Notes: Fuselage keels whose design and structural strength are such as to resist crash impacts associated with the emergency landing conditions of § 7.260 (§ 29.561) without extreme distortion which might tend to rupture the fuel tank may be considered to comply with the requirements of this section (7.261).
Puncture resistant “bladder” fuel cells that are adequately designed and also protected from the stated impact loads imposed on the fuselage may also satisfy the standards.
(8) For rotorcraft with retractable landing gear, alternative landing gear positions and the resulting effects on potential fuel release should be evaluated.
AC 29.561A. § 29.561 (Amendment 29-29) EMERGENCY LANDING CONDITIONS - GENERAL.
a. Explanation. Amendment 29-29 adds or increases the design static load factors of § 29.561 in three different areas. The addition of these load factors eliminates the 5 FPS descent velocity criteria of unamended § 29.561(b)(3).
(1) The design static load factors for the cabin in § 29.561(b)(3) are increased in concert with the dynamic test requirements of new § 29.562.
(2) Design static load factors are added in § 29.561(c) for external items of mass located above or behind the crew and passenger compartment.
(3) The static load factors, which were formerly only referenced in § 29.561(d), are now included explicitly in § 29.561(d) for substantiation of internal fuel tanks which are below the passenger floor.
b. Procedures. The procedures in se ction 29.561 of this AC continue to apply except the new load factors of § 29.561 should be used. Penetration of any items of mass into the cabin or occupied areas should be prevented. In addition, each fuel tank and its installation are subject to specific load factors that are based on the fuel tank location.
(1) The crash impact load factors for the airframe structure surrounding the underfloor fuel tanks are specified in § 29.561(d). The fuselage structure must be designed to resist the specified crash impact loads and to help protect the fuel tank from rupture. If equipped with retr actable landing gear, the effects of the landing gear on fuel system rupture should be considered in both the retracted and unretracted configurations.
(2) Section 29.952(b) (see section 29.952 of this AC) specifies the design load factors for crash resistant fuel systems in an otherwise survivable impact. This section relates to § 29.561(d) as follows. The § 29.952 load factors are for the fuel tanks, other Page C - 53
AC 29-2C, Chg 4 5/1/2014 significant mass items in the fuel system, and their attachment to the rotorcraft airframe for both occupant survivability and retention of fuel in a survivable impact; whereas, the § 29.561(d) load factors only apply to the rotorcraft airframe surrounding the underfloor fuel tanks and their installation for the same reasons. These two sets of load factors are not additive. They are applied separately (as design ultimate load factors) to the portions of the rotorcraft to which they are specified to apply. The application of the § 29.561(d) load factors is described as follows. The loads generated by § 29.561(d) are intended to be applied to the airframe structure surrounding the fuel cell to ensure that the entire airframe structure provides the appropriate level of crash resistance (i.e., stiffness, crushability, crushing rate, energy absorption capability, etc.) and to ensure that the airframe structure’s failure modes (e.g., buckling, creation of sharp edges, structural spears, etc.) are such that fuel cell rupture (and the resultant post crash fire potential) is mitigated to the maximum practicable extent in a otherwise survivable emergency landing. Each fuel cell (and major fuel cell component) creates an applied load on the airframe in an emergency landing condition. These loads are determined by multiplying the worst case mass of the fuel cell (i.e., a full fuel cell) by the load factors of § 29.561(d). These loads are then applied (ut ilizing the appropriate design load paths) to the airframe structure surrounding the fuel cell to help design the structure for optimal crash resistance. Added stiffness effects for both a full and less than full fuel cell should be considered in the design process. A significantly less than full fuel cell will typically not have any significant stiffness effects, since in a less than full condition the fuel cell cannot typically transfer load hydraulically.
(3) The minor crash ultimate load factors for doors and others emergency exits are specified in §§ 29.783(d) and 29.809(e). The related inertial forces are not applicable to cargo or to service doors (not suitable for use as an exit in an emergency).
If any item of mass installed in the cabin can possibly interact with the fuselage and cause higher deformation, then § 29.561 (b)(3) loads factors should be applied to the design of doors and emergency exits.
