What is the visible condition of the main gear tire that is unacceptable?

The visible condition of the main gear tire that is deemed unacceptable is indicated by the appearance of the red cord. In aviation, tires are designed with multiple internal cords that provide structural integrity and strength. When these cords begin to show through, it indicates that the tire is significantly worn and is no longer safe for use.

The presence of the red cord specifically signals that the tire has reached a critical level of wear, to the point where it can no longer provide adequate performance and safety during operations. Tires that exhibit this condition must be replaced immediately to ensure the safety of the aircraft and its occupants. The red cord typically serves as a visual warning to maintenance personnel that the tire is no longer serviceable and poses a risk if used.

The other options refer to the number of cords visible on the tire, which suggests varying levels of wear. While having one, two, or even three cords showing indicates wear, they do not represent the immediate danger that a tire showing the red cord does. Thus, the appearance of the red cord is the clear indicator of an unacceptable condition for a main gear tire.