Even if you should lose the engine with two good generators and the APU, you still have one engine generator available and it can carry the load.
I've not yet seen actual figures that will support this statement. What I have seen is schematics of left side and right side ECS modules, each with 2 compressors; not sure if this means two ECS modules or four. I've seen a figure of 40 or 45 kw or kva for each compressor (a Boeing figure, IIRC, but I've mislaid my bookmark). I've seen a non-Boeing figure of 70 kw per ECS module.
I've seen a statement that the compressors, combined, are the single largest load on the electrical system. I have no CFM output per compressor figure, but I do have a CFM per pax figure. So whether there is extra compressor capacity that would permit reduced input and under what conditions is a question.
I do know that the compressors have to heat their output air to 200*F, and there is supplemental electric heat if necessary, no kw seen for electric heat. This is required by the ozone reversion system to work correctly or adequately. Thus an air-cooling refrigeration plant is required to bring it back to cabin temperature-- no data on the power input to this, but I do know that typically it takes about one-third the energy represented by the heat removed to move it outboard, but I do not know if this system can achieve that.
Some of the larger control surface loads are hydraulically driven. These loads can be of fairly short duration, but of large magnitude-- a Boeing statement there, but no hint at how large. I'd take it to approach the compressor load, depending perhaps on how many surfaces move at once.
It is true that both the alternators and compressors use synchronous motors, which could mean that the power factor in that loop could possibly be adjusted close to 1.0-- meaning one kva generated could supply one kw load, excepting the transformer-rectifier loss in the DC link, and the wiring and motor-controller loss.
I won't add up all the numbers above right now, but I think it's obvious by inspection that one generator (250 kva at the engines, or 225 kva at the APU) will not do it. My considered guess (an engineering term
) is that three generators could do it, that is, I would allow for one of the four remaining to be out. But I am not now sure that Boeing would have allowed that much margin.
I continue to be puzzled as the why the APU battery must remain connected after it has started the APU. I have read that sometimes a DC voltage is used as a reference in controlling the AC voltage out on an altenator generator, but I am uncertain as to why this battery must supply that. I will have no time the rest of this week to follow up on this, or to sort and include all the system data I have run down.
I did listen to the NTSB video of the first morning session; nearly at the end, questioning by a board member elicted a comment by a manufacturer's specialist that while there are many examples of sucessful lithium-ion battery applications-- almost all, perhaps all-- are so only after problems found in the field in use were solved. That is, design instructions as to requirements, by the buyers, never quite reflected what the situation in the field turned out to require of the batteries. I'll ask someone over on the Tech side to ferret out that comment in my absence.
OE