For PC - Airline Simulation
So, I found this game when it was AIRLINE6 (now AIRLINE7), and I was like you, looking for a challenging/realistic airline management game. It offers both offline and online versions, but you pay for them separately (I believe). I only did the offline version. I hate to compare it to AirportCEO in terms of how you adapt to a game (because this is truly well organized, and that can seem like it is not at times), but you definitely learn how to get it to work for you, and have a challenge. I feel like it was created as a college-level course offering for how the game processes from quarter to quarter.
It lets you create your own airline, headquartered wherever you want in the world - any year, realistically speaking. You can choose to start out as domestic only or international too. You pick you airline service level, and what would be you standard onboard offerings. Then you receive your financial backing (you can trick it to give you more if needed after). These parameters you set up will bound your airline to them for the first 5 years of your existence. Then you are all set to go! (During the game you work to pay off the financial backing - truly easy)
Most of the time, I played the game as PanAm, and starting in the 1950s (because I cannot deal with scheduling props long distance). I find that if you do a route like that (start early in the jet era, in an affluent country) you easily succeed, unrealistically, but gave me great leeway to create a global monster. Now, if you started in today’s time, you get a true challenge financially (to someone who has never done this level of airline “game” management).
Some neat aspects of the game are how true it stays to releasing aircrafts into the correct years - and I mean EVERY commercial/cargo feasible from any year/manufacturer no matter how obscure, ever executive jets; you can also elect to have experimental aircraft which never came to fruition be in your save file, releasing in appropriate years (i.e. Boeing 2707 in 1970s/1980s; MD-12 in 90s); you get all engine type variants (also have to purchase maintenance facilities on these individually); airports truly deny you international/heavy capability (i.e. an EK 777 into LGA) - this also evolves as airport runways have over the years; airports don’t exist until appropriate years(i.e. MCO isn’t available until early 80s - also JFK is IDL until President Kennedy was assassinated); cabin offerings evolve over the eras (no tvs>projectors/aisle monitors>personal tvs - same for comfort/cabin levels); you can build HQs/hubs, buy terminals, gates, slots, etc - the flipside being you can also elect to have the game regulate it for you (although it does it infinitely); you can view airport departure screens to compare your flights, and also to your competitors’; oh, you can buy stock into the competitors, and buy them, operate them separately, turn them into a subsidiary, or merge completely (also merges your resources - planes, HQs, routes, flight numbers, etc) - this can also happen to you!; you can operate subsidiaries under different IATA/ICAO codes (best upgrade from v6 to v7 IMO); it tells you when C/D checks are up and you can schedule regular A/B checks or have the game do it for you (another awesome v6>v7 upgrade). It has a decent way of determining easy - medium - difficult play levels, to an extent; I believe difficult should have been easy LOL, but I also understand why easy is easy (for newbies). Their customer service started our really poorly, but back in the day this was the only to satisfy the need, and they have immensely improved, so much that it is a positive. They are Australian based.
Some downsides I found of the game are its negligence to adhere to aviation regulations over the eras - you can start a true Emirates in the 1950s (violate every 5th freedom right you can think of - not saying they do that, but that size in that era was unheard of); you schedule your saves in yearly quarters (positive, IMO), and as the save file grows, so does your wait time in between saves and processes of data; I personally found it difficult to deal with how you handle flight connections (this was the major overhaul from v6 to v7, which I lived, so it was difficult for me); I also personally found it difficult to figure out whether or not you have appropriately scheduled proper times between repeat segments (I created some ludicrous method by using Excel to keep track of my planes and their assigned flight segments - it helped me especially when it came to their maintenance swap outs); all this coupled with an inadequate forum (maybe this one has spoiled me? that or they are definitely not as receptive/collaborative as here) made it a long time to adapt. One miniscule bug I noted (because of playing as PA) is the crossover from IDL to JFK; you need to completely reschedule those flights for your system you act appropriate with flight numbers - you can sometimes repeat flights numbers on a completely different segment (another thing I mitigated with my Excel “fix”). Your license does have an expiration, but I recall it being over a year, and for me, worth the renewal for a while (it’s kind of easy to complain to get a free extension once or twice ).
Overall, I found its positives outweighed its negatives for a very long while. Now that you asked this question, and I checked their website that they are still in existence, I see they are soon to release v8 (how ironic their twitter posted it today (May 10 for me)! I may just have to try it out!!