Electromagnetic Effects

This section presents important design considerations for controlling electromagnetic effects on the aircraft and equipment. These design techniques are presented in elementary form to emphasize the importance of each design. In the real world, various combinations of these techniques should be used.

Two design levels are considered: aircraft level and equipment level.

Aircraft level

Aircraft-level designs may be grouped into two categories:

• • control of electromagnetic effects in general
• • control of effects due to HIRF and lightning

General Design

Equipment location

Two pieces of equipment performing completely different functions may interfere with each other, based on their signal characteristics, operational frequencies, etc.

One of the most important factors in achieving EMC is the bonding and grounding of electrical and electronic equipment cases to the conductive airframe.

The most effective termination is a 360-degree peripheral bond between the braid and a connector back shell. The peripheral termination configuration ensures a low-impedance electrical bond and, therefore, potential EM coupling is minimized.

Use of composite structures on aircraft has become more widespread to reduce weight. In addition to the aperture effects, the use of composites in certain areas of the aircraft can result in other coupling concerns. Specifically, if a carbon composite structure is located on the aircraft skin, the partial conductivity of the material may allow current to flow through. However, because of its high resistance, the resulting voltage drop across the structure can be significant. This voltage can induce a similar voltage on cable bundles installed under the composite structure. If the potential voltages are too high for certification due to composite usage, additional protection is necessary. The protection technique may be incorporated on the wiring (such as shielding), or on the composite material (such as by “metallizing”). Such protection could negate the weight saved by using the composite in the first place, or even add weight over that which would be had if the structure were metal.

When lightning strikes an aircraft, the current flows through the airframe. Typically, many structural members make up the electrical path for the current. Designers should ensure that the method(s) used to join these members (for example, fasteners) will properly complete the electrical path. Otherwise, ionization of the air gap between members occurs, possibly creating an arc and damaging the materials. One must also ensure the method of joining the structure members can carry the lightning current. Otherwise, damage to the interfaces of the members can occur.

As aircraft incorporate more electronic parts and make greater use of nonmetallic structural materials, systems become more vulnerable to the effects of electromagnetic fields and sudden electric surges associated with lightning strikes. As a result, certification requirements are becoming tougher, while standards are still evolving in this field. Therefore, cognizant engineers must be very alert to certain issues including: (1) internal and external environments, (2) qualification and certification, (3) design techniques, and (4) when to visit your friendly EMC lightning engineer.

In aircraft design we consider three different elements of the electromagnetic (EM) environment.

1. 1. Electromagnetic interference and compatibility. Electromagnetic interference (EMI) refers to the electromagnetic emissions of a system or systems. Electromagnetic compatibility (EMC) refers to the condition that no component on the aircraft creates electric or magnetic effects that cause any other component to fail to operate properly.
2. 2. High intensity radiated fields (HIRF). High intensity radiated fields refers to an EM environment generated by high power transmitters, such as the Voice of America. These fields can be extremely strong and there is a real potential for adverse effects.
3. 3. Lightning. Lightning strikes of aircraft occur regularly. Strong currents flow through the airframe during a strike. These currents can damage external structures and create transients in wiring of electrical/electronic systems. Transient currents can flow throughout the aircraft. Therefore, all structures and safety-related electrical/electronic systems must be properly protected.

When addressing the electromagnetic environment of an airplane there are two zones, external and internal, to be considered.

The external electromagnetic environment consists of two components: HIRF and lightning.

The internal electromagnetic environment may be divided into four parts:

1. 1. Intersystem: Emissions existing between systems.
2. 2. Intrasystem: Emissions internal to an individual system or component.
3. 3. HIRF-induced: Interference caused by exposure to an HIRF external environment.
4. 4. Internal lightning transients: Induced transients resulting from a lightning strike.

