The FAA is responsible for all plane crashes and local law enforcement would not be allowed to investigate unless brought in.
Caine has Erik find out why the number 2 engine failed. It was the left engine that failed, which is actually the number 1 engine.
When Horatio finds the fire extinguisher used to strike the victim's hand that produces the distinctive bruising pattern, in the flashback, the murderer is only shown striking the victim's hand once after clearly missing her hand the first time. Earlier in the episode, Dr. Woods and Horatio found two such bruises on the victim's hand.