MOC VTOL.2520 High-intensity radiated fields (HIRF) protection

1.      Scope of this MOC

This MOC proposes simplified methods for addressing High Intensity Radiated Fields (HIRF) compliance demonstration on VTOL capable aircraft. These methods depend on the VTOL capable Aircraft Category; Basic 0 to 1 passenger, Basic 2 to 6 passengers, Basic 7 to 9 passengers and Enhanced.

The topics covered within this MOC are: Minimum Design Requirements, HIRF Group Determination, HIRF Safety Assessment and HIRF Compliance Verification.

2.      Reference Documents

The following references are quoted in different sections of this MOC as a source of additional information or to provide accepted methods and practices:

(a)      Industry Standards

(1)     ASTM

(2)     EUROCAE/SAE/RTCA

(b)     Authorities Guidance

(1)     FAA

(2)     EASA

3.      Definitions

For the purpose of this MOC the following definitions apply:

(a)      Adverse Effect: A response of a system that results in an undesirable and/or unexpected operation of an aircraft system, or undesirable and/or unexpected operation of the function performed by the system.

(b)     Attenuation: Term used to denote a decrease in electromagnetic field strength in transmission from one point to another. Attenuation may be expressed as a scalar ratio of the input magnitude to the output magnitude or in decibels (dB).

(c)      Equipment: Component of an electrical or electronic system with interconnecting electrical conductors.

(d)     External High-intensity Radiated Fields Environment: Electromagnetic RF fields at the exterior of an aircraft.

(e)     Field Strength: Magnitude of the electromagnetic energy propagating in free space expressed in volts per meter (V/m).

(f)      High-intensity Radiated Fields (HIRF) Environment: Electromagnetic environment that exists from the transmission of high power RF energy into free space.

(g)      High-intensity Radiated Fields (HIRF) Test level: The level of Field Strength applied during the Equipment/System Test, it may vary according the RF Band.

(h)     HIRF Certification Level (HCL): The level of an electrical or electronic system that performs a function whose worst Failure Condition classification is catastrophic, hazardous or major.

(i)      HIRF Group: Group of VTOL categories having the same methodology for their HIRF compliance demonstration. 3 Groups have been identified; Group I for VTOL Category “Basic 1” (0-1 passenger), Group II for VTOL Category “Basic 2” (2-6 passengers) and Group III for VTOL Categories “Basic 3” (7-9 passengers) and Enhanced.

(j)      Immunity: The capacity of a system or piece of equipment to continue to perform its intended function, in an acceptable manner, in the presence of RF fields.

(k)      Internal HIRF Environment: RF environment inside an airframe, equipment enclosure, or cavity. The internal RF environment is described in terms of the internal RF field strength or wire bundle current.

(l)      Normal Operation: A state of the system where the system is performing its intended function. When addressing compliance with VTOL.2520 (a) (2), the function whose failure would prevent the continued safe flight and landing for Category Enhanced or a controlled emergency landing for Category Basic should be in the same undisturbed state than before exposure to the HIRF threat. Other functions, performed by the same system, subject to VTOL.2520 (b), are not required to be recovered.

(m)    Radio Frequency (RF): Frequency useful for radio transmission. The present practical limits of RF transmissions are roughly 10 kilohertz (kHz) to 100 gigahertz (GHz). Within this frequency range, electromagnetic energy may be detected and amplified as an electric current at the wave frequency.

(n)     System: The piece of equipment connected via electrical conductors to another piece of equipment, both of which are required to make a system function. A system may contain pieces of equipment, components, parts, and wire bundles.

(o)     Transfer Function: The ratio of the electrical output of a system to the electrical input of a system, expressed in the frequency domain. For HIRF, a typical transfer function is the ratio of the current on a wire bundle to the external HIRF field strength, as a function of frequency.

(p)     Upset: An impairment of system operation, either permanent or momentary. For example, a change of digital or analogue state that may or may not require a manual reset.

