Aeroflot Flight 971

On 26 August 1954, Aeroflot Flight 971, a scheduled domestic flight from Khabarovsk to Yuzhno-Sakhalinsk operated by a Lisunov Li-2 crashed near Yuzhno-Sakhalinsk killing 26 of the 27 occupants on board.
Aircraft
The aircraft involved in the accident , manufactured on 25 December 1949, was a Lisunov Li-2 registered as CCCP-L4679 with serial number 6803.
At 02:37, the crew reported passing the NDB, and the controller granted permission to descend through the clouds according to the established procedure. The Li-2 began its descent on a heading of 192°, and shortly thereafter, the crew reported leveling off at 800 meters. At 02:42, the captain reported that a standard turn had been completed and descent continued towards the outer marker (DPRS), following a magnetic heading of 10°. At 02:45, the crew transmitted: “Altitude 450, descending to outer marker.” This was the last radio transmission from flight L4679. At 02:50 MST (09:50 local time), while flying on a heading of 10°, the Li-2 struck trees on the slope of a 460-meter hill. The aircraft then plowed through the trees for 128 meters before crashing into the ground at the summit of the hill, completely breaking apart up to frame 25 (half its length) and coming to rest upside down. The accident occurred near Mount Kommissarskaya, 13.5 kilometers southeast of Yuzhno-Sakhalinsk and 9 kilometers from the DPRS of the military Korsakov airfield, which belonged to the aviation of the Pacific Fleet, east of the landing pattern for Bolshaya Elan airport. The crash site was found the next day, with only one survivor — a colonel of the Soviet Army born in 1914, who was taken to the hospital in critical condition. The other 26 people (5 crew members, 13 adult passengers, and 8 children) perished.<ref name="AirDisaster.ru" />
Investigation
The examination of the wreckage revealed that at the time of the collision with the trees, the engines were running and the landing gear was extended. The ARK-5 radio compass was switched on and tuned to a frequency of 683 kHz, close to the operating frequency of the beacon at Bolshaya Elan airport — 670.2 kHz. At the same time, the operating frequency of the beacon at Korsakov airfield was supposed to be 687 kHz, but a test measurement showed it was 684 kHz, even closer to the operating frequency of the civil airfield beacon. The beacon of both airfields are only 13.5 kilometers apart, and in combination with the small difference in frequencies, this led to cases where their signals overlapped, causing the call signs to interfere with each other. A minor error in setting the ARK-5 radio compass could easily lead the aircraft to the wrong station. At Bolshaya Elan and Korsakov airfields, there was no continuous monitoring of the nominal frequencies of the NDBs, although crews had repeatedly complained about errors in the operation of the Bolshaya Elan airfield beacon. At Korsakov airfield, control was carried out only once in the last two or three months. Furthermore, the air traffic control service at Yuzhno-Sakhalinsk airport did not properly manage flights during cloud penetration and landing approaches, and did not adequately monitor the movement of airliners in the airport's airspace. Moreover, Bolshaya Elan airport lacked the radio-technical equipment that would have allowed ground controllers to monitor the position of aircraft in the airfield's airspace under difficult weather conditions. In addition, crews were not informed that the NDBs of both the civil and military airfields were operating simultaneously, and the flight supervisor did not require the crew to activate the RV-2. It is worth noting that prior to the crash of flight 971, there had already been two cases in May of that year where civil aircraft mistakenly approached the Korsakov airfield's outer marker, but the command of the Far Eastern Civil Aviation Directorate and the Khabarovsk Aviation Group did not take any measures to prevent such incidents in the future.<ref name="AirDisaster.ru" />
Causes
According to the commission's conclusions, the crash occurred due to a collision with terrain that was outside the crew's line of sight. This was caused by the crew's failure to notice that they had incorrectly set the frequency on the radio compass and subsequently did not listen to the call signs. As a result, the airliner began descending and penetrating the clouds according to the Yuzhno-Sakhalinsk airport approach procedure, but based on the Korsakov military airfield's beacon. Additionally, the crew did not use the available radio altimeter, which led to the aircraft being dangerously close to the ground while flying over the hill, eventually crashing into the trees.<ref name="AirDisaster.ru" />
The following factors contributed to the incident:<ref name="AirDisaster.ru" />
# The crew's mistake in setting the radio compass to the Bolshaya Elan airfield's beacon;
# The very small difference in the operating frequencies of the NDBs of Bolshaya Elan and Korsakov airfields, given that they are located relatively close to each other, and the lack of adequate control by the service personnel over the nominal frequencies;
# When, in May 1954, three months before the crash, there were already two cases of airliners mistakenly approaching the Korsakov airfield's beacon, which, fortunately, ended safely, the Khabarovsk Aviation Group's command and the Far Eastern Civil Aviation Directorate's leadership did not take the necessary measures that could have prevented these violations.

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