C++: Warum kein Zugriff auf protected Member?

template<typename T, int elements> class Fifo {

  protected:

    int             _avl;

  public:

    ...

}

template<typename T, int elements> class TxFifo : public Fifo<T, elements> {

    void setInterruptFlags(void) {

        switch (_avl) {

            ...

        }

    }

  public:

    ...

}

error: ‘_avl’ was not declared in this scope