HOME LATEST NEWS VI

INSTRUMENT PROCEDURES HANDBOOK

DIRECT FLIGHTS

There are a number of ways to create shorter routes and fly off the airways. You can use NACO low and high altitude en route charts to create routes for direct flights, although many of the charts do not share the same scale as the adjacent chart, so a straight line is virtually impossible to use as a direct route for long distances. Generally speaking, NACO charts are plotted accurately enough to draw a direct route that can be flown. A straight line drawn on a NACO en route chart can be used to determine if a direct route will avoid airspace such as Class B airspace, restricted areas, prohibited areas, etc. Because NACO en route charts use the Lambert Conformal Conic projection, a straight line is as close as possible to a geodesic line (better than a great circle route). The closer that your route is to the two standard parallels of 33 degrees and 45 degrees on the chart, the better your straight line. There are cautions, however. Placing our round earth on a flat piece of paper causes distortions, particularly on long east-west routes. If your route is 180 degrees or 360 degrees, there is virtually no distortion in the course line.

About the only way you can confidently avoid protected airspace is by the use of some type of airborne database, including a graphic display of the airspace on the long-range navigation system moving map, for example. When not using an airborne database, leaving a few miles as a buffer helps ensure that you stay away from protected airspace.

In Figure 3-33 on page 3-28, a straight line on a magnetic course from SCRAN intersection of 270 degrees direct to the Fort Smith Regional Airport in Arkansas will pass just north of restricted area R-2401A and B, and R-2402. Since the airport and the restricted areas are precisely plotted, there is an assurance that you will stay north of the restricted areas. From a practical standpoint, it might be better to fly direct to the Wizer NDB. This route goes even further north of the restricted areas and places you over the final approach fix to Runway 25 at Fort Smith.

One of the most common means for you to fly direct routes is to use conventional navigation such as VORs. When flying direct off-airway routes, remember to apply the FAA distance limitations, based upon NAVAID service volume.

RANDOM RNAV ROUTES

Random RNAV routes may be an integral solution in meeting the worldwide demand for increased air traffic system capacity and safety. Random RNAV routes are direct routes, based on RNAV capability. They are typically flown between waypoints defined in terms of latitude and longitude coordinates, degree and distance fixes, or offsets from established routes and airways at a specified distance and direction. Radar monitoring by ATC is required on all random RNAV routes.

With IFR certified RNAV units (GPS or FMS), there are several questions to be answered, including “Should I fly airways or should I fly RNAV direct?” One of the considerations is the determination of the MIA. In most places in the world at FL 180 and above, the MIA is not significant since you are well above any terrain or obstacles. On the other hand, a direct route at 18,000 feet from Salt Lake City, Utah to Denver, Colorado, means terrain and obstacles are very important. This RNAV direct route across the Rocky Mountains reduces your distance by about 17 NM, but radar coverage over the Rockies at lower altitudes is pretty spotty. This raises numerous questions. What will air traffic control allow on direct flights? What will they do if radar coverage is lost? What altitudes will they allow when they can’t see you on radar? Do they have altitudes for direct routes? The easy answer is to file the airways, and then all the airway MIAs become usable. But with RNAV equipment, a direct route is more efficient. Even though on some routes the mileage difference may be negligible, there are many other cases where the difference in distance is significant. ATC is required to provide radar separation on random RNAV routes at FL 450 and below. It is logical to assume that ATC will clear you at an altitude that allows it to maintain radar contact along the entire route, which could mean spending additional time and fuel climbing to an altitude that gives full radar coverage.

All air route traffic control centers have MIAs for their areas of coverage. Although these altitudes are not published anywhere, they are available when airborne from ATC.