Yak-50 Service Bulletins  

The following is intended to clarify the precise position of Yak-50 Service Bulletins relating to wing and centre section structures. 

The main points are:

Ø      The aircraft first flew in late 1972 or early 1973.

Ø      It was issued to the Aerobatic Team of the Soviet Union in 1975, and competed in the World Aerobatic Championships in 1976, and continued in frontline competition until 1984.

Ø      Service Bulletin 16 – dated 24/05/1978, was a stronger centre section, by virtue of reinforcing the vertical wall, and adding a 3.5mm plate riveted to the upper flange.

Ø      The original part number was 500113-00SB before modification, and after 500113-100SB.

Ø      In August 1978 Victor Letsko crashed while practising in a Yak-50.  He had exceeded 12G, this being the limit of the recording instrument installed on the aircraft, so the actual amount is not known.  However the wing broke near its attachment point to the fuselage.

Ø      Service Bulletin 37, was issued December 1978.  This was a reinforcement of the wing spar at the root area, by bolting an plate onto the wing spar vertical surface at the root section.

Ø      In August 1979, Gubenko had a failure of the lower attachment point of the wing to the fuselage, but fortunately parachuted.

Ø      Service Bulletin 42 – dated August 1979 was for the inspection of these rivets along the wing spar attaching the external skin.

Ø      Service Bulletin 43 (shortly after) – required the replacement of these rivets, going from 3.5mm diameter to either 4mm or 4.5mm.

Ø      Service Bulletin 47 – we include this for the sake of completeness, since it is a structural Bulletin, but it is for the fuselage, and not related to the wing area.

Ø      Service Bulletin 53 – issued August 1980 required stronger (but the same size) wing attachment bolts.

Ø      August 1981 Arcischkiavitchus was killed in a crash.  The problem was a fatigue destruction of the lower flange of the wing spar – effectively the lower mounting bolt hole broke.  Again it was clear that this aircraft was being flown at very considerably more than 9G.

Ø      Service Bulletin 61 was introduced in May 1982, which was similar to Service Bulletin 37, and was a further wing reinforcement at the root by installation of a plate along the vertical wall of the main spar, but also a reinforcing plate on the horizontal part of the main spar.  In addition 61 was different to 37 in that the plate is bigger, and its thickness tapers from the root along the spar towards the tip.  The bolt diameters holding the plate were increased to 8.5mm from 8.2mm.  In addition, Bulletin 61 has a reinforcement of the spar flange.

Ø      November 1982 - Designer’s Directive 50-473.  This was the strengthening of the centre section, adding to Service Bulletin 16 by a 3mm plate under the centre section.

Ø      In 1983 Gomyliaev was killed following a centre section failure.  This was the only known case of centre section failure.

Ø      Service Bulletin 79 – dated July 1985, was replacing the Designer’s Directive 50-473 and Service Bulletin 16.  It consisted of a 4.5mm plate on both top and bottom of the centre section. 

Ø      There have been no failures for any aircraft incorporating all the above Service Bulletins.