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I think they often end up underwater? In that case, sound seems to be the more used medium, unless hollywood stories about sonar have led me astray...

Under water, I assume that sound is easier to detect than IR.

Infrared doesn't work underwater, water covers most of the earth surface and it's where the hardest to find crashes are located.

For communication under(salt)water, you're pretty much screwed with every part of the electromagnetic spectrum apart from visible light, and even that won't work in cloudy conditions or if the emitter is occluded. Sound is really the best option.

Because infrared doesn't work very well in water, and crashes in water are the biggest challenge for finding wreckage.

Airliners also carry emergency locator transmitters† (ELTs), which, when activated by a crash, transmit radio signals which can be triangulated via satellite. However, ELTs do not work underwater. No ELT signals from MH370 were heard.

†http://en.wikipedia.org/wiki/Emergency_Locator_Transmitter