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

6. Explosion in the Fuselage

Within seconds of a hypothetical breaking of the wing, telecommunication contact with the plane was lost, and its monitoring and recording device stopped working. This event can be identified with the explosion in the hull, although the military prosecutor's office said:

Recorders of the Tu-154 M stopped working at about 1.5 to 2 seconds before the plane's impact with the ground. The reason (according to experts) could be the damage to the electrical system. Recorders in the Tu-154M do not have emergency power supply. The prosecutor does not know what was the reason for a failure of electric system. This issue could not have been the subject of experts investigating the loggers, because this type of parameter cannot be registered.

and then he tries to downplay the importance of this event. So, going back to Figure 2, an explosion took place on the wing around the point K, while the fuselage exploded near the FMS.

The explosion inside the airplane, which was still in the air, is the only logical explanation of the effects, partially visible on the photographs, namely:

airplane fragments scattered over a large area. (Figure 19), a very large number of very small fragments (most of them a few centimeters in diameter) that were found. (Appendix II, Figure 22), a segment of the hull with the characteristic opening and folding of edges (Figure 3b and Figure 7). The degree of opening of the hull skin is a measure of the energy of a material that exploded, the appearance of fragments of the hull as seen along its axis. The main parts of fuselage appear as though very little was left inside the cabin, besides the structure itself. The contents were "blown out" (Figure 7 and 10). (Lack of sufficient lighting makes it difficult to be more specific.) damage to the rear bulkhead area. (Chapter 7), number of burned and unburned debris. (Appendix IV - Aircraft Fire), rapid vertical acceleration, about 0.78 g, oriented down relative to the craft, as recorded by instruments. (Fig. 20), some passengers’ bodies torn to pieces. A great amount of human remains. Clothes sometimes torn off, sometimes forming characteristic tears. (Fig. 18). All these effects are observed when a HE explosion occurs next to a group of people.

The last point deserves special attention because there is no other physical phenomenon, except for an explosion that could cause such effects. (Severe impact, in terms of speed occurring like here, can cause rupture of clothing, but not it's fraying to such an extent. Witnesses indicate that there is a range of degrees of destruction of clothing. Similarly, with the human bodies. Collision may result in loss of continuity, but not in dismemberment into many fragments).

What could be the nature of the blast? The first thing you need to take into account are the substances present in the plane, oxygen and fuel vapor. The explosion in flight in partially emptied containers can only occur because of a spark, which is unlikely. (The tanks are constructed so as to exclude any possibility of arcing). The fuel vapor entering the cabin in such a quantity that a big explosion can eventuate is difficult to imagine. The amount of gas needed to achieve adequate concentration would be overwhelming to the passengers, who would first raise an alarm.

In addition, explosions of this kind are relatively mild (with the same amount of energy involved). They produce splitting into a few pieces rather than smashing of the vessel wall. This is due to a more uniform distribution of the explosive energy within a container. More details about the behavior of fuel is given in Appendix III.

The explosion could be caused by a solid material such as dynamite, or by another high-energy (HE) explosive. Such an event is associated with the propagation of shock waves, destroying everything in their path. The distance from the source of the explosion is a measure of a potential damage. This means that if the event took place in the central part of the hull, there are people in remote areas that have a greater chance of survival. (They could also be shielded to some extent by the chairs of other passengers). Either way, the nature of hull damage suggests the effect of the shock wave and a strong wind, which follows it. There are other possibilities, but the determination of how the explosives got into the airplane does not fall within the scope of the report.

Can we determine any connection between the above and the explosion in the wing? It is difficult to accept such a relationship, because the locations of the explosions were far apart. A strong shock wave could not easily move from one place to another, even if it was supported by fuel. The center wing has many ribs, which are a serious obstacle. The wave may destroy some, and also get through the relief holes in the ribs, but then it would be severely weakened.
The role of fuel in a disaster is best known in the following situation: a plane hits the ground, the tank cracks, fuel spills out, and often, a moment later, it may be followed by an ignition. What material was used, where it was at the time of the blast, and what was its total mass or energy, can be determined by performing a simulation using Finite Element Analysis (FEA). The results, compared with the actual damage to the structure, should lead an engineer to the correct conclusions. After conducting such a simulation one will know, for example, what transverse velocity of the disintegrating fuselage fragments was acquired. Only then one can closely associate that with the location of debris on the ground.

