In spite of the low reliability, the airline decided to increase overhaul life intervals to reduce the frequency of repairs, which were carried out in Soviet factories and quite expensive. LOT sent a letter to the Ilyushin Design Bureau containing the results of a test in which it was found that the engines could operate normally 8600 hours without maintenance. From the design bureau office came the answer that the Poles could fly as much as they want, but the manufacturer was responsible only for 5000 flight hours. Increased stress on the engines and lengthening of the service interval led to growth failure. There were cases when the IL-62 flew from the United States to Warsaw without passengers, on three engines. Representatives of the technical staff at the John F. Kennedy Airport reported two similar flights in the past two years. The most common causes of failure were bending or breaking of the blades. Consequently, there were not enough serviceable engines. LOT fell into the practice of using three engines within the service interval, and a fourth beyond the interval. The investigation revealed the practice to be widespread. The airline called the fourth engines "leaders". Initially, Mikołaj Kopernik'''s engine 2 was installed on the aircraft SP-LAC Fryderyk Chopin, but after 1,700 hours of flying in 1975, the engine was removed due to damage of the low-pressure compressor's blade and sent for repair to the Soviet Union. After repair, the engine was placed on SP-LAB Tadeusz Kościuszko. After 5000 hours of flight, vibration was detected above the acceptable level and felt noticeably in the back of the aircraft. Therefore, in 1978 the engine was again removed for repair in Poland and subsequently installed on SP-LAA Mikołaj Kopernik. After repair, the engine accumulated 700 flight hours before the accident. In spite of the vibrations, the defective engine was fitted to the aircraft, it being argued that the vibration level was "below acceptable standards." Before the flight to New York the aircraft was checked by mechanic Zdzisław Jarmoniak, who found that the No.1 engine had a defect in one of the turbine blades. This deformation was located in the lowest (and widest) part of the blade. The mechanic wanted to report it, but found that the defect was already marked there (noted), and the plane was subsequently allowed to fly. As he later explained to investigators, the mechanic decided that the defect was