Constructor called twice and destructor causes segmentation fault

Question

I have this code that compiles normally, but when I run it:

a)if I don't include the destructor, the constructor will be called twice b)If I include it the constructor will be again called twice but at the end I will get "Segmentation fault (core dumped)"

I'm new in programming generally and c++ so I can't seem to find the problem...Any help is appreciated and thanks for your time.

This is the main:

int main(){
  aircraft_list<planes> plane_list;
  plane_list.add();
}

This is the constructor of planes:

planes::planes(){
  //initializing some vars
}

This is the code for add():

template <class T> aircraft_list<T> aircraft_list<T>::add(){
  chain_node<T> *y=new chain_node<T>;
  planes obj;
  y->data=obj;
  y->link=first;
  first=y;
  return *this;
}

This is the constructor for aircraft_list:

aircraft_list(){
    first=0;
  }

This is the destructor:

template <class T> aircraft_list<T>::~aircraft_list(){
  chain_node<T> *next;
  while(first){
    next=first->link;
    delete first;
    first=next;
  }
}

**EDIT:**So I changed the del() and add() to return references:

aircraft_list<T>& del(const T &x);
  aircraft_list<T>& add();

and there are no problems in compiling. But after some reading I still don't understand why the constructor is called the first time.