Air India Plane's 'Black Boxes' Found. What Are They? Why Are They Orange?

The Cockpit Voice Recorder, or CVR, of the London Gatwick-bound Air India plane that crashed in Ahmedabad last week - losing thrust 36 seconds after take-off and slamming into residential buildings bordering the airport, killing all 274 people, including 33 on the ground - was recovered Monday.

The Flight Data Recorder, or FDR, had been located earlier.

Data from the CVR and FDR - popularly called 'black boxes', although they are painted bright orange to help locate it amid debris - should allow investigators to understand what happened to flight AI-171 shortly after it took off from Ahmedabad's Sardar Vallabhbhai Airport at 1.39 pm.

The government has launched a detailed inquiry into the crash.

Separately, the United States' National Transportation Safety Board is conducting a parallel investigation since the aircraft - a Boeing 787-8 Dreamliner - was made in that country.

READ | Boeing Experts In Ahmedabad To Analyse Details Of Air India Crash

Experts from Boeing are also in India to assist with the investigation.

So what are the CVR and FDR? And how can they help investigators?

Let's start with the cockpit voice recorder.

A CVR, as the name suggests, records audio from within the plane's cockpit.

This refers to conversations between the pilots and other crew and also ambient noise - aural alarms, control movements, switch activations, engine noises, and airflow noises - which could provide clues about why a plane, in this case the Air India AI-171, may have crashed.

It also records and stores audio signals from the pilots' microphones and headsets, which means crash investigators can also listen to conversations with Air Traffic Controls, or ATCs.

A Cockpit Voice Recorder, or CVR. Photo: SKYbrary.aero

According to SKYbrary, an electronic repository of safety knowledge related to all aspects of aviation and aviation safety, modern CVRs record (in a loop) up to 25 hours of audio signals.

This is to allow for substantial data collection on long-haul flights.

The 25-hour rule was approved by the International Civil Aviation Organization in 2016; it was originally passed by the European Union Aviation Safety Agency in 2015.

However, there is a caveat.

The 25-hour rule is for aircraft built after January 1, 2021. And according to flight tracking website Flightradar24, the crashed AI-171, registration VT-ANB, was delivered to Air India in January 2014. Most widely-used CVRs for commercial planes then recorded for two hours only.

Before that CVRs recorded only for 30 minutes.

That, though, should not be a problem for the AI-171 crash investigators, given the plane was airborne for less than a minute. Horrific visuals show the plane failing to gain altitude and falling on residential buildings bordering the airport, with a hostel taking the brunt of the impact.

Newer CVR models combine recording of critical flight parameters like altitude, airspeed, and control inputs by pilots, leading to it being called Digital Voice and Data Recorder, or DVDR.

A more traditional CVR is usually a small and bright orange coloured metal box, with a cylindrical protrusion on one side and a rectangular one on the other. These are roughly six inches high, five inches wide, and about 13 inches deep, and can weigh up to 10kg.

Now the flight data recorders

On planes where the two are separate devices, the FDR records key flight parameters.

Modern FDRs can record from thousands of data points and systems inputs by the pilot or the plane itself, and there are slight variations between different models. All, though, will record a number of mandatory data points and also radio transmissions, to correlate data from the CVR.

Some of the mandatory data points are:

1. Pressure altitude
2. Indicated airspeed
3. Magnetic heading
4. Normal acceleration
5. Microphone keying, which are the radio transmissions

With the help of all this data investigators can even re-create the flightpath of a crashed aircraft, allowing them to see exactly what went wrong and then figure out why.

A Flight Data Recorder, or FDR, Photo: National Transportation Safety Board

Like the CVR, the FDR can also record for 25 hours in a loop, and is almost identical to the former in its size and weight. Also painted a bright orange, the FDR looks like a small gas cylinder attached to a large rectangular box.

So where are the CVR and FDR stored?

In the tail section of the plane. This is widely considered the area that suffers the least damage from a crash, allowing for the greatest chance of recovering complete flight information.

These boxes are specially designed - specifically the section that stores recorded data - to survive high-speed impact and fire. They are not, though, unbreakable and can be destroyed.

'Black boxes' are storied in the tail section of a plane.

Each also emits a radio signal - which is active for 30 days and can also be located if the box is underwater - that investigators can track in the event of a crash.

What happened to flight AI-171?

There were 242 people, including the pilots and crew, on board the London-bound plane.

Of them only one person - a British-Indian male sitting in seat 11A - survived.

The plane took off June 12 at 1.39 pm, the Civil Aviation Ministry spokesperson said last week, and within seconds, radioed Ahmedabad ATC about a lack of thrust and then went radio silent.

READ | "Pilot's Last Message Was 'Mayday'": Ministry On Ahmedabad Air Crash

"Mayday! Mayday! Thrust not achieved..." were Captain Sabharwal's final words. Ahmedabad ATC immediately tried to contact the flight but there was no response. Seconds later the plane crashed into buildings in the Meghaninagar neighbourhood, including the BJ Medical College.

Initial theories suggested birdstrikes in both engines, a major hydraulic or electronic failure, or an error with the flaps, but there has been no confirmation of the cause as yet, and data from the CVR and FDR will go a long way to establishing just what happened to flight A-171.

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