Airspace classes in the United States
<a href="https://en.wikipedia.org/wiki/Federal_Aviation_Administration" class="extiw" title="w:Federal Aviation Administration">Federal Aviation Administration</a>
2015-12-10
<a rel="nofollow" class="external free" href="http://www.faa.gov/air_traffic/publications/media/AIM.pdf">http://www.faa.gov/air_traffic/publications/media/AIM.pdf</a>