scope:

Description of Driving Event:

The description that follows will demonstrate that pre-flight troubleshooting relied on history of a similar failure and did not check the actual configurations of certain subsystems or their components. Because troubleshooting did not go "outside the box", there was no attempt to think beyond historical failures. This was compounded by a design that did not provide a way to directly verify power was being delivered to the specific component - in this case specific channels to an electronic card.

During the STS-88 (Flt 2A) Node 1 ECLSS equipment checkout, the Node 1 Sample Distribution System (SDS) forward and starboard valves failed to change position when electrically commanded from the ground. Due to a previous failure history, it was incorrectly assumed that the SDS valve failures were attributed to internal binding. A subsequent troubleshooting and repair activity was authorized for STS-96 (Flt 2A.1) to try to repair the valves. This activity was unsuccessful.

However, it was determined from data reviews post STS-96 procedure that the Remote Power Controller (RPC) was open rather than closed during all previous tests. The RPC provides power to the Solenoid Driver Output (SDO) card channels controlling these SDS valves. Analysis has determined that the RPC power feed to the SDO had not been closed for the FWD and STBD SDS valves. Previously, the NADIR and SELECTOR SDS valves had been commanded on both the 2A and 2A.1 missions, with success both times. It turns out that these SDS valves are connected to an SDO card in which the RPC is always kept closed due to SDO card requirement to power the Multiplexer / Demultiplexer (MDM) heaters. A subsequent test conducted at GMT 320/1999 verified that the valves could be commanded to change position from the ground if power was applied to the appropriate SDO card. Flight Note EFCN934 SDS Valve Checkout - Inc B provided the proper power configuration. This problem is documented in PR# 988.

Root cause: An analysis has determined that an RPC power feed to the SDO card had not been closed for the Node 1 FWD and STBD SDS valves preventing the valves from actuating by command. This was due to an oversight of the ISS 2A Operations Checklist to verify that the correct power configuration was enabled to perform the SDS valve checkout procedures. The Operations Checklist indicated what command would be sent to change the position of the SDS valve and the initial / final valve position indications, but no additional commanding to power the appropriate SDO cards. This appears to be a cross-subsystem problem where the ECLSS engineering knew how to command the valves and the EPS engineering knew which RPCs were open or closed, but in the Flight Operational Readiness (FOR) review of these documents, this issue was missed by both.