Originally Posted by
212man
I think it is very probable that the Nr would decay below 80% during the final flare and, yes, in that situation it would be desirable to have the AFDS functioning. I agree that having the AFDS powered by the Batt Bus is probably a desirable feature. However, I do not believe it is at all plausible that this aircraft hit the water with no engine power and Nr below 80% which is why I suggested your statement about the probable loss of life being connected was unfounded.
Regarding the AFDS power logic, I think we need to step back a bit and consider the whole design and philosophy of the electrical system, which is nothing like a Super Puma. There are three key differences:
- The engines are pneumatically started
- The Generators run off the MGB
- It has an APU, which is used to provide the air for engine starting and acts as the emergency electrical supply
So, essentially, the battery is only there to start the APU. Also, most scenarios that will result in a ditching require the APU to have been started. I think, in this context, the idea of not having the AFDS powered by the Batt Bus is much more understandable than it would be in an aircraft with engine starter/generators and no APU.
Well just on your bullet points, the second one is the same as the Super Puma. I don’t know when the APU is routinely started, other than at start up. For example I don’t think it is started prior to a normal landing or a low level SAR hover? Is it just started in response to some first level failure eg a generator failure? My point is that an APU is only any use if it is on, and it takes a while to start up so in the event of a sudden fairly catastrophic issue it is of no use. Of course the emerg checklist will have lots of progressive scenarios that could ultimately result in ditching after several minutes, but the real world is not always so reasonable and predictable.
And surely that is what the float switches are for - not for an emergency checklist led ditching - you press the button for that - it is for when arriving in the water is sudden and unexpected.