"Aeroplanes are at the airport"
Translation:Ndege zipo uwanja wa ndege
I think it's likely due to 'uwanja wa ndege' as a whole translating to 'airport'. If it were just 'aeroplanes are on the field' then it would be 'ndege zipo uwanjani'. Personally I think in this case it would fit more to say 'ndege zipo katika uwanja wa ndege' because you don't have the locative in the object noun