When a recorder cannot explain an accident — because it stopped early, because the event happened faster than it recorded, or because the question is structural — investigators rebuild the aircraft. It is one of the most labour-intensive things done in accident investigation and it answers questions nothing else can.
What it is for
A reconstruction establishes the sequence of a break-up. Recorders tell you what the aircraft was doing; the physical wreckage tells you what came apart first, in which direction, and under what kind of load.
Fracture surfaces are diagnostic. A part that failed in fatigue looks different under a microscope from one that failed in overload, and a piece that failed from the inside out looks different from one that failed from the outside in. That last distinction is how TWA 800's centre fuel tank explosion was separated from a missile or bomb hypothesis.
How it is done
Recovery and documentation. Every piece is located, photographed in place, tagged and mapped. Where it was found matters as much as what it is — a distribution of debris along a path indicates the order in which parts left the aircraft.
Sorting. Pieces are identified against the aircraft's structural drawings and grouped by zone.
Framing. A wooden or metal frame is built in the approximate shape of the airframe, and pieces are hung on it in their correct relative positions. Missing areas are left as gaps, which are themselves informative.
Analysis. With the aircraft assembled, investigators trace soot patterns, heat damage, deformation direction and fracture type across the whole structure rather than piece by piece.
Two examples
TWA 800 (1996). More than 95% of a Boeing 747 was recovered from the Atlantic and rebuilt in a hangar at Calverton, New York. The reconstruction showed the break-up initiating at the centre wing fuel tank and propagating outward, with no external penetration. It is the physical basis on which the missile theory was excluded, and the model for large-scale reconstruction since.
Swissair 111 (1998). Canada's TSB recovered roughly two million fragments from the seabed off Nova Scotia. Mapping heat and soot damage across the reconstructed forward section allowed investigators to trace the fire's origin to wiring above the cockpit ceiling and its spread through the insulation blankets — which identified the flammable material that was subsequently banned worldwide.
Why it is worth the cost
Both of those investigations produced changes affecting every aircraft flying: fuel tank inerting in one case, insulation flammability standards in the other. Neither finding was available from the recorders. The reconstruction was the only way to get it, and in both cases it took years and cost a great deal.
Modern practice increasingly supplements the physical build with three-dimensional scanning and digital reassembly, which is faster and allows analysis to be shared. The physical reconstruction persists where the question requires people to walk around the structure and look at it.