Like many aircraft designed as interceptors, the MiG-21 had a short range. This was exacerbated by the poor placement of the internal fuel tanks ahead of the center of gravity. As the internal fuel was consumed, the center of gravity would shift rearward beyond acceptable parameters. This had the effect of making the plane statically unstable to the point of being uncontrollable, resulting in an endurance of only 45 minutes in clean condition. This can be somewhat countered by carrying fuel in external tanks closer to the center of gravity. Additionally, when more than half the fuel was used up, violent maneuvers prevented fuel from flowing into the engine, thereby causing it to shut down in flight. This increased the risk of tank implosions, a problem inherited from the MiG-15, MiG-17 and MiG-19. The short endurance and low fuel capacity of the MiG-21F, PF, PFM, S/SM and M/MF variants—though each had a somewhat greater fuel capacity than its predecessor—led to the development of the MT and SMT variants. (from the Wikipedia).
The MiG-21 was passed over to the Bulgarian Air Force in the 1960s’ and has since been operated at eight Bulgarian military air bases.
To find the cache, imagine that the plane has taken off unexpectedly and has flown more or less in the direction where its nose cone points. Something went wrong immediately after the departure, and the pilot had to eject.
The place where the pilot landed is unknown, but can be determined using this information.
When the emergency occurred, the aircraft was exactly half-way to the point where the pilot landed after ejecting. This midway point was N42° 39.Z×61 E027° 41.Y24,
where Z is the number of black poles surrounding the monument site, and Y is the 3rd figure in the year, in which the latest “МиГ-21 БИС” variant was passed over to the Bulgarian Air Force (see plaque facing west) minus 1.