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Originally Posted by BeechBoy  When this is depicted why is the PT not required ONLY if the airplane is on an airway? Contrast this with the "T" shaped GPS approaches where the entire "Direct" sector is labeled "NoPT". |
The entire IFR system is built around the idea that you travel via airways to your destination. The stuff such as radar and RNAV has been slapped like band-aids on this airway-oriented system and is the cause of much confusion.
When you arrive at an IAF via an airway, the approach designers protect the airspace around the IAF only to the degree necessary for you to make the published turn. They do not take into account the larger turns that might be necessary when you arrive from unexpected directions. This is something I think wise to consider when you're cleared direct to an IAF.
So the NoPT arrival sectors were created with this mindset, long before RNAV capabilities were widely available. Although anything in the wedge would not cause a turning radius problem, TERPS didn't provide any criteria for authorizing such areas. There are issues to address such as the equipment required to identify when you're in the area, method of course guidance, and the proper altitudes to maintain. The TAA's provide you with all these things.