Japan Air Lines 123, 12 August 1985

On 12 August 1985, Japan Air Lines Flight 123 crashed into Mount Takamagahara after suffering a catastrophic structural failure. 520 people died and four survived — the deadliest accident involving a single aircraft in history.

The aircraft

JA8119 was a Boeing 747SR-46 — a short-range, high-density variant built specifically for Japanese domestic routes, with a structure reinforced for far more takeoffs and landings than a long-haul 747 would ever see.

That reinforcement is central to what happened, because the SR’s whole design premise was surviving an extraordinary number of pressurisation cycles.

The repair

In 1978 the aircraft suffered a tailstrike, damaging the aft pressure bulkhead. Boeing technicians repaired it — incorrectly.

The correct repair called for a single continuous splice plate across the joint. What was fitted was two separate plates, which meant one row of rivets carried load it was never intended to carry. The repair had roughly 70 percent of the intended fatigue resistance, and a calculated life of about 11,000 pressurisation cycles.

The aircraft flew 12,318 flights after the repair.

What happened

The bulkhead failed in flight. The escaping cabin air blew off a large part of the vertical stabiliser and — critically — severed all four hydraulic systems.

The 747 has four independent hydraulic systems, each driven by a different engine, precisely so that no single failure can take them all. But all four ran through the tail. A single event destroyed every one of them, and with them every flight control.

The crew kept the aircraft airborne for 32 minutes using engine thrust alone to control pitch and direction, before it struck the mountain.

What changed

Repair oversight. The accident forced a fundamental review of how repairs are documented, inspected and signed off — and of the assumption that a repair, once made, need not be revisited.

Structural inspection. Inspection regimes for pressure bulkheads and other critical structure were tightened, with more attention to areas that had been repaired.

Common-mode failure. The loss of four “independent” systems in one event became a standard case study in how redundancy can be defeated by routing. It is discussed alongside National Airlines 102, where hydraulics and the stabiliser jackscrew were destroyed together by shifting cargo.

Related

• All 747 accidents
• The 747-100SR — the high-cycle Japanese domestic variant
• 747 systems — how the redundancy is arranged