Minor is the opposite of Severe. Severe is related to Disease. 
Similar is related to Road. Road is related to Transport. Airplane can be used for Transport. you are likely to find Pilot in/at/on Airplane. 
Minor <--Antonym--> Severe <--RelatedTo--> Disease 
Similar <--RelatedTo--> Road <--RelatedTo--> Transport <--UsedFor-- Airplane <--AtLocation-- Pilot 