British Airways Boeing 777-236 G-ZZZB operating BA72 Muscat – Abu Dhabi – London Heathrow diverted via Al Ain on the Muscat – Abu Dhabi sector due to visibility at Abu Dhabi. Crew did not have sufficient duty hours on the delayed Abu Dhabi – London Heathrow sector to reach London Heathrow and the aircraft therefore diverted via Frankfurt for a crew change.