IEEE Spectrum interviewed the experts, and the answer is that hitting an engine (bird strike to a MD11 shown above) is not as bad as hitting the windshield. Key excerpt:
The drone will hit the leading edge of the fan blades and would probably break up into small pieces. The fan blade itself is not likely to break, in Morse’s view. “There’s a good chance it’ll take the engine out at high power,” says Morse, but not necessarily. “It’s absolutely amazing how they will still run.” In one sense, small drones appear to be less threatening than birds because drones don’t fly in flocks, so the chance of crippling multiple engines in one incident is much more remote.
But what about the lithium-ion batteries that these little drones carry? Aren’t they hard enough to create real problems for a turbofan engine? “Ice can be hard, too,” says Morse. And as for the worry about the volatile material from a battery ending up in the combustion chamber: “The engine will probably burn it up.”
In all, this expert was surprisingly calm about the possible consequences should a 1- or 2-kilogram drone strike a turbofan engine. Perhaps that’s what comes of looking at thousands of engines damaged by debris left on the runway or parts of a plane that break up in flight. Still, Morse emphasizes that when an engine ingests any foreign object, the cost of inspection and repair can be considerable, even if the mishap does not put people’s lives in peril.
“I’d be more concerned about hitting the windscreen,” says Morse. And the dangers involved with that, while considerable, hinge on the speed of collision and the mass of the object rather than its composition. Which is all to say, Exponent’s analysis of the dangers drones of this size pose should one strike an aircraft still appears reasonable enough.