"Where are you looking for airplanes?"
Translation:Hol keresel repülőgépeket?
I know Hungarians saying just 'repülö' - shouldn't that be accepted or is this way to informal?
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.