| | Title | Definition |
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| ILS (Instrument Landing System) | Electronic approach aid which enables a pilot to carry out an approach for landing when weather conditions preclude visual contact with the ground. |
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| IMC (Instrument Meteorological Conditions) | Weather conditions in which visibility is less than specified for visual flying, and in which flight is legally possible only under IFR. |
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| Inadvertent | An inadvertent action may be performed by the pilot who did not mean to do it. (Unintended but demanded operation). This term is normally used to consider the consequences of an unintended action by a crew member (ground, flight or cargo crew) [AMC to CS25.1309]. |
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| Incident | A near miss accident with minor consequences that could have resulted in greater loss.
An unplanned event that could have resulted in an accident, or did result in minor damage, and which indicates the existence of, though may not define, a hazard or hazardous condition.
Sometimes called a mishap. |
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| Initiating Events | Initiating Events; initiator; the contributory hazard; unsafe act and / or unsafe condition that initiated the adverse event flow, which resulted in the hazardous event under evaluation; also see Root Cause. |
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| Inspection | A static technique that relies on visual examination of development products to detect deviations, violations or other problems. |
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| Installation Appraisal. | This is a qualitative appraisal of the integrity and safety of the installation. Any deviations from normal, industry-accepted installation practices, such as clearances or tolerances, should be evaluated, especially when appraising modifications made after entry into service [AMC to CS25.1309]. |
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| Integrity Level | A denotation of a range of values of a property of an item necessary to maintain system risks within tolerable limits. For items that perform mitigating functions,the property is the reliability with which the item must perform the mitigating function. For items whose failure can lead to a threat, the property is the limit on the frequency of that failure. [ISO/IEC 15026, 3.9]
"The system integrity level corresponds to the tolerable level of risk that is associated with the system. A system can be associated with risk because its failure can lead to a threat, or because its functionality includes mitigation of consequences of initiating events in the system |
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| Item | Any level of hardware assembly (system, sub-system, equipment, component, etc) including associated software or firmware [University of York]. |
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| Knot | One nautical mile per hour. Equivalent to 1.853km/h. |
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| Latent | Present and capable of becoming though not now visible or active. |
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| Latent Failure. | A failure is latent until it is made known to the flight crew or maintenance personnel. A significant latent failure is one which would in combination with one or more specific failures or events result in a Hazardous or Catastrophic Failure Condition [AMC to CS25.1309]. |
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| Life cycle | All phases of the system's life including design, research, development, test and evaluation, production, deployment (inventory), operations and support, and disposal [MIL-STD-882D]. |
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| Lifeguard | A term attached to an airliner's radio callsign when the aircraft is transporting time sensitive medical materials/supplies, such as blood plasma, organs for transplant, etc. (Example TWA Flight 800 was known as "Lifeguard TWA800") |
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| Likelihood | Likelihood defines in quantitative or qualitative terms, the estimated probability of the specific Hazardous event under study. Likelihood is one element of associated risk (the other being severity). Fault Trees and other models can be constructed and individual Hazard Probabilities estimated, and likelihood can be calculated via Boolean Logic. |