The Cockpit Voice Recorder (CVR) and Flight Data Recorder (FDR)—colloquially called "black boxes" despite being required by regulation to be bright international orange—are the cornerstone investigative tools of aviation safety. For ATP candidates and professional air carrier pilots, these devices represent far more than a checkbox on a pre-flight scan: they are the subject of detailed, testable regulatory requirements found across 14 CFR Parts 91, 121, and 135, supported by technical standards and NTSB notification obligations. A thorough command of these rules is both an ACS requirement and a genuine professional responsibility.
What CVRs and FDRs Actually Record
The Cockpit Voice Recorder captures audio from the flight deck environment on a continuous-loop basis. The microphone channels typically include the captain's audio panel, the first officer's audio panel, an area microphone mounted on the overhead panel, and—where installed—a third or fourth crewmember's station. Everything passing through those channels is preserved: crew-to-crew conversation, radio transmissions on all frequencies, public address system use, interphone calls, aural warnings, and even ambient sounds such as gear extension, engine power changes, and switch clicks. Accident investigators use those ambient sounds to correlate the audio timeline with the FDR parameter data.
The Flight Data Recorder is a parametric device. The minimum number of required parameters has grown significantly as aircraft became more complex. Aircraft certificated before certain amendment dates may be required to record as few as 11 to 17 parameters; aircraft certificated under later amendments of 14 CFR Part 121 Appendix M and related appendices may be required to record well over 80 discrete parameters. Core parameters include pressure altitude, indicated airspeed, magnetic heading, normal acceleration, pitch attitude, roll attitude, radio transmission keying, thrust of each engine, and control surface positions. Together, CVR audio and FDR parameter traces allow investigators to reconstruct the final moments—and the flight history—of an accident flight.
Which Aircraft Are Required to Have Recorders
Part 121 Air Carrier Operations
Under 14 CFR Part 121, essentially all turbine-powered transport category aircraft used in scheduled air carrier operations must be equipped with both a CVR and an FDR. The specific applicability sections within Part 121 (Subpart L and its associated appendices) tie requirements to passenger seating configuration and the amendment standard under which the aircraft was originally certificated. As a practical matter, every modern jet or turboprop airliner operated by a Part 121 certificate holder will carry both devices.
Part 135 Operations
Under 14 CFR Part 135, the recorder requirements are tiered by seating capacity and powerplant type. Multiengine turbine-powered aircraft with more than 6 passenger seats and that are required to have a type certificate under transport category standards must carry both a CVR and an FDR. Single-engine or smaller multiengine aircraft below that threshold are generally not required to carry recorders, though operators may elect to install them. Check-airmen and ATP candidates must be careful not to over-generalize: the specific seating threshold and operational context control applicability.
Part 91 Large Turbine-Powered Aircraft
Part 91 is not a blanket exemption. 14 CFR §91.609 imposes FDR and CVR requirements on certain large turbine-powered civil aircraft operated under Part 91, and §91.609(e) specifies that a turbine-powered civil airplane that is used in operations under Part 121 or 135 must meet the recorder standards of those parts even when temporarily operating under Part 91 on a ferry or positioning flight. Additionally, large civil aircraft (generally those with a maximum certificated takeoff weight above 12,500 lb) may face FDR requirements independent of their air carrier status. The key lesson: never assume that operating under Part 91 automatically eliminates recorder obligations.
Construction and Survivability Standards
Both the CVR and FDR must be built to withstand the most severe accident environments foreseeable. FAA Technical Standard Orders (TSOs) specify crash impact resistance, high-temperature fire resistance, and static crush resistance. Recorders must survive immersion in salt water at significant depth for an extended period—hence the requirement for an Underwater Locating Device (ULD), sometimes called an underwater locating beacon (ULB), attached to each recorder. The ULD activates automatically upon water immersion and emits an acoustic signal on 37.5 kHz, allowing recovery teams to locate the recorder from the surface. Both recorders must be mounted in the aft section of the fuselage, where statistical evidence shows the highest survivability in a crash sequence.
Key Regulatory Numbers and Requirements
- CVR retention duration: At least the last 2 hours of flight deck audio must be retained on the continuous loop. For digital CVRs installed after certain amendment dates, some rules specify extended durations, but 2 hours is the foundational testable number.
- FDR retention duration: At least the last 25 hours of recorded flight data must be retained. This longer window ensures investigators have access to the entire duty day and preceding flights, not just the final approach.
- Post-accident CVR preservation: Under 14 CFR §121.359(g) and parallel Part 135 provisions, the operator must preserve the CVR recording after an accident or incident. The CVR must not be erased for at least 60 days following an accident, or longer if the NTSB or FAA requests extended preservation. Intentional erasure after an accident is a serious regulatory violation and can constitute obstruction of an NTSB investigation under 49 U.S.C.
- Operative before dispatch: Both recorders must be operative before each flight. If either device is found inoperative, the aircraft generally may not be dispatched. Limited exceptions exist—for example, a ferry flight to a repair facility may be authorized under specific conditions set forth in the operator's approved MEL (Minimum Equipment List)—but these are narrow and subject to FAA approval.
- FDR readout check: Operators are required to read out FDR data periodically (at least annually for most Part 121 operators) to verify the recorder is actually capturing accurate parameters across all required channels. A recorder that appears to be powered on but is writing corrupted data is not compliant.
- Recorder color: Both devices must be a distinctive bright orange (international orange) and marked with reflective tape to facilitate visual location after an accident.
NTSB Notification and Preservation Obligations
The CVR preservation rule connects directly to 49 CFR Part 830, which governs NTSB notification and the preservation of aircraft wreckage, mail, and cargo. When a reportable accident occurs, the operator must immediately notify the NTSB and must not disturb the wreckage—including the recorders—until authorized by the NTSB. In practice, the CVR circuit breaker should be pulled as soon as safely possible after landing to stop the loop from overwriting the accident audio; this is a standard post-accident checklist item and a commonly tested operational detail.
Why This Matters for the Working ATP
Understanding CVR and FDR regulations is not academic. A pilot who inadvertently allows the CVR loop to continue running after an incident may overwrite the very audio needed to exonerate the crew. Conversely, deliberately erasing the CVR is a federal violation with career-ending consequences. During normal operations, confirming that both recorders are operative is part of every professional pre-flight flow. A thorough understanding of MEL provisions for inoperative recorders prevents an illegal dispatch and protects the operator's certificate. Familiarity with the parameter requirements also supports conversations with maintenance personnel about recorder health—something a professional ATP should be able to engage in intelligently.
Common Test Traps
- Swapping retention times: CVR = 2 hours; FDR = 25 hours. These numbers are reversed on many incorrect answer choices.
- Assuming Part 91 means no recorders required: Large turbine aircraft and aircraft ferry-flying under Part 91 after Part 121 or 135 service may still have recorder obligations under §91.609.
- The 60-day rule is a minimum: The NTSB can extend CVR preservation beyond 60 days. Answer choices that say the recording can be erased after exactly 60 days are traps—erasure requires explicit NTSB release.
- "Operative" does not mean merely powered on: A recorder that is running but recording corrupt data is not considered operative, making the aircraft non-dispatchable without an MEL authorization.
- ULD is required on both recorders: Some candidates believe only one recorder carries the underwater locating device. Both the CVR and FDR must be equipped with a ULD.
- Color: International orange, not black. The "black box" nickname is colloquial only and has no regulatory basis.
Memory Aid
Use "C-2, F-25": the CVR keeps 2 hours of conversation, while the FDR keeps 25 hours of flight data. The FDR's longer window reflects the need to examine an aircraft's full duty period, not just its final moments.