Left: Fig. 18a Partially burned and torn clothes of the passenger from Lounge 3ⁱⁱ.

The effect of the blast on the fragmentation of the fuselage also depends on the geometry of the latter. The status of human remains indicates that the source of explosion could be near Lounge No. 3, around the middle of the length of the passenger cabin.

Apart from the opening of the ceiling, the shock wave also travelled along the hull. The component of it going towards the tail had few obstacles on its way to the back wall. It seems that the reflection from this wall was strong enough to make the perimeter of the hull crack, and the entire rear part of the aircraft to fly rearward. Having a clean aerodynamic shape, it could fly quite far. Owing to the compact shape it could survive the fall in good condition, as evidenced by pictures. The only exception was that the engines were torn off, along with the elevator and stabilizer.

Left: Fig. 18b Torn and marginally burned uniform of the passenger of Lounge 3ⁱ.

It was different for the wave component travelling toward the front of the aircraft. In the path there were a few partition walls, which reduced the impact, in spite of the open doors. The separation of the cockpit by the wave may be attributed to the fact that all the walls, with the exception of the last one, were destroyed.

The graph in Figure 20 represents the history of acceleration. It shows two peaks of downward acceleration. One of them is a little weaker, but more extended. While the first can be associated with the beginning of an explosion in the hull (the first wave hitting the floor), the second may be a trace of splitting of the hull and the recoil, and then the subsequent detachment of the front and rear of the fuselage. At the first, a sharp peak of acceleration, we have the beginning of the degradation of the structure, but the recording still continued, because the sensors were still powered. Only after this second, longer lasting period, the record terminates, which means the power supply terminates and the structure is practically annihilated.

There is an important question of direct influence of the blast on the movement of the aircraft. If the explosion took place only inside the fuselage, nothing would change the trajectory. When the explosion ripped the top of the hull, the gas escaped up at a high speed. The recoil effect pushed the hull down inducing the acceleration, as recorded during the flight. The disintegration of the hull as described previously, resulted in rapid changes in vertical velocity around the point of FMS. The breakup of the structure in the manner described also had to break the power wiring.

There is also a marginal matter of the audibility of the explosion. If it took place completely inside the hull, the ground observers would have heard nothing. When the upper part of the hull was opened by the shock wave, the latter or whatever was left of it, went mostly up. Whatever reached the ground, was only due to the wave diffraction around the edges of torn fuselage. A terrestrial observer could hear only the gentle sound, compared to the passengers, which would have had their eardrums burst (autopsy would demonstrate this). Slightly better audibility would eventuate in the case of the plane flying upside down. However, the increased roar of engines would not help.

Eyewitnesses present at that time near the scene confirmed that they heard the "thunder", "bangs", "bursts" or "explosions". (Appendix VI)

Above: Figure 19. Spread of larger pieces and debris of wreckage, based on satellite image of April 11, 2010. Resolution 50 cm per pixel.

Above: Figure 20. The history of vertical acceleration at the critical time period. In normal flight, there is an overload of 1.0g. If the value is greater than 1.0g, the acceleration is directed upward. (maximum 0.27g). If less, to the bottom. (maximum 0.78g)

The level of destruction is also affected by the final events, such as impacting of the trees or the ground, or even collisions between fragments. If the hull had previously disintegrated, the number of fragments would multiply.

One of the aspects of the case that is difficult to understand is the duration of certain phenomena. In Figure 20, acceleration much higher than 1.0g takes a much longer time than a trace of the elastic wave pulse coming from the breaking wing. Similarly, the sounds that can be interpreted as the sound of cracking in the video mentioned in Sec. 3 last for a few seconds. A possible explanation that comes to mind may be a number of small, coordinated explosives, whose task is to pre-crack, rather than to break the structure. But this is only a preliminary idea, which is otherwise difficult to justify

Above: Figure 21a. A reconstruction of last phase of the flight of Tu-154M.

Above: Figure 21a, b. A reconstruction of the airframe disintegration. After separation from the rest of the aircraft, the cockpit and the front lounge finally stopped in an upright position. The nature and degree of damage requires the use of advanced numerical methods to simulate the disintegration of the aircraft.

Above: Figure 21c. A reconstruction of the in-flight airframe disintegration.