Airbus Hydraulic Failures
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Because the landing gear (and most other hydraulic consumers) does not have multiple independent hydraulic actuators connected to seperate systems.
It would be stupid to "x-feed" hydraulic pressure and fluid to a system, that may have a leak.
On the 747 one hydraulic system supplies the wing gear, while another supplies body gear and nose gear. However it is not possible to feed among the different systems.
By using the alternate gear extension procedure, hydraulic pressure is locked out. For that reason it is not possible to retract the gear again. On some planes you can revert the alternate gear extension procedure again, after which hydraulic supply is again available. In case of a hydraulic failure, this is however not advisable.
Brgds
SE210
It would be stupid to "x-feed" hydraulic pressure and fluid to a system, that may have a leak.
On the 747 one hydraulic system supplies the wing gear, while another supplies body gear and nose gear. However it is not possible to feed among the different systems.
By using the alternate gear extension procedure, hydraulic pressure is locked out. For that reason it is not possible to retract the gear again. On some planes you can revert the alternate gear extension procedure again, after which hydraulic supply is again available. In case of a hydraulic failure, this is however not advisable.
Brgds
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I think you'll find the reason is that Airbus don't want to compromise the last remaining hydraulic system by a) demanding large flow rates and b) possibly opening up a source of a leak just when you don't want it!
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So I've heard through the rumour mill from the only known double failure is that when two systems are down, even simple manipulation of the controls causes some major hydraulic pressure fluctuations. 
Dream Land
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It would be stupid to "x-feed" hydraulic pressure and fluid to a system, that may have a leak.
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I know, that you can not transfer fluid from one system to the other. That is my point.
On some planes you have a power transfer unit, in which you can transfer pressure from one system to the other.
On some planes you have a power transfer unit, in which you can transfer pressure from one system to the other.
On some planes you have a power transfer unit
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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.
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.