AC 29.561B. § 29.561 (Amendment 29-38) EMERGENCY LANDING CONDITIONS - GENERAL.
a. Explanation. Amendment 29-38 adds a new rearward emergency load factor of
1.5g to both §§ 29.561(b)(3)(v) and 29.561(c)(5). The addition of the 1.5g rearward load factor in § 29.561(b)(3)(v) is to provide an aft ultimate load condition for substantiation of the restraints required for retention of both occupants and significant items of mass inside the cabin that could otherwise come loose and cause injuries in an emergency landing. The addition of the 1.5g rear ward load factor to § 29.561(c)(5) is to provide an aft ultimate load condition for substantiation of the support structure for retention of significant items of mass above and forward of the occupied volume(s) of the rotorcraft that could otherwise come loose and injure an occupant in an emergency landing. Amendment 29-38 also increases the forward, sideward, and downward emergency load factors of § 29.561(c)(2), (c)(3), and (c)(4), respectively, for retention of items of mass above and behind the occupied volume(s) that could otherwise come loose and injure an occupant in an emergency landing.
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b. Procedures. The procedures in secti ons 29.561 and 29.561A of this AC continue to apply except the newly specified load factors must be used. A list of the significant items of mass to be considered should be compiled by the applicant and approved by the certifying authority.
Note: For doors and emergency exit design, when applicable, the rearward load factor to consider is in § 29.561(b)(3)(v).
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AC 29.562 § 29.562 (Amendment 29-29) EMERGENCY LANDING DYNAMIC CONDITIONS.
a. Explanation. Amendment 29 -29 adds new requirements for the dynamic testing of all seats in rotorcraft. This paragr aph is rewritten to incorporate the guidance previously documented in AC 20-137 dated 3/30/92.
b. Background. Improved occupant restraint in civil rotorcraft is addressed in Amendments 29-29 to the airworthiness standards, which add two dynamic crash impact design conditions for seat and occupant restraint systems. This amendment also prescribes a shoulder harness for each occupant and adopts human impact injury criteria as a measure for occupant protection for the dynamic crash impact conditions.
In addition, these amendments significantly improve occupant protection for normal category rotorcraft in a survivable emergency landing. This advisory material addresses the dynamic test conditions and the related pass-fail injury criteria for the dynamic test conditions. This material pertains to single as well as multiple seats and tandem arrangements of the seats in rotorcraft.
(1) Dynamic test methods. This guidance focuses on the use of dynamic test methods for evaluating the performance of rotorcraft seats, restraint systems, and certain related interior systems for demonstrating structural strength and the ability of those systems to protect an occupant from possible injuries in an emergency landing environment represented by the standard. These test methods differ from static test methods, which are limited to demonstrating only the structural strength of the seat or restraint system under ultimate load for at least 3 seconds. This guidance contains sources for appropriate test procedures and provides some insight into the logic of these procedures. It also defines, in part, test facility and equipment characteristics necessary for conducting these tests.
(2) Standardized test methods. Dynamic tests are often conducted at a specially equipped facility, one other than that owned by the designer or manufacturer of the test article. To obtain consistent test results, it is necessary to specify the critical test procedures in detail in the test plan, and then carefully follow these procedures when conducting the tests. This guidance defi nes certain critical procedures for accomplishing the tests of the seat and restraint systems and assessing the data obtained in the tests. Many of these procedures are accepted as standards by government and commercial test facilities and have been modified in this guidance only as necessary for the specific testing of rotorcraft systems.
(3) Relationship of dynamic tests to design standards. This guidance describes test procedures useful in assessing the performance of a rotorcraft seat, Page C - 55
AC 29-2C, Chg 1 2/12/03 restraint system, and interior system. Howeve r, it is impractical to conduct sufficient tests for assessing the performance of the system throughout its entire range of possible uses in unique interior arrangements. The seat, restraint system, and related interior system should be designed for the range of occupants and environments for which it is expected to perform, not just for the dynamic test conditions described in this guidance.
(i) Occupant size. The dynamic tests are conducted with a specific, acceptable, standard anthropomorphic test dummy (ATD) representing a 50th percentile male occupant. Energy absorbing syst ems, restraint system loads and anchorage locations, seat adjustments, seat pitch (for multiple seat rows), head strike envelopes, etc., are typical factors directly influenced by occupant size.
(ii) Seat position and location. T he tests should be sufficient to represent the range of performance expected of a seat and restraint system. A seat, especially an adjustable flight crew seat, should be qualified for those positions approved for take-off and landing. As with static test procedures the seat is also tested to the most critical condition for the dynamic tests. For an adjustable flight crew seat, as an example, the full-up position and longitudinal impact case are expected to be the critical condition.
But these dynamic tests and occupant injury assessment provide a systems approach to qualification. It is therefore necessary to test adjustable seats at the design position for the ATD. Two tests would be required to demonstrate compliance with the strength standards and with the occupant injury criteria. Alternatively adjusting the flight crew seat to its highest position with the interior features, such as an instrument panel shield, raised to maintain the proper perspective or relation to the ATD, is considered an acceptable test procedure for demonstrating compliance with the structural and occupant injury requirements for the seat and its location in a particular cockpit arrangement.
