Reading the AAIB Bulletin S/1 2008, I found the spar valve description a little bit puzzling regarding the examination of the spar valve circuitry and which valves are actually addressed. The spar valves installed on the rear spar of the wing or the engine shutoff valves:
On page 5 of the bulletin:
"Spar valves
On examination, both engine spar valves were found to be OPEN, allowing the fuel leak evident at the accident site.
The spar valves are designed to shut off the fuel supply to the engines . . .etc. . . . .etc . . . .
The wiring on G-YMMM was as originally designed and manufactured, and such that when the fire handle was operated, it isolated the power supply to the run/cut-off relay. When tested, the run/cut-off relays for left and right engines were still in the valve OPEN position, despite the fuel control switches being set to cut-off. The fire handles had also been pulled and the engine fire bottles had been fired. Therefore the fire handles had been operated prior to the fuel control switches.
The left spar valve circuit breaker (CB) had been tripped. This was due to damaged wiring to the valves as a result of the left main gear being forced upward through the conduit at the initial impact. The tripping of the CB meant there was no means of electrically closing the left spar valve. Similar damage was also evident to the right spar valve wiring, however, in this instance the CB had remained set."
On page 6 of the bulletin:
"Examination and tests of the wiring identified that, in the case of the right engine, the valve CLOSE wire from the run/cut-off relay was still continuous. This could have allowed the valve to operate had the fuel switch been operated before the fire handles."
1. First of all, I assume the AAIB refers to the engine fuel spar valves in the aircraft fuel system, not the engine fuel shutoff valves (SOVs) on the engines? As I read the text, it leaves room for misinterpretation as to which valves they actually refer to. No mention is made of the spar valve control relays, which in case of operating the fire switches are the relays directly isolated, down stream of the run/cut-off relays. The run/cut-off relays referred to by the AAIB are multi-function relays. Depending on which relays are actually addressed, other run/cut-off relays control the engine SOVs and are also wired to the fire switches.
2. Furthermore, has the AAIB positively established (proven by evidence/testimony) that the open spar valves were a result of incorrect sequencing or is that an assumption purely based on the pre SB 777-28-0025 configuration? Or is the AAIB also trying to find other possible causal scenarios that may have lead to a similar effect?
3. As I show in
bold text above, and assuming the AAIB refers to the aircraft spar valves in the engine fuel feed system, there is perhaps a contradiction in their findings. On page 5 the AAIB found the run/cut-off relays "were still in the valve
OPEN position." However, on page 6, in the case of the right engine, "the valve
CLOSE wire from the run/cut-off relay" was still continuous?
I assume the AAIB means the run/cut-off relays (Reset/Fuel Spar L-(R-) Eng) located in the left (right) power management panel. From that relay onward the normal wire paths have yet to pass through the spar valve control relays before reaching the spar valves. If so, how could the right wire path still be continuous when the right run/cut-off relay was found in the OPEN position? Under these circumstances this would imply that the right spar valve control relay should also have been found in the OPEN position, hence no continuity. The bulletin, however, does not mention any examination regarding condition/position of those control relays. My assumption is that they must have electrically closed them in order to test continiuity and subsequently found the right valve wire to be continuous. Or were the control relays (latched) closed without having to move them to the closed position for the continuity test. . . .?
The final report may perhaps answer that question.
Green-dot