Inverted impact of aircraft fuselage on semi-soft ground
Analytical Service Pty Ltd
TECHNICAL NOTE 66B
TECHNICAL NOTE 66B
The reason for this investigation was to learn more about some effects of the crash that took place near Smolensk airport in April 2010. The airplane involved was Tu- 154M carrying 96 passengers. In the simulation presented below the fuselage is assumed to drop with its axis parallel to the ground. This allows us to model only a short segment of the entire fuselage.
Two impact velocities are simulated, 10 m/s and 20m/s. The first is the upper bound of the estimated vertical speed. The other number could be used for estimates where the falling fuselage axis makes a significant angle with the ground level.
There are two parts to this Note: In Part A the normal orientation is the ship is considered, while in B the inverted position, with the ceiling striking the ground is investigated.
In this part B the difference between the faster and the slower impact is not only the degree of damage. In the 10 m/s case the contact between the chairs and the stowage as well as the frame is quite minor, while during the faster impact that contact is very strong, as evidenced in the velocity plot. The fuselage model remains the same as in Part A.
DETAILS
Typical acceleration record of impact events (real or virtual as here) has many local sharp peaks. When read off the graph, such peaks have little meaning, as acceleration has to last for some amount of time to have a definite effect. This is especially true with regard to the influence of accelerations on passengers. For this reason we have taken 0.1s or 100 ms to be the reference time span. The acceleration as defined in this work is the maximum change of velocity during the event divided by 0.1s.
The reference point for reading velocities and calculating accelerations is the geometrical center of the floor. The acceleration is calculated as a multiple of g, where ‘g’ is the acceleration of gravity. For 20 m/s vertical speed the extremes of velocity are -24.55 and 12.07 m/s. Thus we have
a = (-24.55 – 12.07)/0.1 = - 366.2 m/s2 = - 37.33g
Similarly, for 10 m/s we get 17.92 g. (This for the interval between t= 20 and t = 120 ms. At the latter point the velocity is 3.02 m/s.) For our event, where the estimated speed is 8.5 m/s, this reduces to 15.23 g.
The results do depend on the assumed stiffness of stowage and chairs. They were taken in this simulation to be reasonably soft, but no special study of their properties was conducted. The softer those elements are, the larger deformation of the fuselage section will eventuate.
Fig. 70 Full model with impacted ground. The details as in TN66A.
Fig. 71 Early impact phase (10 m/s) with ovalization and flattening at the bottom.
Fig. 72 Advanced impact phase, near beginning of rebound. Permanent distortion of the shell is visible in several places.
Fig. 74 Velocity history shown here is the result of intense vibrations caused by impact plus the translation of the reference point. (The former is marked in the early phase.) This is 10 m/s drop.
Fig. 81 Early phase of impact with 20 m/s. A joint is forming in the plane of symmetry.
Fig. 82 Falling at 20 m/s. Advanced deformation and breakage. The chairs and the overhead stowage are interfering.
Fig. 83 Maximum deformation, near the rebound point. The frames are broken at spots, but the skin remains intact.
Fig. 84 The status after the early rebound phase of 20 m/s drop.
Fig. 85 The disturbance between 40 and 80 ms is due to the floor beam and the frame below coming in contact
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