LOT Flight 7 - Causes of Disaster

Causes of Disaster

According to the Polish government's Special Disaster Commission, the crash was caused by defects in materials, faults in the manufacturing process of the Kuznetsov NK-8 jet engine’s shaft, and weaknesses in the design of its turbine.

During the manufacturing of the low pressure shaft, at a location where its section diameter increases, a sharp, 90-degree step was made, resulting in a sudden diameter change over a very short linear length. In engineering terms, such sharp step instead of a radiused fillet causes a significant concentration of stress. This became a tragic, textbook example of stress concentration increase of a "stepped shaft", which in-turn initiated the formation of micro-cracks through the material core, a phenomenon known as material fatigue. Additionally, the metallurgical analysis found, that alloying impurities were present such as non-metallic inclusions in the alloy of the shaft, in addition to the incorrectly performed heat treatment process, which further reduced the shaft's ability to carry the torsional loads as designed. The improperly machined engine shaft, coupled with metallurgical alloy impurities, facilitated an accelerated fatigue of this key engine component via unmitigated formation of micro-cracks through the shaft's core, ultimately leading to its failure.

Over time, the magnitude of the structural defects in "Kopernik"'s shaft reached a critical point, and the shaft broke, resulting in the physical separation of the low pressure turbine from the low pressure compressor. As a result, the low pressure turbine explosively disintegrated. Ejected with enormous force, pieces of turbines damaged 3 of the plane's 4 engines and cut through the hull, causing the failure of the vertical and horizontal flight controls (rudder and elevator), and a massive failure of numerous systems of the airplane. The sudden loss of control of the flight control surfaces caused a steep, unrecoverable dive, and resulted in a crash, 26 seconds from the time of the original failure.

A press article, released in Poland in 2010 and based on the review of archival documentation kept in IPN claimed that PRL authorities contributed to the crash by demanding savings from PLL LOT and excessive exploitation of engines. The article also claimed that, when the engine that disintegrated on 14 March 1980 was earlier found to have an unusually high level of shaft vibrations, the only solution implemented by Polish ground personnel was switching the engine between different Il-62s; also, the turbine disc exhibited evidence of some crude attempts to repair it, against the producer's regulation. Part of the claims of the article was subsequently dismissed by the ground personnel and aviation experts: the mechanics claimed that they reported the high vibration level to the engines' manufacturer, but the only response they received was the paper stating that "the level of vibrations reported does not exceed the levels found by the manufacturers as ordinary." (The Polish TV documentary exhibits those documents in their original form.) Having no technical possibility of getting inside the engine and check it (this also contradicts the claims that the turbine disc was tampered with by the ground personnel), the mechanics decided to switch the suspicious engine between different aircraft, to ensure that it gets on the aircraft where the other three engines are in the best condition so, in case it fails, the aircraft could continue flight on three remaining engines - no one suspected such failure could lead to loss of flight controls and two of three other engines.