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Are there sensors that can detect a rip, crack or damage in the fuselage skin by not using air pressure differential on aircraft?

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    $\begingroup$ Something down that alley discussing control of damaged aircraft. I don't think exact damage location knowledge is a priority though since it will be hard to estimate and evaluate; that can be done on the ground. In theory I guess you could go sticking strain gauges or optical fibre sensors on the fuselage, but it will be hard to do it effectively. Aircraft do record forces that can give a good prediction if problems are to be expected. $\endgroup$ – Thunderstrike Mar 26 '15 at 23:27
  • $\begingroup$ I'm guessing you are asking whether it can be done in flight. What do you mean by "not using air pressure differential"? $\endgroup$ – fooot Mar 27 '15 at 2:25
  • $\begingroup$ Actually should say air pressure loss. You're right.. $\endgroup$ – Fabrizio Mazzoni Mar 27 '15 at 2:27
  • $\begingroup$ Ever heard oh [Aloha 243][en.wikipedia.org/wiki/Aloha_Airlines_Flight_243]? It does not exist then, it does not exist now. Proper maintenance and preflight helps detect these damages before they fail in flight. $\endgroup$ – vasin1987 Mar 27 '15 at 3:18
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    $\begingroup$ @vasin1987 Aloha 243 happened in 1988. Plenty of things that didn't exist in 1988 do exist now (the world-wide web, for example) so that's not really a very good example. $\endgroup$ – David Richerby Mar 27 '15 at 17:45
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Such sensors certainly exist. The sensor is as simple as can be: usually a piece of wire. The bonding of the wire to the skin or structural elements dictates what kinds of damage it's sensitive to.

The "wire" can be in the form of a thin metallization on a thin non-conductive substrate that's intimately glued to the skin - such gives the most sensitivity to cracks, and is a crack detector.

If you wish to detect whether the rocket is in the process of breaking up, you simply need to run a thin, insulated wire along a longitudinal structural member, such as a tank, and attach it in a few places, but certainly rigidly at the ends. The input is used sometimes for automatic flight termination.

The major problem with pressure differential sensors is that you really need a big hole to overwhelm the pressurization systems on turbine-powered transports. A pencil-sized or even coin-sized hole is noisy but essentially irrelevant, and no, you won't get sucked out through it.

Why is it so? For cabin ventilation to work to begin with, there must be an outflow valve. Such valves, in steady state, have openings way larger than a coin. The more unplanned-for holes you add to the pressurized section, the smaller the outflow valve's opening will be. Only when the outflow valve completely closes and the pressure is still insufficient, you've got a problem...

So yes, the outflow valve's position in steady state can be used as a measure of air leakage elsewhere.

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    $\begingroup$ It's not typically used for automatic flight termination in airplanes, though. :) We tend to avoid putting automatic flight termination systems in airliners. Pax don't like them... pilots don't either. $\endgroup$ – reirab Mar 27 '15 at 18:15

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