Wjen is the best time to sell plaines?

Hello all
I wonder when is the nest time to sell the plaine when its said sell plus golden star or waiting untill it will finish the d tretment?

There are 2 decisions you can make
1)If you want to get golden planes without donating to the Devs, then you can buy planes and once they get past maintenance C, you can sell them.
2)If you want to buy a plane and keep it, then keep it till the last flight of maintenance D. If you pass Maintenance D, the distance traveled by your plane will start all over again so better to sell it before maintenance D for golden planes.