One is FDR, flight data recorder. It can record flight data, such as altitude, speed, heading, climb rate, descent rate, acceleration, fuel consumption, speed, landing gear retraction, aircraft system working state and engine working parameters. Early recorders can only record more than 20 kinds of data, but now they can record more than 60 kinds of data. Among them, 16 are important required data. The number of parameters has gradually developed from the initial five to hundreds or even thousands. The electronic recorder uses stacked memory chips, so there is no movable equipment, which reduces the probability of sudden interruption and accidents in daily maintenance. Through the special data playback software, the flight process of the aircraft can be reproduced intuitively and truly with black box data, and the possible hidden dangers and abnormal operation of the aircraft can be automatically analyzed to prevent faults or accidents, which greatly improves the safety level of aviation flight.

The second is CVR, cockpit voice recorder. It is actually a radio energy call recorder, which records all kinds of calls on the plane. The four tracks on the recorder record the conversation between the pilot and the ground command organization, the conversation between the pilot and the co-pilot, the conversation between the captain, the flight attendant and the passengers, threats, explosions, abnormal engine sounds and the sounds in the cockpit. It can save the parameters before the plane stops working or crashes and the sound in the cockpit for more than 20 hours. Its working principle is similar to that of an ordinary tape recorder. The tape turns round and round, constantly washing the old and recording the new, and always recording various sounds in the last half hour. A flight usually goes through eight stages (take-off, initial climb, climb, cruise, descent, initial approach, final approach and landing), and each stage can't escape the "ears" of the black box.

In order to withstand the violent impact and high temperature flame when the plane crashes, the black box has a thick steel plate and multiple layers of heat insulation and anti-collision materials. Moreover, in order to be as safe as possible, the black box is usually installed in the safest place at the tail of the plane, that is, the part that is least likely to be damaged when it crashes. When the plane crashes, the black box can withstand 30 minutes of barbecue in the flame of 1 100℃, 5 minutes of extrusion by a 2-ton object, and can be soaked in gasoline, engine oil, oil essence, batteries, acid solution and seawater for several months. Even with this protection, the black box is still damaged in some air crashes, so the international aviation agency has formulated stricter standards. The recording medium has been improved from magnetic tape to static storage recorder, which can withstand greater impact, similar to the memory chip in the computer.

? ? In most air crashes, the equipment that can be preserved is the flight data recorder and the crash survival recording unit (CSMU) of the cockpit sound recorder.