Boof02671 wrote:NW747-400 wrote:Boof02671 wrote:Once again
At LUS our 767s were equipped as follows:
“It does produce electricity but not directly. The hydraulics that the RAT produces turns a special hydro-electric generator that supplies the electricity.”
Not all 757/767 ships have an HDG. They are only installed on ETOPS capable tails. Non-ETOPS birds have a RAT for hydraulic power and batteries for emergency electrical power. LUS 767’s were all ETOPS capable, so you’re referencing a small sample.
The Atlas 767 was used and was an ETOPS airplane when it flew revenue passengers.
Not that it is likely relevant to the accident but N1217A was an ETOPS certified airplane so it would have had the HDG installed.
Systems wise the activation of the RAT and HDG are two separate scenarios, the RAT will be deployed automatically (assuming it’s not manually deployed) with a dual engine failure. The HDG will automatically activate with a loss of both AC busses. So while one can lead to the other they are two completely separate systems. Our manuals also state that the RAT provides “hydraulic power to the flight controls portion of the center hydraulic system.” The HDG on the other hand relies on center air demand pump to ensure sufficient pressure and if you have no engines turning you have no bleed pressure so the HDG wouldn’t work. This is a long winded way of saying that if I’m reading this all correctly by the time the RAT automatically deploys you have no way of powering the HDG and your backup power is from the batteries.