AC 29-2C, Chg 4 5/1/2014 (B) Particular Risk Analysis. Particular risks are defined as those events or influences that are outside the systems concerned. Examples are fire, leaking fluids, bird strike, tire burst, high intensity radiated fields exposure, lightning, uncontained failure of high energy rotating machines, etc. Each risk should be the subject of a specific study to examine and document the simultaneous or cascading effects or influences that may violate independence.
(C) Common Mode Analysis (CMA). This analysis is performed to confirm the assumed independence of the events that were considered in combination for a given Failure Condition. The effects of specification, design, implementation, installation, maintenance, and manufacturing errors, environmental factors other than those already considered in the particular risk analysis, and failures of system components should be considered. The CMA considers many aspects of design; one is design dissimilarity for both hardware and software. Without dissimilarity for redundant applications, there is a possibility that a hardware or software failure or malfunctions could occur in the same flight for all redundant subparts of a system with a Catastrophic or Hazardous or Severe-Major Failure Condition Category. Another major contribution from the CMA is the determination of which failure or malfunction combinations inputs to the Fault Tree Analysis (FTA) must be independent for events that are Catastrophic or Hazardous or Severe-Major. This analysis is iterative in nature as it will be employed early in design to identify possible common modes for failures or malfunctions and then is used after the design is complete to determine if the FTA goals have been met.
(6) Documentation Software Airborne Electronic Hardware (AEH). For airborne complex custom micro- . All laboratory, ground and flight tests, and failure analyses, must be documented in sufficient detail to show compliance with § 29.1309 and be included in the type design file. Section 21.31(a) provides the regulatory basis for requiring this documentation. If the applicant elects to use a numerical reliability and probability analysis, it must also be documented in sufficient detail.
(7) . RTCA Document DO-178C, “Software Considerations in Airborne Systems and Equipment Certification,” dated December 13, 2011, is the latest standard and is recommended to be used for qualification and subsequent approval of airborne software. See AC 20-115C for guidance on using DO-178C and earlier standards.
(8) coded electronic components assessed as level A, B, or C in the safety assessment, RTCA Document DO-254, “Design Assurance Guidance for Airborne Electronic Hardware,” dated April 19, 2000, provides certification guidance for qualification and subsequent approval of these components. Per DO-254, the component will need to be identified as either “simple” or “complex” and tagged to one of the five failure condition classifications. Based on these identifiers, DO-254 provides the guidance to show compliance to 29.1309. Although RTCA DO-254 applies specifically to complex custom micro-coded components, applicants are highly encouraged to apply DO-254 to other hardware components up through to the LRU and systems level.
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(9) Environmental Qualification.
(i) Laboratory Tests.
(A) Environmental Standards. In order to assure that the components and systems under consideration will function properly when exposed to adverse environments, they should be tested in the laboratory under a simulated adverse environment. If a TSO exists and it is appropriate in environmental range and performance for an equipment installation, it is preferable the equipment be TSO approved. If there is no applicable TSO or an existing TSO does not provide for a sufficiently adverse environment, the latest revision of the Radio Technical Commission for Aeronautics (RTCA) document DO-160 is an acceptable environmental standard for laboratory qualification of aircraft equipment.
(B) Adverse environmental variables for all types of required and critical equipment include, but are not necessarily limited to temperature, humidity, vibration, shock, altitude, overpressure, and power source transients.
(C) For electrical and electronic equipment, adverse environmental variables include all of (b) above plus overvoltage and undervoltage. Electronic equipment should also be tested for electromagnetic interference (EMI). These tests should include both emission and susceptibility evaluations of both conducted and radiated EMI.
(D) Explosion Tests. Those items of electrical and electronic equipment that are to be located in areas subject to flammable fluids and vapors, as a result of any single probable malfunction or failure, including failure of couplings or lines should be tested as an ignition source. These tests consist of normal operation of the equipment in a physically contained explosive atmosphere. The explosion test procedure in the latest revision of DO-160 will satisfy this requirement. Section 29.863 of this AC provides further guidance on safety from explosion. If another standard is used that is at least as good as the latest revision of DO-160, it may also be accepted to satisfy this requirement.
