If we look back in history with large swept wing jet transport aircraft, hydraulic systems were 'protected' from one another while airbourne by system design...IE: not allowing the pilots to transfer
hydraulic fluid from one system to another.
PTU's use the hydraulic power of one hydraulic system to pressurize another, which clearly is not the same.
With the first swept wing jet transport (Boeing 707) there were two hydraulic systems, the utility and the aux systems.
There was a fluid interconnect for use on the ground, but this interconnect was disabled while airbourne.
Except on 707's for one specific airline...PanAmerican.
With PanAm aircraft, if the MLG truck was not level prior to landing gear retraction, the system interconnect was used, together with the aux system electric hydraulic pumps (after depressurizing the engine driven pumps) to
very slowly raise the landing gear and while doing so, allow the rails in the MLG bay to carefully level the trucks.
With this procedure, 10+ hours of fuel could be saved (instead of dumping) and the flight could be continued as scheduled.
Of course, big RED letters in the QRH advised...if a hydraulic leak was suspected, this special procedure was absolutely not allowed.
Boeing...leading the way with technology, since the beginning.
Autoland excepted.
