에러가 나네요 casting에러요  air 객체와 car 객체를 casting 했는데 오류 납니다.

소스코드 수정본

REPORT  z14_08.

CLASS vehicle DEFINITION INHERITING FROM object.
   PUBLIC SECTION.
    METHODS: create,
                       drive.
   PROTECTED SECTION.
    DATA speed TYPE i VALUE '100'.
ENDCLASS.                    "vehicle DEFINITION

CLASS plane DEFINITION INHERITING FROM vehicle.
   PUBLIC SECTION.
    METHODS: fly.
ENDCLASS.                    "plane DEFINITION

* Class IMPLEMENTATION
CLASS vehicle IMPLEMENTATION.
   METHOD create.

      ENDMETHOD.                    "vehicle
 METHOD drive.
          speed = speed + 100.
          WRITE : 'Driving is possible, ', 'Current:', speed.
      ENDMETHOD.                    "vehicle
ENDCLASS.                    "vehicle IMPLEMENTATION

CLASS plane IMPLEMENTATION.
   METHOD fly.
    speed = speed + 1000.
   ENDMETHOD.
ENDCLASS.                    "plane IMPLEMENTATION

DATA : car TYPE REF TO vehicle,
  air  TYPE REF TO plane.

START-OF-SELECTION.
  CREATE OBJECT: car,
         air TYPE plane. " 비행기 클래스의 인스턴스 생성
*  air ?= car.

CALL METHOD: car->create. " 차량 생성
  CALL METHOD: air->drive.
  CALL METHOD: air->fly.

casting 주석 풀면 에러나네요. 수정 부탁드려요