The comment from L-Dutch states:
"The first line is at most a comment to the OP's question. The rest, being based on nothing that the OP asked, is not an answer"
However, this ignores what the OP said:
"Say hypothetically, an entire American airbase with many extremely modern aircraft is instantaneously transported to the 1400s."
Now, my answer was to first point out that the choice of aircraft for what was being described was a poor choice - a Fleet Defense fighter (that was designed to stop Russian Heavy Bombers from getting within Anti-Ship Missile range of a Carrier group) was the wrong choice of Aircraft.
However, since the frame of the question was 'Many extremely modern aircraft' - this gives the answerer (me) scope to recommend what would be appropriate - which is any ground-attack aircraft in the US Arsenal.
This gives us a choice between the A-10, the F-35, F15 Strike Eagle, Apache Attack Helicopter yadda yadda yadda - you get the picture.
In the scenario - the best choice would be an A-10, since you don't need to fly high or fast or stealthy - flying low and slow and creating as big a bang as possible is as per the story requirements what was needed.
Then outlining what would be the likely effects - First a dry run, then a gun run and if the Army still held their nerve, a Cluster Bomb run - which would break any medieval army's will to fight.
TL;DR - I disagree with the reason it was deleted - I think the deletee didn't read the question properly and so thought I wasn't answering the question asked. and I'm Salty and want it re-instated cause I liked that answer.