The Comet’s fatal crack began at a roof antenna cutout, not a window

The Comet’s fatal crack began at a corner of an automatic direction finder antenna cutout on the cabin roof — not at a passenger window. That cutout was an operations opening, sealed with opaque panels rather than passenger glazing.

The Royal Aircraft Establishment‘s water-tank test later reproduced the failure under repeated pressurization, and the test fuselage’s fracture point matched the reconstructed wreckage of G-ALYP.

Where the Comet’s crack actually started

Investigators had to rebuild the G-ALYP structure before the fracture path could be traced, and it led back to a rivet hole beside the ADF aerial cutout. The opening existed for the antenna that gave the crew their bearing readings — it carried no view, no sunlight, and no obvious drama.

Which is precisely why it gets forgotten. Investigators have a term for the spot: a stress concentration. It is not a place a traveler can point to from a seat.

Passenger windows, by contrast, are the part of an aircraft anyone can see and picture. Whoever first turned the Comet’s fatigue failure into a square-window story is not named anywhere in the retrieved material, and no date for it appears either. What the material shows is that popular retellings turned a wider finding — fatigue at openings in the pressure boundary — into a single suspect: the windows.

One caveat on the geometry. The retrieved descriptions are not fully consistent about the cutout’s exact position — they place it on the cabin roof and variously describe a forward or rear aerial opening — so this piece does not assert a precise coordinate.

Why corners and rivet holes concentrate stress

Both the accident and the test failures started at the same class of feature: a rivet hole or a cutout corner. Engineers’ original design calculations underestimated how much stress would gather at those geometric discontinuities.

An opening cut into a pressurized skin forces load to route around it. A sharp corner gives that load nowhere smooth to go.

Riveting added a second problem. Imperfect holes from punch riveting became initiation sites — small flaws where a crack could take hold and then weaken the cabin’s structure further. One source in the reviewed set raises punch riveting against drill riveting as the reason the ADF site initiated first. That comparison is not confirmed by the retrieved primary-oriented material, and it stays an open question here.

There is a deeper gap too. The material supports that both the test and the accident failures began at rivet-hole or cutout locations. It does not support any verified comparison showing the passenger-window corners carried the same local stress. Why the ADF cutout gave way ahead of the window sites is therefore unresolved — not settled.

The design still cleared its safety factors. The Comet’s pressure cabin met the requirements of the British Civil Airworthiness Requirements it was measured against, yet those standards contained no provision for the cumulative effect of repeated cycling.

The tank at Farnborough

To break the cycle safely, the Royal Aircraft Establishment turned to water. Investigators submerged a complete Comet fuselage — G-ALYU, a sister aircraft to the lost G-ALYP — in a tank at Farnborough and cycled it, pressurizing and releasing over and over. Water does not store energy the way compressed air does, so a rupture releases its violence gently instead of tearing the rig apart.

When the test fuselage failed, its fracture was set alongside the reconstructed damage from G-ALYP — and the two matched.

That correspondence is the whole argument: a single peak-pressure event could not explain a failure the cycling reproduced. Metal fatigue — cumulative damage from repeated pressurization — emerged as the cause of the fleet’s mid-air break-ups.

One number remains contested. Some retrieved material puts the test failure at 3,057 simulated cycles, while FAA figures report 1,230 pressurized flights before the tank test plus 1,830 tank flights — which add to 3,060.

The original RAE document would settle which figure is right.

What this means for you

The Comet investigation did not end with a rebuilt wreckage. It ended with rewritten rules.

FAA training material dates the shift to 1956, when CAR 4b.270 first allowed fail-safe design as an alternative to safe-life — a structure built to tolerate a failure rather than one assumed never to have one. Later rules went further still: manufacturers now have to prove, through full-scale fatigue testing, that widespread fatigue damage will not show up before an aircraft reaches its design service goal.

That lineage now runs through 14 CFR 25.571, which the FAA’s AC 25.571-1B names as the rule for fatigue and damage tolerance in transport-aircraft structure.

For a traveler, the practical read is blunt. The safety of the cabin you’re sitting in was never meant to rest on a single peak-pressure margin. It rests on evidence that repeated cycling won’t open a crack first — proof the Comet made mandatory, and proof whose verified failure point sits on the roof, not in the window a passenger can see.

Key terms

Stress concentration

A stress concentration is a point in a loaded structure where stress rises sharply, usually at a corner, notch, or hole cut into the surface. An opening cut into a pressurized skin forces load to route around it, and a sharp corner leaves that load nowhere smooth to go. On the Comet, it explains why the investigation’s conclusion rests on geometry rather than visibility: the feature that failed first was one no passenger could see from a seat.

Automatic direction finder (ADF)

An automatic direction finder is navigation equipment that gives a flight crew their bearing readings, fed by an antenna mounted on the aircraft. On the Comet 1, that antenna sat in a cutout on the cabin roof, closed with opaque fibreglass panels rather than passenger glazing. Both the ADF cutout and the passenger windows were pressure-boundary openings, but only one of them has a verified failure origin in the retrieved evidence.

Punch riveting

Punch riveting is a fastening method that forms rivet holes by punching through sheet metal instead of drilling them, which can leave small imperfections at the edge of the hole. On the Comet, imperfect holes of this kind became sites where a crack could take hold and then weaken the surrounding structure further. Whether punch riveting put the ADF cutout ahead of the passenger windows in the failure order remains an open question in the retrieved material, not a settled finding.

Damage tolerance

Damage tolerance is the design principle that a structure should be able to carry its load with a crack or other damage present, and keep doing so until an inspection finds the flaw. It replaced the older safe-life assumption that a component would not develop a fatigue crack at all within its service life. For a passenger, it is the difference between a cabin certified on a single peak-pressure margin and one certified on evidence about how the structure behaves across a lifetime of repeated cycling.

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