(i) Dynamic impact tests may be voluntarily proposed by the applicant.
At least two conditions should be used to be representative of impact cases. Report No. DOT/FAA/CT-85/11, Analysis of Rotorcraft Crash Dynamics for Development of Improved Crashworthiness Design Criteria, June 1985, may be obtained for reference from the National Technical Information Service, Springfield, Virginia 22161.
(ii) Advisory Circular 21-22, Injury Criteria for Human Exposure to Impact, June 20, 1985, may be used for part of the acceptance levels or performance criteria in developing a proper dynamic test proposal. The static design conditions contained in the present standards shall be satisfied also.
c. Procedures.
(1) Each seat with its belts and harnesses are to be substantiated for the flight, ground, and emergency landing loads of § 27.561 by structural test or stress analysis.
Approval can be gained by Technical Standard Order (TSO) approval or by accomplishing sufficient structural substantiation to gain certification approval of the seat and its belt(s) and harness as part of the type design of the rotorcraft.
TSO No. C-39a concerns standards for aircraft seats, including rotorcraft seats. If TSO No. C-39a is used as an approval basis for a specific rotorcraft seat, the seat and harness should be checked to ensure it has been substantiated for the vertical (up and down) and side loads imposed by installation in the aircraft. For example, TSO No. C-39a (and NAS 809) specifies an ultimate down load of 4.0g which is in agreement with the 4.0g emergency landing load factor of § 27.561, but it may be less than the design maneuver load factor (which can be as high as 3.5g limit or
5.25g ultimate).
(i) The 1.33 factor is specified for substantiation of attachments of each seat to the structure and each safety belt or harness to the seat or structure and the head rest, if used, for § 27.561 loads, whether analysis or test is used.
(ii) If static testing of seats, belts, and harnesses is used, the body block of NAS 809 may be used. The corners of the NAS 809 body block may be radiused and padded if it is found that the small radii cause premature, unrealistic crippling of thin wall tubing or other structure used in the static seat.
(iii) The substantiation of the pilot seats is required to include pilot forces of § 27.397 in conjunction with normal flight and ground loads. For example, the pilot foot force (195 pounds ultimate) must be reacted by the seat.
(2) The head rest, if used, shall be substantiated for a head weight of 13 pounds, § 27.561 inertia load factors, and a factor of 1.33 whether by test or analysis.
(3) The following criteria have been found satisfactory for preventing occupant head injuries:
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(i) Whenever a harness is used, it should support the shoulders without applying hazardous loads to the side or front of the neck. It should be easily donned and removed. A single point release with the seat belt is required for each pilot’s seat and preferred for other seats. If a separate release is provided, it must be simple, compatible with the seat belt release, and near the seat belt release. The harness should be tested in conjunction with the seat belt using a “body block” similar to that of NAS 809, if possible. It shall be tested to 60 percent of the § 27.561 minor crash loads for the entire occupant weight of 170 pounds. TSO -C114, Torso Restraint Systems, dated May 27, 1987, was recently issued.
(ii) During certification TIA testing, the pilot shall ensure that all of the pilot’s necessary functions may be performed with the seat in the most adverse adjustable position and the belt and harness fastened. Each belt and harness shall also be secured, when not used, if necessary, to comply with § 27.785(c).
(iii) Elimination of injurious objects within striking distance of the head and other vital parts can be accomplished by removal of objects with sharp edges or rigid surfaces from within striking distance of vital parts of the occupant. Dimensions and weights for typical occupants are available in U.S. Army USAAVLABS Reports 70-22 (August 1969) and 66-39 (June 1966) and NACA Report TN 2991 (August 1953).
Because of the range of occupant head striking distance, a combination of “elimination of injurious objects” and “cushioned rests” may be required for some interior configurations. If only a belt or a belt-harness which allows use of only the belt is installed, the minimum arc or strike sphere requirement may be met by establishing a 35-inch minimum radius strike-free zone from the seat back and bottom cushion junction. The cushions may be assumed to be normally compressed.
