provided. If pilot reset is to be allowed, the procedure should be described in the RFM.
(viii) Calculated failure rate (with unannunciated faults present): Before a calculation of the failure rate can be attempted, the failure should be defined. The determination of failure rate, using the definition of failure, can be the product of a Failure Mode Effects Analysis (FMEA) combined with a reliability analysis, using individual part reliability figures. The figures should come from some recognized data base. The failure rate calculations should consider the worst case application limitations such as flight operation, environmental considerations, and time of operation. The flight operations to be considered for the worst case scenario include all flight segments (take off, cruise, hover, landing, etc.) together and separately for the various missions the aircraft is expected to be used in. Another way to determine failure rate is to use service history. However, service history is applicable only if a high degree of similarity exists for the FADEC and its installed application. The calculated failure rate is the direct result of the FMEA, and should meet the integrity level requirement determined by the Functional Hazard Assessment.
(6) Certification Approach:
(i) Analysis Requirements: Functional Hazard Assessment:
Compliance to the requirements of FAR paragraphs 27.1141 and 27.1309 for a single channel FADEC in a single engine application should be based on criticality of application for the system under consideration. This criticality of application may be determined by performing an aircraft level hazard assessment that starts with the type of possible failures and ends with the results of these failures. The results can be categorized into criticality levels and the required integrity levels can be obtained by matching the required integrity level to the criticality level. The main emphasis should be on determining the higher levels of criticality (Major and above) and their source. This process should include consideration of failures seen at the operational level and interaction of the failures with the airframe and crew as well as the system itself. The following subject areas are related to this assessment.
(A) Assumptions: Assumptions should be made about the airframe/crew interface in order to perform the aircraft level hazard assessment.
These assumptions are prerequisites to perform an aircraft level hazard assessment and must be listed in this hazard assessment and validated by airframe testing when the airframe is available. If the assumptions cannot be validated, the actual airframe test data must be substituted for the invalidated assumptions (assumed prerequisites) and the hazard assessment re-evaluated with the new data supported prerequisites. The results of this new assessment would be the deciding factor for acceptance of the FADEC system for the installation as designed or provide the necessity for design changes.
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(B) Criteria: Acceptance of an engine fitted with a single channel FADEC system in a single engine rotorcraft application requires that the integrity levels of the FADEC system be compliant with the criticality levels determined by the aircraft level Functional Hazard Assessment (FHA). In addition, final acceptance of the system at the aircraft level for the application is based on the integrity level(s) that match the criticality level(s) determined by the hazard assessment that uses data that has been validated during the aircraft flight test program. These assumptions/prerequisites would include operational aspects associated with the possible FADEC failures and would include as a minimum the following:
(1) Crew/aerodynamic response to failure.
(2) Worst case flight operation for failure to occur. (Landing, IFR, etc.)
(3) Duration of flight operation (exposure time).
(4) System interaction with shared Inputs/Outputs with other systems and/or with back-up systems.
(5) Adequate annunciation of failure.
(ii) Validation Criteria:
(A) General:
(1) Validation of the assumptions/prerequisites made by the engine manufacturer in developing the SSA, using aircraft level FHA requirements, must be validated by conducting flight testing during the certification of the installation. The possibility exists that if the assumptions cannot be validated during flight testing, then engine and/or FADEC redesign may be required.
(2) Failure management methods that are related to operational characteristics should be addressed. It should be determined that the FADEC/engine manufacturer's envisioned failure management is desirable and compatible with the operational requirements. Therefore, the following basic FADEC related information should be identified:
(i) The detected failures.
(ii) The failures that are not detected.
(iii) The action that the FADEC takes when failures are detected.
(iv) The failures that are annunciated to the crew and in what manner.
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(v) The anticipated operational action required as a result of detected failures.
(vi) Possible operational results of the undetected failures.
(vii) Verification that the assumed worst case flight operation is the worst case.
(B) Manual Backup: Additional aircraft operational testing is required to specifically evaluate the manual backup system for compliance with the FAR requirements. The acceptability of the manual backup system depends substantially on its installation and interface with the airframe. The following items need to be demonstrated in accordance with § 27.927 and §27.939 or accomplished on each application prior to the acceptance of the manual backup system:
(1) It should be demonstrated by flight test with the failure of the primary engine control, that the aircraft can be flown and a safe and controlled power-on landing executed without creating an undue pilot workload.
