Ask Captain Lim - Why Boeing 777 has no critical engine and what is an aft loaded wing?

Unlike propellers aircraft, there is no critical engine in the right sense of the word because both the engines of the Boeing 777 are equally important. In a propeller aircraft, usually one engine is more critical than the other because both the propellers turns in the same direction. As a result, when an engine fails, it is more critical and difficult to control on one side than the other. That is why they have a critical engine, usually the right engine, I believe (I have not flown a propeller aircraft for quite sometime now!).

Aft loaded wing refers to where the lift force acts on a wing. If you were to look at a side profile of a wing called an airfoil, you could see the distribution of the pressure acting over the shape. On most traditional airfoil shapes, this distribution tends to be concentrated towards the forward portion of the airfoil.

While these sorts of airfoil shapes worked well on planes up through the start of World War II, they were not as efficient on newer aircraft that approached the speed of sound. On high speed flight, shock waves start at about Mach 0.7 and they result in large increases in drag. They may interfere with control surfaces thereby drastically degrading a plane's performance. A new kind of airfoil called the supercritical airfoil was thus designed. As the pressure is distributed over the surface of a supercritical airfoil, the pressure is distributed much more evenly with a large portion of the lift concentrated towards the aft end of the shape.

This supercritical airfoil is thus termed as 'aft-loaded' because the lift force is more significantly 'loaded' onto the aft portion of the shape. When Boeing refers to the Boeing 777 as having aft-loaded wings, what they are implying is that this aircraft use supercritical airfoils. This is because the cruise speeds of the aircraft is around Mach 0.84. These aft-loaded wings are more efficient than a conventional wing because they reduce or delay the adverse effects of shock waves in transonic flight.