(iv) An acceptable cushioned rest can be provided by use of a 1-inch thickness of foamed polyvinyl chloride (PVC) or equivalent energy absorbing material.
The density of material should be in a 5- to 10-pound per cubic foot density range. PVC foam has the property of absorbing energy efficiently with negligible rebond effects.
PVC foam recovers slowly to the original configuration after deformation. If PVC foam is used, however, care must be taken in its application relative to its flammability characteristics (reference § 27.853).
(4) Handholds for the occupants are generally provided by transport aircraft seat backs adjacent to the aisle. If the seat backs fold, the amount of support provided by the seat backs before they fold must be evaluated in a furnished interior or mockup.
To provide adequate support, the seat back may use an easily disengaged latch or adequate friction in the hinge mechanism to obtain adequate support. Handholds along the aisle are, of course, not needed for rotorcraft with no aisles or where seat belts must be fastened during flight according to the operating rules.
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AC 27-1B, Chg 1 2/12/03
(5) Projecting objects which could injure occupants in normal flight should be padded. The amount of padding required depends on the location, size, and minimum radius of the projecting object. In general, this requirement may be met by padding sharp edges with one-half inch of PVC foam or equivalent energy absorbing material (5 to 10 lbs. density). Objects with edge radii in excess of 1 inch may meet the requirements of § 29.785(e) with a lesser amount of energy absorbing padding, if it can be contacted only by persons while “seated or moving about in the rotorcraft in normal flight.”
AC 27.785A. § 27.785 (Amendment 27-25) SEATS, BERTHS, SAFETY BELTS, AND HARNESSES.
a. Explanation.
(1) The title of § 27.785 now includes berths (which would include litters).
(2) Section 27.785(a) has been revised to include reference to the new § 27.562, “Emergency Landing Dynamic Conditions”.
(3) Section 27.785(b) has been revised to include a reference to the new § 27.562(c)(5) head injury criteria and to describe a torso restraint system that is contained in TSO-C114.
(4) Section 27.785(f) has been revised to change the percentage of load distribution of a combined safety belt and harness from 60-60 to 60-40.
(5) A new § 27.785(i) has been added which provides a list of “seating device system” components.
(6) A new § 27.785(j) provides for deformations of the seat energy absorption device system installed to meet the requirements of § 27.562 but requires that the system “remain intact and not interfere with rapid evacuation of the rotorcraft.” Further “structural” performance standards are contained in §§ 27.562(c)(1) and (2).
(7) A new § 27.785(k) provides static strength and restraint requirements for litters and berths. Litters may be oriented laterally as well as longitudinally in the rotorcraft. Dynamic tests of litters are not required. For longitudinally oriented litters, features should be provided to protect the occupant from the increased loads in § 29.561(b) of Amdt. 27-25.
b. Procedures. The procedures of paragraph AC 27.785 still apply to static substantiation of the seats, berths, safety belts, and harness. In addition:
(1) Compliance with § 27.562 (except litters are not included) and § 27.561(b) is required.
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(2) Section 27.562 includes a specific pass fail criteria, which includes head injury criteria.
(3) Shoulder harnesses need only be substantiated for 40 percent of total occupant load rather than the former 60 percent adopted by Amendment 27-21.
(4) AC paragraph 27.562 provides guidance for evaluating the functioning of a seating energy absorption device system under dynamic test conditions. Stroking is generally associated with the vertical-horizontal impact case and is recognized in the static strength substantiation.
(5) Berths or litters installed within 15° or less of the rotorcraft longitudinal axis (oriented longitudinally) shall use a combination of restraint devices, such as a padded end-board, cloth diaphragm, or equivalent means to withstand and distribute the occupant loads resulting from § 27.561(b) requirements. Other berths or litters may be equipped with straps or safety belts to withstand the forward reaction of § 27.561(b) as well as other loads, including flight loads.
(i) Berths/litters may be substantiated by static load tests, analysis, or a combination thereof and need not be substantiated to the 1.33 fitting factor of seat installations.
