An earlier post mentioned the use of 'aeroplane' by the English weekly FLIGHT, later renamed FLIGHT INTERNATIONAL. The poster looked at an issue for 1920. He did not go back to the start. If you look at the editorial of the first issue of FLIGHT, that of January 2, 1909, you will see that 'aeroplane' was then the common and accepted term in use.
The various patent applications made by the Wrights in the USA and Europe referred to the design features of their 'flying machine.' It will require further delving into the archives to find when 'airplane' first crept into common usage in North America . It is a fair assumption that because most serious American researchers would have read FLIGHT and it's contemporary sister weekly THE AEROPLANE from their inception, that some years would have passed before 'aeroplane' was corrupted into 'airplane'.
The day that the Australian accent goes further down the gurgler and our
kids start calling 'Mom, mom', and persist with demands to 'listen up',
it will be too bloody late to give their collective little asses a swift kick, metaphorical or otherwise.