AC 27-1B, Chg 2 4/25/06 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.
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 interferencei
• 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.
EMI Installation Testing for Critical Controls:
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.
Page MG 4 - 12
(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.
Page MG 5 - 1
AC 27-1BChg 4 12/15/2009
b. Procedures. The certification basis for agricultural dispensing aircraft equipment 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 tollows:
(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 Rule Section Center of Gravity § 27.27 (Provided an expanded envelope is necessary) Performance (Takeoff) § 27.51 Performance (Landing) § 27.75 Controllability and Maneuverability § 27.143 Static Longitudinal Stability § 27.173 Static Directional Stability § 27.177 Taxiing Condition § 27.235 Excessive Vibration § 27.251 Limit Maneuvering Load Factor § 27.337 Static structural strength at the equipment attachment using § 27.561 emergency landing loads Fatigue (apply forward airspeed restriction to prevent increasing § 27.571 mast bending and oscillatory loading on dynamic components) Design and Construction (material strength properties, part 27, Subpart D protection of materials from environmental conditions, use of aerospace grade hardware, etc.)
Pilot Compartment Areas § 27.771 thru 27.779 External Loads § 27.865 Equipment Installations § 27.1309 Electrical Equipment and Installations § 27.1351 Circuit Protection Devices § 27.1357 Airspeed Limitations § 27.1503 * Instruction for Continued Airworthiness § 27.1529 Rotorcraft Flight Manual § 27.1581 Operating Limitations § 27.1583 Operating Procedures § 27.1585 Page MG 5 - 2
*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 1.5g 4.0g 2.0g 4.0g - ...
In Or Near The Fuselage 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.
Page MG 5 - 3
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 tuselage 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.
Page MG 5 - 4
(10) Recent service history has shown that external equipment and external fixture modifications that generate high drag loads in forward flight can affect main rotor mast bending loads. In lieu of a mast bending survey, a pre and post modification flight test may be conducted at identical weights, center-of-gravity (CG), power, and density altitude to compare a critical control position parameter (typically longitudinal cyclic stick position) at pre and post modification VNE airspeeds.
(i) If required, the post modification VNE should be reduced so that the post modification longitudinal cyclic stick position is slightly aft of (or less than) the premodification stick position. This alternative procedure assumes that the static longitudinal stability of the helicopter has not been altered by the modification. For helicopters with neutral static stability, a more comprehensive investigation may be required.
(iI) In some cases, a control position parameter other than longitudinal stick position may be critical. For example, a heavy external device mounted to the side of the helicopter that gives a lateral CG close to the limit and an asymmetric yaw component would require pre and post modification lateral cyclic stick and pedal position measurements. Operating limitations other than VNE may need to be established, or reduced from pre-modification limitations, to ensure pre-modification mast bending is not exceeded.
(11) For rotorcraft certificated in dual categories, the inspection requirements of § 21.187(b) must be observed when converting from restricted to normal category.
c. Acceptable Means of Compliance.
(1) Analysis Method. Static structural analysis may be used provided a methodology is applied that has been shown to be reliable for analyzing the type of structure. Structural substantiation of tanks that are designed to contain liquid materials may be accomplished by pressure testing. For tanks or hoppers designed to contain dry material (e.g., dust or fertilizer), static load tests may be used to verify structural integrity. The tank/hopper, mounting hardware, and support structure should all be substantiated to the load conditions specified and should consider the effects of internal fluid pressures, when applicable, in Figure AC 27 MG 5-1.
(2) Static Tests. Static tests of tank/hoppers, mounting hardware, and support structure for each critical load condition may be accomplished using conventional techniques; such as, dead weight loading, whiffletree systems, and hydraulic rams. If tests of the tank and its mounting hardware are conducted using a test fixture representing the rotorcraft, the rotorcraft support structure may be substantiated independently by means of test or analysis, or both. Static test loads should be applied in combination with associated internal fluid pressure loadings. The ultimate loads specified in Figure AC 27 MG 5-1 should be sustained for at least 3 seconds without Page MG 5 - 5
AC 27-1BChg 4 12/15/2009
(3) Dynamic Tests.
(i) If the applicant elects to test to the loading conditions in Figure AC 27 MG 5- 1, the maneuvering and gust loadings will be considered to be adequately substantiated. For each condition, the critical volume and density of fluid should be used.
(ii) The tank and mounting hardware should support ultimate loads without permanent elastic deformation failure, respectively. The rotorcraft support structure may be included in the dynamic tests, or it may be substantiated separately via static test or analysis, or both, for each condition specified in Figure AC 27 MG 5-1.
(4) Pressure Testing. Internal pressure loads may be applied using the water standpipe technique. Standpipe water height should be accurately computed for each critical spray tank static test loading. Pressure testing of spray tanks is not absolutely essential but is recommended for safety reasons. This testing will also determine whether the joints and connections are tight and will not leak in addition to determining any weak spots in the construction. Where spraying is done with highly volatile and flammable liquids, or where the tank has a return line, such as in an engine oil tank where the fluid is pumped back into the tank, it is recommended that the tank be tested for a pressure of 5 pounds per square inch. For other liquids, and where no fluid return line is used, testing to 3 ½ pounds per square inch should be satisfactory. There are many ways of pressure testing a tank, however, it is believed that the simplest and easiest method is to fill the tank with water and use a standpipe filled with water. A 1 1/8-inch pipe can be connected to the venting tube or one adapted to the filler opening. In either case, the height of the pipe would be the same. For a 3 ½ PSI test of the tank, the height of the water in the pipe would only need to be 8 feet and for a 5 PSI test only an 11 ½-foot height of water will be needed. (See Figure AC 27 MG 5-2 below.)
Page MG 5 - 6
Pipe 11 ½ TANK FIGURE AC 27 MG 5-2 SKETCH OF TANK PRESSURE TEST Page MG 5 - 7