4.      Means of Compliance

(a)      Minimum Design Considerations

(1)     In order to utilise the methods described in this practice, the following minimum design considerations should be addressed.  If deviations from these minimum design considerations are desired, the acceptability of the methods described should be agreed to by the Agency.

(2)     The airframe should incorporate low impedance electrical conductors to allow induced current to flow through the aircraft. The low impedance conductors should be incorporated into the basic structure of the aircraft.

(i)      For aircraft with primarily metal structure, the metal skin provides a low impedance electrical conductor. Standard rivets and bolts should provide adequate electrical bonding between permanent structural joints. Electrical bonding straps or jumpers should be installed on moving parts or for removable panels or parts.

(ii)     For aircraft with primarily carbon fibre or fiberglass structure, metal mesh, metal foil, or expanded metal foil should be incorporated onto the external surfaces of the aircraft composite structure. This mesh or foil should be joined together electrically and provide a continuous electrical conductor between the extremities of the aircraft. Metallic components that are internal to the structure of the aircraft may also be used to provide similar shielding for equipment and its wiring.

(iii)     For aircraft constructed of tube and fabric, the tube skeleton can be considered to be the low impedance electrical path through the aircraft. The bonding also may be achieved by the use of bonding straps or jumpers where required to electrically bond other metallic sub-structure that might be relied upon to provide bonding for equipment.

(3)     Electrical bonding specifications and verifications should be developed and implemented on the production drawings and instructions for continued airworthiness.

(b)     HIRF Group Determination

The HIRF Group should be identified by using Table 1; the relevant Group will determine the HIRF Compliance Verification method given in paragraph (d).

Table 1 – HIRF Group Allocation

(c)      HIRF Safety Assessment

(1)     The VTOL capable aircraft systems that require a HIRF Safety Assessment should be identified.  The elements of the system that perform a function should be defined, considering the use of redundant and/or backup equipment that constitutes the system. The process used for identifying these systems should be similar to the process used for showing compliance with VTOL.2510.  This requirement addresses any system failure that may cause or contribute to an effect on the safety of flight of a VTOL capable aircraft.  The effects of a HIRF encounter should be assessed to determine the degree to which the aircraft and its systems safety may be affected.  The operation of the aircraft systems should be assessed separately and in combination with, or in relation to, other systems.  This assessment should cover:

(i)      All normal VTOL capable aircraft operating modes, stages of flight, and operating conditions;

(ii)     All failure conditions and their subsequent effect on VTOL capable aircraft operations and the flight crew; and

(iii)     Any corrective actions required by the flight crew

(2)     A safety assessment related to HIRF should be performed to establish and classify the equipment or system failure condition.  Table 2 provides the corresponding Failure condition classification and system HIRF certification level for VTOL.2520. The HIRF safety assessment determines the consequences of failures, due to HIRF, for the aircraft functions that are performed by the system.  The HIRF Certification Level (HCL) classification assigned to the system and functions can be different from the Design Assurance Levels assigned for equipment function and/or item (software, and complex electronic hardware).  This is because HIRF is an environment that can cause common cause effects.  The term ‘Design Assurance Level’ should not be used to describe the HIRF Certification Level because of the potential differences in assigned classifications for software, complex electronic hardware, and equipment function

Table 2 – HIRF Failure Conditions and System HIRF Certification Level

(3)     The HIRF safety assessment should consider all potential adverse effects due to system failures; loss, malfunctions or misleading information caused by a HIRF threat.  The HIRF safety assessment may show some systems have different failure conditions in different phases of flight; therefore, the HCL corresponds to the most critical Failure Condition.

(4)     In addressing the Failure Condition in Table 2, the nature of HIRF should be considered.  The potential for common causes of failures across multiple equipment/systems performing the same or different functions due to the simultaneous exposure to the HIRF threat should be considered.  Mechanical systems can be considered inherently immune to HIRF and may be used in the safety assessment. 

