Plasma is pulled by gravity. If you fly too close to a planet, the plasma will be pulled inside of it's field to have a disproportionate amount of plasma facing the gravity source. This could cause a variety of effects depending on how you envision your ship working such as causing your ship to be pushed toward or away from the planet, or causing your ship to go into an uncontrollable spin that rips it apart.
If you want ships to be able to get closer than the outer limits of the gravity well, consider that moving through an atmosphere at more than a few thousand kph can incinerate pretty much anything. Trying to fly through an atmosphere at warp speeds will experience the relativistic baseball affect as per the most commonly cited meem here on world builder: https://what-if.xkcd.com/1/