7/30/97 АС 27-1A (I) Electrical Power Availability for Avionic and Instrument Systems.
Minimum avionic and instrument systems should remain operative after electrical power failures in relation to IFR operation. The lists that follow suggest the minimum Avionic and Instrument Systems that should remain operational after a single failure of the generating system and after failure of all but the emergency power source. These lists do not address the basic equipment required for non-IFR related operation. These basic equipment requirements are addressed by the appropriate paragraph of this AC.
(A) Avionic and instrument systems that should remain operational for JER approved rotorcraft, after a single failure of the electrical generating system.
The rotorcraft must be capable of IF flight for one-half the maximum cruise duration.
The suggested minimum avionic and instrument systems are as follows:
(1) Flight Instruments. Same as § 27.1303 requirements, except as defined by Subparagraphs 775(12)(i)(A) and (C).
(2) Communications. One VHF radio.
(3) Navigation System. One navigation system, including necessary sensor inputs such as directional gyros.
(4) Transponder.
(5) ICS System. Required for two pilot approval.
(Q) Instrument Lights (or equivalent).
(B) Avionic and instrument systems that should remain operational, for IFR 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.
(ії) 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 Par 775 1039
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 reference.
(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 unslaved).
(13) ER 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 VFR operation but unacceptable for IFR operation.
(B) Provisions should be made for a capability to continue flight for one-half the maximum cruise duration in the event of a single failure in the electrical system. Paragraph 652 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 since failure of equipment and components are the most probable.
(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. Also, wiring for each power system should be separated to the maximum extent practical from the wiring associated with other required power systems.
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(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 that are being upgraded 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 extra power supply capability described above.
Review of Regulations. The airworthiness regulations concerning electrical systems begin with § 27.1301 (Ref: 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 original VFR approval.
(її) 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 IFR 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. Also, 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 Par 775 1041
АС 27-1A 7/30/97 IFR operation, the rotorcraft's basic electrical system should be very carefully reviewed for this capability.
(C) Power Adequacy Indication. Most flight instruments that use a power supply have a visual means integral with the instrument to indicate the adequacy of the power being supplied. For those 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.
(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 to be 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 considered:
() 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. Some areas, such as pedestals, junction boxes, and equipment racks bring systems close together, and in these areas physical separation may be minimal.
(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 to be 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. The location of the generator 1042 Par 775
field protective devices has been a problem in some rotorcraft. The protective devices that can result in the loss of a required power system should be accessible in the cockpit. 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 Paragraph 655b(4) of this advisory circular.
(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 will result in the loss of all audio for the rotorcraft. Check for common grounds, common connectors, etc. Power inputs should also be disabled.
(14) Rotorcraft Flight Manual Material.
(i) In addition to other required information, the limitations section of the Rotorcraft Flight Manual (RFM) or RFM Supplement 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 which 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.
The limitations section of the Rotorcraft Flight Manual 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 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.
(15) Rotorcraft Flight Below Instrument Flight Minimum Speed.
The advent of steep angle, decelerating precision instrument approach procedures will necessitate flying at airspeeds below the instrument flight minimum speed (VINI) established for most rotorcraft under FAR 27 Appendix B, Paragraph 11(c).
(ii) Applications for findings of equivalent safety to approve instrument flight below VMINI, will be considered by the FAA/AUTHORITY for rotorcraft meeting at least the following criteria:
(A) The rotorcraft is certified for FR flight.
(B) For constant airspeed approach approval: a minimum approach airspeed is specified by the applicant, at which the rotorcraft is demonstrated to be safely controllable and capable of instrument flight without undue pilot effort for the Par 775 1043
duration of the approach and transition to missed approach, including acceleration to an airspeed above VMINI- (C) For decelerating approach approval: a two or three cue flight director is provided as required equipment, and the rotorcraft is demonstrated to be safely controllable and capable of instrument flight without undue pilot effort for the duration of the approach and transition to missed approach, including acceleration to an airspeed above VMINI- (D) The rotorcraft is demonstrated to be safely controllable following single failures of aircraft systems not shown to be extremely improbable at the minimum approach airspeed specified by the applicant or encountered during a decelerating approach.
