Table of Contents
4. Aircraft Accident Investigation
Aircraft accident investigation requires a wide range of technical knowledge that makes it possible for the person(s) in charge of an investigation to utilize the many different fields, coordinate them, and evaluate the results. It is not necessary for the investigator in charge to be an expert in the individual technical fields, but he or she must be able to understand the “language” of the experts.
This wiki attempts to assist in the learning of technical terms and impart experience in fields related to turbine engines. These fields include engine technology with typical problems and weak points, strength and vibration technology, material technology, manufacturing technology, and non-destructive testing.
Useful reports are a prerequisite for analysis of accidents (Chapter 4.1). Investigators who know where to look in order to recognize important characteristics have a deciding advantage. This means that, if possible, the investigator should have experience with the affected engine type.
Consultations with the OEM and, if applicable, with any involved repair shops, are an important source for understanding technical relationships and any characteristics specific to the engine parts (Ref. 4.1-1). Information from overhaul handbooks is also valuable since it refers to, for example, weak points and typical damages, as well as possible use limitations due to these weak points and damages. Important
aspects can be understood from technical discussions with specialists from the manufacturer. Controversial discussions should be especially sought, since they present the opportunity for critical revision of one`s own hypotheses. Of course, the investigator must also be aware of the interests of the discussion partners, which is important to consider when evaluating their statements. Since “physics is the same for everyone”, in technical discussions, the presence of influences and effects is less debatable than the level of their contribution to the damage.
Determining the operating data of an engine is made easier by the usual flight recorders and the extensive sensor systems (at least in commercial aircraft). In military aircraft, e.g. with fighters, this is often not the case. In these situations, the investigation must be based entirely on inspection of the damage symptoms of the engine (Chapter 4.2), (if available) new resistant electronic memory, and the testimony of the pilot.
Reconstruction of the damage process (Chapter 4.3) is done through evaluation and assessment of the findings, determining if, for example, the engine was actually the cause of the accident, formulation of damage hypotheses, and testing these hypothesis against the facts of the reports.
If they are necessary for understanding the damage process, reconstruction tests for single effects can be very useful. These can be special engine part tests (Chapter 4.4) and/or computer simulations.
In order to impart experience, Chapter 4.5 gives suggestions for reports in connection with damage sequences, and also uses examples to explain special engine part behavior.
At the end, there are several remarks for the investigator responsible. These suggestions apply to all damage analysts and are a prerequisite for achieving the most accurate possible results within the framework of the findings.
In addition to the above-mentioned technical expertise, an investigator must be critical and self-critical. Data and findings that have been supplied, as well as the investigator`s own findings, must be intensively checked for coherence. It is best to take all possible explanations, even the seemingly improbable ones, and to evaluate the probability and plausibility (from a necessarily subjective standpoint) of each in a comprehensible manner. If discrepancies or inconsistencies are found, then these must be listed even if they put the investigator`s “favorite” hypothesis at risk (often, small discrepancies are decisive). An engineer who was otherwise excellent in his area of expertise once told the author, “I have never had to retract any hypothesis.” This type of statement disqualifies him as a damage analyst or accident inspector. It is vital to continually critically check the hypotheses against the current state of the findings. This should be done with the following adapted judicial maxim in mind: if in doubt, the hypothesis is not correct. Just as with evidence in a court case, experience has shown precisely that often apparently minor discrepancies are, in fact, important indicators of the actual damage sequence. When checking the hypotheses with regard to the facts, an important step in every systematic damage analysis (Fig. "Analysis") is to disqualify any hypothesis that contains a (!) contradiction to the facts at hand.