Accident An-24 in Cherkessk

An-24RV (serial number 37308502, serial 085-02) was released by the Kiev Aviation Plant on March 28, 1973 and sold to the USSR MGA ( Aeroflot airline), which assigned the registration number CCCP-46516 to the liner and sent it to Stavropol United on April 2 air squad of the North Caucasian Civil Aviation Administration; On January 20, 1993, the Stavropol OJSC was transformed into the Stavropol Joint-Stock Airline (SAAC) . It is equipped with two AI-24 turboprop engines manufactured by the Progress ZMKB named after A. G. Ivchenko .

It was a fairly old aircraft, 24 years old with a designated resource of 25 years. In total, the liner underwent 7 repairs, the last of which was September 26, 1991 at ARZ No. 412 ( Rostov-on-Don ). After the last repair, the overhaul life was set to 5 years and 5,000 flight hours, later it was increased to 6 years and 6,000 hours with 5,000 take-off and landing cycles (FAS decision 252.33-503 of October 24, 1996). On the day of the accident, the liner completed 27,628 take-off and landing cycles and flew 41,181 hours and 46 minutes; the time after the last repair was 3660 take-off and landing cycles and 5912 hours 46 minutes ^{[1] [2]} .

The aircraft was controlled by an experienced crew, its composition was as follows:

• The commander of the aircraft (FAC) is 41-year-old Viktor Vasilievich Goncharov. Born April 7, 1955. 1st Class Pilot. 12 641 hours flew, 11 600 of them on the An-24 (9600 of them in the post of the FAC) ^[2] .
• The co - pilot is 34-year-old Igor Evgenievich Osipov. Born on October 6, 1962. 3rd Class Pilot. Since the end of the flight school, 4563 hours 25 minutes flew, 2657 hours 8 minutes of them on the An-24 ^[2] .
• Navigator instructor - 40-year-old Valery A. Nikiforov. Born July 12, 1956. Navigator 1st class. From the moment the navigational school graduated, 11,582 hours and 55 minutes flew, 11,230 of them flew on the An-24 and An-26 ^[2] .
• Bortmekhanik - 38-year-old Rustem Nurievich Asanov. Born October 23, 1958. 2nd class mechanic. From the moment of graduation, 3952 hours and 23 minutes flew, all on the An-24 ^[2] .
• The checking flight instructor instructor is 47-year-old Alexei G. Zherdev. Born March 31, 1950. First-class mechanic. 11 639 hours and 40 minutes flew, all on the An-24 ^[2] .

One flight attendant worked in the airplane cabin - 28-year-old Olga Nikolaevna Spivakova. Born July 8, 1968. Since the end of training, 3057 hours and 40 minutes flew into the position, 2,000 of them on the An-24 ^[2] .

Also in the cabin there were three official passengers to service the aircraft at the Trabzon airport ^[2] :

• Vladimir Aleksandrovich Pivovarov - aircraft technician,
• Alexander Vasilievich Gavrilov - engineer for AEREO;
• Felix Shamilyevich Asanov is a representative of SAAK Airlines at Trabzon Airport.

At the time of the crash, the flight crew had a working time of 3 hours 2 minutes, a flight attendant - 2 hours 57 minutes ^[2] .

An-24RV aboard RA-46516 performed a charter passenger flight SZHL-1023 (SVL-1023) from Stavropol to Trabzon for the transportation of a group of tourists. At 04:00 UTC ^{[* 1] the} crew normally underwent a medical examination, after which it began to carry out pre-flight training. According to the received weather forecast, a strong wind of 260 ° was expected throughout the flight route, and turbulence was expected at levels from 5 to 7000 meters. 41 passengers with 170 kilograms of hand luggage boarded the plane. The take-off weight of the airliner was 21,523 kilograms with a centering of 25.3% of the MAR, which did not go beyond.

At 06:30, flight 1023 departed from Stavropol and, at the direction of the approach manager, took a height of 2700 meters. At 06:37, passing Severny , the crew contacted the dispatcher in Mineralnye Vody and, following the route Severny - Puzar - Abela - Peredovaya under the leadership of the Minvody dispatcher, first took the train 4800 meters, and then 5400 meters. Further, in the region of Abela, the crew requested an increase in height from the dispatcher, for which he received permission to rise to a height of 6000 meters, which he took at 06:55 ^[2] .

The flight took place at a cruising altitude of 6000 meters with an instrument speed of 320-330 km / h. At 07:03 (10:03 local time), when the flight SZHL-1023 flew over the eastern outskirts of Cherkessk , 82 kilometers from Mineralnye Vody Airport and 99 kilometers from Stavropol Airport , the flight information on the radar screen suddenly disappeared, but instead two more marks appeared at the plane’s mark, after which the plane disappeared from the radar. No distress signals from the plane were received. Subsequently, as shown by the data of the flight recorders , the destruction of the fuselage structure began at the echelon, and after a couple of seconds the tail part was separated. Having lost control, the airliner rushed down at a vertical speed of 80–90 m / s and at 07:02:32 (10:02:32 local time) with an instrument speed of 480 km / h with a negative pitch of 25 ° and a left roll of 55 ° crashed into land just 1 kilometer east of residential areas. From the blow, the plane completely collapsed and caught fire. All 50 people on board (6 crew members, 41 passengers and 3 service passengers) were killed ^[2] .

