1.0 Purpose and Scope
The purpose of an aircraft accident safety investigation is to prevent future accidents and incidents—not to assign blame or legal liability. ICAO Annex 13 provides the international standards and recommended practices for this work. ICAO Annex 13 overview
This document explains the general investigative sequence rather than serving as a field manual or legal compliance checklist. National procedures, investigative scope, and resources vary. The U.S. National Transportation Safety Board (NTSB) process is used as a practical example. NTSB investigative process
2.0 Investigative Authority and Participation
For international civil aviation accidents, the country where the accident occurs—the State of Occurrence—normally leads the investigation and controls the release of investigative information. When U.S. registration, operation, design, or manufacture is involved in a foreign accident, the NTSB may participate through an accredited representative supported by technical advisers. NTSB international participation
In NTSB investigations, designated organizations may contribute technical expertise through the party system. Safety investigation is distinct from criminal investigation; if an event is determined to be a criminal act, law enforcement becomes the lead investigative body. NTSB roles and party system
3.0 Notification and Initial Response
The investigation begins with notification to the responsible authority and a decision about the scope of the response. In the United States, NTSB leadership determines whether to launch investigators or a specialized team. The nature and complexity of the event influence the resources assigned. NTSB launch decisions
Early investigative actions emphasize personnel safety and preservation of evidence. The aircraft, wreckage, and contents must be protected against further damage, unauthorized access, loss, and deterioration. Evidence that could disappear or be disturbed should be documented by photographs or other appropriate means. ICAO planning and site safety guidance · ICAO evidence protection guidance
4.0 On-Site Examination and Fact Gathering
Investigators collect evidence at the accident site and from sources away from the scene. NTSB examples include flight logs, maintenance records, interviews, weather information, and engine-performance data. ICAO technical guidance also addresses wreckage, structures, systems, flight recorders, and aircraft performance. NTSB fact gathering · ICAO technical investigation guidance
Principal Evidence Areas
Aircraft and wreckage: Examine structures, systems, and relevant components to support reconstruction of the event.
Recorded information: Recover and arrange prompt specialist readout of available flight recorders.
Operational records: Collect flight logs and maintenance documentation.
Interviews: Gather accounts from people with relevant information.
Environment and performance: Analyze weather and aircraft or engine-performance information.
These evidence areas summarize NTSB and ICAO guidance; the applicable work depends on the accident and available evidence. NTSB process · ICAO investigation manual topics · ICAO recorder readout guidance
5.0 Technical Analysis and Event Reconstruction
Specialists analyze the collected evidence to reconstruct the sequence of events and determine why the accident occurred. Analysis may identify gaps that require additional records, examinations, or manufacturer information. Fact gathering and analysis therefore overlap rather than proceeding as a strictly linear sequence. NTSB analysis phase
In some NTSB cases, an investigative hearing is used to establish additional facts. Findings then support a draft report describing the accident, the analysis, and the determination of probable cause. NTSB hearings and draft reports
How Investigators Determine Probable Cause
Probable cause is the investigation authority’s conclusion about what caused the accident, based on its analysis of the facts, conditions, and circumstances. The NTSB may identify one or more probable causes and factors that contributed to the outcome. This is a safety finding, not a determination of blame or legal liability. NTSB investigation objectives and findings
The following steps explain the reasoning process. They combine the NTSB’s description of event reconstruction and report approval with the Australian Transport Safety Bureau’s (ATSB) published analysis methodology; they are not a universal checklist or a statement that both authorities use identical terminology.