The Rotorcraft Flight Manual Supplement (RFMS) contains the following information in addition to the requirements of Paragraph IX of Appendix B to FAR 27:
(1) Minimum approach airspeed, if applicable.
(2) Additional aircraft equipment requirements for flight below VMINI and/or the minimum approach airspeed, if applicable.
(3) Maximum approach angle.
(4) Maximum allowable surface wind for safe conduct of the approach.
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Par 775 7/30/97
ACCEPTABLE 15 10
UNACCEPTABLE 5
L L L -0.3 -0.2 -0.1 0.05 0.1 0.15 AC 27-1A DAMPING RATIO (5) 1045 FIGURE 775-1. ROTORCRAFT DYNAMIC STABILITY REQUIREMENTS FOR IFR
776. CERTIFICATION PROCEDURE FOR ROTORCRAFT AVIONICS EQUIPMENT
a. Pretest Requirements.
(1) General. This test guideline has been prepared as an aid in the evaluation of rotorcraft avionics (aviation electronics) equipment installations. The criteria presented are not to be considered exclusive but are offered as one method of evaluating design practice and pertormance. The testing and qualification of an electronic installation should be considered as consisting of three phases:
preinstallation, ground, and flight. The amount of testing necessary during each phase will vary with the amount of testing performed on previous phases. For example, if a system is TSO'd, the preinstallation performance is probably substantiated, and therefore the ground and flight testing can be reduced accordingly. Also, a thorough ground testing program should result in reduction in necessary flight testing. When the operating or airworthiness regulations require a system to perform its intended function, the use of TSO'd equipment or the submission of data substantiating the equipment performance is strongly recommended.
(2) Regulatory References. Sections 27.1301 and 27.1309 (through Amendment 27-19).
(3) System Design. Systems or equipment presented for installation approval, when not qualified by TSO or other approval means, should be accompanied by sufficient data to substantiate their design acceptability.
(i) Operation of Controls. The operation of controls intended for use during flight, in all possible position combinations and sequences, should not result in a condition that would be detrimental to the continued safe performance of the system.
(ii) Electrical Shock. Systems should be designed so that under all probable conditions, the risk of dangerous electrical shock is minimized.
(Ш) Fire Hazard. The design of the system should be such that all components meet the applicable fire and smoke protection requirements of §§ 27.853 and 27.863. Cables and equipment to be installed in designated fire zones that are used during emergency procedures should be at least fire resistant.
(iv) Plugs and Cables. Connector pins for sensitive signal circuits should not be adjacent to pins used for ac power circuits. If redundant wiring is used to comply with systems regulations such as § 27.1309, the wires should be routed through separate plugs and/or cables with as much physical separation as practicable. The system should be designed so that incorrect mating of plugs is not possible. Cable grounding and shielding techniques should be used to minimize electromagnetic interference.
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(4) System Performance. Where the operating or airworthiness regulations require a system to perform its intended function, and when the equipment is not qualified by TSO or other approval means, performance data furnished to the FAA/AUTHORITY can reduce the installed performance testing. The appropriate TSO minimum performance standard may be used as a guide.
Environment. An appropriate means for environmental testing is set forth in Radio Technical Commission for Aeronautics (RTCA) document DO-160. The applicant should submit test reports showing that the laboratory tested categories such as temperature, vibration, altitude, etc., are compatible with the environmental demands to be placed on the rotorcraft.
(ii) Failure Analysis. Section 27.1309(b) requires consideration of system malfunctions or failures.
(5) Installation Design.
