AC 27.1307 § 27.1307 MISCELLANEOUS EQUIPMENT.
a. Explanation. This rule provides a listing of several items of required miscellaneous equipment. Each item seems to be self-explanatory except the one requiring a master switch arrangement. The purpose of a master switch arrangement is to allow rapid removal of sources of electrical power from the rotorcraft in an emergency situation.
b. Procedures. When reviewing possible solutions to the master switch arrangement requirement, the following considerations should be included.
(1) System separation. Since wiring from each electrical system will be brought in close proximity to each other, extra care should be taken to maintain some separation. As examples, common connectors, common grounds, and common wire routing should be avoided.
(2) Installation of switches. A single switch should be avoided since it introduces the possibility of a single failure turning off the entire electrical system. One solution that is commonly used provides a close grouping of the switches such that the pilot can easily reach all switches and turn them all off with one action. This solution requires a cockpit evaluation to ensure the installation will be suitable for different hand sizes. Another solution involves a gang bar that can be moved with a single motion to turn off all sources. This solution has been found to be acceptable in several instances. Other solutions should be evaluated on their own merits, and the primary emphasis should be on maintaining some minimum system separation and conducting a cockpit evaluation by flight test personnel.
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AC 27.1309. § 27.1309 (Amendment 27-21) EQUIPMENT, SYSTEMS, AND INSTALLATIONS.
a. Types of Equipment. Section 27.1309 of the Title 14 CFR regulations covers, but is not limited to, electrical, pneumatic, and hydraulic power sources, distribution, and corresponding utilization systems.
b. Environmental Qualification.
(1) Laboratory Tests.
(i) Environmental Standards. In order to ensure 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 that 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 Radio Technical Commission for Aeronautics (RTCA) Document DO-160 is an acceptable environmental standard for laboratory qualification of aircraft equipment.
(II) 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.
(I) For electrical and electronic equipment, adverse environmental variables include all of (b)(1)(i) and (ii) 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.
(iv) 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. 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.
(2) Installed Environmental Tests. After the environmental ratings of the components and systems have been established, it should be ensured that as installed, these ratings will not be exceeded. Normally, installed equipment need not be instrumented and tested in flight, and it is not necessary to instrument the compartment or rack where the equipment is installed. Satisfactory environment and equipment compatibility are ensured by selection of the proper environmental category of Page F - 13
AC 27-1B, Chg 4 3/31/2014 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;
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.
(i) Temperature Tests.
(A) Temperature Survey to Determine Proper Cooling of Electrical and Electronic Equipment.
(1) Equipment Requiring Cooling Test. As with any avionic equipment, good engineering judgment may deem that all components of the electrical or electronic equipment should have an in-flight temperature survey performed to ascertain that the thermal environmental tolerance of the system components is not exceeded. Usually, the following general guidelines may be used to aid in determining when an in-flight temperature survey is warranted.
(i) Components which contain a CRT (e.g., EDS) require the temperature survey testing outlined in paragraph b.(2)(i)(B) below.
(II) Equipment that does not contain a CRT, but is specified by the manufacturer to require forced air cooling (by an airframe mounted system), usually requires temperature survey testing.
(I) Equipment that does not contain a CRT and is not specified as requiring forced air cooling may usually have its critical thermal environment substantiated by laboratory testing.
B) Temperature Survey Testing. The temperature tests for the electrical or electronic equipment units should include a short-term test of approximately 30 minutes, which accounts for an aircraft that has heat-soaked on the ramp. A factor of 25° F should be added to the maximum corrected temperature to account for "greenhouse effect." A long-term test should be accomplished at various altitudes and limiting (low and high) airspeeds. All avionic equipment should be turned on during this test, and the cockpit panel lights should be operated at full intensity. The environmental control unit (ECU) or air-conditioning system should not be operating during these tests;
however, any windows or vents that are part of the "basic" TC rotorcraft may be utilized to ventilate the pilot's stations. Both of these tests should be corrected to the maximum temperature for which the rotorcraft is certified and a standard lapse rate for altitude as specified in this guidance. If an airframe cooling system is necessary to display units within acceptable temperature limits, then the pilot(s) must be made aware of a failure Page F - 14
or malfunction of this cooling system. Some type of cockpit visual annunciation with the capability to perform a preflight test is usually utilized to fulfill this requirement.
(C) Temperature tests may be accomplished by instrumenting the installed equipment 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 ensure 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 27.1351 of this AC.)
Section 27.1043(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 27.1043(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.
(D) The § 27.1043(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 ensure compliance.
(E) 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.
(F) 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.
Torh = The high reference outside air temperature. It varies with 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 Page F - 15
AC 27-1B, Chg 4 3/31/2014 altitude. It can be no less than 100° F (reference § 27.1043(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 (ITEST 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. × 3.6°/1,000 ft. = 64° F @ 15,000 ft., Torh = 100° F - 15,000 ft. × 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:
Th mar = 295 - (270 + (82 - 80)) = 23° F At 10,000 feet:
Th mar = 295 - (285 +(64 - 60)) = 6° F Page F - 16
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 ensure the generator is operating within its approved temperature limit of 295° F.
(G) 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.
Tor = 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.