(iii) Test conditions. Only two minimum impact tests are described in the dynamic test procedures discussed in this guidance. These procedures address the tests needed to demonstrate compliance for a typical seat and restraint system installation. Additional tests may be nece ssary to demonstrate compliance for other types or variations of seat and restraint system installations. For example, while only one lateral load direction is specified in the tests, the system should perform properly when similarly loaded from either side.
(iv) Floor deformation. The test procedures require evaluating the effect of certain sidewall or floor deformation. The seat and its attachments and the restraint system should also perform properly if no floor deformation is present.
(v) Head impact. Should such contact occur, head impact with a seat back or the interior of the rotorcraft is evaluated by using a Head Injury Criterion (HIC), which can be measured directly in the tests discussed in this guidance or in supplementary tests of the interior. The design of the inte rior should protect the head Page C - 56
(vi) Emergency egress. Standards for emergency evacuation of the rotorcraft are contained in FAR Part 29. The objective is to allow each occupant to leave the seat and rapidly evacuate the rotorcraft using an exit on either side of the rotorcraft.
c. Dynamic Test Methods and Facilities.
(1) General. A minimum of two dynamic tests are used to assess the performance of the rotorcraft seat, restraint system, and related interior system. The seat, the restraint, and the nearby interior all function together as a system to protect the occupant during emergency landing. The specific test conditions are shown in Figure AC 29.562-1. Ex planations of the test conditions are as follows:
(i) Test 1. The test determines the protection provided when the impact environment is such that the resulting predominant impact load component (vertical) is directed along the spinal column of the occupant in combination with a horizontal (longitudinal) component. Protection against spinal injury is important and it may be necessary to provide energy absorbing (load limiting) or attenuation capability in the seat system in order to comply with the human injury criteria specified in § 29.562 (c)(7).
(ii) Test 2. The test determines the protection provided in an impact where the predominant impact load component is in the longitudinal direction in combination with a lateral component. Evaluation of head injury protection is important in this test if the head could strike some interior portion of the rotorcraft or a forward seat. Chest or spinal column injury, which might result from the upper torso restraint, is also evaluated in this test.
(iii) Tests 1 and 2. These test conditions are also significant for the structural strength of the system. Both tests should be used to assess submarining (where the seat belt slips above the ATD pelvis) and rollout of the upper torso restraint system particularly with single, diagonal torso restraint belts. Since external crash forces frequently cause significant structural deformation, simulated floor deformation is specified for the tests to prove the seat design can accommodate the relative deformation between the seat and the floor or sidewall and still function without imposing excessive loads on the seat, the attachment fittings, or floor tracks.
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AC 29-2C, Chg 1 2/12/03 Illustration shows a forward facing Test 1 Test 2 seat ATD inertial load shown by arrow:
1030° Min. Vi, fps 30 42 Max. tr. sec. - 031 • 071 Min. G 30 18. 4 Deform floor:
degrees roll 10 10 degrees pitch 10 10 deceleration tr = rise time V1 = impact velocity FIGURE AC 29.562-1. Seat Restraint System Dynamic Tests Normal and Transport Category Rotorcraft Page C - 58
(2) Test facilities. A test proposal is prepared for certification authority approval and should reflect the capability of the facility. It should be noted that a number of test facilities could be used to accomplish dynamic testing. Test facilities can be grouped into categories based on the method they use to generate the impact pulse (i.e., accelerators, decelerators, or impact with rebound) and whether the facility is a horizontal (sled) design or a vertical (drop tower) arrangement. As in all certification compliance tests, a test proposal, which may refer to certain specific or generic test equipment, is approved prior to testing. The test may be conducted anywhere, within certain availability or mutually convenient constraints, as long as the test is conducted in accordance with the approved test plan and properly witnessed.
(i) Facility Characteristics or Features. Each of the facilities has characteristics that may have advantages or disadvantages with regard to the dynamic tests discussed in this guidance. One concern is the rapid sequence of acceleration and deceleration that must take place in the tests. In a landing impact, the acceleration phase (flight) is gradual and usually well separated in time from the deceleration (crash impact) phase. In a test, the deceleration usually closely follows the acceleration.
When assessing the use of a facility for the specific test procedures outlined in the recommendations, it is necessary to assess the possible consequences of this rapid sequence of acceleration and deceleration on the test articles and ATD. The standard accommodates the different facilities that are or may be available for the applicant’s use. That is, the standards dictate the peak acceleration with a tolerance as stated in this AC. The “decay” in deceleration with respect to time is not dictated, thereby allowing for the different test facility equipment characteristics.
