CVR and DFDR
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the cvr records the last 30mins of flight i believe. Which i think is standard across all types. As this is the most important part of the flight/crash.
The dfdr records the last 25 hrs of flight and once the 25hrs are up, the new data starts recording over the oldest recordings.
The dfdr records the last 25 hrs of flight and once the 25hrs are up, the new data starts recording over the oldest recordings.
Join Date: Mar 2005
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Direct from the Airbus manual
So last 2 hours is what we have 
The Solid State Cockpit Voice Recorder (SSCVR) is designed to record crew conversations and communications into memory block unit in flight and on ground, when at least one engine is running or up to five minutes after the last engine is shut down irrespective of which engine is shut down first. The system can also operate in manual mode on the ground. The recorder is a four-track system and all tracks are recorded simultaneously.
The SSCVR provides storage for 2 hours of consecutive recording for each of the four audio input channels.
When the memory block unit is fully recorded, the system progressively erases recordings made in the previous 2 hours and simultaneously records new information; thus only information recorded in the last 2 hours of operation is retained. The recorded information can be intentionally erased when the aircraft is on the ground with the parking brake on, locked and electrically powered. Bulk erasure is also possible during manual operation of the system.
Recording of conversations and communications must comply with standards specified by the CAA (Ref. ATA 23-51, Audio Management, para. CAA Recording Function).
The SSCVR provides storage for 2 hours of consecutive recording for each of the four audio input channels.
When the memory block unit is fully recorded, the system progressively erases recordings made in the previous 2 hours and simultaneously records new information; thus only information recorded in the last 2 hours of operation is retained. The recorded information can be intentionally erased when the aircraft is on the ground with the parking brake on, locked and electrically powered. Bulk erasure is also possible during manual operation of the system.
Recording of conversations and communications must comply with standards specified by the CAA (Ref. ATA 23-51, Audio Management, para. CAA Recording Function).
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I think it's quite simple.. the CVR records the last 30 mins do you even know the difference between the two??
Cockpit Voice Recorder
Time recorded 30 min continuous, 2 hours for solid state digital units
Number of channels 4
Impact tolerance 3400Gs / 6.5 ms
Fire resistance 1100 degC / 30 min
Water pressure resistance submerged 20,000 ft
Underwater locator beacon 37.5 KHz; battery has shelf life of 6 years or more, with 30-day operation capability upon activation
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I don't believe you can put a definite number on the DFDR recording duration. DFDRs use proprietary data compression routines so the actual amount of time before they have to re-record varies depending on what the aircraft is actually doing - if it's sat powered on the ground with nothing much happening except the clock ticking you'll get a LOT more than 25 hours I'd expect. (I'm not familiar with the details of A320 DFDRs, but that's certainly true for some other types, and I'd be shocked if the Airbus units were vastly different)
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Mad Flt Scientist, the same is true for the Airbus DFDRs.
25 hours is the minimum that they have to record, with the right combination of capacity and data rate they can actually record 50 hours.
From the manual again.
The correct answer to the Original poster I guess is
DFDR - 25 hours (rising to 50 hours theoretical)
CVR - 30 mins
SSCVR - 2 hours (although SSCVR are still referred to as just CVR).
25 hours is the minimum that they have to record, with the right combination of capacity and data rate they can actually record 50 hours.
From the manual again.
The DFDR is a solid-state flight-data recorder in compliance with ARINC 717. The DFDR stores all aircraft information in CMOS bulk erasable EEPROM (Flash Memory IC) devices. Being a solid state device, the DFDR has no moving parts. The recorder can store all data which the FDIMU has collected over the last 25 hours. It is possible to store more than 25 hours of data if the correct combination of DFDR capacity and data rate are used.
Data is stored in shock protected solid state nonvolatile memory device (CMOS). The recorder receives 64,128 or 256 words/sec series messages from the Flight Data Interface Unit (FDIU). The write mode is a single channel incremental block recording with inter-record gaps between each data block. One data block contains data received during one second (768 BITS). Throughout this recording phase the DFDR continuously monitors its correct operation. If an abnormality occurs during this time it is automatically memorized (BITE function).The recording duration of the recorder is 50 hours under normal flying conditions. After the aircraft has landed, the memorized data can be transferred for analysis.
DFDR - 25 hours (rising to 50 hours theoretical)
CVR - 30 mins
SSCVR - 2 hours (although SSCVR are still referred to as just CVR).
Moderator
While there are design/certification minimum values, the answers really depend on the kit installed. For instance, the DFDR on the aircraft with which I am involved typically runs to 90-120 hours.
