Big, but lots of folks do it successfully and very few who try fail at it. So obviously not too hard. As well, plenty of airline types still fly fast jets in the Guard or Reserve, switching between their Superman and Clark Kent personas on a day-to-day basis. So it’s apparently not that big a deal.
Military pilot training from the git-go is all about operating in instrument weather conditions to instrument flight rules (IFR) tolerances. In training aircraft that are broadly equivalent performance wise to airliners. Speeds, top altitudes, power to weight, etc., are all broadly comparable. Which makes sense since about 2/3rds of those pilot trainees are destined for military heavies. There are differences between military flight regulations and civil regs. But they’re differences in detail, not in fundamentals since both groups have to interoperate in the national airspace of whichever country.
Then in military fast-jet training the fast-jet stuff is layered on top of the instrument jet pilot basics: formation, weapon systems operation, dogfighting, bomb dropping, aerial refueling, etc. All of which is irrelevant to airline ops except insofar as it makes one real good at precision control of the machine in space. Which is certainly an airline-useful skill. Especially when the weather’s down to crap, the runway has snow, the wind is howling, and of course we’re still going.
Upon making the jump to the airlines there are a few broad areas to learn.
Civil specific regs and airline policy are of course all new. The stuff related to flying itself is similar, but the BS about when and how many life jackets are required, duty and rest requirements, maintenance logbook procedures, etc. are all fundamentally new.
The administrative side of airline airplane ops is also all new. How one plans and prepares a flight, coordinates with the ops command center at corporate HQ and with the local airport ops command center are all new. And not stuff tactical pilots ever messed with. Military heavy pilots dealt with all of these same concepts albeit with different details.
The biggest difference is operating as a crew versus as a solo. I had the now-obsolete privilege of starting out a flight engineer. Who has a vital job, but the job isn’t piloting. He’s crazy busy getting the aircraft ready for departure, getting it started and moving, then he just sleeps and eats until near landing. Whereafter he’s once again real busy until the jet is shut down and handed off to either the next crew or the maintenance team.
The “sleep and eat” part is an exaggeration, but the real point is the in-between time provides plenty of opportunity to observe two pilots in action working together to operate the aircraft, it’s interaction with ATC, with the company, and with our fellow workers to hand-craft the perfect airline flight. It’s an excellent apprenticeship into being an airline pilot.
Sadly that’s gone now. We now have newly hired trainees ride a few flights in the jumpseat of their assigned type when partway through their training. The goal is to meld all the part-task pieces they learn in school into a coherent whole. And to see the whole thing work in conjunction with real ATC, real other traffic, and real airports. Even so their first few months on the line as co-pilots can be a trying experience for the lucky captains who get to mold them into real line pilots.
Learning to work as a team takes practice. Yes, there are procedures and scripts and such. But there’s also the fuzzier human factors stuff of how and when does one speak up when one’s feeling behind, or believes the other guy/gal is starting to screw up? Or is well down the road to confusion and error?
Us single-seat guys never gave stuff like that a thought. In airlining there’s always 4 versions of what’s happening now and what’s about to happen next. What the airplane and its computers think; what the ATC guy/gal thinks; what I think; and what the other pilot thinks. Keeping all 4 tightly synced no matter what is key to physical and regulatory survival. Depending on what military mission you used to fly, there may have been only one or two of those before. Learning to stay coordinated with the other pilot when stuff’s going wrong and you’re task-saturated or nearly so is not natural at all. If there’s one built-in weakness in using former single-seat pilots as airline pilots it’s this; their instinct is to go solo when overloaded.
OTOH, many of the folks who grew up flying civil turboprops and now jets have never flown fast solo can get overwhelmed if they don’t have the help of another good pilot. Being able to juggle *all *the balls, and knowing how and when drop them in the correct priority order isn’t easy or instinctual either. Lack of that skill and deep mindset has risks too.