Appendix 2 APPENDIX 2. DEFINITIONS The following definitions apply to this AC:
TERM DEFINITION Actual Transient Level (ATL) The level of transient voltage or current that appears at the equipment interface circuits because of the external environment. This level may be less than or equal to the transient control level, but should not be greater.
Aperture An electromagnetically transparent opening.
Attachment Point A point where the lightning flash contacts the aircraft.
Component Damage A condition in which transients permanently alter the electrical characteristics of a circuit. Because of this, the component can no longer perform to its specifications.
Continued Safe Flight and Landing The aircraft can safely abort or continue a takeoff, or continue controlled flight and landing, possibly using emergency procedures. The aircraft must do this without requiring exceptional pilot skill or strength. Some aircraft damage may occur because of the failure condition or on landing.
For transport airplanes, the pilot must be able to land safely at a suitable airport. For Part 23 airplanes, it is not necessary to land at an airport. For rotorcraft, the rotorcraft must continue to cope with adverse operating conditions, and the pilot must be able to land safely at a suitable site.
Direct Effects Physical damage to the aircraft or electrical and electronic systems.
Direct attachment of lightning to the system’s hardware or components causes the damage. Examples of direct effects include tearing, bending, burning, vaporization, or blasting of aircraft surfaces and structures, and damage to electrical and electronic systems.
Display Systems Flight, navigation, and power plant instruments required by 14 CFR §§ xx.1303 and xx.1305.
Equipment Transient Design Level (ETDL) The peak amplitude of transients to which you qualify your equipment.
External Environment The natural lightning environment, outside the aircraft, for design and certification purposes. See AC 20-155, which references documents that provide additional guidance on aircraft lightning environment and related waveforms.
Indirect Effects Electrical transients induced by lightning in aircraft electrical or electronic circuits.
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Appendix 2 APPENDIX 2. DEFINITIONS (CONTINUED)
TERM DEFINITION Internal Environment The potential fields and structural IR voltages inside the aircraft produced by the external environment.
Lightning Flash The total lightning event. It may occur in a cloud, among clouds, or between a cloud and the ground. It can consist of one or more return strokes, plus intermediate or continuing currents.
Lightning Strike Attachment of the lightning flash to the aircraft.
Lightning Strike Zones Aircraft surface areas and structures that are susceptible to lightning attachment, dwell time, and current conduction. See AC 20-155, which references documents that provide additional guidance on aircraft lightning zoning.
Lightning Stroke (Return Stroke) A lightning current surge that occurs when the lightning leader (the initial current charge) makes contact with the ground or another charge center, A charge center is an area of high potential of opposite charge.
Margin The difference between the equipment transient design level and the transient control level.
Multiple Burst A randomly spaced series of bursts of short duration, low amplitude current pulses, with each pulse characterized by rapidly changing currents (that is, high di/dt). These bursts may result as the lightning leader progresses or branches, and are associated with the cloud-to-cloud and intra-cloud flashes. The multiple bursts appear most intense when the initial leader attaches to the aircraft. See AC 20-155.
Multiple Stroke Two or more lightning return strokes during a single lightning flash. See AC 20-155.
Return Stroke see Lightning Stroke Structural IR Voltage The portion of the induced voltage resulting from the product of the distributed lightning current (I) and the resistance (R) of the aircraft skin or structure.
Swept Channel The path lightning travels. Because of the aircraft’s motion, the lightning flash causes successive attachments as it sweeps across the aircraft.
System Functional Upset A permanent or momentary problem that affects how the system performs (for example, a change of digital or analog state). Electrical transients cause these problems. Upsets may or may not require manual reset.
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Appendix 2 APPENDIX 2. DEFINITIONS (CONTINUED)
TERM DEFINITION Transient Control Level (TCL) The maximum allowable level of transients that appear at the equipment interface circuits because of the defined external environment.
Upset see System Functional Upset Page A2-3 (and A2-4)
Appendix 3 APPENDIX 3. ACRONYMS We use the following acronyms throughout this AC:
AC Advisory Circular ACO Aircraft Certification Office AFM Airplane Flight Manual ARP Aerospace Recommended Practice ATL Actual Transient Level CFR Code of Federal Regulations ETDL Equipment Transient Design Level FAA Federal Aviation Administration FHA Functional Hazard Assessment ICA Instructions for Continued Airworthiness IR Current (I) x Resistance (R) SAE Society of Automotive Engineers TCL Transient Control Level
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