Quoting Green-dot
“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).”
In my experience, any PED (or domestic kit) more than around 10 years old is unlikely to have been built to any EMC standard. Less than 10 years, most kit sold in the EU will conform to an EMC requirement – usually sufficient for domestic use, not necessarily suitable for aviation use. E.g. A computer with bubble jet printer was installed in an un-pressurised light twin for some monitoring work. As the A/C climbed through FL100 the printer burst into flames. It transpired the spec for the printer bubble jet cartridge had not considered extreme altitude in un-pressurised A/C and would instantaneously combust with no ignition source at around FL100. This manufacturer was responsible and modified the design. To emphasise your point, the kit was made to a spec, but the spec did not take account of all operational conditions. Similarly with the kettle, it still boiled water fine and did not emit any RFI and was within spec. “Our” radio station emitted high levels of RF energy, again within spec. However, an unusual combination of circuit elements in the kettle decoded “our” radio signal. The space inside the kettle (old type, not a jug kettle) acted like a guitar sound box and amplified it so it was audible. Again, everything in spec, but the spec did not consider all possible uses of the kit.
Green-dot:
“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.”
I agree totally.
Green-dot:
“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.”
Also worth noting anything physically mounted on the engine is not in a faraday shield (like the fuselage is). It has 2 large holes at either end, 1 for air in and the other for exhaust out. I accept it will be shielded around the sides.
Green-dot:
“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?”
An electromagnetic pulse may have caused the logic states to reverse, at least until everything re-set itself – however, I would have expected that to be recorded in the FDR somewhere or some other anomaly would make the AAIB think “what caused that reading” on the FDR?
Green-dot:
“Since EMI has occurred sporadically in the past as i have explained, why couldn't it happen again on other aircraft designs?”
I think EMC issues are happening to A/C of the FBW type with some regularity now. Often it is not possible to reproduce the event and it goes unexplained, so “something” was changed as a precaution. As more A/C types become FLB and more PED’s are taken on board the chances of an adverse interaction increase – that is simple mathematics.
Maybe its like the micro-burst situation. Years ago it was not known about and any incident/accident was put down to pilot error. Now it is recognised and specifically looked for. Maybe its time for the AAIB or others to start looking for EMI issues in accident reports?
Green-dot:
“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.”
Evidence of a source pointing in that direction is positive logic. How about negative logic? If, I repeat, if, the AAIB can’t find the positive source, then should they consider EMI (and other) sources? It is clear from the AAIB information so-far (in laymen’s terms) fuel that met the spec did not get to the engines when the computers requested more and it was available on-board. Usually, the fuel system works well, but on this occasion it did not perform as expected. In the absence of other sources being positively identified, I would say that is a classic case of EMI awaiting investigation.
Regards
3db