DH Comet what if

I seem to remember the issue also being linked to the physical method of riveting used during construction. Rather than pre-drill the rivet holes and then following up with riveting the factory apparently used punch riveting to speed up production. There has been suggestion that the punch riveting process may have caused initial fractures or created imperfections in the rivet holes which subsequently propagated.

When I read the RAE /AAIB reports on the Comet accident investigation I was surprised to see that some of the cracks had been stopped drilled DURING production.
The real issue wasn’t the shape of the cutouts rather than one of metallurgy and the lack of understanding of failsafe structures. How many pressurised airliners had been built before the Comet? How many all metal aircraft had de Havilland built before the Comet? Being a pioneer means living with a steep learning curve.

Spot on with the comments regarding de Havilland understanding of fracture mechanics, if the rf direction finder cut out had not cracked something else would’ve failed. They did actually conduct a fatigue test for cert but had no idea what they were doing, so far from making the A/C safe, it lulled them into a false sense of security.

BTW prior to Comet, the Viscount and Apollo were both pressurised. 400 + Viscount operated for years without any significant problems. AW design manual at the time of the Apollo contains a very practical/workable understanding of fracture mechanics, based on a first principles development of Miners Rule derived in about 1940. Metal fatigue was first characterised in the 1920’s.

The basic data to produce a safe aircraft was available within the UK industry and de Havs screwed up.

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These are the pics I refer to above showing the failure cracks.

Cosford have a test specimen by the side of their Comet1 which shows an example of the decompresson. Worth looking at for this sort of discussion.

When I read the RAE /AAIB reports on the Comet accident investigation I was surprised to see that some of the cracks had been stopped drilled DURING production.
The real issue wasn’t the shape of the cutouts rather than one of metallurgy and the lack of understanding of failsafe structures. How many pressurised airliners had been built before the Comet? How many all metal aircraft had de Havilland built before the Comet? Being a pioneer means living with a steep learning curve.

The ‘rectangular’ windows had rounded corners, which should have reduced the stress issues. As noted above, it was not a passenger window that failed.

I think it it was the redux bonding of the window frame doublers that was the issue and it was decided to use rivets instead, to speed up production. Subsiquently the rivet holes were found to help propergate the cracks. If I have understood correctly, the origin of the crack was from a round cutout on the top of the fuselage. It was the fact that the fuselage skin was too thin to cope with the pressure cycles that led to the failures. Of course changing the window shape helped to restore public confidence later.

First I’ve heard of that.
Remember, it wasn’t one of the passenger windows that failed.
Let’s remember that stresses of pressure vessels was already well known…and had been for about a century. Look at the cutouts (doors and inspection portals) of steam boilers.

It’s hard to believe that a significant change would be done based on one complaint, then again it’s hard to believe no one recognized the issue. But similar things have happened throughout aviation history, and probably will again.