NK-8-4 engines were not particularly reliable. Warranty service life was only 5,000 hours, and about half of LOT engines failed after 2000–3000 hours. Because of this, Polish pilots often called the Il-62 "flying coffins". 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.