A 50 caliber shot straight head on into the spinning turbine blades is all it would take, believe me.
Titanium turbine blade spinning at a gazillion RPM are NOT made to take 50 caliber rounds in stride."
You are ignoring the part about WHAT PART OF THE ENGINE BEING HIT WOULD CAUSE IT TO TAKE FOUR MINUTES TO FAIL?
You keep repeating the idea that the engine would be destroyed if shot by a .50 caliber firearm. I will grant you that. BUT if you shoot the engine (as in your scenario) as it is coming towards you in a takeoff, you will likely not survive the impact of the crippled aircraft hurtling into your sniper position at several hundred miles per hour.
Please, if you want to argue the point any more, contact an engineer and ask him what part of the engine needs to be hit to cause a 4 minute delay in the catastrophic destruction of an aircraft engine.
Also, FWIW, a timer on this aircraft, let's say of 60 minutes or so, would have put the plane about an hour out of JFK had it departed on time. As it was I think I heard that it had a delay of about 56 minutes or so (Now these are just numbers I heard, I haven't verified them). Let's see...60 minute timer minus 56 minute delay equals....OHMYGOSH!!!! FOUR MINUTES!!! I'm also suprised that you think they (the bad guys) wouldn't use a timer. They really don't CARE when or where the aircraft crashes, just so long as it crashes. Have you spent any time checking with the aircraft bombers to see what their methods and motives are?
Who said it took four minutes to fail, and the failure happened in one second after four minutes? The (hypothetical) shot into the engine on take off could hit a secondary system which leads to a chain reaction.
I am only positing a hypothesis which has not been mentioned, but which is realistic, hard to prevent, and which could happen tomorrow even if it didn't today.
An arab moslem jihadist jet sniper is not going to care if the jet kills him on crashing, in fact he would welcome his chance to collect his 72 virgins sooner rather than later.