There is no FAA requirement to discharge LiPo batteries before flying on airlines. DJI's "must" statement shown below
is wrong. You are required to PROTECT each battery INDIVIDUALLY from shorts, such as installing covers over the terminals (there are various aftermarket solutions) if stored loose in the bag with other items, or having individual non-conductive compartments that protect each battery from touching anything else. Lithium battery bags are nice and super safe if you're paranoid about this, but I don't use them due to the inconvenience of the added bulk, they won't fit in my backpack. I don't see that many issues with DJI batteries spontaneously catching fire, but I know it's not impossible. I personally think this happens to people who mistreat their batteries.
Discharging alone doesn't help safety when flying on airlines. When discharged to 20%, there is still PLENTY of power in a DJI battery to catch on fire if it were to fail for various reasons. The results from a battery catching fire when fully discharged in the overhead compartment of an airplane, compared to a discharged battery @ 20% is not much different. This will all result in the same thing with smoke, chaos, and the flight crew taking care of the problem to protect passengers and flight integrity. Keep in mind, there are hundreds of rechargeable lithium batteries on every plane, contained in iPods, iPhones, laptops and tablets. Any of them can potentially catch fire to cause chaos on an airplane.
If you recall, Samsung Galaxy 7 was banned from flying due to threat of spontaneous fire in that design. Laptops had a similar problem, using
Sony batteries about 10yrs ago. Exercising common sense is key to safety in many things, and treating your batteries like EGGS helps prevent mishaps. Don't throw your drone stuff to the ground (like Casey N.) with no respect. Inspect the batteries regularly, take care for them by storing them in cool places half charged when not in use a while. Never let your batteries bake in the car on a hot day, never. Slight abuses over and over can add up to cause trouble in the long run. Watch your voltage differentials between cells. Take mental note of the differentials on every flight, try to make that a habit. Most guys don't bother check their cell differentials very often, but it's good practice. It only takes 10 extra seconds during flight prep to look at the battery screen in Go4. Checking differentials AFTER the flight is good practice too.