Creative Speculation

... what on earth difference does it make if the plane's engine was leaving the smoke trail or something else was responsible.

Here is a website with over 100 eyewitnesses & excerpts from their testimony.
http://911research.wtc7.net/pentagon/evidence/witnesses/bart.html

Not less than 9 of those witnesses (on my quick scan) explicitly state that the plane hit light poles in the last seconds before impact. The chance of FOD from these impacts is very high, although not certain.

It's also likely that the impacts with the poles struck the wings instead of the engine. In which case, the smoke might well be a trailing plume of unignited fuel. It is a near certainty that Jet Fuel will not ignite merely from impact [ETA] with the lamp post.

One person, Frank Probst, said:
"On either side of him, three streetlights had been sheared in half by the airliner's wings at 12 to 15 feet above the ground. An engine had clipped the antenna off a Jeep Grand Cherokee stalled in traffic not far away."

Another witness:
Evey, Walker Lee
"The plane approached the Pentagon about six feet off the ground, clipping a light pole, a car antenna, a construction trailer and an emergency generator"

So, again, please explain to me what you deem to be the implications of the smoke trailing behind the airplane. Especially in view of the numerous persons who unequivocally described it as "a passenger jet", and "an American Airlines passenger jet"?

Was it a lamp or antenna or other material in the engine? Or fuel trailing from a wing?

Please provide me with the slightest reason to give a damn. Tell me what changes about it being AA77 being flown into the side of the Pentagon.