(A) Deceleration sled facilities. In an aircraft crash, the impact takes place as a deceleration, so loads are applied more naturally in test facilities that create the test impact pulse as a deceleration. Since it is simpler to design test facilities to extract energy in a controlled manner than to impart energy in a controlled manner, several different deceleration sled facilities can be found. The deceleration sled facility at the FAA’s Civil Aeromedical Institute (CAMI) was referred to in developing the test procedures discussed in this and similar AC’s related to airplanes.
(1) The Acceleration Phase. Sufficient velocity for the test impact pulse acquired in this phase can distort the test results if the acceleration is so high that the test articles or ATD are moved from their intended pre-test position. This inability to control the initial or onset conditions of the test would directly affect the test results.
This can be avoided by using a lower acceleration for a relatively long duration and by providing a coast phase (in which the acceleration or deceleration is nearly zero) prior to the impact. This allows any dynamic oscillation in the test articles or the ATD that might be caused by the acceleration to decay. To guard against errors in data caused by pre-impact accelerations, data from the electronic test measurements (accelerations, loads) should be reviewed for the time period just before the test impact pulse to make sure all measurements are at the baseline (zero) level. Photometry film taken of the Page C - 59
AC 29-2C, Chg 1 2/12/03 test should also be reviewed to make certain that the ATD’s used in the test and the test articles were all in their proper position prior to the test impact pulse.
(2) Orientation of test article. The horizontal test facility readily accommodates forward-facing seats in both tests discussed in this guidance, but problems can exist in positioning the test ATD in Test 1 if the seat is a rear or sidefacing seat. In these cases, the ATD’s tend to fall out of the seat due to the force of gravity and must be restrained in place using breakaway tape, cords, or strings. Since each installation will present its own problems, there is no simple, generally applicable, guidance. Attention should be given to positioning the ATD against the seat back and to proper positioning of the ATD’s arms and legs. It will probably be necessary to build special supports for a breakaway restraint so that the restraint will not interfere with the function of the seat and occupant restraint system during the test. Photos of the test from “side of track cameras” should be reviewed to make sure that the breakaway restraint did break (or become slack) in a manner that did not unduly influence the motion of the ATD or the test articles during the test.
(B) Acceleration sled facilities. Acceleration sled facilities, usually based on the Hydraulically Controlled Gas Energized (HYGE) accelerator device, provide the impact test pulse as a controlled acceleration at the beginning of the test. The test item and the ATD are installed facing in the opposite direction from the velocity vector, opposite from the direction used on a deceleration facility, to account for the change in direction of the impact. There should be no problem with the ATD or the test items being out of position due to pre-impact sled acceleration, since there is no sled movement prior to the impact test pulse. Because of this characteristic the applicant may prefer this type of a facility.
(1) Test pulse. After the impact test pulse, when the sled is moving at the maximum test velocity, stop the sled safely. Most of the facilities of this design have limited track length available for deceleration, so that the deceleration levels can be relatively high and deceleration may begin immediately after the impact test pulse.
Since the maximum response of the system usually follows (in time) the impact test pulse, any sled deceleration, which takes place during that response will affect the response and change the test results. The magnitude of change depends on the system being tested, so that no general “correction factor” can be specified. The effect can be minimized if the sled is allowed to coast, without significant deceleration, until the response is complete.
(2) Test results. If the seat or restraint system experiences a structural failure during the test pulse, the post impact deceleration can increase the damage and perhaps result in failures of unrelated components. This will complicate the determination of the initial failure mode and make product improvement more difficult.
One other consideration is that the photometry film coverage of the response to impact test pulse must be accomplished when the sled is moving at near maximum velocity.
Onboard cameras or a series of trackside cameras are usually used to provide film coverage of the test. Since onboard cameras frequently use a wide- angle lens placed Page C - 60
close to the test items, it is necessary to account for the effects of distortion and parallax when analyzing the film. The acceleration sled facility faces the same problems in accommodating rearward-facing or side-facing seats in Test 1 as the deceleration sled facility, and the corrective action is the same for both facilities.