(ii) The berth/litter occupant’s head, neck, and spine should be protected from (landing) impact forward loads by appropriate design means; e.g.,
• non-longitudinal orientation of the berth/litter; or
• “feet forward” orientation; or
• distribution of an appropriate percentage of forward loads on the shoulders (not solely to the head and spine).
(iii) Recommendations for litter occupant
• If the occupant’s head is oriented forward, a shoulder harness should be provided, in conjunction with body and leg straps, that prevents the occupant’s head from falling off the litter. A padded end board, diaphragm, etc., may be used, provided head and spinal loads are alleviated or prevented.
• If the occupant’s feet are oriented forward, the padded end board may also be used in combination with body and leg straps or other such restraints.
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AC 27-1B, Chg 1 2/12/03
• Multiple or combinations of devices should be used to distribute the occupant loads as well as protect the occupant from possible neck and spine compression.
AC 27.785B § 27.785 (Amendment 27-35) SEATS, BERTHS, LITTERS, SAFETY BELTS, AND HARNESSES.
a. Explanation. Amendment 27-35 revised the title of the rule to now include litters to distinguish from berths. Additionally, § 27.785(k) was revised to require that the 1.33 fitting factor of § 27.625(d) must be applied to the emergency landing loads of § 27.561(b)(3) for the substantiation of attachments of each berth and litter to the structure.
b. Procedures. All of the advisory material pertaining to this section remains in effect with the following additions:
(1) A fitting factor of 1.33 must be applied to the emergency landing loads of § 27.561(b) when evaluating the berth or litter attachment and the occupant restraint system to the structure.
(2) The 1.33 factor is required whether analysis or test is used.
AC 27.787. § 27.787 (Amendment 27-11) CARGO AND BAGGAGE COMPARTMENTS.
a. Explanation.
(1) This standard concerns the strength or structural integrity of either a cargo or baggage compartment. For purposes of this paragraph, baggage and cargo compartments are synonymous. Other design standards are also included.
(2) Fire protection standards of these compartments are contained in § 27.855, paragraph AC 27.855.
(3) The compartment must contain the maximum (design) weight cargo for maximum landing and flight load factors. The minor crash conditions noted in § 27.561 are not applied to cargo compartments. However, a forward ultimate load factor of 4 is applied to the contents of cargo compartments. This forward load condition is related t o occupant protection. Compartments forward of the occupant’s compartment may be designed to the appropriate landing load factor (landing with drag and side load).
(4) Features such as straps, nets, ropes, and possibly other means of restraint may be used when necessary to prevent hazardous shifting of cargo as prescribed under flight and landing loads.
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(5) Compartment lamps must be protected from possible lamp bulb and cargo contact.
(6) Other than the standards in this section, specific standard design features for cargo compartment doors are not contained in FAR Part 27. The following are recommended design features.
(i) Door latch or lock mechanism should not fail and allow the door to open and should not open as a result of cargo shifting.
(ii) Crewmembers should by visual means such as handle positions and markings determine, when on the ground, that the door is fully locked. A separate signal system may also be used to show a door unlatched condition.
(7) Compartment marking standards such as maximum weight, floor loading, possible tiedown instructions and other appropriate compartment markings or placards are prescribed in § 27.1557(a).
b. Procedures.
(1) The compartment design allowable load, including distributed loading, is determined during the initial design phases of the rotorcraft. For an example, the compartment may have a placarded maximum allowable load of 250 pounds, with an allowable distributed load of 100 pounds per square foot. The compartment maximum load and floor distributed load (allowable pounds per square foot) should be included in a stencil, placard, or equivalent durable marking per § 27.1557(a).
(2) Static tests or analyses may be used for substantiation. Light weight rotorcraft configurations typically should be associated with the most severe flight and landing load factors.
