The ILS Failure Review Oversight Board (FROB) has concluded the May 16 failure of the Express AM4R satellite was down to a loss of structural integrity of a bolted interface that attaches the Stage III steering engine turbopump to the main engine structural frame.

In a detailed review of reports from the Russian State Inter-agency Commission (IAC) and Khrunichev (KhSC) investigations into the probable cause of the failure the FROB said the loss of integrity led to an excessive steering engine turbo pump vibration environment that damaged a fuel inlet line to the oxidizer gas generator, resulting in a fuel leak. The loss of fuel led to the premature shutdown of the turbopump and loss of stage control authority and ultimately loss of mission approximately 545 seconds into the flight.

The FROB agreed that the identified corrective action plan would address the identified probable cause and contributors to the failure.

“We thank all of the FROB participants—our customers, insurance underwriters, technical experts and all others–for their diligent work and generous time; their continued support is sincerely appreciated,” said ILS Chief Technical Officer and Vice President of Programs and Operations, John Palmé.

Proton returned to work on September 28 with a launch on behalf of the Russian Federal Government.