Originally Posted by
larryboy
Guys, there seems to be an assumption of double hydraulic system failure, this is very unlikely. It looks to me like left system failure only, no emergency was declared, which would have been required after double hydraulic system failure, ie "declare an emergency and plan to land etc...."
You are probably correct in your assumption of dual hydraulic failure occurring, however I do recall a "spate", well at least two instances of dual failures on B757 in the late eighties period and each for the same rather odd reasons. Been a while but something like failure of the left system triggered the PTU to run, driven by the right system to pressurise the left system if fluid available. i.e. it was there to cover a left engine or EDP failure.
A low pressure switch on the PTU output side which would shut the PTU down if no fluid available in the left. This switch failed and so PTU continued to run trying to compress air instead of oil. This in turn caused an overheat in the right system which required pumps to switched off on that side. It may have only been the EDP which overheated but the little 6 gpm electric pump wasn't much use under that load and voila dual hydraulic failure. At least two identical events and although not perhaps a dual hydraulic failure in the conventional sense, it had the same effect at least for a while. Right system could be used for approach if overheat was cleared.