(ii) Installed Environmental Tests. After the environmental ratings of the components and systems have been established, it should be assured that as installed, these ratings will not be exceeded. Normally, installed equipment need not be instrumented and tested in flight nor is it necessary to instrument the compartment or rack where the equipment is installed. Satisfactory environment and equipment compatibility are assured by selection of the proper environmental category of laboratory tests. The category is determined by the type of aircraft (reciprocating or turbine) and flight envelope (altitude and temperature). Exceptions to normal installations are (a) Alternator or generator cooling, where radiated and conducted heat is almost always uncertain, also cooling air temperatures and flow rates are uncertain;
(b) Where flight tests reveal excessive instrument panel vibration. In this case, the panel should be instrumented, tested, and, if necessary, design improvements made;
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AC 29-2C, Chg 4 5/1/2014 and (c) Any other cases where good engineering judgment and application of sound engineering principles indicate a high likelihood that the installed environment is more severe than the equipment is capable of operating within.
(A) Temperature Tests.
( ) Temperature tests may be accomplished by instrumenting the installed equipmen 1 t environment with a recorder that provides a permanent record of time, altitude, and temperature. The pertinent temperature should be recorded as the rotorcraft is operated throughout its altitude range, including ground operation. The maximum and minimum temperatures recorded should be corrected degree for degree to assure the equipment under test remains within its temperature rating while the rotorcraft operates throughout its approved ambient temperature envelope. (For generator or alternator cooling test procedures, refer to section 29.1351 of this AC.)
Section 29.1043(b) requires the maximum approved operating OAT to be at least 100° F for powerplant-mounted accessories such as starter generators, vacuum pumps, etc. Due to the impracticality of the 100° F hot day temperature limit, rotorcraft systems mounted on the powerplant are normally evaluated for at least 115° F hot day sea level conditions with corresponding 3.6° F/1,000-foot correction. The maximum hot day OAT at sea level must be specified in the rotorcraft flight manual. Section 29.1043(b) is the regulatory basis for the lapse rate of 3.6° F/1,000 feet. This lapse rate should be applied regardless of the hot day sea level temperature the applicant chooses to certify for operation.
(2 3 4 ) The § 29.1043(b) maximum ambient temperature definition should not be confused with operating temperatures in closed areas. Closed equipment rack areas can easily reach temperatures of 140° F when sitting on the ramp in the southern United States in midsummer. Normally, proper selection of the altitude temperature category in the latest revision of DO-160 will assure compliance.
( ) In some cases, the equipment manufacturer furnishes temperature limits for internal critical parts. For example, brushes, bearings, or field windings on DC generators. In these cases it is better to record the critical component temperature rather than equipment or equipment environment temperature.
( ) The following will illustrate an acceptable high temperature evaluation method:
TOAT MAX = Maximum outside air temperature at which temperature tests are conducted.
TMAX = Maximum temperature to which the installed equipment has been tested in the laboratory.
TTEST MAX = Maximum installed equipment temperature recorded during tests.
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TORH = The high reference outside air temperature. It varies w ith altitude starting at the highest sea level temperature at which rotorcraft operation is to be approved and decreases at
3.6° F/1,000 foot altitude. It can be no less than 100° F (reference § 29.1043(b)); however, it can be as high as the applicant wants.
TH MAR = Temperature margin between the maximum equipment temperature substantiated in the laboratory and the maximum installed equipment temperature when the rotorcraft is operating in the highest available OAT and approximately corrected at the altitude under consideration. If the margin is zero or positive, the equipment passes. If the margin is negative, the equipment fails the test.
TH MAR = TMAX - (TTEST MAX + (TORH - TOAT MAX)) Example #1: Assume the applicant is seeking approval for rotorcraft operation at the lowest acceptable OAT, at sea level, of 100° F and TMAX for Generator Brush = 295° F at maximum load current throughout the altitude range. In- flight test data are:
Measured Altitude (ft, MSL) Cylinder Temp (TTEST MIN) OAT = TOAT MAX sea level 275°F 90°F 5,000 270°F 80°F 10,000 285°F 60°F 15,000 294°F 42°F 20,000 290°F 20°F First, TORH must be calculated for each altitude test point.
