Comets are actually working extremely well for me. Of course they scale, like other jets, faster when fuel rises - however, they also scale better with fuel low and average. Basically a Comet 4 can do what 3x Connies or 2x DC-6B were able to do but with reduced operating costs. The only downside is, DC-6B are making profits longer when fuel is high. Still, break-even for my Comets is somewhere above $135.