T1 mar = - (Tmin - (Ttest min + (Tor - 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 Page F - 17
AC 27-1B, Chg 4 3/31/2014 rejection. Temperature in the zero 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) OAT = TOAT MAX sea level -0°F -25°F 5,000 -9°F -45°F 10,000 -21°F -59°F 15,000 -32°F -65°F 20,000 -40°F -69°F (Tor must be calculated for each altitude test point) @ sea level, Tor = -25° F @ 5,000 ft., Tor = -25° F -5,000 ft., x 3.6°/1,000 ft. = -43° F @ 10,000 ft., Tor = -25° F - 10,000 ft. x 3.6°/1,000 ft. = -61° F @ 15,000 ft., Tor = -25° F - 15,000 ft. x 3.6°/1,000 ft. = -79° F* @ 20,000 ft., Tor = -25° F - 20,000 ft. x 3.6°/1,000 ft. = -97° F* According to § 27.1043(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:
T1 mar = -(-40 - (-32 + (-69.7 - (-65)))) = 3.3° F At 20,000 feet:
T1 mar = -(-40 - (-40+ (-69.7 - (-69)))) = -0.7° F Page F - 18
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.
(ii) 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
(ili) 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.
(3) Lightning Protection.
(i) 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 normal category rotorcraft are utilizing the same advanced technology systems and materials as airplanes. Because of this fact, a specific requirement was added by Amendment 27-21 for the consideration of lightning strike protection of required systems, equipment, and installations. The addition of paragraph (d) to § 27.1309 further defined the consideration required for the foreseeable operating condition of a lightning strike encounter on the rotorcraft.
(II) Procedures.
Section 27.1309(d) requires when showing compliance to § 27.1309(a) and (b) that the effects of lightning strikes on the rotorcraft must be considered. Guidance to show compliance with § 27.1309(d) is available in a dedicated AC 20-136B.
(B) Detailed means of compliance should be agreed with the authorities taking into account the effects on the rotorcraft and minimum considerations are as follows:
(1) Any combination of analysis and testing should be agreed to by the authority.
(2) Margins account for uncertainty in the compliance method. As confidence in the compliance method increases, the margin can decrease. A factor of Page F - 19
AC 27-1B, Chg 4 3/31/2014 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.
(C) 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 § 27.1309(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 strike). Section 27.610 contains some methods which may be utilized for less complex mechanical systems; however, a great deal of difficulty will be experienced in trying to use these criteria to demonstrate that a very complex avionic system complies with § 27.1309(a). These avionic systems usually only require protection from indirect effects of lightning, and therefore a method such as that outlined in paragraph b.(4) below is recommended. This method may be readily adapted to other avionic systems performing critical functions. Also, this identifies an acceptable quantification of the expected airborne environment. The next step involves expanding the fault/failure analysis to determine what probable malfunctions or failures of the installed equipment systems and installation may occur as a result of a lightning strike to the rotorcraft. For a twin-engine rotorcraft, these systems must be designed such that a failure or malfunction will not cause a hazard to the rotorcraft. For a single-engine rotorcraft, these equipment systems and installation should be designed to minimize hazardous effects of failures or malfunctions. Note that the analysis which determines what are probable malfunctions or failures should consider that the encounter with a worst-case lightning strike is a given event.
(4) Lightning Strike Protection of Full Authority Digital Engine Controls (FADEC)
(i) Explanation.
(A) The following discussion is written specifically for FADEC with an alternate technology backup fuel control installed on rotorcraft with part 27 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 Page F - 20
electrical transient, ceases proper operation in accordance with its embedded software while suffering no apparent component or device damage.
(B) 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 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.
(C) 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 conditioni 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 § 27.1309 as installed.
(D) 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 used in a multiengine rotorcraft have a common failure caused by a lightning strike which 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 OE ratings are approved, cumulative loss of power from all engines must be limited to allow flight manual performance based on OE ratings.
(i1) 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:
(A) 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).
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AC 27-1B, Chg 4 3/31/2014 (B) Obtain FAA/AUTHORITY concurrence that the certification plan is adequate.
(C) Obtain FAA/AUTHORITY detail part conformity of the test articles and installation conformity of applicable portions of the test setup.
(D) Schedule FAA/AUTHORITY witnessing of the test.
(E) Submit a final test report describing all results and obtain FAA/AUTHORITY approval of the report.
, 1999, has beefinition of Environment. (iii) 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 Advisory Circular AC 20-136B, "Aircraft Electrical and late Synew desins a pration ons this red seersedion previe ersions of
(iv) Certification Plan. The following subjects are not intended to provide a complete list of the items which should be included in the certification plan, but rather highlight some of the areas which should receive consideration. The certification plan should address the total protection which is required to allow the FADEC to continue to operate properly when the rotorcraft experiences a worst-case lightning strike encounter.
(A) 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 Page F - 22
not exist for all lightning strike encounters due to the fact that the rotorcraft may be airborne with little or no airspeed.
(B) Establish the Lightning Environment. Establish the components of the total lightning event to be considered. These are the currents and voltages which are described in the definition of the environment.
(C) 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 § 27.1309(a). Because of the difficulties involved in utilizing this type of an approach, it is generally not used.
(D) Analytical Processes. A description should be given in the certification plan of 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.
(1) The verification testing should be conducted on a system which simulates as closely as possible the installed configuration. As few items as possible of actual hardware should be simulated.
(2) Unless the applicant has opted to follow the guidance in AC 20-136B as a means of showing compliance with § 27.1309(d), 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 SAE 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.
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