@ sea level, TORH = 100° F @ 5,000 ft., TORH = 100° F -5,000 ft., x 3.6°/1,000 ft. = 82° F @ 10,000 ft., TORH = 100° F - 10,000 ft. x 3.6°/1,000 ft. = 64° F @ 15,000 ft., TORH = 100° F - 15,000 ft. x 3.6°/1,000 ft. = 46° F @ 20,000 ft., TORH = 100° F - 20,000 ft. x 3.6°/1,000 ft. = 28° F Then at sea level:
TH MAR = 295 - (275 + (100 - 90)) = 10° F At 5,000 feet:
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- AC 29-2C, Chg 4 5/1/2014 TH MAR = 295 - (270 + (82 - 80)) = 23° F At 10,000 feet:
TH MAR = 295 - (285 +(64 - 60)) = 6° F At 15,000 feet:
TH MAR = 295 - (294 + (46 - 42)) = -3° F At 20,000 feet:
TH MAR = 295 - (290 + (28 -20)) = -3° F Since TH MAR comes out negative at the 15,000- and 20,000-foot points, the generator fails. It will be necessary for the applicant to reduce the maximum load current, improve cooling, or otherwise change the design to assure the generator is operating within its approved temperature limit of 295° F.
(5) In most cases, the equipment is laboratory tested to minimum temperatures as severe as that of the rotorcraft’s maximum certified altitude on a minimum temperature day. Therefore, unless equipment minimum temperature is affected by refrigeration or other temperature reducing environments, actual installed instrumented minimum temperature tests are unnecessary. If low temperature evaluation is necessary for the installed equipment, the following is an acceptable method:
TOAT MIN = Minimum outside air temperature at which temperature tests are conducted.
TMIN = Minimum temperature to which the installed equipment has been tested in the laboratory.
TTEST MIN = Minimum installed equipment temperature recorded during tests.
TORL = The low reference outside air temperature. It varies with altitude starting at the lowest sea level temperature at which rotorcraft operation is to be approved and decreases at 3.6° F/1,000-foot altitude.
T1 MAR = Temperature margin between the minimum equipment temperature substantiated in the laboratory and the minimum installed equipment temperature. If the margin is zero or positive, the equipment passes. If the margin is negative, the equipment fails the test.
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T1 MAR = - (TMIN - (TTEST MIN + (TORL - TOAT MIN))) Note: This equation assumes all temperatures are negative. It is necessary to place a (-) in front of the right side of the equation in order to convert the T1 MAR value to the conventional positive answer for acceptance and a negative answer for rejection. Temperature in the 0 to 32° F range can be handled by conversion to the centigrade scale.
Example #2: Assume the applicant is seeking a low temperature operating limit at sea level of -25° F. Assume the hydraulic control cylinder has been substantiated in the laboratory to operate at a cylinder temperature of -40° F. The in-flight test data are:
Measured Altitude (ft, MSL) Cylinder Temp (TTEST MIN) sea level 0°F 5,000 -9°F 10,000 -21°F 15,000 -32°F 20,000 -40°F OAT = TOAT MAX -25°F -45°F -59°F -65°F -69°F (Torl must be calculated for each altitude test point) @ sea level, TORL = -25° F @ 5,000 ft., TORL = -25° F -5,000 ft., x 3.6°/1,000 ft. = -43° F @ 10,000 ft., TORL = -25° F - 10,000 ft. x 3.6°/1,000 ft. = -61° F @ 15,000 ft., TORL = -25° F - 15,000 ft. x 3.6°/1,000 ft. = -79° F* @ 20,000 ft., TORL = -25° F - 20,000 ft. x 3.6°/1,000 ft. = -97° F* *According to § 29.1043(b), the lowest temperature to be considered is -69.7° F.
Then at sea level:
T1 MAR = -(-40 -(0 + (-25 -(-25)))) = 40° F At 5,000 feet:
T1 MAR = -(-40 - (-9 + (-43 - (-45)))) = 33° F At 10,000 feet:
T1 MAR = -(-40 - (-21 + (-61 - (-59)))) = 17° F At 15,000 feet:
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AC 29-2C, Chg 4 5/1/2014 T1 MAR = -(-40 - (-32 + (-69.7 - (-65)))) = 3.3° F At 20,000 feet:
T1 MAR = -(-40 - (-40+ (-69.7 - (-69)))) = -0.7° F It can be seen that there is an acceptable margin at all altitudes up to and including 15,000 feet. However, at 20,000 feet, the margin is negative and the system fails.
