Aeroflot Flight 6515. Aeroflot Flight 6515 was an scheduled flight operated by Aeroflot that crashed on October 23, 1978, in Sivash Bay due to engine failure caused by icing and crew error, resulting in the deaths of all 26 people on board.

Accident 
The crash of the An-24 over Sivash Bay was an aviation accident that occurred on 23 October 1978, in the Sivash Bay with an An-24B aircraft operated by Aeroflot, resulting in the deaths of 26 people.

Aircraft 
The An-24B with tail number 46327 (factory number 97305504, serial number 055-04) was produced by the Antonov plant on August 26, 1969. At the time of the accident, the aircraft had a total flight time of 15,851 hours and had performed 13,457 landings.

Crew 
The aircraft was piloted by a crew from the 83rd flight squadron, which included the following members:
 Pilot in command — Vladimir Mikhailovich Dubinin;
 Co-pilot — Vladimir Vladimirovich Yunkin;
 Navigator — Vladimir Borisovich Latalin;
 Flight engineer — Vladimir Alexandrovich Lisitsyn;
 Flight attendant — Nina Rudolfovna Vorobyova.

Accident 
Earlier that day, the crew had already completed flights on the Stavropol—Grozny—Stavropol route. They were now scheduled to operate Flight 6515 from Stavropol to Simferopol—Chișinău—Lviv. There were 21 passengers on board. At 18:14 Moscow time, the An-24 departed from Stavropol Airport, and after reaching cruising altitude, the aircraft maintained a flight level of 2,400 meters, following an alternate route 29B (Gelendzhik—Tobechik—Lenino—Yemelyanovka—Simferopol).

The sky over Stavropol was partly cloudy, with visibility exceeding 10 kilometers. However, near Kerch and further on, the cloud cover was continuous with a lower boundary of 600–800 meters and an upper boundary of 3,000–3,500 meters. The wind at 2,400 meters was strong and westerly (40–50 km/h). Additionally, as the aircraft approached Simferopol, the intensity of icing increased.

At 19:39, when the An-24 was flying at an altitude of 2,450 meters in clouds for about 15 minutes, the crew was contacted by the dispatcher of Simferopol Airport. In response to the dispatcher's inquiry, the pilots reported, "Flying in clouds, experiencing icing," and followed up with "The icing is quite substantial." A minute later, the crew contacted the dispatcher again and reported that the left engine had failed. Fourteen seconds later, they clarified that both engines had failed, and the crew decided to descend toward land. After this, the crew did not make further contact.

At 19:40, after 1 hour and 25 minutes and 31 seconds from the time of departure from Stavropol, both engines of the An-24 failed almost simultaneously, with a 3.5-second interval, and the propellers were automatically feathered. The crew made a sharp left turn toward land, during which the speed dropped to 250 km/h. They then attempted to restart the left engine. The engine did not restart, but the attempt resulted in reverse thrust, causing the speed to drop to 186 km/h, and a left roll of 45° occurred. The aircraft entered a critical angle of attack and began a deep spiral descent, with its forward speed increasing. A pitching moment occurred, causing the crew to pull the control yokes fully back, which increased the angular velocity. At an altitude of about 500 meters, the aircraft made a complete 360° roll around its longitudinal axis. A minute and a half after the engines stopped, at 19:41, the An-24 crashed into the surface of Sivash Bay with a forward speed of 490 km/h and a vertical speed of 90 m/s at an angle of about 50° with the nose and left wingtip. All 26 people on board were killed.

The aircraft was found the next day, submerged at a depth of 2.5 meters and located 24 kilometers southeast of the village of Yemelyanovka. Although the water layer was only 2.5 meters deep, a layer of soft silt extended down another 4.5–5 meters. Recovery operations began on the day the wreckage was found and were conducted in almost zero visibility until November 18, when the left engine was recovered. All major wreckage pieces and the flight recorders were brought to the surface.

Causes 
According to the data extracted from the MSRP-12-96 flight recorder, it was established that for 79 minutes of the flight (except for the first 7 minutes), the crew did not activate the wing and tail anti-icing system (AIS), and the engine AIS was not activated until the last 3 minutes before the end of the recording (crash into the water). It was impossible to determine whether the system was activated in the remaining 1.5 minutes before the engines stopped. The air bleed valve for heating the inlet guide vanes and air intakes of the left engine was found in the open position when recovered from the water, but the flight recorders did not record its activation, indicating that it might have been opened either during the first 7 minutes or between 3 and 1.5 minutes before the end of the recording.

To determine the causes, the commission conducted flight and bench tests of the engines under icing conditions. During flight tests, the first ice shedding (40 mm thick) into the engine occurred 1 minute or more after heating was activated and did not cause automatic feathering of the propeller. On the test stand, ice shedding led to automatic feathering of the propeller 40–45 seconds after the engine's AIS was activated. A flight test with 65 mm thick ice led to immediate engine shutdown with automatic feathering of the propeller. It was also noted that no deformations or dents remained on the IGV blades or the first-stage compressor blades after ice ingestion.

Based on these findings, the commission concluded that the simultaneous engine shutdown that led to the crash was likely caused by the icing of the air intakes and IGVs, followed by ice shedding into the engines' gas-air ducts due to the crew's late activation of the heating system. The commission also noted that the AFM lacked recommendations for actions in the event of dual-engine failure during flight, and there was no crew training program for actions in such conditions (night, flight in clouds, and icing), which contributed to the emergency situation escalating into a catastrophic one.

References 

6515
Nyzhniohirskyi Raion
October 1978
Accidents and incidents involving the Antonov An-24
Aviation accidents and incidents caused by ice