The Investigation Process Research Resource Site
A Pro Bono site with hundreds of resources for Investigation Investigators
Home Page Site Guidance FAQs Old News Site inputs Forums
. . . .. . . . . . . . last updated 8/8/09


to advance the
State-of-the-Art of
investigations, through
investigation process

Research Resources:

Search site for::

Launched Aug 26 1996.

  Extracts from ICAO Document 6920 Manual of Accident Investigation 1970
for Experimental On-line Investigation Research Project study.
Document available from ICAO at

Note focus on what caused the accident, rather than who caused it(p1).
Note how scope of investigation is defined(p2,4,5).
Note chain of events framework for accident phenomenon (p2).
Note restatement of purpose (P3)
Note investigation structure prescribed in Purpose chapter(p3):
Note Cause determination by exclusion approach (p6)
Note Quality Assurance approach (p6)
See also Annex 13 reference to <investigation objectives



The fundamental purpose of inquiry into an aircraft accident is to determine the facts, conditions and circumstances pertaining to the accident with a view to establishing the probable cause thereof, so that appropriate steps may be taken to prevent a recurrence of the accident and the factors which led to it. An equally important purpose is to determine the facts, conditions and circumstances pertaining to the survival or non—survival of the occupants, and the crashworthiness of the aircraft. The nature of the inquiry into an aircraft accident should not be accusatory as the object is to take remedial rather than punitive action; similarly the assessment of blame or responsibility should not be included in the duties of an aircraft accident investigation authority since this function is normally the prerogative of the judicial authorities of the State concerned. Nevertheless, it is unavoidable that acts or omissions, by individual persons or organizations, are sometimes clearly revealed and in such instances it is the duty of the inquiry to say so. Any such statement should not confuse the purpose of the aircraft accident investigation which is primarily to indicate what caused the accident rather than who caused it: this should rightly be for others to decide.

Aspects of safety totally unconnected with the circumstances or chain of events leading to the accident are often revealed during the investigation with a resultant benefit in terms of effective accident prevention. Investigators should not he inhibited from investigating these matters or from drawing attention to them merely because they are not related to the cause of the accident.

Reduced to simple terms, the investigator has to determine what happened, how it happened, and why it happened, applying these questions not only to basic cause but to all aspects relating to safety including survival of occupants. In doing this he must seek out, record and analyse the facts, draw conclusions and, where appropriate, make recommendations.

The basic cause of an accident and the remedial action necessary to ensure that it will not recur does not always emerge from the physical facts of the case. For example, a failure of some mechanical part may be due to a failure to inspect or faulty inspection technique in a factory or a maintenance shop where the defective part should have been detected thereby preventing its failure in service. Similarly, if human error appears as a possible cause of the accident all factors which may have influenced the actions should be examined. The inquiry should not cease if or when it is established an error has been made: the inquiry should endeavour to establish why the error occurred. Poor design, indifferent human engineering, inadequate or improper operational procedures could well have confused or misled the person. Experience has shown that the majority of aircraft accidents have been caused or compounded by human error, often by circumstances which were conducive to human error; this applies to design, manufacture, testing, maintenance, inspection and operational procedures both ground and air. Identification of this element is frequently difficult but it may be revealed by careful, skillful and persistent investigative methods

Some aircraft accidents have resulted from organizational defects or weaknesses in management: for example, an operator may have prescribed or condoned procedures not commensurate with safe operating conditions. In practice, similarly ambiguous instructions, and those capable of dual interpretation may also have existed; these factors may well have stemmed in the first instance from uncritical scrutiny by regulating authorities. It may therefore be necessary to inquire closely into other organizations or agencies not immediately or directly concerned with the circumstances of the accident but where action, or lack of it, may have permitted or even caused the accident to happen.

Where the cause of an accident is obscure it may be necessary to pursue as many hypotheses as could seriously be regarded as possibilities and each pursued to the limits of its usefulness, or to the limit where it can be excluded as a possibility. This approach will often result in some degree of speculation and prolonged exploration but it may be the only course open to the investigator. By carefully considering each possibility in the light of the evidence adduced, and the existing state of aeronautical knowledge, a number of the hypotheses will be eliminated: the credibility of those which survive the process is thereby increased and experience has shown that these will generally relate to one particular area or group of possibilities. Findings which have been arrived at by more than one line of inquiry, by more than one person each reasoning independently, are more likely to be correct than those conclusions arrived at by pursuing one narrow field of activity.