Explainer: what will MH17’s black boxes reveal?

A separatist soldier hands an MH17 black box to a Malaysian delegation. EPA/Robert Ghement

The news that the black boxes from Malaysia Airlines flight M17 has been handed over to Malaysian authorities, will lead to many questions about what information they have to offer.

The two recorders have been in the hands of pro-Russia rebels in the Ukraine for at least some of the four days since the aircraft was shot down over Ukraine. The data inside should give a strong indication about what happened to the aircraft but the question now is whether that data has been tampered with.

What’s in a black box?

All commercial aircraft with a take-off weight over 5,700kg are required to carry a flight data recorder (FDR) and a cockpit voice recorder (CVR) – referred to as the black boxes.

The equipment is designed to withstand the forces of any crash impact, subsequent fire damage and water contamination from being submerged at the bottom of an ocean. That means that there is a good chance of recovering the data at the MH17 crash site.

Nowadays, the black boxes on most aircraft are digital, so that all recorded data is stored in a memory, not unlike a modern camera that stores its images in an internal memory or on a memory card. Before take-off, the equipment is reset and typically stores two hours of data which is then progressively overwritten during the flight, so that it will contain two hours of data stored prior to an accident.

The cockpit voice recorder records all communications on the flight deck, including all transmissions with air traffic controllers, discussion between the flight crew, cabin announcements and conversations with any other crew entering the cabin. This information is carefully time-stamped and after an accident can provide valuable insight into the procedures followed and, to some degree, the thinking of the flight crew as they deal with an emergency.

The flight data recorder records the essential flight data parameters. The data is recorded at least 10 times per second and defines the aircraft’s flight path and motion. There are between 50 and 100 parameters recorded including the primary information: position, altitude, airspeed and heading to enable the flight path to be reconstructed. The data also includes other essential aerodynamic and engine parameters, including information derived by the air data computer from sensors and the engine data from the FADEC engine control subsystem.

The pilot primary controls, secondary actuator positions (selectors for flaps, undercarriage and fuel) and avionics settings are also stored, defining the complete state of the aircraft prior to an accident.

Investigating the scene

In most countries, following an accident, an air accident investigation team is contacted immediately to take control of the investigation, which is potentially a crime scene. In countries lacking such facilities, they will often request the NTSB from the USA or the AAIB from the UK to undertake the investigation.

Apart from initial efforts of emergency teams to recover casualties, the site should not be touched until the investigation starts.

Clearly, recovery of the FDR and CVR is critical to any investigation. Normally, the recorders are closely guarded and sent to an approved laboratory where the data is carefully recovered. Once this data is copied the recorders have done their job, but confidence in the integrity of the raw data and the copying process is essential. The data extracted from the recorders is analysed to play back the last few minutes of a flight or monitor critical parameters or crew activity.

Trusting the data

A major area of concern is the integrity of the process to recover the data, particularly, as in the case of MH17, the outcome of an investigation may have political ramifications. The reputation of the airline or even the manufacturer may also be at stake. There are several points at which the process of data recovery is vulnerable.

For a start, it’s possible to substitute the black box for another, identical recorder which holds a different set of data. This would be difficult for the rebels to do in such a short period of time but they could carefully swap labels and plates bearing serial numbers to make a substitute box indistinguishable from the original.

More simply though, the the data stored in the memory of a recorder can be erased, overwritten or modified to give an alternative sequence of events. Data in a black box is not encrypted so overwriting its memory would be a straightforward operation.

The flight data recorder on flight MH17 should show, quite conclusively, if the aircraft had a major structural failure, an engine failure, a fire, an internal explosion or was struck by a missile. It may also show reveal the force and direction of any such impact.

However, even though the rebels could in theory tamper with the black boxes, it’s difficult to see what benefits there would be from either hiding the data or tampering with it, other than to add to the overall confusion. The rebels have faced intense scrutiny and were suspected of removing the recorders from the crash site without authorisation so they must know that the data in these boxes will come under extremely detailed analysis.

But while a huge amount of attention has been given to the black boxes in this case, it may, in the end, not come down to the information they yield. The damage done to the airframe of MH17 could tell us exactly who is responsible for this tragedy. The explosive force of the impact, the missile fragments and the explosive material embedded in the airframe will be analysed by forensic scientists who should be able to identify the type of missile used to bring it down and who manufactured it. Tampering with evidence on that scale is a much harder task.

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