Locator beacons after aircraft disappears.
I'd love to devote a good 50kilos of mass to a battery for a locator beacon on my aircraft that flies entirely around European airspace because sometimes those searches in the various valleys can take ages in winter...
But then again a larger battery pack would mean more protection on the device and more mass and pretty soon your entire usable load is taken up by ELT and associated equipment.
Let's face it, if whatever happened to MH370 hadn't happened, then it would never have been lost in the first place. And if the Wrights had designed a Cessna and had formal flight training, they wouldn't have killed a passenger in a crash...need I go on?