(2) It should be demonstrated by flight test that switching between control modes will not create an unsafe condition during any phase of operation within the aircraft operating envelope.
(3) The pilot action required as a result of a failure of the primary control and used as an assumption in the FHA and FMEA should be validated during flight tests and listed in the emergency procedures section of the flight manual.
c. FADEC RELIABILITY REVIEW DUE TO INCREASED ROTORCRAFT
ENDURANCE
(1) Background. This advisory material is to provide guidance for reevaluation of the FADEC control system reliability due to extension of the aircraft mission endurance. During the initial type certification of an aircraft, an analysis is normally conducted on systems to determine their criticality category (e.g.
catastrophic, hazardous, major, etc.) and reliability requirements. To establish a system’s reliability, an exposure time is determined by making certain assumptions.
In most cases, the exposure time is the average endurance based on the various flight scenarios in which the aircraft is to be used. When an aircraft’s expected mission endurance is increased by adding fuel capacity, a new analysis for system reliability should be conducted taking into account the new increased mission endurance.
(2) Requirements.
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AC 27-1B, Chg 2 4/25/06
(i) If the applicant has access to the initial analysis used for the type certification, one method to accomplish the new reliability analysis is by multiplying the exposure time used in the original reliability analysis by the ratio of the increased maximum endurance to the original maximum endurance. That is, if the aircraft endurance increases by 50 percent due to additional fuel capacity, the assumed exposure time should also increase by 50 percent. The applicant should then rework the analysis using this new exposure time.
(ii) If the applicant does not have access to the initial analysis it will be incumbent upon them to provide the rationale used for determining the new exposure time and to provide a complete analysis for the systems determined to be critical.
The FAA engineer should compare this new analysis to the original.
d. CERTIFICATION GUIDELINES FOR COMPLIANCE TO THE
REQUIREMENTS FOR ELECTROMAGNETIC COMPATIBILITY (EMC) TESTING FOR
NON-QUALIFIED EQUIPMENT AND EQUIPMENT KNOWN TO HAVE A HIGH
POTENTIAL FOR INTERFERENCE WHEN INSTALLED ON ROTORCRAFT WITH ELECTRONIC CONTROLS THAT PROVIDE CRITICAL FUNCTIONS.
(1) Background.
(i) Rotorcraft operations are varied and use a wide assortment of equipment. While some of this equipment is qualified to aircraft standards, particularly environmental standards, some of the equipment not qualified to such standards may be the source of harmful electromagnetic interference. Rotorcraft typically have not had electronic controls that perform critical functions, such as engine controls and flight controls; therefore, there was no real concern about requiring equipment to be qualified to aircraft standards. Typically, this equipment was installed with only a cross -matrix operational check for EMC. These tests consisted of operating the equipment in question and checking visually for an indication of interference. The equipment was, for the most part, non-required equipment and the primary concern was that interference might be emitted from the equipment.
(ii) Unqualified equipment and their effects on critical systems is of particular concern due to the recent increase in the number of rotorcraft with electronic engine controls and the implementation of fly-by-wire technology. Additionally, the physical close proximity of installed equipment to electronic controls that provide critical functions is inherent due to the smaller size of most rotorcraft and represents a greater potential for interference than for larger fixed wing aircraft.
(2) Requirements.
(i) The rules to assure that required functions are not subject to interference are provided in the certification basis for the rotorcraft. Although the certification basis may differ between aircraft, the requirements that address electromagnetic interference Page MG 4 - 10
are similar and result in the same methods for compliance. A note has been added to the type certificate data sheets for rotorcraft that employ FADECs. This note was added to remind all modifiers that the requirement for addressing interference exists and that special EMI test considerations must be addressed to show compliance. Most EMC considerations can be addressed by the operational interference checks addressed in the background discussion. However, when a critical control function is provided by some electronic means, special EMI test considerations must also be addressed, in addition to the previously described EMC tests. The determination of when these other, more rigorous tests are required is a simple concept, but complex in practice. More rigorous testing is required to satisfy the concern for the installation of equipment that would interfere with the critical control (e.g., FADEC, Fly-By-Wire, etc.) or failure management of the critical control. This class of equipment is “equipment known to have a potential for interference,” which may or may not be qualified to an aircraft standard, such as high frequency (HF) radios, high powered radars, hoists, transmitting antennas located near the controls systems, etc. The concern associated with this class of equipment is the possible interference with the critical electronic controls.
