Originally Posted by
Mike Cross
Am I the only one who's puzzled by the POH?
You have flat batteries
You start one engine using ground power
You disconnect the ground power
You now have flat batteries being charged by one alternator
You get enough charge into the batteries to crank the other engine, that depletes the charge, you have flat batteries again.
You take off.
Who's to say that the charge in the batteries is now sufficient to maintain voltage under load?
It's the same principle as a car battery - starting takes a lump out of the charge but running the engine puts it back.
After starting the first engine, the GPU is disconnected so that the pilot can be sure that the aeroplane alternator is capable of powering the electrical system. The use of the battery to start the second engine proves that the battery is recharging properly - and therefore you can be confident that the battery is serviceable. Having decent alternators means that the battery charges quickly - unlike those on some conventional aeroplanes where there is insufficient alternator output at idle to prevent battery depletion.
Originally Posted by
Mike Cross
I don't buy the arguments of moggiee et al. For years aircraft crashed because non-return valves were manufactured so that they could be inserted either way round. Writing documentation that said they had to be inserted the right way round didn't fix it. Making it mechanically impossible by having differing threads on each end did.
Sorry, I may not have made myself 100% clear. I am not saying that there is nothing wrong with the design - clearly there is. However, what I am saying is that it's a known problem which has a perfectly adequate, simple, reliable procedure in place to counter it. This applies to just about every aeroplane on the planet - certainly, for example, to every retractable undercarriage aeroplane where the SOP and landing checklist are designed to prevent a wheels up landing (if followed correctly).
The problem with even an almost-idiot-proof-SOP is that there is an SOP-proof idiot out there somewhere. Despite SOPs, landing checklists and green lights (lack of) there are still people who get it wrong and land wheels up.
My reference to the Kegworth B737-400 crash, for example, showed that despite FOUR indications of an engine problem on the LEFT engine (fluctuating N1, high EGT, low & fluctuating fuel flow and finally a full-scale vibration indication) the crew still managed to shut down the RIGHT hand engine by mistake because they did not follow their SOP.
The DA42 does need a re-design to prevent this happening again, and I'm confident that it will come. In the meantime, if crews follow the SOP and POH then this accident will not recur. However, as sure as the sun coming up each day, someone, somewhere today will be breaking an aeroplane because he did not follow the correct drills - either in normal or non-normal circumstances.
As crew, you are there as the last line of defence against the failings of the designers. No train, plane or automobile is 100% perfect, and most of us know that. The dangerous people are those who believe that a) the aeroplanes ARE perfect and that they will be safe no matter how little they know or b) that they know better than the designers/manufacturers who have flight tested and certified the aeroplane.