Aircraft Black Boxes: How do they work?

What are those all important “black boxes” in an aircraft? Why are they so important after any accident? How do they work? I explore.


Since the recent mysterious disappearance of Malaysian Airlines Flight MH-370, there has been a lot of din about recovering the so-called “black boxes” of the aircraft.

In this blog, I set about to explore what exactly is this mysterious “black box”.

The Need for a Black Box

Aircraft are engineered to perfection. So are the engines, by the aircraft turbine manufacturers. It is highly unlikely for catastrophe to strike.

Therefore, aviation accidents are indeed rare. But when they do happen, hundreds of questions are raised as to how that happened.

Unlike the car accident on the street, there are no CCTV cameras, or human witnesses to experience an accident up in the air. That when the “black boxes” come in the picture.

These things provide valuable information of how various aircraft systems were functioning, and what conversations were going on in the cockpit.

Black Box: A Media Phrase

The term “black box” is indeed a media phrase. There is nothing “black” about this “black box”.

In fact, it might be so due to the apparent mysterious nature of the “black boxes” to outsiders. Not many, except those in the aviation industry, are aware of the functioning of these things.

But that, of course, is speculation at best.

To go with the popular jargon, there are two “black boxes” on commercial aircraft.

Flight Data Recorder

Shortly known as the FDR, this little piece of instrument records all the tiny details about the functioning of various systems of the aircraft, such as the flap settings, airspeed, attitude, engine pressure ratio, fuel flow, engine performance, outside temperature and pressure, cabin pressure and temperature, etc.

How is it done? Quite simply. There are several sensors in an aircraft that monitor and note down the performance of various systems. 

Modern aircraft can record hundreds of flight parameters on an FDR. For example, the FDR of a Boeing 787 can log 146,000 flight parameters!

Cockpit Voice Recorder

The CVR chiefly records all the conversations going on in the cockpit. Be it within the flight crew, or between the flight crew and Air Traffic Control.

The microphone inputs are taken from the microphones on the headsets of the Captain and the First Officer. In many cases, these headset microphones can also pick up background noise in the cockpit, such as levers being shifted, switches being thrown, etc.

Besides, cockpits of some modern aircraft are equipped with an area microphone, whose main job is to pick up all background noises in the cockpit.

Types of FDR and CVR

Initially, data was recorded on magnetic tape. There was a drawback with this, however. 

There’s a joke that magnetic tape FDR and CVR were known, within the aviation industry of course, as: the most unreliable piece of instrument on an aircraft.

Such was their rate of failure! However, you need not despise. Modern FDR and CVR use solid-state drives, known as SSDs. In these, data is stored within memory chips.

Since there are no moving parts, there are less chances of these newer varieties of FDR and CVR failing to do their job.

Advantages of having the CVR and FDR

When the FDR and CVR were first made standard in all commercial aircraft, the aim was to aid in the investigation of aviation accidents.

Flight Data Recorder

Data from the FDR reveals the performance and functioning of various critical systems on board the aircraft. This can often help ascertain which flight system did not function as expected.

With this deduction made, investigators can get a lead to hold on to, on the basis of which they would proceed with the investigation of determining what or who is responsible.

Cockpit Voice Recorder

The CVR records background noise and crew conversations. Aircraft Crew are typically trained to comment on every decision they take on the cockpit. Whether you push the flaps lever up by a notch, or reduce engine power to 50%, or you are leaving the cockpit to go to the washroom, the flight crew is trained to comment. And they do so even in critical situations.

Thus, the CVR recordings often provide a first-hand (audio) picture of the scenario in the cockpit, and the situation the aircraft is in, from the house of the most well-informed people in the whole scenario: the people flying the plane. This often helps investigators ascertain what was going on.

The data from the FDR and CVR is a shot in the arm for aviation disaster investigators. Very often, all you need is the “black boxes”, in order to answer the million-dollar question: what happened, and how?

Data Acquisition

The data of the performance of various aircraft systems is picked up by various sensors situated all over the aircraft. All the data is then routed to the Flight Data Acquisition Unit, or the FDAU. 

A schematic diagram indicating the main parts of a typical, modern Flight Data Recorder.
Image Credit:

The FDAU is typically located in the electronic equipment bay. The electronic equipment bay is located below the cockpit. The FDAU collects together all the data, and then routes the respective data to the FDR and CVR.

Locating of the recording units

The FDAU, the FDR, and the CVR are kept in different parts of the aircraft. There’s a very good reason for doing so. But first, let’s take a look at their positions.

An infographic depicting the position of the main flight parametre sensors, along with the recording instruments.
Image Credit:

The “black boxes” are situated in the tail section of the aircraft. The reason is a structural and engineering advantage.

An aircraft cannot fly backwards! So, if an aircraft crashes head-on, the front and middle section of the aircraft would absorb the shock of the impact. This would leave the least shock to reach the “black boxes”.

