21.77—Duration.
        
        (a) 
         Unless sooner surrendered, superseded, revoked, or otherwise terminated, provisional type certificates and amendments thereto are effective for the periods specified in this section.
    
    
    
    
        
        (d) 
         An amendment to a Class I or Class II provisional type certificate is effective for the duration of the amended certificate.
    
    
        
        (e) 
         A provisional amendment to a type certificate is effective for six months after its approval or until the amendment of the type certificate is approved, whichever is first.