PEE may incorporate intentional emitters whose emissions may exceed the certified thresholds for aircraft systems interference. However, some larger Defence aircraft may support the establishment of a PEE safe zone that permits the operation of PEE, with only a limited evaluation to confirm that the emissions of the equipment do not exceed the maximum levels associated with the zone. Establishing this zone could reduce the effort required to approve PEE for use. To support the operation of PEE on Defence aircraft, where relevant, PEE safe zones can be established. This Annex provides guidance on design considerations for establishing PEE safe zones to support operational approvals.
Defence aircraft are normally designed to be more immune to electromagnetic radiation than civilian aircraft. Despite this, Defence aircraft may in fact be more susceptible to interference from PEE for the following reasons:
Physical Separation. Electromagnetic energy decreases rapidly with distance, and therefore physically separating PEE from aircraft equipment is an effective means of minimising interference. In civilian aircraft, PEE are almost always used in clearly defined areas (that is, while the passenger is seated), and therefore the physical separation between PEE and aircraft equipment/wiring can be incorporated into the aircraft design. Military aircraft, on the other hand, do not always have clearly defined seating positions, they often have equipment racks and looming within the passenger compartment, and they allow passengers to move freely throughout the entire passenger compartment.
Differences in Role. Unlike civilian aircraft, the mission effectiveness of Defence aircraft is dependent on a wide array of electromagnetically sensitive equipment, for example, electro-explosive devices (EEDs), a profusion of antennas, sensitive role equipment and so on. Interference from PEE can therefore be more than just a ‘nuisance’ factor, and its acceptability may depend on the configuration of the aircraft or even the particular mission phase.
Modifications. Military aircraft are likely to undergo many more modifications than civilian aircraft during their lifecycle. Each modification has the potential to reduce the overall electromagnetic ‘hardness’ of the aircraft. For example, the lack of space within military aircraft often leads to non-optimal equipment locations and routing of wires, new capabilities may incorporate additional antennas which increase the likelihood of interference, and any one poorly designed or incorporated modification can introduce vulnerabilities both in itself and also to other systems.
Notwithstanding the above factors, larger Defence aircraft are likely to support the definition of PEE ‘safe zones’; that is, areas where the risk from PEE is equivalent to commercial aircraft, and therefore commercial airline precautions for PEE may provide a basis for confirming compatibility with the host aircraft (eg non-transmitting PEE may be used freely during all aircraft operational phases). The remaining areas of the passenger compartment may be considered PEE ‘danger zones’ and PEE should not be used unless specifically authorised. These PEE danger zones probably only account for a small portion of the available space in larger Defence aircraft.
Once the PEE safe zone for an aircraft has been established, the operation of PEE during critical phases of flight (if an operational requirement) should be evaluated to confirm that such operation will not result in interference to aircraft systems.
When assessing the interference potential between PEE and aircraft systems, three modes of coupling need to be considered, namely PEE-to-equipment, PEE-to-wiring and PEE-to-antennas. Once the safe distances for each of these modes have been established for a particular aircraft, a PEE safe zone can be defined.
For illustration purposes, worst-case PEE emissions of 1 V/m (within a few centimetres of the PEE) will be assumed.11
PEE to Equipment Coupling. In civil aircraft, the possibility of PEE coupling directly to critical aircraft equipment is discounted, probably because they cannot be collocated on civil aircraft. However, within military aircraft, electronic equipment is often located within, or immediately adjacent to, the passenger compartment. PEE-to-equipment coupling should therefore be considered.
Safety-critical equipment on an aircraft will normally be rated to withstand between 100V/m and 200V/m. Even allowing for significant degradation of the equipment over its lifecycle, susceptibility to PEE emissions for this equipment is still exceedingly unlikely. However, some categories of non-critical aircraft equipment only require testing to 1V/m. In this case, even without allowing for some equipment degradation over the years, there is potential for PEE interference. Consequently, some separation between equipment and PEE would be prudent.
To determine a ‘PEE safe zone’ from equipment, a survey of the equipment in the aircraft passenger compartment would be required, followed by an assessment of the electromagnetic immunity of each piece of equipment. Some testing of the shielding effectiveness of the equipment racks or bulkheads could also be incorporated into the assessment. Adequate safety margins would then be added, and the resulting safe zone defined. If deemed necessary, some limited testing might be conducted to confirm the analysis.
PEE-to-Wiring Coupling. While civil research22 has concluded that harmful interference from PEE is unlikely to couple onto cables, the methodology used to arrive at this conclusion was questionable, and may be a reflection of the limited scope of analysis undertaken in the study. For Defence aircraft, the conducted susceptibility rating of equipment, the shielding effectiveness of applicable wires and the worst-case PEE emissions should be used to derive an appropriate separation distance.
PEE-to-Antenna Coupling. The extremely small signal levels used by aircraft antennas (including connectors and associated cabling) makes them a prime target for interference from PEE. While it would be possible (with substantial difficulty) to predict how PEE might couple to an aircraft’s antenna systems, the results are unlikely to be particularly useful since they rely on a number of independent conditions that are difficult to establish. Realistically, the chance of a PEE insidiously affecting aircraft systems via antenna coupling is extremely low, since there would normally be some outward sign of interference (eg erratic pointer movement, ‘buzzing’ or ‘hash’ over communications and so on). Given that banning all PEE is not a realistic option, the original civil alternative was to ensure that PEE were used only during non-critical phases of flight. However, recent research conducted by the FAA and EASA has concluded that such restrictions are unnecessary, provided that the PEE can be operated in a non-transmitting mode. This may provide an abbreviated approach for assessing the suitability of PEE for use on civil-derivative aircraft.
If transmitting PEE is to be used in a host aircraft, the safe distances from safety-critical and mission-critical equipment and wiring (based on the immunity ratings of the equipment) can be calculated. In addition, a frequency band assessment may be conducted to determine likely interference with the aircraft’s radios and navigation systems. A suitable safety margin should then be added to all analysis figures, and an appropriate area of the aircraft assigned for the transmitter. Source/victim testing should then be conducted, first on the ground and then airborne, to verify the validity of the analysis.
The following provides guidance for assessing PEE for use in the PEE ‘danger zone’:
If the approximate emission levels of the PEE are known (eg it is a common brand name, and marked as tested to a recognised commercial E3 standard) and it will not be used in the vicinity of safety-critical equipment/wiring, little further analysis would probably be required. Source/victim testing might be used if there was thought to be potential for interference with mission-critical equipment or wiring.
If there is any uncertainty in the device’s emission levels it may require testing to a known standard, either commercial or military.
If the device is to be used immediately adjacent to safety-critical equipment or wiring then it should be tested to a stringent level that is known not to interfere with any aircraft systems (eg MIL STD 461E). Similarly, if there is any uncertainty as to where in the aircraft the device might be used, such testing may also be required.
Defence capability needs may require the operation of a PEE of unknown characteristics in a host aircraft. In these circumstances, there may be limited opportunity to conduct even a limited technical evaluation to determine whether the device can be safely operated on the host aircraft. Nevertheless, an operational assessment of the device (using an abridged source/victim test methodology for example) should be conducted to provide some measure of confidence that the device will not pose an unacceptable risk to safe flight.