It seems that even most reliable rocket can be affected with “bad day” syndrome.
According to TASS news agency during last launch of Soyuz-2.1B, which took place on May 29, 2016, some kind of problems appeared before separation of Fregat-M upper stage with the rocket. Due the unspecified anomaly, Fregat-M upper stage was forced to exceed burn time to reduce effect of the problems which appeared in previous phase of flight. Similar situation appeared during last launch of Atlas V with Cygnus OA-6 on atop on March 23, 2016; Centaur upper stage was keeping its propulsion working for prolonged time to overtake insufficient burn time of first stage of the rocket. During Soyuz-2.1B/Glonass-M launch performed from Plesetsk cosmodrome, first three stages (or one of the stages) of the rocket were not able to provide enough thrust to reach designated orbit for Fregat-M. Upper stage started its S5.92 engine to provide 19.85 kN of thrust and with extended burn time helped Glonass-M in reaching correct orbit. It was not unveiled how big was difference between ordinary burn time of S5.92 engine comparing to ordinary flight. This anomaly not direct threat for mission mainly thanks to Fregat-M features: ability for performing up to 25 burns, propellant capacity of 5600 kg (Fregat-M has increased fuel reserve from 5250 kg to 5600 kg comparing to previous Fregat version), independent guidance, navigation, attitude control, tracking and telemetry systems. TASS claims that special State Commission already started investigation on anomaly. In spite of problems during flight, Glonass-M reached its orbit and is operating correctly.
Fregat-M was designed and is manufactured by NPO Lavochkin. Its dry mass is 980 kg with height at 1.5 m and diameter of 3.358 m. Around fuselage fuel tanks are attached, each tank has installed on the top two additional smaller tanks. Single S5.92 engine is fueled with 5600 kg of N2o4/UDMH and gives 19.85 kN of thrust; 3- axis stabilization is using twelve additional thrusters. Main engine can be used with two to thrust levels: 13.96 kN/6.85 MPa chamber pressure and 20.01 kN with chamber pressure at 9.8 MPa.