"Where are you looking for airplanes?"
Translation:Hol keresel repülőgépeket?
Other sentences never really are a strong argument, different sentences may express different relation to the pieces of information.
Here, it's a de facto rule that all interrogative words that can be answered with a part of speech (basically all of them but the likes of "why") are always immediately followed by the conjugated verb.
It can't be keresed because that would be the definite conjugation, but the object here ("aeroplanes") is not definite (it's not "the aeroplanes" or "those aeroplanes" or "my aeroplanes", etc.).
The normal indefinite-conjugation ending for te would be -sz, but because it would be difficult to say keres'sz, the ending becomes -el after s sz z dz.