Why can the black box record the situation before the plane crash?

The so-called black box is only a part of the flight data recording system, or a special instrument. Generally speaking, there are two orange instruments installed in the THS cabin at the tail of the aircraft. Because plane crashes usually land with the nose first, the damage to the tail is relatively small, which can better protect the recorder. These two instruments include digital flight data recorder DFDR and cockpit voice recorder CVR. When any engine is started, the flight data recording system runs automatically. The former can extract thousands of data such as the speed, attitude, position, lift, engine thrust and fuel quantity of the aircraft for more than 20 hours from the flight management computer, while the latter can record all the voice data of the cockpit in the last half hour, mainly the dialogue between the pilot and the co-pilot and the dialogue between the cockpit and the ground. According to the European standard, the black box must be able to withstand the impact force of 2.25 tons, and it will not be damaged at the high temperature of 1 100℃ for 30 minutes, and it is resistant to electromagnetic interference. Moreover, with its own battery and underwater positioning beacon, the instrument will automatically start when it falls into the water, which is convenient for search and rescue personnel to find it at the first time.