Can a plane crash in mid air?

Answer 1

Yes, it can deteriorate and lead to a crash. Or hit another object, i.e. another plane, helicopter, bird, etc.

Two of the deadliest three air crashes (not including the attack on the WTC on 911) involved two plane collisions:

On March 27, 1977 at Tenerife Airport in the Canary Islands, a Pan American World Airways and a KLM (both Boeing B747s) were waiting for take off, fully loaded with fuel. The wait had been long and the pilots were directed to their waiting area by the tower. The KLM arrived at the takeoff point first and the Pan Am pilot did not see his turnoff where he was to wait for the next slot and was still on the runway when the KLM plane started his takeoff roll without clearance. When the KLM pilot saw the Pan Am, he attempted to lift off, but it was too late. The KLM tore into the top of the Pan Am and turned them both into a ball of fire. 583 of the 644 on board both planes perished.

On November 12, 1996, a Saudi Arabian Airlines and Kazastan Airlines planes collided over New Delhi. The cause was attributed to a misunderstanding of directions between air traffic control and the Kazastan plane. Total fatalities 349.