(B) Vibration tests. Normally, installed vibration tests are not necessary for equipment qualified in accordance with the latest revision of RTCA document DO-160. This paper categorizes vibration tests according to installed rotorcraft equipment location such as fuselage, engine compartment, instrument panel, equipment rack, etc. However, installed equipment vibration tests may be necessary when it appears the equipment location environment may exceed the laboratory-tested equipment vibration limits.
(C) Altitude tests. If the equipment has been laboratory tested to the maximum certified altitude of the rotorcraft, installed altitude tests are unnecessary.
The installed equipment must be either laboratory tested or tested in the rotorcraft to the maximum certified altitude of the rotorcraft.
(iii) Lightning Strike Protection of Full Authority Digital Engine Controls (FADEC).
(A) Explanation.
(1 2 ) The following discussion is written specifically for FADEC with an alternate technology backup fuel control installed on rotorcraft with category A engine isolation. The requirement for increased consideration of lightning strike encounter effects on avionic equipment and systems has been brought about by the increased use of avionics to perform functions, the failure or malfunction of which could result in a hazard to the rotorcraft. The susceptibility of current high technology avionic systems is increased by the use of large scale integration, very large scale integration, and complementary metallic oxide silicon technologies, which exhibit a greatly reduced tolerance to large amplitude, low energy electrical transients as compared to conventional bipolar technology, and the reduced physical protection and electromagnetic shielding afforded aircraft avionic systems by the advanced technology composite airframe materials. Additionally, processor -based systems have the failure phenomenon of digital upset. A digital upset occurs when a system, perturbed by an electrical transient, ceases proper operation in accordance with its embedded software while suffering no apparent component or device damage.
( ) Since elements of electrical and electronic engine subsystems are typically spread throughout much of the rotorcraft, transients caused by lightning are coupled into the subsystem interface cables and may damage the system or cause Page F - 34
upset. Effective lightning protection must be designed and incorporated into these systems. Reliance upon redundancy as a means of protection against lightning effects is generally not adequate because lightning electromagnetic fields and structural IR voltages usually interact (to some extent) with all electrical wiring aboard a rotorcraft.
(3 4 1 2 3 ) The testing and analysis outlined in this discussion are methods by which the FAA/AUTHORITY may be assured that when the rotorcraft experiences “the foreseeable operating condition” of a worst-case lightning strike encounter that the electronically controlled engines will continue to “perform their intended function” and therefore be in compliance with § 29.1309(a) as installed.
( ) The definition of what constitutes a full authority engine control is not at this time clearly defined. However, it has been accepted in past certification that any control which relies upon the electronics for the function on which Civil Certification or Military Qualification is based (e.g., rotor speed governing) is a full authority control, regardless of the backup control mode provided. If engine certification or qualification can be achieved without the electronic control, which is subsequently added to achieve improved operational efficiency in the aircraft, the control is “supervisory.” However, if the controls are used in a multiengine rotorcraft, a common failure caused by a lightning strike could result in simultaneous failures, which would cause a reduction in power greater than the loss of one engine. This would also be considered “full authority.”
Note: If OEI ratings are approved, cumulative loss of power from all engines should be limited to allow flight manual performance based on OEI ratings.
(B) Procedures. Although not a regulatory requirement, it is recommended that a formal written certification plan be used to assure regulatory compliance. The use of this plan is beneficial to both the applicant and the FAA/AUTHORITY because it identifies and defines an acceptable resolution to the critical issues early in the certification process. These are the usual steps to be followed when utilizing a certification plan:
( ) Prepare a certification plan that describes the analytical procedures and the qualification tests to be utilized to demonstrate protection effectiveness. Test plans should describe the rotorcraft and FADEC system to be utilized, test drawing(s) as required, the method of installation that simulates the production installation, the lightning zone(s) applicable, the lightning simulation method(s), test voltage or current waveforms to be used, diagnostic methods, and the appropriate schedules and location(s) of proposed test(s).
( ) Obtain FAA/AUTHORITY concurrence that the certification plan is adequate.
( ) Obtain FAA/AUTHORITY detail part conformity of the test articles and installation conformity of applicable portions of the test setup.
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AC 29-2C, Chg 4 5/1/2014
(4) Schedule FAA/AUTHORITY witnessing of the test.
(5) Submit a final test report describing all results and obtain FAA/AUTHORITY approval of the report.