General FAA/JAA Qualification/Certification Requirements

The following basic statements apply:

• EMC
  Electrical equipment, controls, and wiring must be installed so that operation of any one unit or system of units will not adversely affect the simultaneous operation of any other electrical unit or system essential to safe operation.
• HIRF
  Systems performing critical and essential functions must operate safely during and after exposure to an HIRF environment.
• Lightning
  An aircraft must be able to continue its flight and land safely after being struck:
  • • Fuel vapor must not be subjected to potential sources of ignition resulting from a lightning strike.
  • • Systems performing critical functions must operate safely during and after a lightning strike.
  • • Systems performing essential functions must operate safely after a lightning strike.
  • • Damage to structural members must not affect flight safety.
  
  The SAE committee has been working on the requirements for HIRF certification for many years. The proposed environment and requirements are still evolving.
  The SAE Draft Advisory Circular outlines some of the compliance requirements:
  1. 1. Appropriate (safe) operations of electrical and electronic systems must be maintained during aircraft exposure to HIRF.
  2. 2. Critical systems must be evaluated with respect to the severe environment envelope.
  3. 3. Essential systems must be evaluated with respect to the normal environment envelope.
  
  Aircraft fuel systems represent one of the most critical lightning protection concerns. This section addresses important design considerations to minimize the effects of a lightning strike.
• Flame arrestors
  Flame arrestors exist to prevent ground fire; they do not ensure protection from ignition during lightning strikes. However, if they are used, the best location for a flame arrestor seems to be at the surge tank end of the vent outlet pipe.
• Structural joints
  Structural joints can be subject to arcing. It is possible for such a lightning arc to remain attached to a single rivet or fastener. When that happens, the arc can melt the rivet or fastener and the surrounding skin that, in turn, can ignite fuel vapors. The following protection guidelines should be considered during the design stage of the integral tank joints:
  1. 1. Keep the current density in the fastener low. This may be accomplished by using fasteners as large as possible, to maximize the contact area between the fastener and the joined surfaces.
  2. 2. Arcing may be reduced or eliminated if a current path is allowed to be shared by several fasteners. With a large number of fasteners in the current path, the current in any one fastener will be lower. It is important to note, though, that the current will not distribute equally, since the overall lightning current density will diminish as the distance from the attachment point increases.
  3. 3. As a back-up measure, coating joints and fasteners per DPS with fuel tank coatings and sealants can contain any arcs or sparks that may occur.
  
  An improved design for lightning protection is use of good electrical bonding practices at the interfaces. This bonding can be accomplished by treating the surfaces according to Douglas Process Standard (DPS) 1.834-25 and/or by providing bare metal-to-metal contact via rivets or fasteners. The optimal design is to break the current path to the fuel tank by inserting a section of nonconductive tubing between the aircraft skin and the fuel tank wall. This forces all of the current to flow on the aircraft skin and thus eliminates the possibility of sparking at the fuel tank interface.
• Access doors
  To prevent sparking from a lightning strike, an access door should contain the following design features:
  1. 1. Avoid metal-to-metal contact between the door and the door-land interfaces that are exposed to fuel vapor.
  2. 2. Provide adequate current conduction paths between the interfaces shim and the surrounding skin, away from fuel vapors.
  3. 3. Apply sealant to other potential arc or spark sources to prevent contact with fuel vapors.
• Pipes and couplings
  The mating surfaces in pipe couplings are generally anodized coated. These anodized surfaces would seem to prevent arcing across the pipe interfaces. However, relative motion between this surface can wear through the coating and form a poor conductive path that could cause arcing.
  Electrical bond straps or jumpers are generally installed across conducting pipe couplings. These bond jumpers can be adequate for static discharge, but they should not be relied upon to prevent sparking due to lightning currents.
  One solution to the problem of arcs and sparks at couplings and plumbing interfaces is to insert electrically nonconductive isolation links into these lines. This method can eliminate lines as current-carrying paths. Some couplings have now been designed and tested to verify that they can withstand currents up to thousands of amperes. These couplings should be adequate for use in most metal tanks.