B747, double engines lost...

[MagentaLine]

two engine separation

You didn't mention in what phase of flight and your problems are directly related to configuration. The most critical phase would be immediately after V1 and the a/c (B747-100, -200, -300) would typically be very heavy due to enormous fuel and cargo loads (worst case scenario 750,000# to 835,000# max t/o). With the loss of #3 & #4 hydraulics, pneumatics, generators, etc. many things aren't going to operate normally but will via alternate means. You'll be returning to the airport and a heavy cargo classic will most likely need to begin immediate fuel jettison. The leading edge flaps will still be extended so no need to worry about them. This IS a problem if you were to lose both right engines late in the flight (cruise configuration) and have ruptured pneumatics on one wing (right) and go to configure to flaps 5. If you fail to arm the alternate leading edge flaps (electrical override) the left wing leading edge flaps will extend pneumatically while the right ones won't (flap asymmetry). This can require large aileron correction and increased drag (speed brakes) if greater than 8 degrees. Since we are in t/o configuration we also don't have to worry about not having hydraulics on 3 & 4 for outboard gear extension (they'd still be down & locked). If you were in cruise during the ME out, you simply need to be 230-250 knots for proper alternate outboard gear extension. Back to our t/o situation and the outboard trailing edge flaps would be set to flaps 10 or 20 for t/o. Normal landing flaps are 25 or 30 but electric extension only takes about 5 minutes from flaps up to flaps 10. We'd already be at least at flaps ten so it wouldn't take long to drive them electrically to a landing configuration. Because we wouldn't have #3 hydraulics we don't get to use the A autopilot and the stabilizer trim rate & spoiler capability is reduced. Because we lost both #3 & #4 hydraulics, some flight control response will be slower than normal but redundant systems keep us ok otherwise. The primary brakes are also on #4 hydraulic system but we can use #1 instead and still have a third option with #2 hydraulic reserve brakes but they won't hold the parking brake. The loss of #4 hydraulic also means we can't arm the speedbrakes for landing. Boeing builds an incredible plane and the B747 is the definition of redundancy. Just don't try to pull a UAL and use ailerons to compensate for yaw - that's what a rudder is for. Other simple tricks to remember for B747 classics are the #1 hydraulics are for inboard gear, inboard flaps, body gear steering and secondary brake source. The #4 hydraulic system is for the outboard gear, outboard flaps and primary brake source.

Hope this helps.