Over the past several months I have devoted this column exclusively to GPS use for IFR operations. It has been a twisting road, reflecting the considerable difference between the new and the old.
MISSED APPROACHES
Picking up where we left off last time, we are at the missed approach waypoint (MAWP). Up to this point on the approach, automatic waypoint sequencing has been the order of the day, with each procedure waypoint gracefully giving way to the next once passed.
Quite sensibly, auto sequencing stops at the MAWP: after all, the purpose of an approach is to provide a transition to landing, not a go-around. In consequence, the GPS automatically goes into “suspend” mode as you pass the last fix, and if you find you cannot land, some interaction with the unit is required to resume waypoint sequencing to get you started toward the missed holding fix: in other words, the missed approach procedure must be activated.
Activation can be as simple as pushing a single button to re-enter leg mode and start sequencing to the missed waypoint. However, if the first track is not direct to a fix, as in “…fly heading 145° to intercept the 058° radial inbound to...” the situation becomes more complicated. Here is how the AIM puts it:
“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 (AIM 1-1-20 p. 2.).”
I'll say. We are at the most critical portion of any IFR flight-missed approach, IMC, close to the ground, unfamiliar procedure-and the GPS unit requires special hand-holding.
SUSPEND
The required action may be to hand fly the designated heading with the GPS in “suspend” mode until you are ready to intercept the first missed approach leg. At that point, toggling the GPS out of “suspend” will make it possible to proceed direct to the next waypoint on the missed approach.
Once you arrive at the missed approach holding fix, OBS mode may become necessary. Although missed approach holding patterns are held in the database, manufacturers differ on how to automate the actual procedures. As you cross the holding fix and turn outbound for the first leg of the entry, the simplest solution may be to switch to OBS mode and then fly the holding pattern exactly as you would at a VOR. Pilots who heed my advice of a couple of months ago to invest time in OBS mode practice will find their effort now pays a handsome dividend.
PROCEDURE TURNS
There is one more time when OBS mode provides the key to GPS use: the outbound leg of a procedure turn (PT). Keep in mind that GPS units in leg mode can track only toward waypoints. PTs require outbound legs, so the unit must be put into OBS mode for the first portion of the procedure.
At the conclusion of the outbound leg, after starting the PT itself, there are two critical required actions: the unit must be taken out of OBS mode and the approach activated. Failure to do both equals no approach, no sequencing, and no RAIM check. Oh yes, and a missed approach.
WRAP UP
That concludes a broad overview of the use of IFR GPS in the major stages of IFR flight: departure, enroute, approach and missed approach. Throughout, I have made no attempt to detail how to actually use individual units built by different manufacturers: today's interfaces vary too much to make a generic instruction manual practical. Further, there is every reason to believe the differences among the units will continue to proliferate over time: there are no signs of a movement to standardize.
I hope I have made it clear throughout that the decision to use GPS in non-visual flight involves a major commitment of time and effort. While the system responds very well to an on-the-job-training approach for VFR use, IFR is another matter altogether.
The last area that requires review and consideration is GPS failure. Here again, there are marked differences from earlier systems.
FAA RESTRICTIONS
The most outstanding visual element in every modern GPS installation is the moving map. Please keep in mind that it is only intended to provide information, not guidance. After getting past that initial whiz-bang, the next thing to catch the attention of serious GPS users should be the pile of restrictions the FAA has heaped around the system.
I wasn't flying when VOR was the new kid on the block, but no one I know has reported similar restrictions put in place during the introduction of that system. In a word, despite its many limitations, VOR has always been reliable. GPS, by contrast, is not.
GPS is adversely affected by the atmosphere. It is subject to predictable geometry problems. It is vulnerable to shifts in military and political policy. Finally, it does not yet provide the accuracy of equipment developed 50 years ago.
In its defense, you can say GPS shows great promise, sort of like an elementary school student.
RELIABILITY
With reliability in question, IFR GPS users need to become very familiar with FAA accuracy standards and the specifics of how their own GPS equipment is designed to check and warn about pending or existing failures. The core concept is Receiver Autonomous Integrity Monitoring (RAIM), a self-check required of all IFR approved units.
RAIM needs a minimum of five satellites, four for accurate position and one more to perform the accuracy cross-check. In the absence of a fifth satellite, a barometric altimeter setting can be entered to provide what is called “baro-aiding.”
Although the GPS system is designed so that at least five satellites are always above the horizon for every user, there is no guarantee that their respective positions (“geometry”) will make it possible to determine position accurately. A primary function of RAIM is to check for “bad” geometry and warn of impaired accuracy.
Receivers differ in how RAIM alerts are presented, so pilots must be clear on exactly where and when to look in order to see them. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position (AIM 1-1-20 a. 9.)
ENROUTE vs. APPROACH
In the enroute phase over the continental United States, lack of integrity is seldom a problem: satellite coverage can be counted on to deliver almost 100% of the accuracy required. While operating enroute, IFR GPS units look ahead fifteen minutes to predict integrity.
If the unit produces an integrity warning (details differ with manufacturer), the pilot is expected to switch to an alternate form of navigation until the problem is resolved. This explains why IFR GPS use requires “an approved and operational alternate means of navigation appropriate to the flight (AIM1-1-20 e. 1. (b).” It also explains why you are not going to see the nation's VOR backbone dismantled anytime soon.
As explained several columns back, GPS receivers automatically cycle from enroute to terminal to approach sensitivity as you near your destination. Accompanying this change in sensitivity is an increased demand for receiver accuracy: RAIM becomes more picky, and loss of integrity more likely.
Once an approach procedure has been loaded and armed (“activated” in some units), RAIM is checked two miles from the final approach fix (FAF). A RAIM warning received here or at any later point on the approach requires that you abandon the procedure.
PREDICTION
An integrity warning received two miles from the FAF will require major re-shuffling. Unless a pilot is industrious enough to have an entirely independent means of approach navigation already up and running, a missed approach and a brand new start seem inevitable.
To avoid this necessity, the AIM suggests that “RAIM availability should always be checked (1-1-20 j. 1.).” Canny pilots do this by performing a RAIM prediction-sometimes several on long flights--before entering the general approach environment. Again, the method differs among manufacturers, but prediction capability is built into all receivers, making it possible to look at specific points in the future at specific locations. If RAIM is not successfully predicted, an alternate navigation method, airport, or schedule must be adopted until a RAIM prediction is again available.
It is important to keep in mind that a RAIM warning does not mean “loss of signal.” Instead, the likely causes are insufficient satellites or bad geometry, situations which can both be predicted. The pilot who fails to obtain such a prediction when an IMC approach seems likely at the conclusion of the flight is making a serious mistake.
A discussion about RAIM warnings and loss of GPS integrity may seem like a pessimistic note on which to end this series, but it's appropriate. We've lived for years with VOR/Loc systems and learned how to prepare for their failure modes. GPS has different problems and different ways to alert users of their existence; to survive, pilots must concentrate on learning emergency data just as thoroughly as they do on mastering standard operational details.
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