Cover letters
A cover letter should always accompany every resume and update you send to any company. Not writing a cover letter implies a lack of interest and sincerity on your part.
A good cover letter should be targeted at the individual company. You don't have to reinvent the wheel every time, but you should design a form that can customized for each situation.
Try something like this:
(name of pilot recruiter)
Mediocre Commuter Airlines
Address
City, State, Zip
Dear Mr./Ms. whoever:
I should like to update my file with the enclosed resume. I have added flight time.
I have flown an additional 200 hours to raise my total time to 1200 hours. My multiengine time now stands at 400 hours. I expect to add to these totals on my current flying schedule.
The Sunday Podunk News business section had an article that said that Mediocre will be adding flights and hiring staff. I am doing everything possible to become a strong candidate for Flight Officer employment with Mediocre. I hope that you will keep me in mind as you recruit pilots.
Thank you again for your time and consideration.
Respectfully,
KingAirer
Something like that.
Hope that helps. Good luck with your job search.