Comment by tzs

1 day ago

> British Airways 5390: An incorrect repair causes the windshield of a plane to be blown out mid flight. A pilot is nearly sucked out.

This one is a good illustration of how better design can help prevent accidents or make them less severe.

The error the maintenance people made was that when they replaced the window and the 90 screws that hold it on 84 of the screws they used were were 0.66 mm smaller in diameter than they should have been.

The window on that model plane was fitted from the outside, so the job of the screws was to hold it there against the force of the pressure difference at altitude. The smaller screws were too weak to do that.

If instead the designers of the plane had used plug type windows which are fitted from the inside then the pressure difference at altitude works to hold the window in place. Even with no screws it would be fine at altitude. Instead the job of the screws would be to keep gravity from making the window fall in when the plane is not high enough for the pressure difference to keep it in place.

My vague memory of the Air Emergency episode on this (AKA Air Crash Investigation, Air Disasters, Mayday, and maybe others depending on what country and channel you are watching it on) is that after this accident many aircraft companies switched to mostly using plug windows on new designs.

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tzs

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t0mas88  1 day ago

Aviation is full of those design choices. Similar to how a multi-engine propeller plane will use oil pressure to keep the props in the flying angle, which means that when oil pressure is lost (catastrophic engine failure) it will feather giving the other engine the best chances of keeping the plane flying with the least amount of drag. While on a single-engine plane it's installed exactly opposite, in case of oil pressure loss the prop goes to fine pitch giving you the best hope of creating some trust in case the engine may still be working.

Most of these things were figured out over 100 years of carefully analysing accidents and near accidents to continuously improve safety.

throw310822  1 day ago

> the pressure difference at altitude works to hold the window in place

Curious, is the pressure difference actually greater than the force of 800km/h wind pushing on the window? Or is it just for side windows?

  • tzs  1 day ago

    Dynamic pressure of wind is 1/2 p v^2 where p is the air density and v is the velocity.

    At sea level p = 1.225 kg/m^3. It goes down as altitude goes up. At sea level the dynamic pressure at 800 km/hr would be about 4.4 PSI.

    At 20000 ft the air density is about half that of sea level, so around 2.2 PSI wind pressure. It would be around 1.4 PSI at 35k ft.

    At cruising altitude planes are typically about 8 PSI above the outside pressure.

    It would be maybe an interesting project for someone more ambitious then me to get a speed vs altitude profile of a typical airline flight and an altitude vs cabin pressure profile and figure at what part of a typical flight the screws on a plug window are resisting the most force.

  • Rastonbury  1 day ago

    The outward pressure is about 5-6x greater than the force of air resistance at cruising altitude