(C) Definition of Environment. The SAE AE4L Committee report dated June 6, 1999, has been and remains acceptable criteria to define the worst-case lightning strike that may be encountered by the rotorcraft in service. An additional explanation of the lightning environment could be found in FAA Report DOT/FAA/CT-89/22, “Aircraft Lightning Protection Handbook.” This handbook will assist aircraft design, manufacturing, and certification organizations in protecting aircraft against the direct and indirect effects of lightning strikes, in compliance with 14 CFR regulations. It presents a comprehensive test criteria to provide the essential information for the in-flight lightning protection of all types of fixed wing, rotary wing, and powered lift aircraft of conventional, composite, and mixed construction and their electrical and fuel systems. The handbook contains chapters on the natural phenomenon of lightning, the interaction between the aircraft and the electrically charged atmosphere, the mechanism of the lightning strike, and the interaction with the airframe, wiring, and fuel system. Further chapters cover details of designing for optimum protection; the physics behind the voltages, currents, and electromagnetic fields developed by the strike; and shielding techniques and damage analysis. The handbook ends with discussion of test and analytical techniques for determining the adequacy of a given protection scheme. On September 7, 2011 , FAA AC 20-136B, “Aircraft Electrical and Electronic System Lightning Protection,” was issued, superseding previous versions of that AC. For new designs and applications, this revised definition of the lightning environment should be used.
(D) Certification Plan. The following subjects are not intended to provide a complete list of the items that should be included in the certification plan, but rather highlight some of the areas that should receive consideration. The certification plan should address the total protection that is required to allow the FADEC to continue to operate properly when the rotorcraft experiences a worst-case lightning strike encounter.
(1) Determination of Lightning Strike Attachments. Determine the locations on the rotorcraft where lightning strike attachment is likely to occur and the portions of the airframe through which currents may flow between attachments. The main and tail rotors are recognized as likely attachment points; however, consideration should be given to all possible attachment points. The swept stroke phenomenon may not exist for all lightning strike encounters due to the fact that the rotorcraft may be airborne with little or no airspeed.
(2) Establish the Lightning Environment. Establish the components of the total lightning event to be considered. These are the currents and voltages that are described in the definition of the environment.
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(3) Full-Level, Complete Vehicle Testing. In accordance with traditional FAA/AUTHORITY policy, the demonstration that the FADEC installed in a complete type design rotorcraft will continue to operate properly when exposed to a worst-case lightning strike is sufficient to demonstrate compliance with § 29.1309(a).
Because of the difficulties involved in utilizing this type of an approach, it is generally not used.
( ) Analytical Processes. A description should be given in the certification plan of 4 the analytical process and certification tests to be utilized to demonstrate protection effectiveness. Typically, the certification plan will include a combination of analysis and tests. (Analytical techniques are most often utilized to predict the levels of lightning-induced transients in interconnecting wiring.) In most cases, successful analyses are based upon well-defined geometrical or electrical parameters such as structural dimensions and materials resistivity. When electrical characteristics of structural materials are not well established, development tests are often utilized to obtain this data which is subsequently utilized in an analysis. In more complex structures or electrical and electronic system installations, it is sometimes difficult or impossible to define the problem in terms that can be analyzed. In these cases, development or verification testing is often relied upon. The purpose of the certification plan is to show how developmental tests, analyses, and verification tests are combined to demonstrate protection design adequacy. In certain cases, previously verified designs can be incorporated and their adequacy confirmed by reference to previous verifications. Such reference should also be incorporated in the certification plan.
(i) The verification testing should be conducted on a system that simulates as closely as possible the installed configuration. As few items as possible of actual hardware should be simulated.
(ii) Unless the applicant has opted to follow the guidance in AC 20-136B as a means of showing compliance with § 29.1309(h), then the following or other acceptable guidance should be followed. The use of various analytical processes usually requires that the system component tolerance is established. The S AE AE4L Committee Report No. AE4L-81-2 is one example of an acceptable reference to be used for the testing accomplished to determine these tolerances. The testing which is performed to determine the tolerance level of the control computer should include a consideration for the occurrence of a non-recoverable digital upset. One method to provide this consideration is to have the unit powered and the processor operating normally under software control (usually this should be the exact software for which approval is sought) when the test is performed. If strike testing is used, then several shots should be made to develop enough data to provide a reasonable confidence level.
