Navigation Method

2 Answers

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   John D Collins on Sep 15, 2017

   RNAV is area navigation where the navigation equipment allows point to point navigation. There are several systems that can be used to provide RNAV capability, LORAN (obsolete), VOR/DME rho-theta (KNS80), DME-DME (airline class equipment), Inertial Navigation with and without DME/DME (Corporate Jet and Airline class), and GPS. Now a days the GPS is the primary method used for point to point RNAV. T routes are similar to Victor airways, but they require RNAV 2 PBN navigation equipment. Most GPS can fly these routes and you will see them on the IFR Low Enroute charts as blue airways with T and a number such as T201. These routes apply below 18000 MSL. Above 18000 MSL, the routes are called Q routes and they appear on the IFR High Enroute charts. AC 90-100A describes the equipment requirements to fly the Q and T routes and RNAV SID/STAR/ODP. It also includes a listing of the pilot knowledge items that should be understood. You can also read about this topic in the AIM section 1-2-1.

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   Russ MacDonald on Oct 16, 2017

   John is correct, but I would like to add a few points.

   You can actually legally request a direct IFR route without a certified GPS. In fact, the controller doesn’t care what equipment you have, because when he gives you direct, he is actually giving you a radar vector with own navigation. As such, he will sometimes give you a higher altitude, because you must always remain in radar contact for a radar vector. He doesn’t care how you get there, and he is required to watch closely to make sure you are staying on radar vector he has determined on his equipment. If you get off course, he will issue a heading. Then, you have to stay on his assigned headings for the rest of that leg.

   This is actually a very old practice. Back when VORs were the only means of IFR navigation for small aircraft, I used to request IFR direct between VORs that were sometimes as far as 300 miles apart (plains states USA). I would fly outbound from the first VOR until I lost the signal, and then hold that heading until I received the second VOR. The controller knew I would be dead-heading for quite a bit of the center part of that leg, but, as long as I was in radar contact, and stayed on his calculated vector he didn’t care. I was required to climb to 11,000 feet to stay in radar contact. If I couldn’t climb to 11,000 for any reason, he would reroute me on the airways, and that nearly doubled the distance of that leg.

   Today, it is still the same. You can request and receive a direct route IFR clearance from ATC even if you are using a non-certified GPS, as long as you can accept an altitude that will keep you in continuous radar contact. You also have to remain within comms contact. Position reports are not normally required or allowed.

   However, as nextgen and freeflight are implemented, soon there will be times when the controller will assign direct only if you file the correct equipment suffix for a WAAS GPS on your flight plan, and then the direct clearance you receive will not be a radar vector, since you may be out of radar contact. We are already seeing some of this as T and Q Routes are being published. You must have a WAAS GPS to fly those routes.

   And in the midwest plains states some freeflight clearances are being assigned with direct routing down low and out of radar contact. In those areas, the controller will check your flight plan to make sure you have filed the proper equipment before he will issue a direct freeflight IFR clearance. Otherwise he will still make you climb up high to stay in radar contact and keep you on a radar vector. Also, with ADS-B ground stations being implemented, radar coverage is much better, so they don’t have to climb you as high as when VORs were king.

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