(3) Structural substantiation of the fuselage for flight and landing loads must include the baggage and cargo restraining devices and associated attachment structure. Structural substantiation of the compartment structure must include the 4g ultimate forward load condition of § 27.787(c) in addition to the flight and landing load conditions. These can be handled as separate conditions if the structure is substantiated by analysis. If static tests are conducted, all load conditions must be accounted for. A test plan should be approved and conformity inspections conducted prior to FAA/AUTHORITY witnessing of tests.
(4) Cargo nets or straps installed for compliance with § 27.787(b) must be substantiated for the maximum flight and landing loads. The forward load condition of § 27.787(c) must be proven also. Nets or straps should be adjustable.
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(5) Lamp bulbs should be guarded, recessed, or placed in upper inside corners and guarded to prevent contact with cargo and possible bulb breakage or excessive heat.
(6) If the door design recommendations in paragraph AC 27.787a(6) are accepted, these features should be confirmed by design data review and during a compliance inspection. Index or alignment marks with respect to handle (door locked) position are also recommended. If a signal system is used, a switch at the door latch that would signal “door open or unlatched” to the flightcrew is recommended.
AC 27.787A § 27.787 (Amendment 27-27) CARGO AND BAGGAGE COMPARTMENTS.
a. Explanation. Amendment 27-27 adds two subparagraphs to § 27.787(c) which clarify that cargo and baggage compartments should be designed to protect occupants from injury by the compartment contents during emergency landings. This may be done by location or by retention provisions. The new paragraphs also add a requirement that the compartment contents not cause injury when subjected to the loads of § 27.561.
b. Procedures. The procedures of paragraph AC 27.787 are still applicable. In addition to the forward load, the cargo and baggage compartment should be designed to withstand loads in other directions as specified in § 27.561. Also, the compartment may be shown to provide protection of occupants by location; i.e., cargo and baggage compartments may be shown to be located in a position where loose contents will not endanger occupants in an emergency landing impact. If the compartment is located above or behind the occupied area, § 27.561(c) may apply. If a compartment is in the occupied area, § 27.561(b) applies.
AC 27.801 § 27.801 (Amendment 27-11) DITCHING.
a. Explanation.
(1) Ditching certification is accomplished only if requested by the applicant.
(2) Ditching may be defined as an emergency landing on the water, deliberately executed, with the intent of abandoning the rotorcraft as soon as practical. The rotorcraft is assumed to be intact prior to water entry with all controls and essential systems, except engines, functioning properly.
(3) The regulation requires demonstration of the flotation and trim requirements under “reasonably probable water conditions.” A sea state 4 is representative of reasonably probable water conditions to be encountered. Therefore, demonstration of compliance with the ditching requirements for at least sea state 4 water conditions satisfies the reasonably probable requirement.
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(4) A sea state 4 is defined as a moderate sea with significant wave heights of 4 to 8 feet with a height-to-length ratio of:
(i) 1:12.5 for multiengine rotorcraft with Category A engine isolation (reference paragraph AC 27 MG 3).
(ii) 1:10 for all other rotorcraft.
NOTE: The source of the sea state definition is the World Meteorological Organization (WMO) Table. (See figure AC 27.801-1.)
(5) Ditching certification encompasses four primary areas of concern: rotorcraft water entry, rotorcraft flotation and trim, occupant egress, and occupant survival.
(6) The rule requires that after ditching in reasonably probable water conditions, the flotation time and trim of the rotorcraft will allow the occupants to leave the rotorcraft and enter liferafts. This means that the rotorcraft should remain sufficiently upright and in adequate trim to permit safe and orderly evacuation of all personnel.
(7) For a rotorcraft to be certified for ditching, emergency exits must be provided which will meet the requirements of § 27.807(d).
(8) The safety and ditching equipment requirements are addressed in §§ 27.1411, 27.1415, and 27.1561 and specified in the operating rules (Parts 91, 121, 127, and 135). As used in § 27.1415, the term ditching equipment would more properly be described as occupant water survival equipment. Ditching equipment is required for extended overwater operations (more than 50 nautical miles from the nearest shoreline and more than 50 nautical miles from an offshore heliport structure). However, ditching certification should be accomplished with the maximum required quantity of ditching equipment regardless of possible operational use.
