Phantine Air Corps
"We're going to bomb them back into the Stone Age."
- – General Curtis E. LeMay
In case you haven't picked up on it by now, the Imperium is full of such heaping amounts of stupid rules that you could write for millenia and still only record a fraction of these fucked up restrictions. Probably because GW wrote 40k and then realized there was no possible way in their setting for the Imperium to have not wiped everyone else out already, so GW had to compensate somehow. One that does make some relative sense is that the Imperial Guard and Imperial Navy should be kept separate. After the Horus Heresy, you can't risk another self-sufficient force
(justifiably) rebelling against the Imperium. Fortunately, this keeps all aircraft under Navy control. It simply wouldn’t do for the Imperial Guard to be able to defend against a ship should the vessel turn traitor, after all. Nothing says “balance of power” like rendering one group utterly at the mercy of the other.
Enter the planet of Phantine, which had an issue: all their population is forced to live at the peaks of their homeworld. This meant there was a problem for the Imperial tithe, since Phantine PDF forces were primarily pilots. Deciding they were already worse hypocrites than most popes, the Imperium decided to disobey the rule that the Imperial Guard shouldn't have an air wing. While deploying them with a dedicated force of drop-troops, the Departmento Munitorum soon revealed their newest regiment: The Phantine Air Corps had been formed.
Of course, not simply having a dedicated Air Force for the Imperium was proving to be a bad decision. The Admirals tended to remove any air support from their ships - particularly ships dedicated and specifically built for the sole purpose of providing air support for the Imperial Guard (this is technically both normal heresy for violating the ship’s doctrine and tech-heresy for violating the ship’s sacred purpose) - and replace them with attack craft because the Navy always needs more of them, while refusing to just build more dedicated attack craft carriers because they're "useless". Almost as useless as the ship commissars, who for some reason don’t get trigger-happy at this gross deviance from the air support ship’s specific purpose. In fact, why isn’t the Mechanicus pissed at this virtual (pun) raping of these ships’ sacred purpose?
Why This Was Hated
It's already bad when neckbeards find out their favorite armies are getting changed. Imagine how the Phantine were received by the Guard and the Imperial Navy. For millennia, the Navy had been the go-to force for atmospheric transport and air superiority (pfft, air superiority, riiiiight). Now there was a Guard regiment to take on that role? Never mind that the common guardsman is probably pissed at the Phantines in their nice personal aircraft as they're forced to slog through the mud, blood, and crud that is a 40k battlefield. Although, real soldiers like anyone that brings death from above to kick the shit out of whatever is shooting at them, so the Phantine likely would find themselves quickly accepted after blasting the hell out of artillery, heavy weapons nests, and shredding enemy infantry and tanks for the infantry to mop up. Best of all, the Guard hates the Navy because when they need air support the Navy tells them to go die in a ditch. When they call for air support from the Phantine, they get their damn air support from some of the best pilots in the entire galaxy.
Why They Aren't Common
Have you fucking seen the prices to buy Imperial fighter models? However, in the new areonautica Imperalis game, you can paint your thunderbolts in Phantine color schemes so it is slightly more common.
Why They Should Be Common
The great part about the Guard is that it's about the common soldier (and his Baneblade) in the face of overwhelming odds, and even in the darkness of 40k this definitely extends to the Phantine. They're the same bog-standard humans as the Guard on the ground, facing down Tyranid flying hives, Eldar super-fighters, titan-sized Tau bombers, Necron anti-aircraft laser spam, Chaos planes that eat other planes (and whose mere wake in the air is corrupting and can cause their aircraft to eat them alive if they fly through said wake), and Orks guzzling jet fuel because they can. The Phantines are the pilots up against the most overwhelming odds, facing death in the skies knowing that their efforts protect the ground forces to do their jobs without being bombed to hell. To attack the enemy behind the lines with precision strikes on vital points and fortifications. To face the same horrors in the sky as their brothers on the ground with little more than their wits and reflexes and a jet strapped to their ass. So that when they land and see that their efforts allowed others to succeed, they may know they are just as much Imperial Guard as any other in the service of the God-Emperor.
It's also worth pointing out that since they just fly air craft as compared to to the 'proper' navy's space craft they don't break the original function of splitting the navy and imperial army, making it harder for rebellions to have both a ground force and a Navy force. So while yes, there is now a risk of rebellions having air support, it won't be able to leave the planet and cause chaos. You additionally gain closer corporation between air and ground forces, useful for naval ships space to surface weapons, vital for weapons which are going to be operating in close support to ground force, and yes that closer ties mean higher chance of a joint air/land rebellion but it may be worth it in this case, (not to mention since there guard you can have Commissars around to BLAM the pilots if need be, though given the cost of training a skilled pilot this may not be as common as it is for the ground-pounding cannon fodder but it is still an option.) So even by Imperial logic it make sense to have more "Imperial Air Corps" like the Phantine's. Cept for that whole "irrationality" which marks the setting. Besides, it isn’t unknown for air groups on ships to turn renegade, anyway.