It is an acceptable procedure for the engine manufacturer, while he is obtaining his type certificate, to accomplish this bench testing to determine the level of tolerance of the FADEC system components to lightning encounter indirect effects. This approach has the advantages that the bench tests are not necessarily required to be repeated when the engine is installed in a different airframe. This recommendation is not meant to add Page F - 37
AC 29-2C, Chg 4 5/1/2014 a requirement to the engine manufacturer but to propose a more efficient method of certification. If this tolerance was not determined by the engine manufacturer, the applicant installing the FADEC in a rotorcraft would be expected to furnish this data.
(iii) As with any analytical method, it is prudent to include a margin of safety to account for the uncertainties involved in the analytical and testing processes. Margins account for uncertainty in the compliance method. As confidence in the compliance method increases, the margin can decrease. A factor of two is an acceptable margin for systems with catastrophic failure conditions, if this margin is verified by aircraft test or by analysis supported by aircraft tests. For other verification methods, the margin should be agreed upon with the cognizant certification office.
(iv) When an analysis has no associated full-scale vehicle testing to confirm the analysis, the analysis should be very rigorous. Additionally, it should be expected in this situation that this analysis indicates a very large margin of protection.
Many factors must be considered in determining what constitutes an acceptably large margin. The specific additional margin required should be based on an assessment of the inherent uncertainty of a given analysis.
(5) Pass or Fail Criteria. The certification plan should address a pa ss or fail criteria for the testing and analysis to be performed. The following items should be satisfied to assure acceptable system performance:
(i) No immediate crew action must be required.
(ii) Automatic control of the engine cannot be lost for any appreciable period of time. The engine must not be allowed to be out of control for a period of time that will result in a hazard in a worst-case flight condition. Obviously, any rapid, uncontrolled divergence is not acceptable.
(iii) No crew action should be required to reset the system. This is not to imply that the system cannot be designed with a manual reset, but the manual reset cannot be used to show compliance to recover from a digital upset.
(iv vi ) The resumption of engine control after an upset must be reasonably within the range that existed before the upset.
(v) No critical data can be lost.
( ) After the system recovers, if the performance of the system has been degraded in a noncritical manner which would reduce the capability of the rotorcraft or the ability of the pilot to cope with adverse operating conditions, then the crew must be alerted to this system degradation.
(E) System Installation Considerations. In most cases, the installation of the system components is a constituent part of the lightning protection. This is Page F - 38
particularly true in the use of shielding techniques. If these installation features are required for adequate lightning protection, consideration should be given to ensure that their effectiveness is not derogated in service. ICAs be provided to the parties who service and operate the rotorcraft to allow them to take actions necessary to ensure the continued effectiveness of the system lightning protection.
(iv) Lightning Protection.
(A) Background. During the original design and development of rotorcraft and the development of regulations concerning these aircraft, little attention was given to protection from the meteorological phenomenon of lightning. This was, in part, because the early aircraft were constructed mostly of metal and had little, if any, dependence on advanced technology systems. Contemporary design transport category rotorcraft are utilizing the same advanced technology systems and materials as transport category airplanes. Because of this fact, a specific requirement was added by Amendment 29-24 for the consideration of lightning strike protection of required systems, equipment, and installations. The addition of paragraph (h) to § 29.1309 further defined the consideration required for the foreseeable operating condition of a lightning strike encounter on the rotorcraft.
(B) Procedures.
(1 2 ii 3 ) Section 29.1309(h) requires, when showing compliance with § 29.1309(a) and (b), that the effects of lightning strikes on the rotorcraft be considered.
Guidance to show compliance with § 29.1309(h) is available in a dedicated AC 20-136B.
( ) Detailed means of compliance should be agreed with the authorities taking into account the effects on the rotorcraft and minimum considerations are as follows:
(i) Any combination of analysis and testing should be agreed to by the authority.
( ) Margins account for uncertainty in the compliance method. As confidence in the compliance method increases, the margin can decrease. A factor of two is an acceptable margin for systems with catastrophic failure conditions, if this margin is verified by aircraft test or by analysis supported by aircraft tests. For other verification methods, the margin should be agreed upon by the cognizant certification office.
( ) Flight and engine controls are examples of “critical” functions. With these critical functions defined, an analysis and testing should be performed to show compliance with § 29.1309(a) (i.e., equipment, systems, and installations performing critical functions should be designed and installed to ensure they continue to perform those intended functions subject to the conditions encountered in a worst-case lightning Page F - 39