As a result of work performed at the accident site to assess and analyze the technical condition of the destroyed and damaged structural elements of the aircraft, as well as special laboratory tests conducted at the GosNIIERAT MO, the following was established:

The destruction of the side of the RA-46516 began in the air. An analysis of the trajectories of the development of cracks and the structure of their fractures showed that the focal (initial) zone of structural destruction was located on the starboard side of the fuselage under the toilet and sideboard in the area of ​​frames No. 31-34 between stringers No. 6-8, bordering the drain panel of the bathroom below. The focal zone was a through corrosion destruction of the casing in the form of 7 longitudinal cracks with a length of 100 to 330 millimeters and 2 transverse cracks with a length of about 160 millimeters each. In addition, there were numerous through relatively short cracks from 5 to 30 millimeters in length. The total length of the longitudinal through cracks in this section was about 1340 millimeters. The formation and growth of these cracks was of a corrosion-fatigue nature. The length of the section with cracks in the longitudinal direction, taking into account the bridges separating them, was 1523 mm. Merging (merging) of cracks occurred by means of a static dolom of the jumpers of the lining of the repair lining.

There were 12 repair pads on the outer surface of the skin of the initially separated fragment, 4 of which were located directly in the focal zone of destruction. It was determined that one of the 12 linings was installed in the penultimate overhaul of the aircraft and another to eliminate defects identified during the extension of the aircraft overhaul life in August-October 1996. It was not possible to determine when and by whom the remaining overlays were installed.

Comparing the indicated data on the critical size of the cracks with those actually on the crashed plane, it can be argued that before the last flight the state of the lower tail of the fuselage was critical. Therefore, in flight under the influence of normal operating level loads, the bridges were destroyed and a main crack was formed, which caused catastrophic (instantaneous) destruction of the fuselage, namely: the formation of a longitudinal crack forward and backward along the flight along stringers No. 6-8, respectively, to frames No. 28 and 40 s the destruction of lowlands affected by corrosion of frames No. 31-35 in this area. Thus, the destruction of the fragment of the lower part of the fuselage that was originally separated from the aircraft was initiated by the unsatisfactory corrosion state of the clandestine fuselage zone, weakened by corrosion damage to the skin and power elements in the area of ​​frames No. 31-34 (under the toilet and the sideboard).

Corrosion of the panels of the underside of the fuselage between the frames No. 26 and 40 (especially in the area of ​​the toilet and the sideboard) is a massive and well-known defect that is noted on all aircraft arriving for repair. The study and analysis of this design defect was carried out repeatedly. It was found that the corrosion of the panels is caused by the action of an aggressive environment (accumulation of atmospheric condensate, ingress of liquids from the toilet and the cupboard) with insufficient anti-corrosion protection of the lining material and power pack elements (aluminum alloy of the type D16), which is prone to intergranular and delaminating types of corrosion; the processes proceeded according to the mechanism of electrochemical corrosion in the presence of a corrosive medium (moisture with various aggressive components) on the casing in the area of ​​panel destruction for at least 2 years (intergranular character), and on the frames for at least 6 years (exfoliating character).

The specified corrosion condition was the result of poor-quality performance of work on anticorrosive protection and repair of areas damaged by corrosion, as evidenced by:

1. Low quality and unsatisfactory condition of the paintwork; lack of traces of protective preventive compounds.
2. Inconsistency of repair linings on the casing with the requirements of regulatory documents and violation of the technology of their installation.
3. Inconsistency of the required work during the last major overhaul of the aircraft to replace sections of the frames affected by corrosion by the records in the repair and actual work performed.

Factors contributing to intense corrosion damage were:

1. Leakage of the toilet tank due to through cracks and corrosion ulcers in the places of the body weld, performed in violation of the welding technology (burnout of the weld material).
2. Operation of the aircraft for 16 months in a tropical climate (Africa).

The laboratory study allows us to make one more conclusion - the work on the detection and repair of corrosion damage to the fuselage side of the RA-46516, associated with the extension of its overhaul life in August-October 1996, was performed poorly. In particular, according to the assessment of the age of corrosion at the time of the indicated work, the depth of corrosion damage to the fuselage structure elements in the area of ​​the toilet was at least 0.85 millimeters, which is unacceptable for continued operation of the aircraft. In addition, a through corrosion crack in a 50 mm-long casing revealed during inspection in the area of ​​frame No. 31, contrary to the current regulatory documentation, was not cut out with the subsequent installation of the repair lining, but was only cleaned from corrosion and drilled with the subsequent installation of the lining.

