Aviation Investigation Classification

​The NTSB investigates all civil aviation accidents and select incidents to:

  • Determine the facts, conditions, and circumstances relating to an accident;
  • Determine one or more probable causes; and
  • Issue safety recommendations to prevent or mitigate the effects of a similar accident.  (See 49 CFR 831.4.)​

We begin by assessing available facts to determine the appropriate investigative response. Following an initial assessment, our investigators determine the level of response and work with those who may be affected by, involved in, or have pertinent information about the accident or incident. An investigation will result in a report of the facts and the NTSB's analysis and probable cause of the accident or incident. We may also produce other products designed to improve transportation safety such as safety recommendations, safety alerts, or other safety information.

The NTSB's primary focus is to improve safety for the travelling public. We do not conduct investigations for the purpose of determining the rights, liabilities, or blame of any person or entity. The aim of all NTSB investigations is to prevent the recurrence of other accidents and incidents.  We carefully consider the level of detail necessary for each investigation with the aim of concentrating resources on investigations that are most likely to enhance aviation safety while still fulfilling our mandate to investigate all civil aviation accidents. Because many accidents have similar causes and may not provide new safety information that would result in further action, investigating these in detail may not be justified given our limited resources. Thus, our investigators do not launch to the scene of every accident, conduct an in-depth investigation, or produce an extensive report for many accidents with known circumstances.  

We have a five-part classification system for investigation depth and final report for each event (accident or incident). The initial classification of an investigation is subject to change depending on the safety issues that are identified throughout the course of the investigation. 

Investigation Classes and Description

Investigation Class  Description  
​Class 1  
  • Class 1 investigations are reserved for very significant accidents and are likely to involve significant NTSB and external resources. They generally involve transport category aircraft and commercial operations.
  • A large team led by an investigator-in-charge conducts the investigation. NTSB subject matter experts form and lead multiple investigative groups to identify and address safety issues. The investigative team will likely travel to and spend considerable time at the scene of the accident and will likely conduct follow-on investigative activities.
  • Investigation updates or interim reports may be released during the investigation. Following a final technical review of the gathered facts and analysis, a final comprehensive report will be made available to the public. The Board members will deliberate the findings, probable cause, and recommendations accompanying the report at a public “sunshine" meeting. 
Class 2  
  • Class 2 investigations generally have a broad scope and involve a significant effort collecting evidence across several investigative areas and a substantial investment of resources. These investigations may involve:
    • very complex systems and/or processes,
    • multiple organizations, or
    • poor risk controls implemented by the operator, manufacturer, maintainer, and regulator.
  • Class 2 investigations may have a response similar to a Class 1 investigation but upon further examination, the safety issues may be more limited, specific to a certain airplane type or operation.
  • Following a final technical review of the gathered facts and analysis, a final comprehensive report will be made available to the public. The board may deliberate the findings, probable cause, and recommendations accompanying the report. 
Class 3 
  • Class 3 investigations seek to identify safety issues that reveal underlying cause(s) of the accident.
  • The investigation is led by an investigator-in-charge, who may be assisted by other NTSB subject matter experts if the investigation requires an in-depth focus on a specific area. The investigative team may travel to the scene of the accident or to other follow-up activities, or the entirety of the investigation may be conducted remotely.
  • Following a final technical review of the gathered facts and analysis of those facts, a final report will be made available to the public following Board approval that identifies the probable cause(s) of the accident and factors that contributed to the outcome of the accident, if any.
  • Findings from these investigations may be used to support recommendations, or the investigative team may work with industry stakeholders directly to resolve safety issues identified during the investigation. Accidents that involve recurring safety issues may be accompanied by a safety message that includes practical strategies to avoid future accidents.  
Class 4   
  • Class 4 investigations seek to identify the cause of the accident.
  • An investigator-in-charge leads the investigation. In some cases, the investigator-in-charge may travel to the scene of the accident, but class 4 investigations are generally all conducted remotely.
  • Class 4 investigations are of the shortest duration and a final report will be made available to the public that identifies probable cause(s) following Board approval and within 6 months. The report only examines the actions and conditions directly relating to the accident, and the documented sequence of events and probable cause reached is simple and straightforward.
  • The investigator-in-charge may work with industry stakeholders to develop solutions to safety issues identified during the investigation. Investigations that involve well known circumstances may be accompanied by a safety message that includes practical strategies to avoid potential future recurrence. ​