(ii) Accomplishment. In addition to the following special testing considerations addressed in paragraph (2)(iii), “EMI Installation Testing for Critical Controls” and “Installation Test Conditions,” all installed equipment should undergo a cross-matrix operational check for EMC considerations by operating all equipment under consideration and determining if an interference hazard is created.
(A) Class of Equipment - Equipment Known to Have a High Potential for Interference: This class of equipment should be tested in the installation as described in the “EMI Installation Testing for Critical Controls/Installation Test Conditions,” paragraph (2)(iii). Since the concern of this class of equipment is its high potential for interference, its EMI laboratory qualification does not preclude the EMI installation testing.
Equipment that meets any one of the following criteria is considered to be “equipment known to have a high potential for interference.”
• Equipment that requires 25 amps or more to operate,
• Equipment that transmits 30 watts or more,
• Equipment with an antenna located 0.5 meters or less from the FADEC, or
• High Frequency (HF) Transmitters of any power.
The types of equipment in this class include HF radios, high-powered radars, hoists, high-powered radios, installations where radio transmission antennas are in close proximity to the controls, and equipment that require large currents to operate or radiate strong electromagnetic fields. Examples of this type of equipment are some Emergency Page MG 4 - 11
AC 27-1B, Chg 2 4/25/06 Medical Service (EMS) equipment, night sun lights, some air conditioners, video and sound systems that require large currents (25 amps – up) to operate, Forward Looking Infrared System (FLIRS), some forward looking radars, some weather radars, some communication systems that transmit 30 watts or more, some data link transmission systems, etc.
NOTE: Equipment that does not meet this criteria is considered to be “equipment not known to have a high potential for interference.”
(B) Class of Equipment – Equipment Not Known to Have a High Potential for Interference: Once it has been established that the equipment being proposed to be installed is not in the class of “equipment known to have a high potential for interference,” per the criteria stated in paragraph (2)(ii)(A), there is no requirement to conduct the EMI installation tests described in the “EMI Installation Testing for Critical Controls/Installation Test Conditions” paragraph (2)(iii). However, the cross-matrix operational checks for EMC considerations described in paragraph (2)(ii) are still required.
(iii) EMI Installation Testing for Critical Controls:
(A) EMI installation testing is no longer required for unqualified equipment that does not have a high potential to cause interference. However, EMI installation testing is the only method of testing to show compliance for interference considerations, for the class of “equipment known to have a high potential for interference.” The criteria for determining whether the “equipment is known to have a high potential for interference” is stated in paragraph (2)(ii)(A).
(B) To accomplish the EMI installation tests, there must be an FAA- approved test plan that requires the high interference potential equipment to be operated through all reasonable modes of operation, in order to determine if electromagnetic interference is entering the electronic control system. EMI installation testing consists of interrogating the control, if it has such a feature, to determine if the critical electronic control system is adversely affected (identify the recorded faults that occur during the test). Additionally, real -time monitoring of the control’s input/output parameters should be accomplished. The pass/fail criteria is “no detected interference” for a pass state, and conversely a fail state if any interference is detected entering the control. If interference is detected, the source of interference should be investigated to determine if the detected interference is the worst case. In some cases, the detection of interference may result in flight tests being required to determine if the interference is worse in flight. After the worst case interference is defined, the interference must be eliminated at the source, or the interference must be evaluated to assure that the critical electronic controls, its functions, and its related indications do not result in an unsafe condition. For FADECs, special test equipment developed by the engine manufacturer will be required to interrogate and monitor the controls input/output parameters. Other types critical controls may also require special test equipment to perform this type of testing.
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(C) Installation Test Conditions: “Equipment Known to Have a High Potential for Interference” represents the main concern for radiated and conductive interference; therefore, ground and flight tests are usually required. Therefore, when the EMI installation tests described in paragraph (2)(iii) are required, ground and flight tests will usually need to be conducted. Ground tests alone are usually not sufficient since some equipment may pose safety issues if operated on the ground, while other equipment cannot be satisfactorily operated on the ground, or the equipment would provide misleading results if operated on the ground. For example, some equipment is prohibited from being operated on the ground, such as hoists.
(D) If the proposed installed equipment has been tested in relation to the critical electronic control system on another identical installation, then there can be an exception to the EMI installation testing requirements defined in paragraph (2)(iii). The data showing identicality of the equipment and installation with passing test data are acceptable in place of further testing on the same type rotorcraft.
