| * Click on CENTER for graphical tier information | ||
| ZID (ZID 1) | ||
| FIRST TIER | ZAU / ZDC / ZKC / ZME / ZOB / ZTL | |
| SECOND TIER | ZAB / ZBW / ZDV / ZFW / ZHU / ZJX / ZMP / ZNY | |
| * Click on CENTER for graphical tier information | ||
| ZID (ZID 1) | ||
| FIRST TIER | ZAU / ZDC / ZKC / ZME / ZOB / ZTL | |
| SECOND TIER | ZAB / ZBW / ZDV / ZFW / ZHU / ZJX / ZMP / ZNY | |