When asking people to write letters in support your application, always ask if they can write a strong, favourable letter for you.
Many people can write a letter, but what you really want are strong, favourable letters. There is a big difference between being described as "a good pilot and nice person" and being described as "a skilled pilot, exceedingly professional, courteous to others, conscientious in task, a quick study, and one of the kindest people I have ever met".
When people write letters of support for you, their own reputation is also at stake. Because of this, only people that have great confidence in your ability to succeed are comfortable writing strong letters of support. Others will only write mediocre letters and those are plentiful in applications. Yes, it helps if the people supporting you are known to the person reading your application, but, in my opinion, a strong, favourable letter from an unknown, credentialed person is better that a luke-warm letter from an acquaintance.