BA038 (B777) Thread

[Green-dot]

Quoting 3db:

"In my experience PED are normally built to a price, not a spec; as such. A/C, are generally still built to a spec and that includes all the electronics. If that spec is adequate, is a different question. However, the B777 is a very safe A/C."

I agree, the B777 is a very safe A/C. If PEDs are only built to a price and not to a spec, does that mean there is no quality control? To my knowledge, a spec defines what a device is made of and what it should comply with (e.i. items or details included in a description of requirements or plans of a design).

3db:
"In a different field, I once had to investigate a problem of a little old lady with voices coming from her kettle! Yes, its was true, voices were coming from her kettle. Quickly traced to a unusual combination of resistance/capacitance/inductance in the kettle was de-coding "our" radio transmissions. We provided a new kettle, end of problem. Just to illustrate PED are built to a price, any spec (unless it adds "bells/whistles") is secondary."

Same as above, no quality control? Surely there must be procedures in place defining the allowable limits regarding transmitted radiation of PEDs before they are put on the market (and permitted to be used on a plane).

3db:
"Previous AAIB investigations seem to indicate 3 or more items are required to fail before an accident results. Perhaps, only perhaps, EMI was 1 of the 3?"

I agree, perhaps EMI could have been a contributing factor. I am also convinced that the B777 is a very reliable aircraft design.

Therefore, EMI seems highly unlikely. That said, so is a dual engine rollback on a design such as the B777.

The reason i address the EMI issue is that i have had first hand experience with unscheduled closing of fuel shutoff valves in the past due to EMI. This was on another type of aircraft (not any of the Boeing types). I dug up my fuel system manuals for that particular A/C type and checked the wiring diagram which includes the original valve (before EMI became an issue) and the improved valve with an EMI filter installed (lifting restrictions after the original valves were replaced with the improved valves).

I am aware that on the B777 the engine fuel control switching interfaces with ELMS, ARINC 629, AIMS, OPAS and fire switches which are all well shielded systems. The aircraft i refer to (also a FBW design) has none of the above mentioned interfaces. But when comparing the basic electrical control circuit from the fuel control switches to the spar valve actuators, they are functually similar.

Both are fed by 28V dc and with fuel control switch position in "run" or "cutoff", depending on switch position, either the open or close coil in the control relay is energized which controls the valve actuator. Power on the open or close coil moves the actuator motor to the selected position until limit switches are reached which remove power from the actuator.

There is one physical difference between the valves i refer to and the B777 spar valves. The valve i refer to has the control relay (which was sensitive to EMI, hence the filter) integrated in the shutoff valve. In the B777 the control relay and spar valve are separate components. The spar valve control relays (together with the APU fuel shutoff valve relay) are grouped together on a single panel. However, i do not see any EMI filters on the B777 spar valve schematics. Perhaps not required due to other measures taken, making the system imune for such effects, i don't know. Location of other equipment in the vicinity of the relays or valves (or absence of it) is also a factor determining system sensitivity, which obviously varies between aircraft types.

Another point that may be of interest is that (theoretically assuming EMI was a factor) if the spar valve control relays were switched unscheduled from "run" to "cutoff" for a certain period of time, the APU fuel shutoff valve control relay could have switched from "closed" to "open" during the same unscheduled event due to its close proximity to the former control relays. This may perhaps explain why the APU inlet door was observed in the open position?

Since EMI has occurred sporadically in the past as i have explained, why couldn't it happen again on other aircraft designs?

Summarizing, i have factual information that action has been taken to prevent EMI on fuel shutoff valve control relays in the past on another aircraft type. But regarding this B777 incident, evidence (of a source) pointing in that direction would have to be found in order to determine it as a possible cause for a dual engine rollback.

Regards,
Green-dot