Pilot Courses of Instruction
Navigation
Steve Sconfienza, Ph.D.
Airline Transport Pilot
Flight Instructor: Airplane Single and Multiengine; Instrument Airplane
cell: 518.366.3957
e-mail: docsteve@localnet.com
GPS
Over the past several decades, GPS has gradually been working its way into the civil aviation cockpit. What started as a purely military system has become first the obvious successor to Inertial and Loran for long range navigation and is now the evident successor to the VOR-based Victor and Jet airways systems for regional navigation, along with being the presumed successor to ILS for precision approaches (not to mention VOR and NDB for non-precision approaches).
The greatest benefit of GPS navigation is the ability to go directly between any two points ("waypoints"), which could be point-to-point navigation between two airports or any other identifiable waypoints desired. While this model works well in the enroute phase of flight, for IFR operations in approach phases it does not provide any terrain clearances. In fact, it does not provide any terrain clearances in enroute either, but that is often less of an issue in enroute than in approach phases (a Lear Jet at 45,000 feet is not likely to run into any terrain, nor is a Cessna 172 at 5,000 feet in Florida).
Unlike entirely ground-bases systems, GPS systems can apply, additionally, logic that enhances their usability (not unlike the earlier RNAV systems still in use). As opposed to standard navaids, the airway or approach course dimensions are constant with respect to the phase of flight. For example, the sensitivity of the GPS indicator (i.e., full-scale deflection on the course deviation indicator [CDI]) is constant at any position along a route but varies according to the phase of flight:
VOR tracks are wider with increased distance from the station (and extremely narrow close to the station), and the airway width — four miles wide when within 51 nautical miles of a station — is actually 4.5° wide, the actual charted airway increasing in width when beyond 51 nautical miles from a station.
That is, the width of the airway beyond 51 nautical miles from a VOR is a diverging 4.5 degrees on either side of the center line of the airway with respect to the VORs (at 51 nautical miles from a VOR, 4.5 degrees diverging from the center line equates to 4 nautical miles). For airways with legs between VORs greater than 102 nautical miles, the maximum width of the airway is at the middle point between the two VORs, which is when 4.5 degrees from the center of the radial is the maximum distance for both navaids. Some Canadian airways have 250 mile legs, which produces an airway width of 9.8 miles either side of the center line. In the United States, V-120 in Montana has a 156 mile leg, for an airway 6.1 miles either side of the center line.
On an ILS, full scale deflection of the CDI varies with the length of the runway and is — like the VOR — angular (i.e., like the VOR it widens with distance from the station). The reason it varies that that the localizer antenna array is located off the departure end of the runway, typically about 1000 feet, and is calibrated to produce a full scale deflection of a fixed distance of 350 feet either side of the centerline at the runway approach threshold. This means that a short runway will generate a wider angle and a longer runway a narrower one. (Imagine the angle needed to reach the 350 foot width, the geometry of which is in effect a right triangle, with one leg extending from the localizer antenna to the threshold; the second leg, a fixed 350 feet in length from the centerline offset 90° from the centerline; and the hypotenuse, the line connecting the endpoints of these two lines. The longer the runway leg the smaller the angle between the runway leg and the hypotenuse, which angle makes the width of the approach course [referred to below as Θ (theta)]. Assuming the antenna array is located 1000 feet from the departure end of the runway, the full scale deflection for a 6500 foot runway would be the following:
This is within the standard 3° to 6° width of the localizer course. At five miles out (generally glide-slope intercept point and the outer marker & final approach fix) the course in this example is plus/minus 1832 feet from the centerline. With the GPS in approach mode the width of the approach course is always plus/minus 0.3 nautical mile, 1822 feet.
Oh, and let's not forget the reception range of the localizer,
Which, of course, is not an issue with GPS.
Typically for an enroute segment, a "flight plan" will be entered. This may consist of a series of waypoints or of a destination airport or of a combination. Waypoints may be added or deleted from the flight plan as situations warrant. Waypoints may be created (user-defined) if so desired. A waypoint may be programmed and then "Direct TO" (DTO), or a diversion may be made to the nearest suitable airport in the event of an emergency. In conjunction with the preceding, it is possible to retrieve airport information from the GPS database (useful to determine if an airport is "suitable"). Other routings, such as a STAR, may be included in the flight plan.