(5)     In addressing the Failure Condition in Table 2, the effects of HIRF should not be combined with random failures that are not the result of the HIRF threat.

(6)     Due to the similar approach in the safety assessment process related to IEL and HIRF, the System Certification Levels for HIRF and Lightning are usually the same.

(d)     HIRF Compliance Verification

(1)     By applying the ‘Net Safety Benefit’ approach[6] on the lower HIRF Group, VTOL.2520 (b) is not applicable for level C system of HIRF Groups I and II, it could be removed from the Compliance Verification.

(2)     HIRF Groups I and II

(i)      For  Level A Non-Display Systems:

(A)     Follow  AMC 20-158; or

(B)     Conduct Equipment/System testing using the following default levels:

(a)     Conducted susceptibility testing with the Generic transfer function for aircraft (according to VTOL shape and size) extrapolated to the HIRF Environment III (as defined in Section 5) corresponding to the EUROCAE ED-14G section 20 categories Y or W.

(b)     Radiated Susceptibility testing with Generic attenuation curves (depending on equipment location) extrapolated to the HIRF Environment III (as defined in Section 5) corresponding to the EUROCAE ED-14G section 20 categories L, G or F

(c)      Fail/Pass Criteria; when subjected to the HIRF Environment, it could be acceptable that redundant equipment is/are subject to adverse effects, provided that the Level A function is maintained at the aircraft level and all the Equipment/Systems that are required in normal operation recover manually or automatically, in a timely manner, this function after the threat.

(ii)     For Level A Display Systems:

(A)     Follow the AMC 20-158; or

(B)     Conduct Equipment/System testing using the following default levels:

(a)     Conducted susceptibility testing ?with the Generic transfer function for aircraft (according to VTOL shape and size) extrapolated to the HIRF Environment I (as defined in Section 5) corresponding to the EUROCAE ED-14G section 20 categories O or M.

(b)     Radiated Susceptibility testing with Generic attenuation curves (depending on equipment location) extrapolated to the HIRF Environment I (as defined in Section 5) corresponding to the EUROCAE ED-14G section 20 categories G, F or D.

(c)      Fail/Pass Criteria; when subjected to the HIRF Environment, it could be acceptable that redundant equipment is/are subject to adverse effects, provided that the Level A function is maintained at the aircraft level and all the Equipment/Systems that are required in normal operation recover manually or automatically, in a timely manner, this function after the threat.

(iii)     For Level B Systems:

(A)     Follow the AMC 20-158 (by using Equipment HIRF Test Levels 1 or 2 as defined in Section 5); or

(B)     Conduct Equipment/System testing as defined in (d) (2) (ii) (B) (a) and (b); when submitted to the HIRF Environment, if the Equipment/System subject to adverse effects  does not to recover its level B function after the threat, the method proposed by the AMC 20-158 for Level B systems in (d) (2) (iii) (A) can be used as an alternatively.

(3)     HIRF Group III

(i)      For Level A Non-Display Systems:

(A)     Follow the AMC 20-158; or

(B)     Conduct Equipment/System testing using the following default levels:

(a)     Conducted susceptibility testing with the real transfer function of the aircraft (determined by Low Level coupling test, analysis, combination of both or similarity) extrapolated to the HIRF Environment III (as defined in Section 5).

(b)     Radiated Susceptibility testing with real attenuation curves (determined by Low level Testing, analysis, combination of both or similarity) extrapolated to the HIRF Environment III (as defined in Section 5).

(c)      Fail/Pass Criteria; when submitted subjected to the HIRF Environment, it could be acceptable that redundant equipment is/are subject to adverse effects, provided that the Level A function is maintained at the aircraft level and all the Equipment/Systems that are required in normal operation recover manually or automatically, in a timely manner, this function after the threat.

