Designing a 747 to wear out: the high-cycle problem

An airframe ages on two different clocks. One counts flight hours — time spent cruising, which is gentle. The other counts cycles — every take-off, landing and pressurisation, each of which flexes the structure through a full range of load. A long-haul 747 spends most of its life on the first clock. Japan’s domestic 747s spent their lives on the second.

A different design target

A standard 747-100 was designed for roughly 24,600 flights over 20 years. The 747SR, built for Japanese trunk routes, was designed for about 52,000 flights over the same 20 years — more than double the cycles.

Boeing reached that figure by changing the aircraft in ways that would have looked perverse on a long-haul jet:

• Reinforced structures. The wings, fuselage frames and landing gear were strengthened to absorb many more pressurisation cycles.
• Higher design landing weight. The domestic aircraft landed heavy, again and again, so the structure was sized for it.
• Fuel capacity cut by about 20 percent. Range was not the problem; carrying fuel it would never burn was just weight.
• High-density single-class cabins, with economy on the upper deck — the only 747s routinely configured that way.

One idea, three generations

Boeing built the concept three times over four decades. The 747-100SR arrived in 1973; the 747-300SR applied the same thinking to the stretched-upper-deck airframe from 1987; and the 747-400D closed the line from 1991, deleting the winglets that never repaid their weight on a one-hour sector. Only Japan Airlines and All Nippon Airways bought them.

The repair that broke a high-cycle airframe

On 12 August 1985, Japan Air Lines Flight 123, a 747SR, lost its aft pressure bulkhead in flight. The escaping cabin air blew off the vertical stabiliser and severed all four hydraulic systems, which on the 747 run through the tail. Of 524 people on board, 520 died — still the deadliest single-aircraft accident in history.

The cause ran back seven years. In 1978 the same aircraft, JA8119, suffered a tailstrike at Osaka; the aft pressure bulkhead was damaged and repaired. The manual called for a single continuous splice plate joining the upper and lower halves of the bulkhead. The repair instead used two separate plates, leaving a single row of rivets to carry the entire load — about 70 percent of the intended fatigue strength. Cracks started at those rivet holes and grew over the 12,319 pressurisation cycles the aircraft then flew.

Two high-cycle factors made it worse. The aircraft was inspected on the basis of the short-range cabin pressure setting, but JAL operated it at the higher long-range differential pressure, which costs fatigue life. And on a domestic 747 the cycles simply accumulated faster: a crack that might take decades to matter on a long-haul aircraft advanced in years.

What changed

Investigators in Japan found the immediate cause; the NTSB issued recommendations to the FAA that reshaped rules for every transport aircraft, not just the 747: redesign of the tail so a bulkhead failure could not destroy flight controls, changes so that no single event could disable all hydraulic systems, and a re-examination of how fail-safe criteria were applied to dome-shaped aft pressure bulkheads and to repair procedures on them.

The outcome is that a repair is no longer treated as a local fix. On a high-cycle airframe it is part of the structure’s life, and it is documented as such.

Related

• The 747-100SR · 747-300SR · 747-400D
• Japan Air Lines Flight 123
• What 747 accidents taught aviation