GPS approaches are more complicated and require a lot more thought and pre-planning than enroute segments.
There is currently a mix of GPS "overlay" approaches (approaches based on traditional ground-based navigation aids but with "or GPS" in the title) and GPS stand-alone approaches. While conducting these IAPs, ground-based navaids are not required to be operational and associated aircraft avionics need not be installed, operational, turned-on, or monitored; however, monitoring backup navigation systems is always a good idea when they are available.
All pilots should have a basic understanding of GPS approach procedures and should practice GPS IAPs in VFR flight until thoroughly proficient with all aspects of their equipment (both the general receiver operation and as-installed in the aircraft) prior to attempting flight under IFR and especially in IMC.
All IAPs must be retrievable from the current GPS database supplied by the manufacturer of the GPS or another FAA-approved source (i.e., the database that is stored in the GPS unit). Flying point-to-point on the approach course (i.e., simply programming-in the final approach fix and the airport as waypoints) does not assure compliance with the published approach procedure:
Also,
GPS approaches are requested from and approved by ATC using the GPS title such as "GPS runway 24" or "RNAV runway 35" [the latter only if the RNAV 35 is an RNAV (GPS) and not an RNAV (RNP)]. Using the manufacturer's recommended procedures, the desired approach and the appropriate IAF are selected from the GPS receiver database [know how to do this ahead of time: in-flight, in IMC, in not the time to be figuring it out]. Pilots should fly the full approach from and initial approach waypoint (IAWP) or feeder fix unless specifically cleared otherwise: randomly joining an approach at an intermediate fix does not ensure terrain clearance [just like any other approach].
When an approach has been loaded in the flight plan,
Pilots should pay particular attention to the exact operation of their GPS receivers for performing holding patterns and, in the case of overlay approaches, operations such as procedure turns. These procedures may require manual intervention by the pilot to stop the sequencing of waypoints by the receiver and to resume automatic GPS navigation sequencing once the maneuver is completed. (The same waypoint may appear in the route of flight more than once and even more than once consecutively [e.g., IAWP, Final Approach Waypoint (FAWP), Missed Approach Waypoint (MAWP) on a procedure turn or approach holding pattern].) Care must be exercised to ensure the receiver is sequenced to the appropriate waypoint for the segment of the procedure being flown, especially if one or more fly-over wapoints are skipped (e.g., routing to the FAWP rather than the IAWP if a procedure turn is not flown). The pilot may have to sequence past one or more fly-overs of the same waypoint in order to start GPS automatic sequencing at the proper place in the sequence of waypoints.
When receiving vectors to final, most receiver operating manuals suggest placing the receiver in the non-sequencing mode on the FAWP and manually setting the course. This provides an extended final approach course in cases where the aircraft is vectored onto the final approach course outside of any existing segment that is aligned with the runway. Required altitudes at waypoints outside the IAWP or step-down fixes must be considered in any decision to descent, but ATC should be consulted concerning altitudes on the vectors if there is any concern: pilots should handle this as they would any radar-vectored arrival.
When within 2 nautical miles of the FAWP with the approach mode armed, the approach mode will switch to active, whch results in the following:
Incorrect inputs into the GPS receiver are especially critical during approaches:
A GPS missed approach requires pilot action to sequence the receiver past the MAWP to the missed approach portion of the procedure. The pilot must be thoroughly familiar with the activation procedure for the particular GPS receiver installed in the aircraft and must initiate the appropriate action after the MAWP. Activating the missed approach prior the the MAWP will cause CDI sensitivity to immediately change to terminal (plus/minus 1 n.m.) sensitivity and the receiver will continue to navigate to the MAWP [i.e., this is beginning the miss and the approach is actually aborted as the course is now set to be 1.4 miles too wide].
Missed approach routings in which the first track is via a course rather than direct to the next waypoint require additional action by the pilot to set the course. Being familiar with all of the inputs required is especially critical during this phase of flight.
If proceeding to an alternate airport,
FAA Instrument Flying Handbook
Aeronautical Information Manual
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rev. October 27th, 2012
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Steve Sconfienza, Ph.D.
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