(9) Current practices allow wide latitude in the design of cabin interiors and, consequently, the stowage provisions for safety and ditching equipment. Rotorcraft manufacturers may deliver aircraft with unfinished (green) interiors that are to be completed by the purchaser or modifier. These various “configurations” present problems for certifying the rotorcraft for ditching.
(i) In the past, “segmented” certification has been permitted to accommodate this practice. That is, the rotorcraft manufacturer shows compliance with the flotation time, trim, and emergency exit requirements while the purchaser or modifier shows compliance with the equipment provisions and egress requirements with the completed interior. This procedure requires close cooperation and coordination between the manufacturer, purchaser or modifier, and the FAA/AUTHORITY.
(ii) The rotorcraft manufacturer may elect to establish a “token” interior for ditching certification. This interior may subsequently be modified by a supplemental Page D - 83
type certificate or a field approval. Compliance with the ditching requirements should be reviewed after any interior configuration changes and limitations changed where applicable.
(iii) The Rotorcraft Flight Manual and supplements deserve special attention if a “segmented” certification procedure is pursued.
b. Procedures. The following guidance criteria has been derived from past certification policy and experience. Demonstration of compliance to other criteria may produce acceptable results if adequately justified by rational analysis. Model tests of the appropriate ditching configuration may be conducted to demonstrate satisfactory water entry and flotation and trim characteristics where satisfactory correlation between model testing and flight testing has been established. Model tests and other data from rotorcraft of similar configurations may be used to satisfy the ditching requirements where appropriate.
(1) Water entry.
(i) Tests should be conducted to establish procedures and techniques to be used for water entry. These tests should include determination of optimum pitch attitude and forward velocity for ditching in a calm sea as well as entry procedures for the highest sea state to be demonstrated (e.g., the recommended part of the wave on which to land). Procedures for all -engines-operating, one-engine-inoperative, and all-engines-inoperative conditions should be established. However, only the procedures for the most critical condition (usually all engines inoperative) need to be verified by water entry tests.
(ii) The ditching structural design consideration should be based on water impact with a rotor lift of not more than two-thirds of the maximum design weight acting through the center of gravity under the following conditions:
(A) For entry into a calm sea--
(1) The optimum pitch attitude as determined in AC 27.801(b)(1)(i) with consideration for pitch attitude variations that would reasonably be expected to occur in service;
(2) Forward speeds from zero up to the speed defining the knee of the height-velocity (HV) diagram;
(3) Vertical descent velocity of 5 feet per second; and
(4) Yaw attitudes up to 15°.
(B) For entry into the maximum demonstrated sea state-- Page D - 84
(1) The optimum pitch attitude and entry procedure as established in AC 27.801(b)(1)(i);
(2) The forward speed defined by the knee of the HV diagram reduced by the wind speed associated with each applicable sea state;
(3) Vertical descent velocity of 5 feet per second; and
(4) Yaw attitudes up to 15°.
(C) The float system attachment hardware should be shown to be structurally adequate to withstand water loads during water entry when both deflated and stowed and fully inflated (unless in-flight inflation is prohibited). Water entry conditions should correspond to those established in paragraphs AC 27.801(b)(1)(ii)(A) and (B). The appropriate vertical loads and drag loads determined from water entry conditions (or as limited by flight manual procedures) should be addressed. The effects of the vertical loads and the drag loads may be considered separately for the analysis.
(D) Probable damage due to water impact to the airframe/hull should be considered during the water entry evaluations; i.e., failure of windows, doors, skins, panels, etc.
(E) The ditching maximum demonstrated sea state for water entry is the same or greater than the maximum demonstrated sea state for flotation and trim.
(2) Flotation Systems.
(i) Normally inflated. Fixed flotation systems intended for emergency ditching use only and not for amphibian or limited amphibian duty should be evaluated for:
(A) Structural integrity when subjected to:
(1) Air loads throughout the approved flight envelope with floats installed;
(2) Water loads during water entry; and
(3) Water loads after water entry at speeds likely to be experienced after water impact.
