Passenger aircraft are not supposed to cross red areas on the weather radar. But what if the weather system is too large to circumnavigate and red areas inside the system are too large/prevalent to avoid ?
What are pilots supposed to do if the required detour around red areas would be impractically long ?
- divert to an alternate airport in front of the storm (on the near side) ?
- go back to the departure airport ?
When/where (at what stage of the flight: e.g. PNR minus x minutes, or at a specific distance to the weather front ahead, etc.) is the decision made whether to fly through the bad weather or to abandon the destination airport ? And which criteria is this decision based on ?
I assume this decision takes place between the conflicting priorities of assuring the passengers' safety and the (real-world) pressure exerted by (some) airlines on their pilots to reach their destination airport for financial reasons ?
Please distinguish in your answer flight over land and sea, assuming different considerations apply. For example, the pilot may need to take the decision before reaching the critical point or point of no return. What are the implications of ETOPS for this question ?
The premise ("a weather system too large to circumnavigate and red areas inside the system are too large/prevalent to avoid") may be a particularly common situation when crossing the ITCZ:
a broad band of high total precipitable water along the Intertropical Convergence Zone (ITCZ) (source)
Let me take AF447 as an example – not to say that red areas were unavoidable in this case, but just to illustrate how this flight through the ITCZ had to cross a weather system too large to circumnavigate entirely. Unless turning back, the pilots had to pick and choose "holes":