Assumptions are essential elements of safety determinations for every aspect of the design, operation, and maintenance of the airplane.  These are expected results that, if all goes as intended, a safe outcome is produced. However, if the part, human, or feature does not perform as it was assumed it would under the specific situation, a completely different, and in some cases, catastrophic outcome can result.

This is the most common of all accident themes and exists in one form or another on nearly all accidents. It involves humans that, in the course of doing their work, make errors that are later shown to have caused, or substantially contributed to the accident. These are human actions that, if done correctly, result in a safe outcome, but if done incorrectly, can result in an accident. It also represents one of the greatest opportunities for advancing safety by the application of targeted interventions which are intended to reduce the risks for human error.

This is where an institutionalized process, procedure, or requirement that allows vital tasks or information to be handled in such a way so as to prevent an accident precursor from being recognized or safety intervention from being initiated. This breakdown could be in the form of a fleet-wide activity that is later found to be deficient, gaps in safety information flowing from one person or organization to another, or key personnel being unaware of an issue because of organizational impediments to the information.

This is a failure condition on a single airplane or possibly a fleet of airplanes that exist, either as a latent condition or an active fault. The failure condition itself may not represent a hazard, but in combination with one or more additional failures or malfunctions, an accident can result.

This is a situation where an initiative, change, new process, or other activity intended to improve something actually produces, in addition to the improvement, an undesirable outcome. The undesirable outcome may not manifest itself for many years and may not be related to the condition being improved, but an undesirable outcome occurs that would not have otherwise happened.  It also underscores the complex interdependence that all actions have the potential for when assessing issues concerning safety.