The best way to avoid bad weather is laterally.
Why?
I was playing with the secondary flight plan during a long flight. And I took some pictures, of course.
Normal conditions:
Estimated fuel at destination: 7,900 kg.
We were flying at FL380 (38,000 ft) with a step climb to FL400 planned a few hours later.
Let’s start with an extreme example:
What happens if we descend to 10,000 ft for the rest of the flight?
19.1 tons difference!
The arrival fuel became negative. Also, notice that the ETA (estimated time of arrival, the value left of the circle) changed by more than 2 hours.
Ok. That was too much.
Why don’t we just descend to FL200 (20,000 ft)? We’ll climb back an hour later:
Not as bad. Minus 3.1 tons.
This is still below the minimum fuel to divert to an alternate airport in this particular case. Not nice.
Lateral weather avoidance, even for long periods, is the most efficient course of action.
In some cases, however, it’s a good idea to descend 2,000 or maybe 4,000 feet to avoid moderate turbulence. Your original level may become occupied when you want to climb back, though.
Author – Alejandro Merediz ( Captain )
