if you have a list for example:
List<AiAircraft> PlaneList = new List<AiAircraft>();
PlaneList.Add(aircraft1); // only examples
PlaneList.Add(aircraft2);
aircraft2.Destroy();
So now we have the aircraft2 already in List. Maybe more of destroyed Aircrafts. To remove them from List:
PlaneList.RemoveAll(item => item == null); // removes all null valued Aircrafts from list.