Some Mechanical and Structural Aspects of the Smolensk Crash
By Dr. Gregory Szuladzinski, Ph.D., MSME
SCND2014GS

4. More about the fate of the left wing

Above: Fig. 12 End segment of the left wing ( bottom surface).

The location of the final segment of the wing in Figure 12 was marked with squares in Figure 2 and described as a "wing". Because everything takes time, even the system's reaction to the explosion, so the accident had to start a little before the point of K. Let's say, for simplicity, that it was more or less around the square marked "wing".

What information does this carry about the trajectory of a falling item? If we take into account only the beginning and the end, the wing dropped down like a stone. But a stone flying horizontally along the plane has a speed of 270 km/h, or 75 m/s. To fall from a height of 30m, the stone needs less than 2.5s. Within this same time, the stone should cover about 190m in the horizontal direction.

However, the wing is not a stone and if it flies with a main surface forward, it is subjected to a large drag. Still, despite the chaotic movements being possible, you can expect 90, or 60 meters, but not zero.

Looking at the Figure 12, one can observe, especially with good digital magnification, fairly evenly distributed small wrinkles. Because they look tilted to the axis of the wing, they can be associated with high shear force, related to the separation of this segment of the wing. The jagged edges along a break line are not well seen on this photo.

In Figure 3a, where the remains are assembled, one can see (in digital magnification), a large amount of debris between the edge of this segment and the rest of the wing. That remainder of the wing also suffered great damage, as shown in Figure 10 and 11. Everything points to the symptoms of an explosion. Where could be the center of such an event?

One possible location is just before the leading edge of the wing. (Fig. 13a). The event, which pushes out and breaks up a short piece of the wing, applies shear force Q on both sides of the breakout. (Fig. 13b). (The force Q in the figure can be estimated as the product of the effective cross-section of the wing, and shear strength of its material).

Above: Fig. 13. a) The explosion just before the leading edge. The thin wavy line shows the extent of the torn fragment. (The width should not be taken literally); and b) The end piece of the torn wing flies back, turning/ spinning.

This action on the end segment of the wing causes it to gain a reverse speed with respect to the aircraft movement. (Fig. 13b). An impulse applied by the shear force gives rise to a torque around the center of gravity, causing the wing section to spin around a vertical axis. Such a motion allows the wing segment to achieve a significant range of flight in the same way as a boomerang does. This hypothesis explains why the wing segment "returned" to the point of K. The strength of inertia pushed it forward, while the force of explosion pushed it back, spinning it simultaneously.

The second force Q, applied to the remaining wing part, causes an additional resistance on the left side, and thus produces a tendency to turn to the left. A strong impulse in the middle of the wing causes a tendency to tear the wing base, as shown in Figure 11. Apart from this, the impulse could, to some extent, weaken the joints of the front part of the fuselage with the rest of the craft.

Another possibility is an explosion inside the wing, caused by the load detonating between the longerons. It doesn't explain the kinetics too well, because the horizontal component of the pulse would be too small to send the tip in the direction opposite to the movement of the aircraft.

To change the direction of the wing tip's flight, caused by an internal explosion, it is necessary for the shock wave to open the skin of the wing for the recoil effect to eventuate. During the destruction of the skin, the shock wave loses some of its intensity and it has less (net) strength than an external explosion. However, it is possible that an explosive going off in the front part of the profile resulted in only a somewhat smaller effect than an external explosion.

The destruction shown in Figure 10 does not determine whether the event began in the interior of the wing or in front of its edge. It only shows what happened when a strong shock wave penetrated inside the wing.