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Until recently, I was under the impression that CVRs will start recording as soon as the engines are started and turn off when the engines are stopped.

However, this is not the case given there are only 2 hours of recording time.

In one airplane disaster documentary, the CVR started recording at the gate and when the plane was getting de-iced.

So when does a CVR start recording, is this something that is started manually?

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The CVR (and FDR) are continuously recording while the aircraft is in operation. In the days of magnetic tape, this was done with a continuous loop. In digital systems, the same is emulated by automatically returning to the start of the storage area whenever the end is reached. In both cases, the new data always overwrites the oldest data, so the last N hours is always available. When there is a crash, investigators locate where along the loop the accident occurred, and then read all the way around the loop (physical or virtual) until they reach the accident again.

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    $\begingroup$ @Asynchronous - That was pretty much my understanding as well. If you are on a flight from Houston to Hong Kong that lasts 14 hours (realistic in my experience), only the last 2 hours of flight time prior to the electrical system of the aircraft being disconnected or destroyed is recorded. That means that the last 2 hours of the incident is recorded and the rest of the flight is overwritten. Two hours is a long time in an emergency. Radio & telemetry recordings can supplement that time. But, 2-3 hours should give investigators enough data to start piecing things together. Read an NTSB report $\endgroup$ – Dean F. May 30 at 23:41
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    $\begingroup$ @Asynchronous That is correct. Most accidents end within a matter of minutes, so even 2 hours is probably overkill. $\endgroup$ – StephenS May 31 at 0:35
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    $\begingroup$ "The disappearance of Malaysia Airlines Flight 370 demonstrated the limits of the contemporary flight recorder technology, namely how physical possession of the flight recorder device is necessary to help investigate the cause of an aircraft incident. Considering the advances of modern communication, technology commentators called for flight recorders to be supplemented or replaced by a system that provides "live streaming" of data from the aircraft to the ground" - no company wants to literally air their dirty laundry. Presumably the same reason why body cameras last thirty seconds. $\endgroup$ – Mazura May 31 at 9:09
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    $\begingroup$ There actually have been incidents where key moments happened long before the end of the tape. Here's a contrast. Flight 232 is a case study in managing a disaster. Key parts of the flight were lost because the tape had limited capacity. On the other hand, the sinking of the El Faro gave investigators hours and hours of bridge recording, which gave the NTSB a nuanced understanding of why a ship would sail into a hurricane. $\endgroup$ – Andrew Brēza Jun 1 at 14:52
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    $\begingroup$ So what happens when the critical incident occurs early in the flight but isn't noticed until much later? A famous (spaceflight but still relevant) example of this would be the Space Shuttle Colombia, which experienced unnoticed damage to the tiles during liftoff that resulted in the shuttle's destruction on reentry. It's possible this sort of thing could happen to regular aircraft as well. $\endgroup$ – Darrel Hoffman Jun 1 at 20:02
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CVRs are normally active as soon as one of the basic electrical buses, like the battery bus, or emergency bus, is energized. So if the airplane has a BATTERY MASTER switch or similar, it is recording audio as you as you switch it on, so pretty much as soon as you climb in and start getting things ready. CVRs will include a G switch to stop recording in a crash since they record on a continuous loop (most newer ones are 2 hours; older ones 30 minutes) so it can't overwrite the 2 hours prior to the crash if it survived and kept running. They also normally have an ERASE button that the pilot can theoretically push, but he/she would be in a heap of trouble if they pushed it without some really compelling reason if an incident occurred, and some airlines may have specific policies on when they can be erased.

Flight Data Recorders, on the other hand, are not started up until some action prior to engine start is initiated, typically when the red Rotating Beacon is activated, which is a standard sign to folks on the ground that you are getting ready to start. On the CRJs, the FDR starts with Beacon, Strobes, or when Weight off Wheels in case you forgot both of the other two somehow, and I'm pretty sure that is typical.

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    $\begingroup$ Do FDR stop recording if engine fails? $\endgroup$ – Auberron May 31 at 3:28
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    $\begingroup$ No they are normally powered by a battery bus, essential bus, or emergency bus and will be one of the basic services powered by a battery power no matter what. $\endgroup$ – John K May 31 at 3:47
  • $\begingroup$ If the pilot pushes the erase button when shutting the plane down after an uneventful flight, nobody should ever notice, no? As I understand it, the company is not allowed to read the CVR anyway. so if the pilot pushes it during shut-down and somebody finds out, it will be the company who will be in trouble. $\endgroup$ – Jan Hudec Jun 1 at 20:29
  • $\begingroup$ You brought up an interesting point. I was with a private operator and we weren't supposed to use the erase button and maybe I made too much of an assumption based on that. I know that ALPA has a policy that pilots should be allowed to use it after a flight with no incidents, but I am pretty sure a lot of airlines forbid erasure. I modified my post a bit. Thanks. $\endgroup$ – John K Jun 2 at 1:31
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    $\begingroup$ @JanHudec It would probably depend on how they found out. Another crew member could report seeing them do it. I figure the “no erase” rule arose from the incident on TWA841 (1979) where the aircraft had plunged 34,000 feet and the captain erased the CVR when they landed. That caused controversy as the NTSB decided he was covering up evidence of wrongdoing by the crew. $\endgroup$ – TomMcW Jun 2 at 4:15
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By legal requirement, the CVR must be operated from the start of the first checklist (typically Cockpit Preparation or similar checklist), until after the reading of the final checklist. The CVR can only be erased when the parking brake is set.

If an incident occurs, some aircraft have a marker button on the instrument panel to mark the event, and the CVR circuit breaker will be pulled to preserve the recording, under certain circumstances.

Length of CVR recording may be as little as 2 hours, but most are typically 6+ hours and digital, now. They include area microphones to pick up cockpit sounds such as switches, alarms, etc, as well as microphone channels for the captain and first officer.

Regarding a mishap on a 12 hour flight: if the mishap is severe enough to warrant CVR review, the flight isn't likely to continue for another 6-10 hours; it's probably going to be an air turnback to the point of departure, or a diversion to a different airport.

A mishap in which the CVR will become significant will occuring up until the CVR can no longe record (a crash, for example); the CVR isn't being over-written, because it's no longer recording. The CVR records up to the final moments.

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  • $\begingroup$ I agree with you for airplanes, which is the point of the question. It's worth noting that ships have longer recording periods. Having 26 hours of data was extremely helpful in investigating the sinking of the El Faro. $\endgroup$ – Andrew Brēza Jun 1 at 14:55
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    $\begingroup$ Note that the FDR also has a longer loop, generally longer than the aircraft endurance, so that one will have the complete flight. $\endgroup$ – Jan Hudec Jun 1 at 20:40
  • $\begingroup$ I've been out of it a while, and wasn't aware they are up to 6 hrs. Cool. $\endgroup$ – John K Jun 2 at 1:34

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