Thunder and Stratus worlds can be played in either Classic or Challenging mode.

There are a few economic changes between the Challenging and the Classic mode which are detailed below:

Fleet Management Cost is only applied in Challenging mode
To emulate the increasing overheads from managing a fleet, when you own more than 5 aircraft an additional cost will be charged weekly depending on the number and types of aircraft in the fleet.
For virtual airlines this cost is applied to the virtual airline itself, not to individual members who happen to have access to those aircraft.
This is intended to be a representation of the costs associated with the extra background staff needed to manage a fleet. E.g. Ground crew, weather handling, accountants, etc.

Weekly FBO Management Costs is only applied in Challenging mode
To emulate the increasing cost/overheads of FBO management, when you own more than 3 FBOs, an additional weekly fee based around your total number and class of FBOs will be charged. This is a representation of all the staff working in the background to manage the FBOs.

Job Pay
The amount of credits that individual jobs payout is significantly lower in challenging mode compared to classic mode, i.e. Classic mode jobs are, on average, around 3x higher payout than challenge mode equivalents.