However, there may be another case, where the aircraft belly-lands during a crashWell, in an uncontrolled situation, which is typical during a crash, the front of the aircraft is generally lower than the rest of the aircraft. So, in this case too, the principle holds.

Then, why put the electronic equipment bay? Because, practically most of the electronic equipment is in the cockpit. The very reason for the existence of the electronic equipment bay is to accommodate the huge electronics in the cockpit.

How Long Does the Data Last?

If you have taken care care to note, you’ll have seen this.

On your PC, you should have noticed that audio files occupy a larger space than text files.

The data from the FDR can be compared to the “text files”, and the data from the CVR you can compare to the “audio files”.

Much like on your PC, the FDR will therefore need less space to store data than the CVR. However, both the FDR and CVR have, more or less, equal space. 

Therefore, the amount of data the CVR can record would be less than that of the FDR. 

  • Cockpit Voice Recorder: Modern CVR (ones which used SSDs) can typically record audio continuously for 2 hours, before it must start overwriting data.
  • Flight Data Recorder: FDR which use SSDs (the ones used now) can record flight parameters for up to 25 hours, after which it must start overwriting data.

The Principle of Overwriting

The CVR must start overwriting stored data after 2 hours, and the FDR must after 25 hours of recording flight parameters. A principle is followed for safety purposes.

The oldest data is overwritten first, then the newer ones, then the more recent ones, and so on. This makes sure that in the event of an accident, the latest data before the crash is available. That increases the chances of gaining potential leads, as things normally go wrong immediately before an accident.

Aviation Safety Protocols for FDR Data

The Federal Aviation Administration, according to a mutual international agreement, is taken for granted as the de-facto authority of aviation protocols. Aviation Regulators of other countries are ethically expected to implement the protocols of the FAA in their country.

The Federal Aviation Administration, or the FAA, is the aviation regulator in the USA.

The FAA has laid down a list of 88 flight parameters that are compulsory to be recorded in FDR. Some of them are:

  • Time
  • Barometric altitude (altitude based on barometer reading)
  • Airspeed
  • Vertical Acceleration (acceleration while climbing or descending)
  • Magnetic heading (heading as indicated by magnetic compass)
  • Control-column position (position of the flight yoke, which the pilot uses to control bank and climb, etc.)
  • Rudder pedal position
  • Horizontal stabilizer position (position of the wing flaps)
  • Fuel flow rate (rate of fuel flow into the engines)

However, modern FDR on commercial aircraft as such record so many flight parameters that, very often, data from the FDR is often enough to reconstruct the entire flight. It has often been done by the NTSB.

Built for Survival

Airplane Crashes are very violent affairs, so long as structural mechanics is concerned.

In many cases, the only part that remains intact is the crash-survivable memory unit (CSMU). That is the thing that contains the FDR and the CVR.

In a typical case, the internal components of the CSMU are mangled by the impact of the crash.

The CSMU itself is a large cylinder which is attached to the flat portion of the FDR and CVR. The data recorders reside inside it.

The CSMU itself is a three-layered piece of equipment. This makes the CSMU heat, crash, and pressure-resistant. Just to make sure that the memory units of the FDR and CVR survive the crash. 

In this context, it would be wise to refer back to the structure of the recording units.
  • Aluminium Housing: The memory are coated with a thin layer of aluminium.
  • High-Temperature Insulation: This thick layer is illustrated in the above infographic. This dry silica material is about 1 inch thick. It protects the memory units from a post-crash fire.
  • Stainless Steel Armour: It forms the shell of the CSMU. The shell is a cast of stainless steel, typically about 0.25 inches thick. Often, titanium is used instead of steel.

Better safe than sorry. Therefore, aircraft engineers make sure to test their CSMU against all the possible situations that it may encounter, during and after a crash.

Locating the Black Box

Although called black boxes, they are anything but black.

  • The CSMU is painted bright orange, so that rescue parties find it easier to locate it at a crash site.
  • The exterior of the CSMU also contains reflective tape, which further helps in locating it.
Yes, refer to it once more!

In addition to all these, the “black box” is equipped to help locate it underwater. That bull’s eye like thing of the side of the underwater locator beacon is the submergence sensor. 

Whenever it comes in contact with water, it means that the CSMU is in a water body. The underwater locator beacon (ULB) is then activated. 

The ULB transmits an ultrasonic ping every second, at a frequency of 37.5 KHz. SONAR and ultrasonic sensors on ships and other amphibian vehicles can pick up these pings, in order to help locate the resting place of the “black box”.

The ULB is powered by a battery in the CSMU, which itself has a shelf life of 6 years.

To Sum Up

Once you’ve got your hands on the flight recorders, rest assured the mystery will one day be solved. That’s why they are probably the most important pieces of equipment on an aircraft from an investigation perspective.