I have some airplane footage and would like to remove the voices from the nearby spectators and keep the engine noise. There is maybe 1 or 2 voices during the clips and it's nearby so it could be isolated fairly easily.
How much of the audio contains the voices? If there is plenty of engine noise without the voices then simply slice the voice parts out. Overlap the remaining sections just a bit and line up the 'beat' of the engine so that the gaps aren't noticeable.
Most likely it will be close to impossible to isolate or filter it.
I can't say for sure without listening to a sample. You should certainly try iZotope RX and Spectral Layers. However, my instinct says that Kelly (Chienworks) is right on the money: cut out the sections with the voices, and then mix sound from adjacent sections of the clip to file the gaps. If the airplane noise is fairly constant, you'll be done right away. If the noise is from airplanes at a show, and they are moving across the field of view, creating a change in both volume and pitch (Doppler effect), then it will be trickier.
EQ notch would reduce, but not eliminate vocal frequencies without much effect on engine frequencies, but that would be considered the quick-;n-dirty method.