(B) Rotorcraft handling qualities throughout the approved flight envelope with floats installed.
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(ii) Normally deflated. Emergency flotation systems which are normally stowed in a deflated condition and inflated either in flight or after water contact during an emergency ditching should be evaluated for:
(A) Inflation. The float activation means may be fully automatic or manual with a means to verify primary actuation system integrity prior to each flight. If manually inflated, the float activation switch should be located on one of the primary flight controls. These activation means should be safeguarded against spontaneous or inadvertent actuation for all flight conditions.
(1) The inflation system design should minimize the probability of the floats not inflating properly or inflating asymmetrically. This may be accomplished by use of a single inflation agent container or multiple container system interconnected together. Redundant inflation activation systems will also normally be required. If the primary actuation system is electrical, a mechanical backup actuation system will usually provide the necessary reliability. A secondary electrical actuation system may also be acceptable if adequate electrical system independence and reliability can be documented.
(2) The inflation system should be safeguarded against spontaneous or inadvertent actuation for all flight conditions. It should be demonstrated that float inflation at any flight condition within the approved operating envelope will not result in a hazardous condition unless the safeguarding system is shown to be extremely reliable.
One safeguarding method that has been successfully used on previous certification programs is to provide a separate float system arming circuit which must be activated before inflation can be initiated.
(3) The maximum airspeed for intentional in-flight actuation of the float system and for flight with the floats inflated should be established as limitations in the RFM unless in-flight actuation is prohibited by the RFM.
(4) The inflation time from actuation to neutral buoyancy should be short enough to prevent the rotorcraft from becoming more than partially submerged assuming actuation upon water contact.
(5) A means should be provided for checking the pressure of the gas storage cylinders prior to takeoff. A table of acceptable gas cylinder pressure variation with ambient temperature and altitude (if applicable) should be provided.
(6) A means should be provided to minimize the possibility of overinflation of the float bags under any reasonably probable actuation conditions.
(7) The ability of the floats to inflate without puncture when subjected to actual water pressures should be substantiated. A full-scale rotorcraft immersion demonstration in a calm body of water is one acceptable method of substantiation.
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Other methods of substantiation may be acceptable depending upon the particular design of the flotation system.
(B) Structural Integrity. The flotation bags should be evaluated for loads resulting from:
(1) Airloads during inflation and fully inflated for the most critical flight conditions and water loads with fully inflated floats during water impact for the water entry conditions established under paragraph AC 27.801(b)(1)(ii) for rotorcraft desiring float deployment before water entry; or
(2) Water loads during inflation after water entry.
(C) Handling Qualities. Rotorcraft handling qualities should be verified to comply with the applicable regulations throughout the approved operating envelopes for:
(1) The deflated and stowed condition;
(2) The fully inflated condition; and
(3) The in-flight inflation condition. For float systems which may be inflated in flight, rotorcraft controllability should be verified by test or analysis assuming the most critical float compartment fails to inflate.
(3) Flotation and Trim. The flotation and trim characteristics should be investigated for a range of sea states from zero to the maximum selected by the applicant and should be satisfactory in waves having height/length ratios of 1:12.5 for multiengine rotorcraft with Category A engine isolation and 1:10 for all other rotorcraft.
Model tests in a wave basin on a number of different rotorcraft types have indicated that an improvement in sea keeping, response of the rotorcraft to waves, performance of approximately one sea state can consistently be achieved by fitting float scoops. If the basic flotation system (without scoops) has demonstrated compliance with the minimum flotation and trim requirements, credit for float scoops to achieve stability in more severe water conditions may be allowed. However, the effect of scoops on improved sea keeping must be demonstrated during model testing.
(i) Flotation and trim characteristics should be demonstrated to be satisfactory to at least sea state 4 conditions.
(ii) Flotation tests should be investigated at the most critical rotorcraft loading condition.
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