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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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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.
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(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.
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(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
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(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.)
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Pipe 11 ½ TANK FIGURE AC 27 MG 5-2 SKETCH OF TANK PRESSURE TEST Page MG 5 - 7
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CHAPTER 3
AIRWORTHINESS STANDARDS
NORMAL CATEGORY ROTORCRAFT
MISCELLANEOUS GUIDANCE (MG) AC 27 MG 6. EMERGENCY MEDICAL SERVICE (EMS) SYSTEMS INSTALLATIONS INCLUDING: INTERIOR ARRANGEMENTS,
EQUIPMENT, HELICOPTER TERRAIN AWARENESS AND
WARNING SYSTEM (HTAWS), RADIO ALTIMETER, AND FLIGHT DATA MONITORING SYSTEM.
a. Explanation. This section pertains to EMS configurations and associated rotorcraft airworthiness standards. EMS configurations are usually unique interior arrangements that are subject to the appropriate airworthiness standards, part 27 or its predecessor CAR part 6, to which the rotorcraft was certificated. No relief from the standards is intended except by § 21.21(b)(1) or an exemption. EMS configurations are seldom, if ever, done by the original manufacturer.
(1) The FAA has specified in the operating rules the minimum equipment required to operate as a helicopter air ambulance service provider (identified by an "*" in this guidance). This equipment, as well as all other equipment presented for evaluation and approval, is subject to compliance with airworthiness standards. Any equipment not essential to the safe operation of the aircraft may be approved provided the use, operation, and possible failure modes of the equipment are not hazardous to the aircraft. Safe flight, safe landing, and prompt evacuation of the rotorcraft in the event of a minor crash landing, for any reason, are the objectives of the FAA evaluation of interiors and equipment unique to EMS.
(1) For example, a rotorcraft equipped only for transportation of a non-ambulatory person (e.g., a police rotorcraft with one litter) as well as a rotorcraft equipped with multiple litters and complete life support systems and two or more attendants or medical personnel may be submitted for approval. These configurations will be evaluated to the airworthiness standards appropriate to the rotorcraft certification basis.
(il) Normal category rotorcraft should comply with flightcrew and passenger safety standards, which will result in the need to reevaluate certain features of the basic certified rotorcraft related to the EMS arrangement, such as doors and emergency exits, and occupant protection. Compliance with airworthiness standards results in placards or markings for doors and exits, exit size, exit quantity and location, exit access, safety belts, and possibly shoulder harnesses or other restraint or passenger protection means to be retained as part of the rotorcraft's basic type design. These features, including any placards and markings required as part of the rotorcraft's basic type design, should be retained unless specific replacements or alternate designs are necessary for the EMS configuration to comply with airworthiness standards.
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(2) Many EMS configurations of normal rotorcraft are equipped with the following:
(i) Attendant and medical personnel seats, which may swivel.
(ii) Multiple litters, some of which tilt.
(iii) Medical equipment stowage compartments.
(iv) Life support and other complex medical equipment.
(v) Human infant incubator (isolette).
(vi) Curtains or other interior light shielding for the flightcrew station.
(vii) External loud speakers and search lights.
(viii) Special internal (intercom) and external communication radio equipment.
*(ix) Flight data monitoring system (FDMS).
*(x) Radio altimeter.
*(xi) Helicopter terrain awareness and warning system (HTAWS).
*(3) All helicopter air ambulance service providers are required to operate at all times under a part 135 subpart L certificate. The equipment required to obtain operational approval includes:
*(i) FDMS. The installation guidance is in paragraph b. (13) of this MG.
*(Il) Radio Altimeter (RAD ALT). The installation guidance is in paragraph b. (14) of this MG.
*(iii) HTAWS. HTAWS is required for operation under part 135 subpart L, Helicopter Air Ambulance Equipment, Operation, and Training Requirements;
§ 135.605, Helicopter Terrain Awareness and Warning System (HTAWS). The design standards are in Technical Standard Order (TSO)-C194 and the installation guidance is in AC 27-1 MG 18.
b. Procedures.
(1) General.
(i) Original type design information and criteria may or may not be available from the manufacturer. Availability of this information is dependent on whether the information is considered "public" (i.e., non-proprietary) or proprietary. It may be Page MG 6 - 2
appropriate to reference the helicopter manufacturer's "standard" features, placards, and markings in the applicant's modification design data.
(ii) The EMS modification presented for approval usually contains equipment of one manufacturer's model or design. The type design of the modification will have features to power and restrain the equipment, maintain the rotorcraft systems integrity, and to otherwise protect the occupants. See paragraph b. (17), which refers to equipment substitution.
(iii) All equipment installations in the helicopter must be approved. EMS helicopters typically include operations in which large medical equipment is not installed in the helicopter but instead is carried onto the helicopter, as needed, such as isolettes, large medical equipment, and other medical items. This equipment is not included as part of the rotorcraft type design modification because it is not considered a permanent installation in the helicopter. However, carry-on medical equipment must be evaluated as to how it affects the safety of the helicopter and its occupants (including the occupant in the isolette) while being carried in the helicopter. This carry-on equipment (including the isolette must be properly restrained so it is not a hazard during flight operations.
Consequently, the means to stow or store the medical carry-on items must be evaluated for the appropriate load factors relative to the helicopter such that the means for stowage of the carry-on equipment does not fail. For instance, in the case of a carry-on Isolette, the isolette is typically placed on top of an installed "mount" for the isolette. The "mount" is evaluated for carriage and restraining of the isolette. In some cases, an isolette may be completely self-contained and include other items, such as oxygen bottles required for the isolette occupant, which must also be evaluated for carriage of that equipment.
(2) Evacuation and Interior Arrangements. Access to the emergency exits or doors from any location in the cabin or compartment, access to and use of the exit or door opening means or release device, and the unobstructed area of an exit are potential problems that should be addressed in the early design stage. Multi-litter arrangements may be especially critical for normal category rotorcraft due to their limited space.
(i) The operation or use of devices for locking the position of swivel seats and for rapid installation and removal of litters (isolettes, etc.) should be labeled unless they are simple and obvious, and do not require exceptional effort. The design features of the device(s) and the seat or litter will influence the extent of information in any label necessary to ensure proper and safe installation for routine use and prompt evacuation when appropriate or necessary for the interior arrangement. The requirement for labels or markings (instructions, etc.) that applies to operation of seat or litter features, release devices, etc., is not relieved even if attendants are necessary for an evacuation as discussed in paragraph b.(2)(iii). Placards or instruction cards that contain evacuation procedures do not necessarily contain detailed procedures for individual seats, litters, and so forth. Release devices that are simple and obvious and do not require exceptional effort are recommended. For example, a single central control for litter Page MG 6 - 3