Investigation
The investigation was conducted by a Czechoslovak commission headed by Chief Inspector of Aeronautics Jan Dvorak. Under international law, he coopted representatives of the country in which the aircraft was registered and representatives of the Ilyushin design bureau and the Ivchenko engine design bureau. A Bulgarian commission of enquiry was also formed, headed by State Comptrol Commission Chairman and BKP Bulgarian Communist Party Central Committee member Ninko Stefanov and also included TABSO Director General Lazar Beluhov, air specialist and deputy defence minister Gen Avgust Kabakchiev and Darzhavna Sigurnost (State Security) investigator Col Ivan Ohridski, chief police pathologist Dr Yordan Peychev and enough others to fill two specially chartered aircraft. The Bulgarians requested to lead the inquiry under a provision in international law which allowed this. The Czechoslovak authorities refused this on the grounds of reciprocity: Bulgaria had not signed the clause allowing foreign investigators to inquire into crashes in Bulgaria.
The investigators documented that the local militia (police), Czechoslovak State Security and Bratislava Military Engineer School ranks searched an area of some 350m by 50m (1000 ft by 150 ft) surrounding the accident site. This involved removing snow cover of between 30 and 50 cm (12 to 20 in). The fears that the airliner was carrying radioactive isotopes were confirmed on 8 December 1966 when it was stated that it comprised two steel-lead containers with Iodine-131 for medical purposes. This isotope is, in fact, entirely harmless to human health and permitted for carriage on commercial flights under international law. The airliner's altimeter was found to indicate the correct height above Bratislava airport.
Extreme and escalating tension between the Bulgarian and Czechoslovak authorities and accident investigators was apparent from the outset. An advance TABSO party led by Beluhov had arrived at Bratislava the day after the crash but had been denied any access to the accident site or to any air traffic controllers. The Bulgarian side believed that air traffic controllers in the local Tower and Approach sectors had neglected their duties in allowing a slower aircraft (the Il-14) to depart before a high-performance aircraft (the Il-18), in failing to clear flight LZ101 to a safe height, and in failing to monitor its progress on radar. The Czechoslovak side believed that the flight crew had demonstrated poor comprehension of English by failing to turn right after lift-off, as instructed. The Czechoslovaks were also accused by the Bulgarians of spreading rumours that LZ101's flight crew had consumed alcohol while waiting in Bratislava. Dr Peychev insisted that a test recently developed by NASA be conducted on tissues taken from the Commander and Second Pilot; the results showed that they had not consumed alcohol.
Ultimately, Czechoslovak Minister of Transport Alois Indra assumed overall control of the investigation in order to resolve the conflict between the Bulgarian and Czechoslovak sides. The issues in the conflict touched national prestige and the amount of damages payable to the victims' families: if Czechoslovak air traffic control was found deficient, they would receive 20,000 US dollars each. If the Bulgarian crew was found deficient, they would receive 10,000 leva each.
The Czechoslovak commission's eventual report stated that departure from runway 31 followed by climb to 300 m/1000 ft was possible without colliding with obstacles. While the stipulated height of 300 metres was adhered to, the aircraft departed from the route which would have kept it clear of ground obstacles. This deviation may have been caused by a wider than necessary turn, greater speed, or a combination of both. It was not possible to exclude the possibility that the crew may have doubted the correct operation of their artificial horizon. It was also not possible to exclude the effect of turbulence on maintaining a uniform bank angle in the turn. The crew, however, had sufficient warning of the weather. The situation became critical when the crew failed to execute the stipulated and acknowledged departure manoeuvre and when unpredictable circumstances may have arisen and accrued during the turn. The report concluded that the cause of the accident could not be determined clearly from the established facts. The most likely cause was inadequate evaluation of terrain and weather by the crew, leading them to fly in a manner inappropriate to these conditions.
Read more about this topic: TABSO Flight 101