BA038 (B777) Thread

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Quoting Chris Scott:

What I have deliberately left out so far, in accordance with the AAIB Bulletin, are the spar valve control relays that you are concerned about. Are they relevant? You say that they are downstream of the run/cut-off relays, and are directly controlled by the fire handles. This seems to be at odds with the AAIB’s explanation. Are you saying that they are normally sequenced to the valve-open position by the run/cut-off relays? Are you proposing that they are possible candidates for RF interference?

Chris,
Yes, the spar valve control relays are relevant. The AAIB wrote that, i quote: "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." (Here i assume they mean the spar valve circuit not the engine shut off valve circuit because the header on page 5 refers to spar valves not SOVs)

Without reading anything more into it than what the AAIB wrote here, this could imply that from the run/cut-off relay (which is inside the right power management panel, part of ELMS), the CLOSE wire downstream, right up to the spar valve, had continuity. The relevance here, with the run/cut-off relay in the OPEN position, is that in order to be able to measure continuity on the CLOSE wire right up to the valve implies that the path has to go through the right spar valve control relay which had to be in a closed position in order to measure continuity right up to the spar valve.

Normally, under the conditions described by the AAIB, one would expect that control relay to be open, similar to the run/cut-off relay. They are in the same path (the control relay somewhere between the run/cut-off relay and the spar valve). The fire switch, when selected from "Normal" to "Fire", isolates the above mentioned relay circuit. The wire making the "Normal" circuit is directly connected to the control relay, not the run/cut-off relay. When the switch is selected to the "Fire" position it isolates the "Normal" relay circuit and feeds a close signal directly via a second (separate) wire path to the spar valve if all functions as expected (to be clear, this is the pre SB 777-28-0025 situation valid for G-YMMM as the AAIB explained and makes sequencing important). The 28V dc power source is connected to the fire switch. With the fire switch in "Normal" position, power is supplied to the normal run/cut-off circuit and the second wire path is isolated.

The AAIB, since they mentioned the run/cutoff relay, therefore did not refer to measuring continuity in the second (switch in "Fire" position) wire path but in the "Normal" wire path, which, as explained has to inevitably go through the control relay.

Regards,
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