Consider the following excerpt from the FAA low-altitude IFR enroute chart (L-33 / L-34) in upstate NY.
The light gray, EKG-like line that overlays V293/T295 is appearing more often as the FAA continues the MON program to decommission about one-third of the VORs in the continental U.S.
The zigzag line means that the underlying segment of an airway or route is unusable. That word, unusable, has a specific legal meaning in the FAA regulations. It also affects how you can operate under IFR in the area so depicted on a chart.
The combination of the symbol and its underlying meaning continues to confuse many pilots who fly RNAV-capable aircraft, typically using GPS to navigate even along VOR-based airways and routes.
The Aeronautical Chart User’s Guide describes the zigzag symbol thus:
For clarity, I’ve reproduced the caption below:
Pilots should not file a flight plan for or accept a clearance that includes navigation on any route or route segment depicted as unusable. Pilots using RNAV may request ATC clearance to fly point-to-point between valid waypoints or fixes, even those on routes depicted as unusable (refer to AC 90-108 for RNAV eligibility).
The AIM also addresses this issue, although somewhat obliquely.
For example, a note in AIM 1-2-3 Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes explains:
Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
That’s the legal answer, tied to the legal description of a Victor airway, which is explicitly defined by VOR radials (see 14 CFR Part 71, 14 CFR §91.181 Course to be flown, and FAA Order 7400.11).
For example, if a VOR is decommissioned or temporarily out of service, or if a radial or range of radials that define one or more airways or route segments is unusable, then the Victor airway or route segments that those radials establish are also unusable. An airway or segment might also be rendered unusable for other reasons, including new obstacles.
In other words, even if you fly an RNAV-capable airplane and routinely fly Victor airways using GPS, according to the chart below, you can’t legally file or fly V292 between SAGES and WIGEN. ATC should not even issue a clearance that includes “V292” for that part of the route.
In this case, there’s an easy way around the dilemma. The unusable segment coincides with a T-route, T295. T-routes are designated for use by aircraft with an IFR-approved GPS. And a note below the unusable segment of the airway states: ONLY V292 UNUSABLE.
So you could fly along the black line beneath the zigzag on this segment, provided ATC cleared you via T295.
Another option, point-to-point RNAV navigation, is also legal. ATC could clear you to fly (for example, west-to-east) along this segment via SAGES -> WIGAN.
Sometimes a segment marked as unusable does not have a charted MEA, or the only MEA is a GPS minimum altitude (in blue) associated with a coincident T-route. But ATC can still clear you by using its normal off-route or point-to-point procedures, including the minimum IFR altitudes (MIA) or minimum vectoring altitudes (MVA) established for that area.
This discussion may seem moot in the RNAV era. Most IFR pilots routinely fly victor airways using only GPS. As a practical matter, if you have a navigator that supports airways (for example, a GTN or newer G1000 system) and ATC clears you to fly the “unusable” segment via SAGES -> WIGAN, you would probably still load V292 into the box.
Or you could enter and fly that portion of T295. Either option saves you from having to enter the fixes individually.
As we’ve seen, the preceding example offers a couple of choices, because a T-route coincides with the Victor airway. But that’s not always the case if a Victor airway has been rendered unusable because a VOR has been decommissioned.
Consider this segment of V469:
The chart shows a GPS-based MEA (6900) for one segment marked unusable, but there is no coincident T-route. According to the AIM reference cited above, you can’t file or accept a clearance to fly V469 south of BRUCY. In this situation, legally, if you want to fly the course defined by the black line on the chart (and if the lack of a VOR signal is the only reason for the unusable segment), you must file, and ATC must clear you, via the fixes along that route: BRUCY -> EXRAS -> IFAVU -> BOOME…
Again, you could save time and avoid potential errors by loading the airway into a navigator like a GTN. But technically, your clearance would be direct between the fixes at an altitude assigned by ATC for that portion of the route.
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