(C) Impact-with-rebound sled facilities. One other type of horizontal test facility used is the “impact-with-rebound” sled facility. On this facility, the impact takes place as the moving sled contacts a braking system, which stores the energy of the impact, and then returns the stored energy back to the sled, causing it to rebound in the opposite direction. This facility has an advantage over acceleration or deceleration facilities in that only one-half of the required velocity for the impact would need to be generated by the facility (assuming 100 percent efficiency). Thus the track length can be shortened, and the method of generating velocity is simplified. The disadvantages of this facility combine the problems mentioned for both acceleration facilities and deceleration facilities. Since one of the reasons for this type of facility is to allow short track length to be used, it may be difficult to obtain sufficiently low acceleration just before or after the impact pulse to resolve data error problems caused by significant pre-impact and post-impact accelerations.
(D) Drop towers. Vertical test facilities can include both drop towers (decelerators) and vertical accelerators. Vertical accelerators, which can produce a longer duration/displacement impact pulse, may not be available. However, drop towers are one of the easiest facilities to build and operate and are frequently used.
(1) Acceleration phase. In these facilities, the pull of earth’s gravity is used to accelerate the sled or guided test fixture and test article to specified impact velocity to avoid the use of a complex mechanical accelerating system. Reproducing the required impact pulse may require extensive development tests for the facility.
Unfortunately, these facilities are more difficult to use for conducting Test 2, particularly for typical forward-facing seats.
(2) Test article. In preparing for (longitudinal) Test 2, the seat should be installed at an angle according to the standards such that the ATD’s tend to fall from the seat due to gravity. The restraint system being tested cannot hold the ATD against the seat unless tightened excessively and will not usually locate the head, arms, or legs in their proper position relative to the seat. Design and fabrication of an auxiliary “break-away” ATD positioning restraint system just for this test are usually a complex task. The auxiliary restraint should not only position the ATD against the seat (including maintaining proper seat cushion deflection) during the pre-release condition of 1 g, it should also maintain the ATD in that proper position during the free fall to impact velocity when the system is exposed to zero g, and then it should release the ATD in a manner that does not interfere with the ATD response to impact. The usual sequence of 1 g/0 g impact, without the possibility of a useful “coast” phase, as done in horizontal facilities, causes shifts in initial conditions for the test impact pulse that can affect the response to the impact. The significance of this undesired movement will depend on Page C - 61
AC 29-2C, Chg 1 2/12/03 the dynamic characteristics of the system under test, and these characteristics are seldom known with sufficient accuracy to achieve the response initially.
(3) Other facets. In addition, the earth’s gravity will oppose the final rebound of the ATD into the seat back, so that an adequate test of seat back strength and support for the ATD cannot be obtained. The problems in Test 1, or with rear-facing seats in Test 2, are not as difficult because the seat will support the ATD prior to the free fall. However, the zero g condition free fall that exists prior to impact will allow the ATD to “float” in the seat restraint system, perhaps changing position and certainly changing the initial impact conditions if movement occurs. Again, the development of a satisfactory auxiliary breakaway restraint system to assure correct pre-impact condition is difficult.
(ii) Test Fixtures. A test fixture is usually required to position the test article on the sled or drop carriage of the test facility and to represent the aircraft’s structure floor, sidewall, bulkhead, etc. It holds the attachment fittings or floor tracks for the seat, provides the floor and sidewall deformation needed for the test, and provides a floor or footrest for the ATD, and it positions the pertinent interior items, such as instrument panels, sidewalls, bulkheads, a second row of seats, if required, for successful performance of the tests, and otherwise simulates the rotorcraft for the test.
The text fixture is usually fabricated of heavy structural steel and does not necessarily simulate lightweight aircraft design or construction. The details of the fixture will depend upon the requirements of the test articles, but provisions for the specified floor and sidewall deformation are needed.
(A) Purpose of floor or sidewall deformation. The purpose of using pitch and roll deformation for the tests is to demonstrate that the seat/restraint system will remain attached to the airframe and perform properly for the tests, although the structure and seat may be more severely deformed by the forces associated with a particular crash. Typical design deficiencies addressed by the test conditions include, but are not limited to, the following:
(1) Concentrated loads may be imposed on floor fittings (studs) or tracks by seat leg attachment fittings which fit tightly or are clamped to a track or fitting, and which do not have some form of relief (especially lateral roll relief) incorporated in the design. These joint fittings can concentrate the forces on one lip of the floor and sidewall track or stud and may break the joint (track or the fitting).
(2) Similarly, loads can be concentrated on one edge of a floor track or stud fitting having an “I,” “bulb head” or “mushroom” cross section and may prematurely break the flange or the fitting.
(3) Detents, pins, or collars which lock the seat leg fitting to the floor track can become disengaged, or the mechanism which is used to disengage the detents, pins, or “dogs” can be actuated and release the seat as the seat or airframe deforms.
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