(i) Mechanical Installation. Installations should be made to (1) ensure compliance with the airworthiness regulations, and (2) comply with the equipment manufacturer's recommendations. The designer should observe good engineering practices in specifying material type, thickness, fastener type, edge distance, and attachment to the equipment rack. By analysis or static tests, the mounted equipment should be shown to withstand the inertia forces of §§ 27.561(b)(3) and 27.337. Refer to AC 43.13-2A for static test procedures.
(її) Arrangement and Visibility. The mounting position of all instruments, switches, position labels, and control heads should make them plainly visible to the pilot while in his normal, panel-facing position and under all cockpit lighting conditions likely to occur. TSO approval does not assure instruments will be acceptable in a particular cockpit installation or for all lighting conditions. The instruments, switches, and placarding must be free from reflections. Malfunction annunciation devices should be conspicuous and clearly visible to the pilot. (See AC 20-69 and §§ 27.1321, 27.771,
27.1381, and 27.1555(a)).
(Ш) Load Analysis.
(A) Power Sources. It should be determined whether the electrical power source capacity is adequate for the system installation under all foreseeable operating conditions including engine failure on multiengine rotorcraft. System load reductions should be applied or power source capacity increased, if necessary, to assure compatibility between load and source. If duplicate systems are required, they should be powered from separate buses.
(B) Navigation Course Deviation Circuit Loading. It should be determined that the deviation circuit source impedance is matched by its load and that Par 776 1047
АС 27-1A 7/30/97 the source capacity is not exceeded. When the system is capable of transfer, the transfer loads should also be considered (§ 27.1301).
(C) Malfunction Indicator Circuit Loading. It should be determined that the malfunction indicator source impedance is matched by its loads and that the source capacity is not exceeded. When the system is capable of transfer, the transfer loads should also be considered (§ 27.1301).
(D) Synchro Signal Loading. When parallel loads are added to synchro's, the manufacturers' specifications should be reviewed to assure that the additional loads do not result in an overloaded synchro.
(iv) Interface. In many cases, the mating units of a system are designed by different manufacturers. For example, a brand-X gyro may be designed for operation with a brand-X flight director, but later a modifier decides to operate a brand-Y autopilot with the brand-X gyro. This applies just as well to NAV receivers, AREA NAV units, course indicators, omni bearing selectors, tachometer indicators, transmitters, and many other equipment items. When this is the case, the applicant should provide data, in summarized form, describing those characteristics such as impedance, volts, etc., that are necessary to ensure a compatible and reliable system.
The data should also reference the source of the interface data (§ 27.1301).
(v) Elight Tests. An FAA/AUTHORITY engineering flight test is required during type certification or after modification that changes the established limitations, flight characteristics, or performance of a rotorcraft or any of its required systems or operating procedures. New installations of equipment in the cockpit or modifications that affect existing equipment in the cockpit should be evaluated by appropriate flight test personnel if it is necessary to evaluate operational aspects of the change. Where possible, cockpit arrangement, placards, markings, instrument visibility, and light reflections can be evaluated on the ground if the applicant opts to darken the windows.
Electromagnetic compatibility functional checks, windshield glare, and pilot workload evaluations may be conducted in flight at the FAA/AUTHORITY flight test pilot's option.
(vi) Radio Master Switches. Some installations incorporate radio master switches to control special busses for the avionics systems. If this capability is provided it should be evaluated to assure failure modes are not introduced that will result in excessive or even total loss of all required avionics. One switch that controls all required avionics is not considered acceptable for IFR installations. The evaluation should include an assessment of the loss of the systems to be included on the radio master switches), and the subsequent effect on continued safe flight.
b. Test Procedures. Where the airworthiness or operating regulations require a system to perform its intended function, and/or not create a hazard to other required systems, sufficient testing should be accomplished to assure satisfactory performance.
When ground testing is not sufficient to properly evaluate a system's performance, flight 1048 Par 776
testing should be accomplished. Acceptable flight test criteria for specific navigation and communication equipment are contained herein. If the rotorcraft is to be approved for IFR operations, the additional criteria of Paragraph 775 of this advisory circular should be satisfied.
