Not as elite as they were trained for...?

In the high speed tail chase engagement scene against the UCAV EDI through the Tajikistan mountain range, Lt. Purcell (Jamie Foxx) is the closest to the UCAV's tail and he is foolish enough to lock on and attempt to fire a heatseeker missile at such close range?

It's too dangerous to do so, because of the danger from colliding with the target's debris.

If so, why did he not use his 30mm cannon instead instead of fall for EDI's trick and crash into a mountain side?

If I were the combat pilot, chasing the UCAV EDI at such breakneck speeds through a mountain range, I would pitch up and gun the UCAV from its 6 o'clock high.

What would you have done?