The Deadliest Commercial Helicopter Crash in History

The largest passenger helicopter in the world worked a shuttle run over the North Sea, carrying 44 men at a time. On November 6, 1986, one of them came apart in clear air two and a half miles from the runway at Sumburgh, in Shetland, in sight of land. Forty-five men died. No civil airline helicopter accident anywhere in the world has ever killed more people. A Boeing Vertol 234 carries a rotor at each end, sixty feet across, and the two discs overlap. The blades never touch because they are geared: a shaft runs the length of the aircraft, tying front rotor to back rotor in a fixed relationship, so a blade at the front always crosses the overlap in the gap a blade at the back has left. Lose that relationship and the blades try to occupy the same space. Boeing later calculated that if the front rotor gained the equivalent of one tooth, they could clash in a second and a half. That relationship was set by a ring gear in the nose, bolted face to face onto its shaft with a thin spacer clamped in between. The bolts did not carry the drive. Friction between the clamped faces carried it, and the bolts only made the pressure. So everything that ever went wrong with that joint came back to one question: was it tight enough. By 1984 that tightness had to be checked every 300 flying hours, which is nothing on a daily shuttle. The report calls the check, in its own words, a considerable operating penalty. So the manufacturer set out to design it away, and put a gearbox on a test rig for 150 hours to qualify the redesign. The overhaul life of a forward gearbox was 2,150 hours, so the qualifying run covered about seven per cent of one cycle. And the gearbox bolted to that rig was not a forward gearbox. It was an aft one, from the other end of the aircraft. One run on the aft was taken to qualify both ends. Nobody, at any point before the accident, tested a forward gearbox with a modified gear inside it. The rig ran dry, no salt and no moisture. The modified gear went into this aircraft on October 18, 1985. Just over a year later, four and a half miles out and cleared to land, the co-pilot said something almost conversational. Seems to have got very noisy in here all of a sudden. Investigators found that noise across the entire thirty minutes the recorder held. What changed, 62 seconds before the end, was that one part of the signature became permanent. That was the gear breaking. Chloride from the sea air had reached the gap between the two clamped faces and turned it into a corrosion cell, cutting a steep-sided groove into the steel, in a space no inspection could reach. Fatigue cracks grew out of that groove and around the flange until the rim sprang open about three quarters of an inch. In the geometry of a gear, that is roughly one tooth. The rotors began to drift out of step. The part of this that stays with you is the inspection. The bolts in that gearbox were checked twice after it was modified, the second time thirty-two days before it fell into the sea. No loss of torque either time. And when the wreckage was stripped at Farnborough, all but two of the readings still came off above the threshold Boeing's own manual required for a unit to stay in service. Even laid out in a hangar, by Boeing's own standards, it would have passed. The inspection had not been skipped and it had not been falsified. It was measuring the tightness of bolts while, underneath the spacer those bolts clamped, a groove none of them could reach was cutting the gear in half. Nobody was ignoring a warning. There was no warning to ignore. Chapters: 00:00 The Largest Passenger Helicopter in the World 01:32 Why the Blades Never Touched 04:06 A Test on the Wrong Gearbox 05:30 Sumburgh, 6 November 1986 08:45 "Seems to Have Got Very Noisy in Here" 09:35 Sixty-Two Seconds 11:53 The Coastguard Was Already Airborne 15:21 Seven Per Cent of a Cycle 16:51 The Inspection That Would Have Passed 19:03 Eighteen Days in Edinburgh 24:17 A Stone Horseshoe Above the Water