(1) VHF Systems.
(i) General. Intelligible communications should be provided between the rotorcraft and ground facilities throughout the airspace within 80 nautical miles (NM) of an FAA/AUTHORITY ground facility from radio line of sight altitude to the maximum altitude for which the rotorcraft is certificated. Communication should be provided with the rotorcraft at or above line of sight altitude in right and left bank up to 10° and on all headings.
(ii) Electromagnetic Compatibility (EMC). With all electrical/electronic systems operating in flight, verify by observation that no adverse effects are present.
Antenna Measurement. If satisfactory antenna measurement data are provided, the following flight test may be reduced to checks in right and left turns in the vicinity of the predicted bearings of worst performance. If antenna locations are symmetrical, tests may be conducted using only one direction of turn.
(A) Long Range Reception. Starting at a distance of 80 NM from the ground facility antenna, perform a right and/or left 360 turn at a bank angle of at least 10°. Communicate with the ground facility every 10° of turn to test the intelligibility of the signals received at the ground station and in the rotorcraft. For 80 NM, the minimum line of sight altitude is approximately 4,000 feet.
(B) Approach Configuration. With the landing gear down and with the rotorcraft in the approach configuration (at a distance of 10 NM from the ground station and in an idle power descent toward the station), demonstrate intelligible communications between the rotorcraft and the ground facility.
(2) LF Systems.
Acceptable communications should be demonstrated by contacting a ground facility at a distance of at least 80 NM. Single sideband equipment should also perform acceptably in the amplitude modulation mode of operation.
(II) It should be demonstrated that precipitation static is not excessive when the aircraft is flying at cruise speed (in areas of high electrical activity, including clouds and rain if possible). Use the minimum amount of installed dischargers for which approval is sought.
(3) VOR Systems.
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These flight tests may be reduced if adequate antenna radiation pattern studies have been made and these studies show the patterns to be without significant holes (with the rotorcraft configurations used in flight, i.e., landing gear retracted en route and extended for approach). Particular note should be made in recognition that certain rotor RPM settings may cause modulation of the course deviation indication (rotor modulation). VOR performance should be checked for rotor modulation in both approach and en route operation while varying rotor RPM throughout its normal range.
The airborne VOR system should operate normally with warning flags out of view at all headings of the rotorcraft (in level flight) throughout the airspace within 80 NM of the VOR facility while flying above the radio line of sight altitude to within 90° to 100 percent of the maximum altitude for which the rotorcraft is certified.
(iii) The accuracy determination should be made such that the indicated reciprocals agree within 2°. Tests should be conducted over at least two known points on the ground such that data are obtained in each quadrant. Data should correlate with the ground calibration and in no case should the absolute error exceed +6°. Fluctuation of the course deviation indication should not be excessive.
(A) En route Reception. Fly from a VOR facility along a radial to a range of 80 NM. The VOR warning flag should not come into view nor should there be deterioration of the station identification signal. The course width should be 20° (t5° tolerance, 10° either side at the selected radial). If practical, perform en route segment on a doppler VOR station to verify the compatibility of the airborne unit. Large errors have been found when incompatibility exists.
(B) Long Range Reception. Perform a 360° right and a 360° left turn at a bank angle of at least 10° at an altitude just above radio line of sight (see b(1)(a) for line of sight altitude) and at a distance of 80 NM from the VOR facility.
Signal dropout should not occur as evidenced by the maltunction indicator appearance.
Dropouts that are relieved by a reduction of bank angle at the same relative heading to the station are satisfactory. The VOR identification should be satisfactory during the left and right turns.
(C) En route Station Passage. Verify that the To-From indicator correctly changes as the rotorcraft passes through the cone of confusion above a VOR facility.
(4) Localizer Systems.
Flight test requirements may be modified to allow for adequate antenna radiation pattern measurements as discussed under VOR, Paragraph 776b(3)(i), flight test.
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