Huge planes (747, A380,777,A340, etc.), they will allow you to have even more money:-)

If you have credits, you could also buy an airport. Main airports in Huge cities (Beijing, Los Angeles, London) cost between 60 to 70 credits (and 6,000,000 - 7,000,000k), but they can generate huge profits.

I'm still on 1985 tho

Just save it. Keep it for when you need it.

Jetlover747 has a point, but the aircraft can make you even more profits, when you are in an economic crisis, you just close all the routes that don't generate profits and sell the planes that have low lifespan.

If you are in 1985, you can buy the jumbo jet or DC-10 (or MD-11), those can also generate 5,000k+ profits. By huge planes I mean 300+ full passengers payload (maximum number of passengers). IDR what other big aircraft are there, but just buy'em:-)

I bought 15 B747’s and they are giving me 100,000K profit

Ok, good. But how save it. Expect economic crisis 1993, albeit mine lasted 1 day. But still save it. Find a good route, open it. But only use that money if you really need to. Don’t just randomly buy 8 Concordes just because you can.

My profits always fluctuate. And i'm now losing on ranks