I believe the root cause is Concorde, you see since it has only economy and it's economy ticket fare is usually 3x-5x the price of a Boeing's economy. Further, the fuel efficiency of Concorde is 2.9 while Boeing and non russian planes are above 6, meaning concorde consumers twice the fuel of other aircrafts. That led to a spike in fuel usage and During the 1976-77 period I faced an oil crisis where price shot upto 800. Thirdly, maintenance costs of concorde and 747s are massive. 747s are designed for extreme long haul and high rated cities, so if the route does not have either of them, your profits bleed. Fuel tanks and maintenance depots only help you take a beating but not a bullet, they only serve short term. You should have renovated planes that are in competing routes to give you an edge over satisfaction. Concordes make you a fortune only if its a monopoly route. I had a Concorde for Toronto-Amsterdam rating of both are above 300 each and it initially made me upto 6k monthly and 8k during peak season per turn, with a satisfaction of 100%. A competition came with an A300B4 and boom my concorde struggle to make it past 4k a turn. I eventually had to sell it because it didn't make it 100% occupancy even in routes such as Boston-Dusseldorf It made 4k a turn, something which an A300 or A 310 would do with lesser fuel costs (there fuel efficiency is twice of concorde) and maintenance is cheap and since they cost less than concorde, you guessed it! Depriciation is also lower (atleast half of concorde)