The cargo feature can be unlocked via the general R&D. It will give you the possibility to carry goods on all your routes and you will access a new research category: the Cargo R&D.
Once it is available, the Cargo R&D will enable you to unlock aircraft specialized in the transportation of goods and other diverse services related to cargo.
Cargo as an additional income
Cargo can be considered like a new source of income. In this case, your strategy is still focused on passenger transportation and cargo is a small source of additional income.
For this strategy, is is recommended to unlock services which do not involve a reduction of your passenger demand, but it’s up to you to find the best compromise according to your strategy.
Cargo as a single strategy
Cargo is a new feature which also enables you to have a brand new approach of the game. If an airline chooses to only carry cargo, it can do it, but it will lose a major part of its passenger demand in return.
An airline specialized in cargo can totally compete with a classic airline, especially if the airline is optimized for the transportation of goods (with services specialized in cargo).
The cargo airlines are characterized by a high profitability of their routes (higher maximum turnover per route) and by a slightly lower profitability per aircraft. However, this decrease is balanced by a tax credit called “Cargo bonus” which you can find in your general accounting: