As I understand, there are two black boxes on-board an aircraft. One black box, the Cockpit Voice Recorder holds the cockpit conversations and the other, the Flight Data Recorder holds essential flight parameters.

  • But why are the two black boxes holding separate data?
  • Why don't both black boxes hold copies of both the FDR and the CVR data for extra redundancy in case the other box goes missing or is completely damaged by the crash?
  • Are there any technical reasons for why this isn't possible or hasn't been attempted yet?
  • Is there any benefit of having the CVR and FDR in separate boxes?

5 Answers 5


Black Box(s) is a misnomer, you have correctly mentioned their actual names.

Why are the two black boxes holding separate data?

They have different purposes.

Flight data recorder: This is an electronic device employed to record any instructions sent to any electronic systems on an aircraft. It is a device used to record specific aircraft performance parameters.

Cockpit voice recorder: This device is used to record the audio environment in the flight deck of an aircraft for the purpose of investigation of accidents and incidents.

Why don't black boxes hold copies of both the FDR and the CVR data for extra redundancy?

Since they record different data, they are different. They are built different.



  • Time recorded: 25 hour continuous
  • Number of parameters: 5 - 300+


  • Time recorded: 30 min continuous, 2 hours for solid state digital units
  • Number of channels: 4


  • Impact tolerance: 3400 Gs /6.5ms
  • Fire resistance: 1100 deg C /30 min
  • Water pressure resistance: submerged 20,000 ft
  • Underwater locator beacon: 37.5 KHz
  • Battery: 6yr shelf life, 30 day operation
  • Both recorders are installed in the most crash survivable part of the aircraft, usually the tail section.
  • Each recorder is equipped with an Underwater Locator Beacon (ULB) or pinger to assist in locating in the event of an over water accident.

Are there any technical reasons for why this isn't possible or hasn't been attempted yet?

It has certainly been attempted.


Is there any benefit of having the CVR and FDR in separate boxes?

Probably the reason is that different teams investigate the data obtained from them as they have to recreate the scenario before the accident.

Regulatory Details

CVR: Due to the highly sensitive nature of the verbal communications inside the cockpit, Congress has required that the Safety Board not release any part of a CVR tape recording. Because of this sensitivity, a high degree of security is provided for the CVR tape and its transcript. The content and timing of release of the written transcript are strictly regulated.

FDR: Newly manufactured aircraft must monitor at least twenty eight important parameters such as time, altitude, airspeed, heading, and aircraft attitude. In addition, some FDRs can record the status of more than 300 other in-flight characteristics that can aid in the investigation. The items monitored can be anything from flap position to auto-pilot mode or even smoke alarms.

  • 2
    $\begingroup$ I think that all of the data is highly sensitive and there is a high degree of security surrounding it, not just the CVR's! $\endgroup$
    – Lnafziger
    Apr 12, 2014 at 4:24
  • $\begingroup$ @Farhan, I meant why hasn't the concept of both black boxes with the same data been attempted yet. Not just the one black box :) $\endgroup$ Apr 16, 2014 at 11:04
  • $\begingroup$ @shortstheory You mean one box which records ALL conversation and ALL data? $\endgroup$
    – Farhan
    Apr 16, 2014 at 16:38
  • $\begingroup$ @Farhan ah yes. BOTH boxes which carry all the conversations and all the data. Like, both boxes have copies of all the CVR/FDR data. $\endgroup$ Apr 17, 2014 at 4:33
  • $\begingroup$ @shortstheory IMO, redundancies are added only when they are needed. Black boxes are not easily breakable and are almost always found. $\endgroup$
    – Farhan
    Apr 24, 2014 at 17:44

Similar to Lnafziger, here is what I can think of:

  1. Protecting two recorders in one box would be harder than protecting one recorder. The smaller the container, the easier it is to crashproof it.
  2. Adding a second recorder in each box adds complexity, which is not desirable in a system like this.
  3. Wiring up all of the data to both recorders would add complexity to the airplane, which is also not desirable.

The number of cases where your suggestion would actually benefit investigation is not very many. In crashes that damage the recorders too much, making the recorders larger and more complex would probably not help. There have been many cases where the recorders didn't work properly or at all, so giving more functions to each box makes this more likely to occur more often.

Here is a list of unrecovered flight recorders. Considering the time it spans, there are not many, and in some cases, neither recorder was recovered.

I'll see if I can find any references for the cases where they were recovered but contained no useful data.

  • $\begingroup$ I know of the flight that crashed on asian soil and law enforcement let the plane burn down however the fire exceeded the limits of the blackboxes $\endgroup$ Apr 11, 2014 at 22:50

There are a couple of reasons that I can think of off of the top of my head:

  1. There are different regulations covering when each of the boxes is required. Some aircraft only require a CVR while others require both. With them in different boxes, they can be installed in whatever combination is necessary.

  2. They require connections to different systems in the aircraft, and requiring them both to connect to all systems would result in quite a bit of additional wiring. It would also expose each box to a potential catastrophic electrical event (voltage spike) from any of the connected systems, potentially damaging both boxes at the same time.

  3. There is already some added redundancy by them being in different boxes. Even if one fails or is is badly damaged, there would probably be data available on the other one anyway.

As Farhan pointed out, there are already some units capable of recording all data at once, but since the regulations only require one unit there is no added redundancy. For aircraft that are required to have both a CVR amd a FDR it could be a great option!

  • $\begingroup$ @Lnafziger with todays technology you could stream both sets of data to both boxes and store it on both... $\endgroup$ Apr 11, 2014 at 22:54
  • $\begingroup$ @ratchetfreak Perhaps on some of them, but there are still a lot of old analog recorders out there too! $\endgroup$
    – Lnafziger
    Apr 12, 2014 at 4:17

In my opinion, I think that whatever is present in one black box also will often tell the other half of the story as well.

For instance, if the airspeed indicators are unreliable, the CVR will pick it up by the pilot's talking, even if the FDR may be lost or destroyed, which will point investigators in the right direction how to continue the investigation.

In other words, I'd imagine there's a bit of indirect redundancy by two devices.

However, many aircraft have a quick access recorder which although not armoured will give much the same stuff as a FDR. If I'm not wrong, Flight Data Recorders are mostly non-mechanical devices and are tested annually, so they are kept reliable.

  • $\begingroup$ Ideally, you are right, you can infer some information about one from the other. However, sometimes the crew fails to make any comments about things, which is difficult without the FDR. And even if you have the FDR, the crew response is often critical, which can be difficult without the CVR. $\endgroup$
    – fooot
    Apr 11, 2014 at 19:37
  • $\begingroup$ Indeed there is at least one case where the Flight Data Recorder has been damaged/destroyed and investigators reconstructed some idea of at least engine power settings based on what could be heard on the CVR. (I can't recall which incident it was off hand.) $\endgroup$
    – voretaq7
    Apr 11, 2014 at 20:31

The reasons for TWO boxes are most likely that they came up with the idea for a cockpit voice recorder first, and later decided to add another one for data.

The magnetic tape and other formats first used took up a lot of space and to put both in one box would have been a big heavy box the size of two put together. These are about 20 pounds each, a combined box would have been around 40 pounds.

Today, with digital chips replacing the big bulky tape machines, they can be made smaller and lighter and they indeed do make recording of both cockpit voice and data contained in the same box, such as this one...


And they are also considering have two per plane, one in the tail section as is done now, and the other near the front cockpit, so if one is damaged, info could still be contained possibly from the second unit which would also contain both voice and data.

They are constantly considering all options, from floating boxes, to transmitting live data to satellites as well during flight.


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