You may want to check out this link:
Pilot Document Library on the International Pilot Network website.
Choose 'Pilot Interview' and then look for a file called 'Pilot Resume Cover Letter'. Layout, structure and information contained in this particular one was successful at Thomson, Cathay, Dragonair, and more...Good luck.
Alternatively you may also want to see how others write their Cover Letters
here. But be warned, many cover letters are not up to the requirements...