Why the frequency of aeroplanes is 400 Hz?

Higher the fequency , higher the speed. Similarly,freq is inversely proportional to the the flux , higher the freq lower the flux requirement hence lower the winding req. which will reduce the weight ( this is necessary req. in operation of Aeroplane) of the winding

The higher frequency allows a lighter transformer. Doubling the frequency allows the transformer's iron core to be half the weight. So 400 Hz allows light Transformers (and motors) to be used.