Who is King comparing to a flight's ground crew in this metaphor from his Nobel Prize acceptance speech?

King using the phrase "flight's ground crew" refers to the people who have sacrificed a lot to make the freedom movement take flight. They are the people who have given their all but are not recognized for their sacrifices. These are the people who worked and fought for what is right and just regardless if they are credited for their contributions. Their reward is to make their dream come into fruition.

The people who have worked hard to make the civil rights movement happen but will never be given an award.