You will have to use soft luggage ( like duffle bags)..they say they cram the luggage into all areas of the planes so they need to be soft only..
And, this is from one of the charter plane sites:
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In general, the luggage allowance per flight is approx 15 kgs per person, though the Johannesburg based planes are requiring a maximum of 10kgs per person - and always in soft bags. This is for meeting weight limits and easier stowage of luggage.
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We also read that the weight of the individual is also scrutinized: If you weigh more than 200 ( or maybe it's more than 210) pounds..you need to buy 2 seats on the plane( the upside is you can have double the weight in luggage allowance )