Case design

The de Havilland Comet was the world’s first jet airliner. Entering service in 1952, it stood poised to revolutionise air travel. However, after only a year in service, several Comets suffered problems and crashed.

What lessons can Tritium learn from the de Havilland Comet? Surprisingly more than you expect! The main cause of the Comet’s problems was metal fatigue as well as the method of installing rivets – the riveting caused microscopic cracks to form, invisible to the naked eye. As they were loaded and stressed during flight, these cracks then grew slowly over a year before failing catastrophically. Stress concentrations caused by the square window design was also a source of fatigue failure.

Tritium has a laser-cut case. In this, an infrared laser is used to vaporise plastic sheets in a specific pattern that you see in the Tritium altimeter. However, the stresses caused by the laser can leave the plastic vulnerable to cracking and failure. Fortunately, we have learned from the mistakes of the Comet! To avoid this, the case has been designed as:

  • bottom layer: polyethylene terephthalate copolymer (PETG)
  • all other layers: poly(methyl methylacrylate), or PMMA for short
The bottom layer is a different material to the rest. Can you spot the difference?

The bottom layer is the most highly-loaded structure in the altimeter, therefore it is made out of polyethylene terephthalate copolymer, or PETG, which is tough, strong and crack-resistant but forms ugly residues if engraved by a laser. The other layers are made of poly(methyl methylacrylate) or PMMA for short, which clean, beautiful laser engravings can be made, but are more brittle than PETG. This gives us the best of both materials that results in a very unique-looking, but durable altimeter.

As a testament to this design, we have not had any issues with the case with any of our jumps in the past year. The case has performed really well, and we will continue testing the altimeters to ensure that the cases are more than capable of keeping up with your skydiving!

However, Tritium altimeters are not indestructible, please treat them as carefully as you would any other altimeter! They are precision instruments and although we have put quite a lot of thought in the case design, please try not to smash them against anything. Exit the plane properly guys and gals!

Blue skies!