in the aircraft's tail section, where it is more likely to survive a crash. Following an accident, the recovery of the FDR is usually a high priority for the investigating body, as analysis of the recorded parameters can often detect and identify causes or contributing factors. Modern day FDRs receive inputs via specific data frames from the flight-data acquisition units. They record significant flight parameters, including the control and actuator positions, engine information and time of day. There are 88 parameters required as a minimum under current US federal regulations (only 29 were required until 2002), but some systems monitor many more variables. Generally each parameter is recorded a few times per second, though some units store "bursts" of data at a much higher frequency if the data begin to change quickly. Most FDRs record approximately 17–25 hours of data in a continuous loop. It is required by regulations that an FDR verification check (readout) is performed annually in order to verify that all mandatory parameters are recorded. Many aircraft today are equipped with an "event" button in the cockpit that could be activated by the crew if an abnormality occurs in flight. Pushing the button places a signal on the recording, marking the time of the event. Modern FDRs are typically double wrapped in strong corrosion-resistant stainless steel or titanium, with high-temperature insulation inside. Modern FDRs are accompanied by an underwater locator beacon that emits an ultrasonic "ping" to aid in detection when submerged. These beacons operate for up to 30 days and are able to operate while immersed to a depth of up to . Cockpit voice recorder A cockpit voice recorder (CVR) is a flight recorder used to record the audio environment in the flight deck of an aircraft for the purpose of investigation of accidents and incidents. This is typically achieved by recording the signals of the microphones and earphones of the pilots' headsets and of an area microphone in the roof of the cockpit. The current applicable FAA TSO is C123b titled Cockpit Voice Recorder Equipment. Where an aircraft is required to carry a CVR and uses digital communications the CVR is required to record such communications with air traffic control unless this is recorded elsewhere. it is an FAA requirement that the recording duration is a minimum of two hours. A standard CVR is capable of recording four channels of audio data for a