Bevs could become overweight for reasons other than load. Took one out to Aden to replace another which we brought back for major servicing. Routed through Khartoum, significant for temperature/perf restrictions. Climb-out was painful and struggled to make the cruise. Later heard that, on stripdown, several thousand pounds of sand was recovered from between the freight bay floor and the skin - collected from years of operating 'up-country' strips in the Protectorate.