The investigation into the January 8 plane crash in Iran that killed all 176 people on board is likely to attract more attention than most thanks to the ongoing political conflict in the region. Canadian prime minister Justin Trudeau claimed intelligence evidence suggested Ukrainian flight PS752 was hit by a missile, as did unnamed US officials speaking to journalists. Iran initially denied the plane was hit by its air defences but has since claimed it unintentionally shot down the craft.

The country also made a point of stating that, as is standard practice for air crash investigations, it would not hand over the plane’s black box flight recorder to aircraft manufacturer Boeing or the US authorities. But the US has since accepted an invitation from Iran to take part in the investigation, and Iran’s Civil Aviation Authority head has reportedly said it may need help decoding the black box data.

Aircraft investigations are covered by a set of international standards and recommended practices produced by the UN’s International Civil Aviation Organisation. The aim to understand what has happened and why, but more importantly, to also produce recommendations to prevent future accidents. Here’s how we should expect the investigation to proceed.

Who takes part?

The standards, known in the aviation industry as “Annex 13”, determine that Iran, as the country where the incident occurred, will lead the investigation. It will nominate an investigator in charge, someone who normally works for an independent aviation or transport accident investigation agency and so should be able to conduct an impartial inquiry.

Other countries entitled to take part include those where the plane was designed and made (the US) and where it was registered and operated from (Ukraine). Countries with a significant number of citizens involved in the incident can also participate, which in this case will be Canada, Sweden and Afghanistan. These countries will all send “accredited representatives” from their respective aviation or transport safety investigation agencies.

The investigator in charge can also draw on technical specialists. It would be normal, for example, for investigators from Boeing to be part of the team but always working with the investigators’ national agency, to ensure transparency and fair play.

The investigator in charge controls and decides what counts as evidence. If there is evidence of a criminal act at any point, then the safety investigation may hand over the inquiry to law enforcement. This was the case with the Pan Am 103 bombing over Lockerbie in 1988 where it became clear that it was not an aircraft accident, but rather an act of sabotage. In that case, a safety investigation continued in parallel to the criminal investigation but focused on aircraft and passenger survivability.

Black boxes

Black boxes are a key source of evidence in most aircraft incidents. An aircraft, such as the one involved in the Iranian crash, would carry a cockpit voice recorder covering up to two hours of audio from the flight deck. It would also contain a digital flight data recorder that would record hundreds of parameters over 25 hours of operation. This could include engine settings, the positions of flight controls, warnings and so on.

The recorders are designed to withstand impact, high temperature and immersion in water, but are not indestructible. They also depend on receiving power from the aircraft and data from a range of sources across the aircraft. The first generations of recorders used wire and magnetic tape that could collect a limited amount of data and were quite vulnerable to damage. But modern recorders use solid-state memory chips in a crash-survivable memory unit.

An undamaged recorder can be read out relatively simply using equipment from the manufacturer of the recorders, but in the case of a damaged recorder, investigators need to be very careful to ensure they do not corrupt the data. They also need to be careful to protect their chain of evidence from any accusations of tampering.

For this reason, they would never be sent to the aircraft manufacturer (in this case Boeing) for read-back. This is a task that would be done by a state safety investigation agency. Yet while most states have such an agency, only some have data recorder read-back facilities and even fewer have experience in dealing with badly damaged recorders.

If the country leading the investigation does not have the technical capability in-house then they are likely to ask another country, either one involved in the investigation or an outside country. Or they might ask a “technical adviser”, an organisation or person with specific expertise, who could specialise in, for example, the recovery of data from damaged memory chips.

If a recorder is to be examined in a different country, it will be taken there by one of the investigation team. They will agree on a method of work, record the opening of the recorder’s protective casing, and be present throughout the examination. If data can be recovered easily then it may be possible to examine the recorded data relatively swiftly.

In the case of the cockpit voice recording, there are strict protocols on who can listen. This is partly to protect the pilots, as the recording is solely for safety purposes and should not be used as evidence against them. This is a principle established because many pilots feared the recorder would be a “spy in the cockpit”. Another reason for limiting who listens to recordings is that they may contain very traumatic sounds associated with the last moments of those on board.

Following the loss of flight PS752, it was correct for the Iranian authorities to say that they would not hand over the recorder to Boeing. If Iran has the technical capability within its safety investigation agency, then they would be following protocol to analyse them. If not, then they are entitled to approach, or accept the assistance of another capable safety investigation agency or technical adviser. In this case, the US and Canada both have a high level of expertise in the examination of flight data recorders.

What is of critical importance is that the recordings are not damaged by the read-back process. A chain of evidence must be preserved to ensure the integrity of the investigation and that the recordings are not released without the express permission of the investigator in charge.

This article has been updated to reflect Iran’s admission of unintentionally shooting down the plane.

Graham Braithwaite, Director of Transport Systems, Professor of Safety & Accident Investigation, Cranfield University

This article is republished from The Conversation under a Creative Commons license. Read the original article.