply capability, in addition to the
(ii) Review of Regulations. The airworthiness regulations concerning electrical systems begin with § 27.1301 Subpart F (Equipment) and continue through § 27.1401. Other rules may also concern the electrical system; however, compliance with these sections should have been assured as part of the (Ili) Specific Emphasis Areas. In some previous installations, changes have been necessary in the areas listed below. Future installations should be checked carefully in these areas and other areas that indicate a need for attention.
(A) Systems Affected by Icing. Gross inaccuracies in altitude and airspeed indicators resulting from icing could be disastrous in IF flight. For rotorcraft not equipped with approved alternate static sources, static ports should be carefully evaluated and should either be heated or an analysis verified by flight test data submitted to substantiate leaving them unheated. Static line routing should be carefully evaluated for low spots. In addition, if static ports are on the side of the rotorcraft, the lines should be initially routed upward just behind the static ports, then down to a drain. If the lines are initially routed upward, the lines will not fill with water when the rotorcraft is flown through rain or is washed.
(B) Overvoltage Protection. A few rotorcraft may have this protection, but many do not.
Since overvoltage protection is specifically required for IFR operation, the rotorcraft's basic electrical system should be very carefully reviewed for this capability.
required flight instruments that are not provided with a visual means, the following should be accounted for:
(1) The visual means provided should be at least adjacent to the instrument.
Page Apdx B - 16
(2) The visual means should be adequately placarded.
(3) The power should be measured at or near the point where it enters the instrument.
(4) For electrical instruments, the power is considered adequate when the voltage is within approved limits. The source of power for the visual means of indication must be independent of the source of power for the instrument itself. Independent, in this case, means a separate circuit protective device and a separate distribution system bus.
(D) Multiple System Separation. Multiple systems performing the same function are required in certain instances because it is probable that a single system will fail. Separation of such systems would preclude a single fault from causing a multiple system failure. The following should be
(1) When possible, cable routing should be accomplished to ensure the maximum separation; for example, one system routed on one side of the rotorcraft and the other system on the opposite side. In some areas, physical separation may be minimal, as pedestals, junction boxes, and equipment racks bring systems close together.
(2) Systems that are required to be duplicated should not be routed through one electrical disconnect plug.
(3) System grounds should be evaluated to assure wiring for two required systems is not grounded to the same terminal. If a terminal strip contains grounds for multiple systems, it should be grounded to the rotorcraft's airframe in two places from two separate terminals.
(E) Circuit Protective Devices. All systems that are "required" for IFR operation are considered necessary for safe IFR operation, and the circuit protective devices for those systems should generally be accessible to the crew in the cockpit so they can be readily reset or replaced in flight. Where a capability is provided that is above the minimum certification requirements, accessibility may not be an issue while additional equipment may be required for dispatch in IFR operation. The location of the generator field protective devices has been a problem in some rotorcraft. The protective devices that can result in the loss of a required power system source should be capable of being reset or replaced in the cockpit while in flight. This position is further supported by the occurrence of nuisance opening of circuit protective devices in rotorcraft. Further discussion on this issue is included in section 27.1357 of this AC.
(F) Intercommunication System. All audio for the entire rotorcraft comes together at this system. An evaluation should be made to ensure that no single failure would result in the loss of all audio for the rotorcraft. Check for common grounds, common connectors, etc. Power inputs should also be disabled.
(15) REM Material.
(i) In addition to other required information, the limitations section of the RFM or RFM Supplement (RFMS) must include the approved IFR flight envelope, minimum IFR crew requirements, the minimum required equipment for dispatch into IFR conditions that is not covered by the operating regulations, and the maximum approach gradient that has been approved. If a significant loss of altitude is experienced in any flight regime or maneuver during certification analysis or testing, the emergency operating procedures should include a statement of this altitude loss along with any other appropriate information.
(il) The limitations section of the RFM should not include restrictions prohibiting external cargo operations. These operations are covered by parts 91 and 133, and all external load operations conducted under these parts must be approved by the controlling operations inspector. It is the Page Apdx B - 17
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.
Page Apdx B - 18
(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.
Page Apdx B - 19
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.
Page Apdx B - 20
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.
Page Apdx B - 22