True that when on a 747 (or in general on a 4-enginer) two engines fail to run after V1, but before Vr, the...

Question

.... aircraft would not climb or really take-off?

I mean when on a 2-enginer like the triple-7, one engine fails, the other has enough power to make a climb (so the half of the total power is enough). But when on the 747, two engines fail (so the half, because they are 4), it would not go up?

Answer 1

It's been decades since I was a Flight Engineer on C-141's so I'm out of practice on this but I do remember there are many factors, including runway length,  to establish Vr if that is 'Refusal Speed', meaning when you've reached that point you must continue the take off.

Vmcg. Velocity minimum controllable ground speed is one of the factors.  That describes the condition when you lose an outboard engine but can maintain a straight line.  If you lost two engines on the same side you would be stuffed at almost any speed.

Answer 2

Although I have never piloted a 747, I am very familiar with the 737. In general, V speeds are set by the plane manufacturer and are thereby tested vigorously for consistency and for safety. Since V1 is the critical engine failure recognition speed, and Vr is the rotation speed (that must be greater than or equal to the V1 speed) - the decision to continue or abort the take off would depend on the length of the runway, and the current weight of the aircraft. In general however, I would feel confident that two engines on a 747 would provide ample thrust for a gradual climb. I base this opinion on the premise that if V1 = Vr - the take off speed has been met and the aircraft can climb.

Answer 3

it all depends on the weight of the aircraft but in general 747's are long haul and therefore heavy during takeoff so it wouldn't climb with 2 engines out.

Answer 4

generally no, due to the aircraft only being planned to reach Vr after the failure of ONE engine ( the critical engine).

unless you happened to be on an extremely long runway if possible at all.

this is so because of the extreme unlikeliness of a double engine failiure besides the two engines would have to be on opposite wings because if they were on the same wing the aircraft would be unflyable(untaxiable even) due to the yawing moment

Answer 5

It should..When aircraft companies design aircraft they design for what is called OEI (one engine inoperable).  Basically meaning that if one engine fails at takeoff(i know this is an assumption for a 747 to mean two engines, so TEI i guess), the plane would still be able to take off and clear a 50 foot barrier at the end of the runway.  Then return to the airport for a safe landing.  I may be way off it has been a while since i studied this in school.  

Here is a video of a 757 taking a bird strike right before takeoff, and being able to return to the airport.  

http://flightlevel350.com/Aircraft_Boein...

So if you do the math:

757 max takeoff weight: 270,000 lbs

757 engine thrust: 43,180 x 1 (other engine out)

757 thrust to weight ratio with OEI: 0.16

747 max takeoff weight: 850,000 lbs

747 engine thrust: 62000 x 2 (other 2 engines out)

747 thrust to weight ratio with TEI: 0.145

so they are pretty close, and i guess what i am saying is that if a 757 can do it on one engine, then a 747 should be able to do it with 2.