Reconstruct what happened. Investigators organize the evidence into a sequence of events. They compare recorded data, interviews, physical evidence, operational records, and specialist reports to identify agreements, contradictions, and gaps. This establishes the factual basis for explaining the accident. NTSB event reconstruction · ATSB evidence interpretation
Test possible explanations. Investigators develop hypotheses and examine the evidence supporting and opposing each one. Component tests, detailed data analysis, simulations, and expert review may help assess whether a proposed explanation fits the evidence. ATSB methodology explicitly tests whether a factor existed and whether it influenced the event. ATSB hypothesis testing
Identify causes and contributing factors. The analysis connects supported findings to the accident sequence. It distinguishes factors that contributed to the occurrence from other conditions that increased risk. NTSB reports can identify probable causes and contributing factors; ATSB reports distinguish contributing factors, other risk-increasing factors, and other findings. NTSB report findings · ATSB safety-factor analysis
Address uncertainty and missing evidence. Incomplete or contradictory information can prompt further evidence collection and analysis. Not every detail can always be resolved. For example, in its Montebello accident analysis, the NTSB identified likely occupant incapacitation associated with hypoxia but could not determine why cabin pressurization was lost or how quickly the loss developed. This illustrates the distinction between a supported explanation and an unresolved detail. ATSB evidence limitations · NTSB Montebello analysis
Review and approve the conclusion. The investigative team documents the evidence and reasoning in a draft report. The NTSB’s approval process involves the responsible office director under delegated authority or adoption at a public Board meeting. The published probable cause therefore reflects the authority’s reviewed conclusion, not an early theory from the scene. NTSB report approval process
Key distinction: Event reconstruction explains what happened; causal analysis explains why it happened. The report should make the connection between the evidence and the findings clear, including the limits of what could be established. NTSB analysis phase · ATSB safety analysis and findings
Hypothetical Example: Comparing Two Explanations
Fictional training scenario—not an actual accident or official finding. An aircraft loses engine power shortly before an accident. Investigators consider two explanations: an interruption of fuel supply or an internal mechanical failure. All observations and test results below are invented solely to illustrate evidence-based reasoning.
Evidence supporting fuel-supply interruption: In this fictional scenario, recovered recorded data show a drop in fuel pressure before power decreases. Examination reveals a blockage in the fuel-supply path. A controlled test reproduces the pressure drop and loss of power with the blockage present; removing it restores normal operation. Together, these observations support a link between the blockage and the power loss.
Evidence weakening mechanical failure: Examination finds no evidence of a pre-impact internal engine failure, and the engine operates normally during the controlled test once the blockage is removed. These results weigh against the proposed internal mechanical failure. They do not establish that every possible intermittent fault has been ruled out.
Evidence still needed: Investigators must determine whether the blockage existed before impact or arose during the accident or recovery. If its timing cannot be established, its discovery alone cannot justify identifying it as the cause of the in-flight power loss.
Illustrative conclusion: If the blockage is shown to have existed before impact, the combined recorded, physical, and test evidence would support fuel-supply interruption as the explanation for the engine-power loss and weigh against the mechanical-failure explanation. Without that timing evidence, the explanation remains provisional. Explaining the power loss would also not, by itself, explain the entire accident; the remaining sequence and contributing conditions would still require analysis.
6.0 Final Report and Public Release
For NTSB investigations, the draft report proceeds to approval by the responsible office director under delegated authority or adoption at a public Board meeting. After adoption, the final report is prepared for public release. In a foreign-led investigation, the investigating state—not the NTSB—releases the final report. NTSB report approval · Foreign investigation reporting
ICAO reporting guidance addresses report format and content, consultation, release, distribution, and safety recommendations. Readers should distinguish established findings in an adopted final report from information being developed during an ongoing investigation. ICAO reporting guidance
Understanding the Timeline
There is no single duration for every investigation. The NTSB generally aims to complete investigations within 12 to 24 months, but complexity and investigator workload can substantially affect timing. This is an agency target, not a guaranteed deadline for an individual accident or a universal international rule. NTSB timing guidance
7.0 Safety Recommendations and Follow-Up
The process continues beyond determining probable cause. Investigations can lead to safety recommendations addressed to regulators, manufacturers, operators, and other relevant organizations. NTSB staff track recipients’ responses, monitor progress toward adoption, and advocate for action on the identified safety issues. NTSB safety follow-up
Process in brief: Notification and launch → evidence gathering → analysis → report approval and release → safety recommendations and follow-up. These stages overlap, and new evidence may send investigators back to earlier activities. NTSB process summary
8.0 Reference Sources
The following official sources were consulted on November 10, 2026. This overview is educational; consult the responsible investigation authority and applicable rules for a specific event.
ICAO — Annex 13: Aircraft Accident and Incident Investigation
ICAO — Accident Investigation Section: Documents and Manuals
ICAO — Aircraft Accident and Incident Investigations: Evidence Preservation and Technical Coordination
NTSB — The Investigative Process
NTSB — Role in Foreign Aviation Investigations
NTSB — Aviation Investigation Classification
ATSB — Investigation Phases
NTSB — Montebello, Virginia: Final Report ERA23FA256