(3) Summary. The concern for potential interference to electronic controls that provide critical functions may be addressed by the methods contained within this document. To address the interference aspects of “equipment known to have a high potential for interference,” the equipment must be tested as a part of the installation as described in paragraph (2)(iii), during ground and flight tests.
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AC 27-1B, Chg 2 4/25/06 Page MG 4 - 14
CHAPTER 3
AIRWORTHINESS STANDARDS
NORMAL CATEGORY ROTORCRAFT
MISCELLANEOUS GUIDANCE (MG)
AC 27 MG 5 AGRICULTURAL DISPENSING EQUIPMENT INSTALLATION.
Note: This paragraph has been extensively revised and expanded to clarify the restricted category certification of agricultural dispensing equipment installations on rotorcraft.
a. Explanation. In the early development of the rotorcraft, one of its primary usages was agricultural operation. The FAA recognized that the existing requirements, which were designed primarily to establish an appropriate level of safety for passenger-carrying aircraft, imposed an unnecessary economic burden and were unduly restrictive for the manufacture and operation of aircraft used in agricultural operations in rural, sparsely settled areas. To resolve this, the FAA developed a special document that established new standards for agricultural dispensing equipment and other special purpose operations. This document, Restricted Category CAM 8, became effective October 11, 1950.
(1) During the re-codification of the CAM’s and CAR’s in 1965, CAR 8 ceased to exist as a regulatory basis and selected portions addressing certification were incorporated into 14 Code of Federal Regulations (CFR), part 21. While the specific standards in CAR 8 were not changed substantively when adopted into part 21, the less restrictive philosophy of CAM 8 and the policy material that was stated in the preamble to CAM 8 was not clearly written.
Advisory material published in 1965 and revised in 1975, summarized the information contained in the advisory portions of CAM 8. Unfortunately, this document specified that CAM 8 was to be used only in conjunction with certain airworthiness standards for restricted category certification of small agricultural airplanes.
(3) A survey of restricted category rotorcraft projects related to agricultural modifications indicates that the CAM 8 philosophy was interpreted to allow the use of AC 43.13-2A structural criteria for most STCs issued for rotorcraft through the early 1980’s. Since then, more restrictive guidance based on CAR 6 and part 27 requirements has been applied by some ACO’s to several STC applications. Since the more restrictive guidance imposed a significant economic burden on the industry, the HAI requested a meeting with the FAA during the 1990 annual convention in Dallas. As a result of the meeting, an Action Notice to clarify the interpretation of § 21.25(a)(1) for restricted category aircraft has been issued.
(4) The following advisory material is a result of a reassessment of past and present policy.
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AC 27-1BChg 4 12/15/2009
b. Procedures. The certification basis for agricultural dispensing aircraft equipm ent installations in the restricted category is § 21.25 as interpreted by Order 8110.56. The accountable Directorate guidance for the substantiation requirements for rotorcraft is as follows:
(1) The list of airworthiness standards below is appropriate for most agricultural dispensing equipment installations and is intended to address the key compliance areas for those installations. However, it is not intended to be all inclusive for every type of agricultural dispensing equipment installation, such as those possessing novel or unusual design features.
Compliance List of 14 CFR, Part 27 Airworthiness Standards for
Agricultural Dispensing Aircraft Equipment Installations
Airworthiness Standard Center of Gravity Performance (Takeoff) Performance (Landing) Controllability and Maneuverability Static Longitudinal Stability Static Directional Stability Taxiing Condition Excessive Vibration Limit Maneuvering Load Factor Static structural strength at the equipment attachment using emergency landing loads Fatigue (apply forward airspeed restriction to prevent increasing mast bending and oscillatory loading on dynamic components) Design and Construction (material strength properties, protection of materials from environmental conditions, use of aerospace grade hardware, etc.)
Pilot Compartment Areas External Loads Equipment Installations Electrical Equipment and Installations Circuit Protection Devices Airspeed Limitations * Instruction for Continued Airworthiness Rotorcraft Flight Manual Operating Limitations Operating Procedures Rule Section § 27.27 (Provided an expanded envelope is necessary) § 27.51 § 27.75 § 27.143 § 27.173 § 27.177 § 27.235 § 27.251 § 27.337 § 27.561 § 27.571 part 27, Subpart D § 27.771 thru 27.779 § 27.865 § 27.1309 § 27.1351 § 27.1357 § 27.1503 § 27.1529 § 27.1581 § 27.1583 § 27.1585 Page MG 5 - 2
______________________________________________________________________
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12/15/2009 AC 27-1B, Chg 4 *Requires acceptance by the cognizant Flight Standards District Office Note: Some rotorcraft manufacturers have qualified certain locations on the underside of their aircraft for mounting external equipment. The manufacturers will typically specify external equipment weight and dimensional limitations at those locations. The applicant should contact the manufacturer to see if this information is available as it could be used to reduce the applicant's certification effort.
