The movement of a 'plane using Herkybird's solo rules is based on three things:
The position of the nearest hostile aircraft
The manouvering capabilities of the plane itself
And a D6 choice of viable options.
This is how it works:
Take a bearing on the nearest hostile – 3 o'clock, 6 o'clock, etc.
Consult the manoeuvre chart which corresponds to the aircraft's manoeuverability class.
For example: a Nieuport 17 (class I) with an enemy aircraft at 10 o'clock.
The chart will give offer three possible manoeuvre options – all sound tactical options, nothing silly or unrealistic.
Roll a D6 to choose one of the options, & you're good to go.
We usually agree that – unless relevant to the scenario – any aircraft well out of anybody's effective range is allowed to just fly straight towards or away from the action as appropriate.
(edited because I can't count.)