From what I understand, the 744 has 4 independent hydraulic systems that provide quadruple redundancy capabilities. Different systems on the aircraft are powered by 1 or the other of the 4 hydraulic systems. Last week I heard about a 744 that had a number 4 hydraulic failure on takeoff and as a result they couldn't get the right (I think it was) wing gear to extend. The crew was able to return and land without scraping the outside engine. Now my question is, with 4 hysdraulic systems how come the aircraft wouldn't simply switch the required functions from the broken #1 system to one of the three functional hydraulic systems? Thanks a lot for the time guys...

From what I've read, the gear problem wasn't a result of the hydraulic problem.An uplock motor failed to activate when they switched to the alternate extension mode, causing the gear to stay in the up and locked position.

I use this as a memory aid. The number 1 system powers systems that are laid out like the numeral "1." The #1 system powers the nose gear and the 2 body gear extend/retract functions (plus nose gear/body gear steering and inboard flaps). The #4 system powers the wing gear extend/retract functions (and the otbd flaps). That kinda looks like a 4 . . . well if you use a little imagination.However, if #4 system completely failed, the wing gear (and all the gear ) has a non-hydraulic backup free fall system. That should have let the gear free fall to down-and-locked. There's a big skid paddle in the gear well to help the wing gear wheels to push open, then skid by the door. (It's greased up like a semi hitch. If the boss doesn't think you've been working hard enough, just go out and give that that thing a big hug. Works like a charm!) There must have been a double failure somewhere, but I've seen it happen. Generally, they will just stop out on the runway and we have to go out on a rescue mission. Towing that big boy in on only the wing gear is a real hazard. Even with the all the fuel inboard, it can get a little tippy. That nose gear really gets bouncy (light). If the thing starts to go, just jump off the tug and run for your life. It's your only chance for survival! With just the body gear, go easy 'roud corners. That narrowed wheel base is a hazard too. Could get to be a bad day real quick. (Ahhh . . . Just a helpful tip!)

Electric/gravity backup is a viable alternative... but not in this case, it seems :)Having a dual hydraulic system per gear would probably be too complex (and weighty). Even if different hydraulic systems fed common actuators (for weightsaving), a broken actuator might result in fluid leakage from two hydraulic systems (far more serious).Cheers.Q>