(ii)     For Level A Display Systems:

(A)     Follow the AMC 20-158; or

(B)     Conduct Equipment/System testing using the following default levels:

(a)     Conducted susceptibility with the Generic transfer function for aircraft (according to VTOL shape and size) extrapolated to the HIRF Environment I (as defined in Section 5) corresponding to the EUROCAE ED-14G section 20 categories O or M.

(b)     Radiated Susceptibility with Generic attenuation curves (depending on equipment location) applied HIRF Environment I (as defined in Section 5) corresponding to the EUROCAE ED-14G section 20 categories G, F or D.

(c)      Fail/Pass Criteria; when subjected to the HIRF Environment, it could be acceptable that redundant equipment are subject to adverse effect, provided the Level A function is maintained at the aircraft level and all the Equipment/Systems required in normal operation recover manually or automatically, in a timely manner, this function after the threat.

(iii)     For Level B Systems:

(A)     Follow the AMC 20-158 (by using Equipment HIRF Test Levels 1 or 2 as defined in Section 5); or

(B)     Conduct Equipment/System testing as defined in(d) (3) (ii) (a) and (b); when submitted to the HIRF Environment, if the Equipment/System, subject to adverse effects,  does not to recover to its level B function after the threat, the method proposed by the AMC 20-158 for Level B systems in (d) (3) (iii) (A) can be used as an alternative.

(iv)     For Level C Systems:

(A)     Follow the AMC 20-158 (by using Equipment HIRF Test Level 3 as defined in Section (5); or

(B)     Conduct Equipment/System testing as defined in (d) (3) (ii) (a) and (b); when submitted to the HIRF Environment, if the Equipment/System, subject to adverse effects,  does not to recover to its level C function after the threat, the method proposed by the AMC 20-158 for Level C systems in (d) (3) (iv)(A) can be used as an alternative.

(4)     HIRF Testing for Level A systems considerations;

(i)      The Test Levels for upper HIRF Group can also be used for lower HIRF Groups; for instance the use of real transfer function and attenuation curves and/or more severe External HIRF Environment can be used for Level A Systems of HIRF Groups I/II.

(ii)     Equipment testing is acceptable when it is shown that the interdependencies between equipment performing a function are understood and each equipment is tested and monitored to verify that there are no unacceptable upsets of the function.

(iii)     If similar equipment are used to perform the same function; the test can be limited to a single equipment.

(5)     Level A System architecture consideration; when a Level A system comprises redundant channels/equipment that perform the same level A function, it is  permitted to limit the system to the channels/equipment that are required in normal operation provided that they are not susceptible when they comply with VTOL.2520(a); for instance if it is demonstrated that the primary channels comply with VTOL.2520(a) without the support of the back-up channel, this channel is not requested to be exposed to the HIRF Environment I/III, however this back-up channel should be considered to be a level B system.

5.      HIRF Environments and Equipment HIRF Test Levels

This Section specifies the HIRF environments and equipment HIRF test levels for electrical and electronic systems under VTOL.2520.

(a)      HIRF environment I is specified in the following Table 3:

Table 3 — HIRF Environment I

In this table, the higher field strength applies at the frequency band edges.

(b)     HIRF environment II is specified in the following Table 4:

Table 4 — HIRF Environment II

In this table, the higher field strength applies at the frequency band edges.

(c)      HIRF environment III is specified in the following Table 5:

Table 5 — HIRF Environment III

In this table, the higher field strength applies at the frequency band edges.

(d)     Equipment HIRF Test Level 1.

Equipment Level Test ED-14G (or later Revision) Cat R for both conducted and radiated susceptibility.

(e)     Equipment HIRF Test Level 2.

Equipment HIRF test level 2 is HIRF environment II in table 4 of this Section reduced by acceptable generic aircraft transfer function and attenuation curves. Testing should cover the frequency band of 10 kHz to 8 GHz.

(f)      Equipment HIRF Test Level 3.

Equipment Level Test ED-14G (or later Revision) Cat T for both conducted and radiated susceptibility.