Originally Posted by
Recidivist
My apologies if it's simplistic, but could you point out why it's flawed?
Both flights departed from the same airport, close together, heading in identical directions (diverging very late in the flight). If wake turbulence falls over time (as I understand it does), having the trailing aircraft at a higher flight level should prevent the trailing aircraft being upset by the wake turbulence of the lower, leading aircraft (unless, of course, the second aircraft overtakes the first). Where is the logical fallacy? Genuine question.
From an ATC point of view while the idea initially sounds like a good idea as you are correct that it would eliminate wake issues in the cruise, it’s not practical... especially when you take into consideration these were 2 supers, both on 14/15hr legs and most likely close to full.
First issue you would have to take into account is getting the 2nd aircraft above the second early on in the flight and then continually reverse step climbing them all the way to their final cruise level. With 2 aircraft this could have a moderate impact on workload.. but scheduling often has 5 or 6 jets departing LA for Oz, or departing SY for the USA in succession. The workload required in order to stack 6 aircraft in the opposite order to which they depart would be too much.
Then you have to take into account weight vs available level. An A380 loaded for 15 hours won’t be able to accept much higher than 30,000 ft initially.. so if #6 is that aircraft then 5 gets 29,000, 4 gets 28,000, 3 gets 27,000.. etc. The first few to depart get penalised heavily, have to mix it with turboprops.. and probably won’t make it across the pacific because of the extra fuel burn at lower levels. Then you have the issue as someone else has mentioned of possibly doing all this work then the last one gets to the highest level, hits some natural turbulence and needs to descend.. so you have to push every back down.
The logical way to run this is get #1 as high as they can go, if #2 is too close behind to get the same level they get one below, #1 will take higher once they can, then in turn you can keep climbing #2 and so on..
If the first one hits turbulence at a level it’s easier to return him to where it was smooth.
To put it into perspective, in a TMA we can legally have an A380 following another A380 in the climb 3miles behind. An A330 behind an A380 needs 6miles, a single engine light and you need 8 miles.. these were 2 supers with 20 miles between them.. 6 and a half times further away then you can legally put them at the same level. Having a wake turbulence issue with this kid of spacing is not the norm.. it’s a rare occurrence caused by the perfect weather/atmospheric conditions. You can’t run impractical, workload heavy, penalising procedures to safeguard against something that may happen 1 in a thousand flights.. we put them as close as we can legally and safely to get everyone where they are going as quickly as possible.. if it happens that on the odd chance you’re copping wake turbulence from the aircraft 20 miles ahead and above then the pilot will let ATC know (as they did), they’ll be cleared to deviate a little to the right or change level, and it will all be over in 20 seconds.