How does one control a plane in GTA San Andreas on Android?

The only way you can do this is if you use a compatible bluetooth controller with seperate analogue sticks, these unfortunately arent widely available and also arent always compatible with every device.

Just using the touch interface there is no way to control them seperately, which does make flying somewhat awful i will agree with you.

The company that makes the openPandora console have also made such a controller which i have used on android but like i had said previously good controllers are not widely available.

Can also say with iOS that when Jailbroken you can use certain bluetooth controllers like PS3 and Wii which is quite fun