(2) The critical structural loading conditions for substantiating the installation of agricultural dispensing equipment can be developed by using the associated occupant protection load factors provided in Figure AC 27 MG 5-1. These load factors are prescribed to prevent dispensing equipment from causing injuries to occupants in the event of an emergency landing. To ensure this, adequate margins of safety should be used in the structural design consideration of dispensing equipment and dispensing equipment installations.
FIGURE AC 27 MG 5-1
ACCEPTABLE ULTIMATE LOAD FACTOR FOR
AGRICULTURAL DISPENSING EQUIPMENT DESIGN
UP DOWN SIDE FORWARD AFT Tanks & Equipment Mounted In Or Near The Fuselage
1.5g 4.0g 2.0g 4.0g Note 1
- - - - Spray Booms 1.5g 2.5g - - - - Note 1 2.5g Note 2 Note 1: An ultimate load factor of 2 G’s is acceptable for externally side or under fuselage mounted tank and forward mounted spray booms where failure in a minor crash landing will not create a hazard to occupants or prevent an occupant’s exit from the rotorcraft.
Note 2: The aft loads for spray booms may be developed by the applicant based on the 111 percent of VNE for which certification is requested or the load factors of Figure AC 27 MG 5-1, whichever is greater.
(3) The applicant may elect to substantiate their product by either static or dynamic testing, by analysis, or any combination thereof.
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AC 27-1BChg 4 12/15/2009
(4) Lower load factors may be used only when justified by manufacturer’s data, rational analysis, actual rotorcraft flight data and ground load demonstrations, or any combination of these approaches.
(5) Tank pressure testing, while not mandated, is recommended for safety reasons. An acceptable procedure is included in paragraph c.(4) under “Acceptable Means of Compliance.”
(6) Dispensing equipment installation attach points that are an integral part of the rotorcraft and have been certified to the appropriate airworthiness standards, need no further substantiation. This applies provided a load analysis indicates the dispensing system does not impose loads at the attach points which exceed those approved as part of the rotorcraft certification.
(7) A 5-inch ground clearance for skid gear equipped, newly manufactured rotorcraft has typically been used when installing dispensing equipment, such as belly mounted supply tanks/hoppers or when installing dual side mounted supply tanks/hoppers. This applies provided the rotorcraft design incorporates cross tubes or other skid gear reinforcing structure below the fuselage and the cross tubes have not experienced in-service permanent elastic deformation. For rotorcraft equipped with wheels and/or landing gear struts, the maximum system deflections should be considered when determining the 5 inches of acceptable static ground clearance. A 3-inch ground clearance has been found acceptable and may be approved for skid gear equipped rotorcraft to account for the in-service permanent elastic deformation allowed for skid gear members (i.e., cross tube deflections allowed per the maintenance manual). Cable supported systems (e.g., cargo hook installations) or dispensing systems utilizing flexible ducts, such as water snorkels, have been approved even though portions of the systems contact the surface during a normal landing. A determination should be made that these systems do not interfere with the safe landing of the rotorcraft.
(8) A number of rotorcraft are approved for external cargo operations that allow a gross weight higher than the approved internal gross weight limit. This difference is usually due to the allowable weight limit restriction of the landing gear. (The gear is not approved for the higher weight.) Those types of dispensing equipment, which can be loaded in flight to a weight that exceeds the allowable limit of the landing gear, should incorporate a reliable means that rapidly reduces the total aircraft gross weight to within allowable landing gear limits. In most cases, this will involve jettison of the disposable load. The time interval for this operation should be demonstrated, and should not exceed a recommended 3 seconds from a level flight condition.
(9) A flight check or demonstration of the agricultural dispensing equipment installation is normally conducted. This flight check should also qualitatively determine that no hazardous deflection or resonance in the rotorcraft or dispensing system exists.
For FAA flight operations approval, this flight check must be conducted under the requirements of § 133.41.
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