From the investigation materials:

1. The failure of the engines of systems and on-board equipment (according to MSRP-12-96) was not in flight.
2. The destruction of the tail of the aircraft occurred in the air.
3. There were no signs of a missile strike, explosion or fire inside the aircraft during the flight.
4. The destruction of the lower tail of the aircraft fuselage when separating a panel fragment between frames No. 28-40 and stringers No. 8-13 on the left has a multi-focal corrosion-fatigue character that began in the panel area in the area of ​​the toilet and occurred from operating level loads (horizontal straight flight, excessive pressure not more than 0.3 kg / cm 2).
5. The reason for the destruction of the panel was a decrease in its bearing capacity due to the extensive area and significant depth of corrosion damage to the skin and power elements from the inner surface of the underground part of the fuselage.

By the time of the disaster, the corrosion state of the aircraft fuselage should be assessed as catastrophic, namely:

1. At the beginning of the destruction (toilet area), the total length in the longitudinal direction of a series of through cracks in the fuselage skin, separated by 10 jumpers in a section of 1523 millimeters, was 1340 millimeters.
2. In the same area, the walls and lower frames of the frames No. 32 and 33 and the shelves of the upper frames of the frames No. 31, 32 and 35 were almost completely destroyed by corrosion.
3. The material of the casing and power set in the vast area between the frames No. 28-36 had corrosion damage that was unacceptable in depth and area.
4. Corrosion processes proceeded according to the mechanism of electrochemical corrosion on the casing in the area of ​​destruction of the panel for at least 2 years (intergranular nature), and on frames for at least 6 years (delaminating nature).

Significant corrosion damage to the fuselage in the area of ​​destruction was the result of poor-quality performance of work on anticorrosive protection and repair of areas damaged by corrosion, as evidenced by:

1. Low quality and poor condition of paintwork.
2. Inconsistency of repair work during the last major overhaul of replacing sections of frames affected by corrosion, records in the repair case and actually performed work.
3. Lack of traces of protective preventive anticorrosive compounds.
4. Inconsistency of repair linings on the casing with the requirements of regulatory documents and violation of the technology of their installation.

The increased intensity of corrosion damage contributed to:

1. Leakage of the toilet tank in the toilet of the aircraft due to through cracks and corrosion ulcers in the places of welding of the body, performed (most likely) during its repair.
2. The formation of material discontinuities in the weld areas is due to a violation of the welding technology, which consists in burning out the weld material.
3. Downtime for 4 months and operation of the aircraft for a year in a tropical climate (Africa), where the rate of development of corrosion damage increases by 1.5 times.

Commission Summary

The cause of the SZhL-1023 flight accident was the destruction of the tail of the fuselage in flight within the permissible operational loads due to the exhaustion of static strength caused by the accumulation of corrosion-fatigue structural damage during operation with an extended overhaul life and service life .

The catastrophe is due to a combination of the following adverse factors:

1. A superficial (without the use of instrumental control methods) assessment by the commission of the technical condition of the side of the RA-46516 and unreasonable issuance of a subsequent conclusion and decision on the possibility of extending the overhaul life and service life.
2. Violation of the requirements of the applicable documents when extending the overhaul life on board RA-46516 and extending the established overhaul life without taking into account its long-term operation in humid and hot climates.
3. Inconsistency of regulatory documents governing the organization of work to establish and extend the resources and life of civil aircraft (Regulation 1994) to flight safety requirements in modern conditions.
4. Violation of the requirements of the technology for repairing aircraft and household equipment during repairs in ARZ conditions.
5. The imperfection of the technical documentation for periodic maintenance regarding the determination of the corrosion and corrosion-fatigue state of the aircraft structure in hard-to-reach areas.
6. Insufficient control in the operation of hard-to-reach areas of the clandestine fuselage in terms of determining the state of structural elements and the presence of corrosion damage.
7. Failure to comply with the prescribed anti-corrosion measures for aircraft construction during repair at the ARZ and in operation ^[2] .

The investigation into the crash case lasted almost six years. At first, 12 people were accused, including employees of the Antonov Design Bureau in Kiev and airline executives. But in the end, only six technicians (Andrei Andreichuk, Yuri Gladkov, Andrei Kondrashov, Aleksandr Kudinov, Dmitri Mezelsky and Nikolai Romanenko) appeared on the dock, who signed a document in August 1996 that extended the aircraft’s flight life ^[3] .

The defendants pleaded not guilty and, together with their lawyers, claimed that a rocket could have hit the plane. To prove this, they claimed that a few seconds before the disappearance of the aircraft from the radar screens, two unidentified objects appeared on them, traveling at an average speed of 2400 km per hour.

As a result, in January 2004, the jury acquitted all the accused ^[4] .

• The crash of a Boeing 737 near Miaoli

Comments

Sources

• Description of the disaster on the Aviation Safety Network
• Accident of An-24RV a / c 'SAAT' in the area of ​​Cherkessk (board RA-46516), March 18, 1997.