> I think they were asking about changing heading in addition to altitude
Heading and altitude are NOT independent. When an aircraft makes a turn, it banks (it's not like a flat turn you make in your car), and inevitably loses some speed because of extra drag, and some altitude because of loss of lift.
It can be compensated by increasing the engine power, but then you really need to predict that as well.
Yes, I know how planes make turns. It is valid to point out that a TCAS instruction that included a heading change would, at least for the plane whose TCAS instruction was to increase altitude, also have to include an engine power increase, which might not be feasible depending on the altitude (cruise power might already be pretty close to max power). Or else make the turn a very slow one, which would decrease its usefulness in collision avoidance.
Also that bank means you have a wing tilted up and a wing tilted down, increasing the vertical height of the plane at a time you’re trying to increase vertical separation.
Doing both reeks to me of a solution in search of a problem that’s going to end up the target of some later NTSB report when you hit some weird edge case where the dynamics and dimensions of the two planes aren’t fully accounted for.
> bank means you have a wing tilted up and a wing tilted down, increasing the vertical height of the plane at a time you’re trying to increase vertical separation
Yes, but it's also increasing horizontal separation (from a predicted value of zero), which might still be a reasonable tradeoff to consider.
Heading and altitude are NOT independent. When an aircraft makes a turn, it banks (it's not like a flat turn you make in your car), and inevitably loses some speed because of extra drag, and some altitude because of loss of lift.
It can be compensated by increasing the engine power, but then you really need to predict that as well.