When BWoS built the VC10K, the load distribution was changed so that the HDU motor starting load back EMFs wouldn't upset the aircraft instruments. This meant that all attitude instruments were powered by the #1 and #3 busbars. In order to provide at least something in the event of an electrical fire on the normally-commoned #1 and #3 busbars, they included a '#3 Bus Tie Breaker Smoke Trip Switch', so that the offending source could be isolated. But any AC supply failure would normally be resolved by the system commoning the remaining supplies.
So it was theoretically impossible to have a #1 and #3 AC bus failure....wasn't it?
Riiiiiiiiiiiiiiigggggggghhhhhhhhht!
One fine day I was conducting an air test on 'The Lizard' which had just been retrieved from Scrapheap Challenge St. Athan in order to participate in one of Bliar's 'bring a bottle' Balkan wars. It had been left in open storage in the balmy Welsh climate for several months, until the gingers popped some engines back in, gave it a quick wash and brush up and invited me to come over and fly it.
All went well until it came to the shut-down and relight part of the test. After shutting down no. 3 engine, the system behaved as advertised - for a while. Then the #1 alternator decided it couldn't quite cope. But rather than come off line cleanly, the rust and corrosion in the alternator control panel refused to allow a normal disconnect. So various time delay units and bus tie breakers came out in sympathy to prevent a cascade failure and the next thing I knew was the bright red lights of the #1 and #3 busbar failure warnings, alert lights, lots of warning flags, failure of several PCUs and to add to the fun, the cabin pressure began to climb because whatever controlled it had now found itself without power. A few rude words followed from the Air Eng as he contemplated a panel full of warning lights which he'd never even seen in the simulator.
This was a bit of a bugger. My co-pilot advised ATC that we had a problem and wouldn't be able to change heading for a while, as every single gyro instrument was in the process of winding down....
The no.3 engine relit at the first attempt, so we soon recovered all the instruments and PCUs, regained control of the cabin pressure - and decided to call it a day.
In the case of multiple alternator failure, it would have been normal to drop the Electrical Ram Air Turbine. But it was just as well that we didn't, as when I took it up on the next air test, the conclusion of which required the ELRAT to be dropped, we had another exciting time as the ELRAT went well out of limits, shortly followed by a smoke warning. So we landed and evacuated with Trumpton in close attendance. The cause? Yet more corrosion, this time in an alternator rectifier which had turned green thanks to the lovely Welsh climate.....
So if an engineer claims it cannot happen, it very well might!
As a postscript, our wonderful Sqn Cdr was more concerned that I'd declined the offer of donating to the Med Centre's leech-handlers than the fact that we'd had such an entertaining day. Why had I declined? Well, we'd been sheltering by the fire wagons waiting for a crew bus for about 10 minutes and had probably ingested more crud from their thick black diesel exhaust than we could possibly have done in the aircraft on 100% oxygen.