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

7. Separation of the Tail Section of Fuselage

Figure 23 is one of the pictures available that shows the tail segment at the place of fall. Construction drawings show that the bulkhead, around which there has been a rupture and separation from the rest of the hull, was strong, well built and had a concave (as viewed from the front of the plane) metal wall.

If the fuselage fell at this spot in one piece and if the rear part hit the ground, the bulkhead would buckle, or even crack, but it would remain as a part of this section of the hull. To achieve this condition, the perimeter would have to ovalize. However, the profile, partly visible in the photographic documentation of the scene, is round, at least to the naked eye. The hull does not look distorted by a strong external impact. But the bulkhead itself appears damaged. Only high internal pressure can explain this. This state fits the hypothesis of an explosion.

Apart from the above it should be noted that the left part of the beam connecting the two motors in Figure 23 is bent inside the tail section of the hull. This may be associated with hitting the ground, although it seems that in this case it should be bent in the opposite direction.

Again, we face the question of fuel vapor explosion versus HE material. There is a difference between the nature of the shock wave in both cases. The latter gives a thinner, but more intense wave. When reflected from obstacles such as wall, suddenly the pressure increases near the wall. This causes the effect of the axial force to be combined with the local bending (axisymmetric) around the entire circumference of the affected area. (A simple example is the well-known configuration of a cylindrical pressure vessel wall near the closed end.) The formation of this secondary bending made it easier for the wall to break and the tail part to separate.

In the case of fuel exploding, the wave is less intense, but wider and therefore not so much amplified by reflection. The secondary bending is weaker and the axial force plays a relatively larger role. In this situation, a better place for a separation may be the section through the last window in the hull. Although the cross-sectional area loss caused by the windows is not great, the windows induce stress concentrations, the effect of which is greater under dynamic loads than under the application static pressure.

Could the separation of the tail section occur after the fall? Unfortunately, there is no discernible trace of the explosion on the ground.