if you ever refueled and rearmed a tank , like I've done at many occasions, you know it take lots of time, putting shells into place through top hatch, mg-ammo, but fixing tracks / whatever is REALLY hard work, and imagine you sit exposed out of the battlefield with 1/2 crew outside , second half inside replenishing / fixing stuff.
Engines repair are not easy stuff in tanks too, I spent endless hours fixing with them damned things.
i say IF field tank repairs are being made out on the battlefield as AH have now , some aircraft repairs should be enabled at hangar.