A SUKHOI SU-27M development prototype, flown from the Ahktubinsk flight-test centre, suffered a near-catastrophic failure of its fly-by-wire system in April, but was recovered by the pilot.
The incident involved what is described as a "double malfunction" of the flight-control system (FCS). The Su-27M (Su-35)was being used in a test-drop of a KAB-500Kr television-guided bomb.
Su-35 test flights are being conducted from Ahktubinsk, and involve several airframes, including aircraft 706 and 708, although it is not known whether it was one of these which suffered the failure.
Although Sukhoi is working on a digital FCS for the Su-27M, the aircraft at Ahktubinsk are thought to be fitted with analogue systems. The latest Su-27M to be shown publicly, aircraft 711, may be the first to be fitted with a digital FCS.
The aircraft was piloted by Sukhoi test pilot Oleg Tsoi and carried one 500kg bomb under the left wing. The test flight was intend to verify weapons accuracy against a small ground target.
While approaching the test range Tsoi reported bank oscillations of increasing magnitude coupled with cockpit-FCS warnings.
The pilot was commanded to drop the bomb and eject, but Tsoi opted to return to the airfield, 150km (80nm) away, on the back-up "rigid-link" FCS, although the Su-27M is unstable without the analogue FCS.
Tsoi landed successfully, but, during the landing run, the left undercarriage collapsed and the aircraft, carrying the bomb under its wing, veered off the runway and ploughed into a field. Minor damage occurred, but the prototype is to be repaired to rejoin the tests.
Source: Flight International
