Originally Posted by
HeliComparator
I've never done an engine off landing in an S92, but surely it is not beyond the bounds of possibility that a frantic hauling up of the collective with no engine power could not dip the Nr below 80% just before water impact? I suppose there is also a question of how long it takes for the alternators to go off line and bus ties to open once the Nr gets below 80%. It won't be instant but it might be quite quick.
It is all speculation of course, but I maintain that it is bad design to have an ultimate last resort life saving device not powered by the ultimate last resort electrical power source. What justification would there be for not having a supply from the battery to the float switches? It is not as if there could be any significant power drain in normal operation. And it does seem odd that there is a CB for manual float inflation on the battery bus, but the blurb posted suggests that this doesn't include powering the float switches. It all seems rather odd.
Do you know what the logic is for the float switches? Does it require any one switch, or both switches to be activated?
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.