Firebird Commander 2 RTF Electric - Use ACT : Two sensors on the fuselage, one on the top and one on the bottom, monitor the position of the plane in relation to the ground. When the plane is flying level, the top sensor sees the sky while the bottom sensor sees the ground, and they tell the onboard computer (receiver) that the plane is flying correctly. If the plane enters a severe dive, the sensors detect the change in orientation and react to the danger that the plane is then in.