Flying GPS Approaches

A new aviation era began February 17, the day the Federal Aviation Administration declared the Global Positioning System operational and granted technical standard order approval to the Garmin 155 GPS receiver. The decree came with little fanfare, yet it will affect how we fly for decades to come.

The announcement surprised Garmin personnel who weren't expecting the approval until March. For once, the FAA was ahead of the game.

With TSO-C129 Class A1 approval, the Garmin 155 may be used to fly IFR in the oceanic, enroute, and terminal areas and also to execute some 2,500 nonprecision "overlay" approaches. According to Jeppesen Sanderson, by the end of the year the number of available approaches is projected to grow to about 4,600 as it completes the process of entering all the approach waypoints into the databases it supplies to the GPS receiver manufacturers.

The Garmin 155 is the first GPS receiver to meet the standards for approaches. Trimble Navigation late last summer won TSO-C129 Class A2 approval, allowing its TNL-2000T to fly IFR in oceanic, enroute, and terminal areas but not on approaches. AlliedSignal's Bendix/King KLN 90A receiver won the same approval a few weeks later. The KLN 90A became the first to actually fly under IFR when AlliedSignal was first to receive a supplemental type certificate to install the KLN 90A in an airplane.

Honeywell got the whole approach process rolling last December when it received approval to use its GPS aboard Continental Express ATR 42 and 72 and de Havilland Dash 7 commuter airplanes for nonprecision approaches into Aspen and Steamboat Springs, Colorado. The GPS approaches are available only to Continental Express airplanes equipped with the Honeywell system. The GPS approaches replace a VOR/DME approach that was not approved for night operations and an aging microwave landing system. The Category A and B minimums on the VOR/DME approach are 2,385 feet agl. With GPS, Continental Express is permitted to fly 716 feet lower for Category A and 556 feet lower for Category B. Visibility minimums are reduced to 1.5 miles from 2 miles. The MLS minimums are 1,255 and two for both categories, but that system has been difficult to maintain.

At least in the near term, general aviation pilots flying GPS approaches will not see minimums lower than those published for the conventional nonprecision procedure. That may change as experience is gained with the system. Approaches designed specifically for GPS may eventually have lower minimums than conventional procedures. The first stand-alone GPS approaches were charted in April, and the FAA says it plans to add 500 new GPS-only approaches a year. Currently, the GPS procedures are overlaid only on VOR, VOR/DME, NDB, NDB/DME, tacan, or RNAV approaches. Localizer, SDF, and LDA approaches are not approved.

Flying a GPS approach is much like flying a VOR/DME, for example. In some ways, the new way is easier; in others, it's not.

A few weeks after Garmin received its TSO approval and an STC to use the equipment in a Mooney, we flew a series of GPS approaches to Topeka, Kansas, with Ralph F. Fisch, the company's regional technical sales manager. Garmin is located in Lenexa, Kansas, a suburb of Kansas City about 35 nautical miles east of Topeka.

We approached Topeka's Forbes Field from the south to set up for an overlay approach of the VOR/DME RWY 21 procedure, passed over the airport and the Topeka VOR northeast of the airport, turned inbound, crossed the VOR again, and turned to the inbound heading, and minutes later, the GPS brought us absolutely to the numbers at the end of the runway. With the needle on the horizontal situation indicator (HSI) centered and the runway end below us, the VOR indicator, which we also had been monitoring, was more than a dot off to the right, which, at 12 miles from the VOR, would equate to about 530 yards, enough to at least momentarily confuse a pilot breaking out at minimums.

The price to be paid for such precision on a nonprecision approach is a bit more manipulation of the receiver than pilots are used to when flying VOR and NDB approaches. This is not an indictment of the Garmin 155 at all. I had never used the receiver before, but after a 20-minute session with the box in the simulator mode, I was able to easily spin the dials and punch the buttons during our flight. The 155 is a well- engineered receiver with a plethora of handy features, not the least of which is a battery backup that can power the unit and guide you through an approach even if the ship's alternator and battery have gone to electron heaven.

The performance goals of an IFR GPS are spelled out in TSO-C129, and the 155 meets them in a logical way and in a similar manner that all other approach-certified receivers will meet them. We'll find out how well the other manufacturers do once their receivers are approved. Trimble expects approach certification around July; AlliedSignal this fall. Others will likely occur in about the same time frame.

Here's how it works: Once the destination airport is selected, either by going direct to it or by virtue of it being the last waypoint in a programmed flight plan, the pilot must select the desired approach from the nonprecision procedures available. At Topeka, for example, the 155 offered VOR/DME approaches to runways 3 and 21, NDBs to runways 13 and 31, and RNAV to Runway 13. The 155 shows two approaches at once; additional ones can be accessed by turning a knob. With the approach selected, the approach annunciator on the panel glows. The TSO requires four status annunciators remote from the receiver be placed in the pilot's scan area.

Next, the pilot chooses the initial approach fix that is most convenient to the arrival route. In our case, arriving from the south, we chose the Topeka VOR, but the approach has two others. The GPS identifies those as d094g and d258g, which equate to the VOR radials for the initial approach fixes (IAFs).

In order to navigate under IFR with a GPS, the TSO requires that an external indicator be used, either a VOR-type indicator or an HSI. In the enroute mode, the course deviation indicator (CDI) sensitivity is automatically set at 5 miles for full deflection either side of center. At 30 miles from the IAF, the receiver automatically ramps the sensitivity down to 1 mile either side of center.

Within 30 miles of the IAF, the pilot enters the local altimeter setting into the receiver. The computer compares the pressure altitude with the signal from the airplane's encoder and with the computed GPS altitude to further refine the position information and to detect any possible errors in the satellite signals. The pilot must also hit the "Arm" annunciator to prepare the receiver for the approach.

The TSO mandates that all the waypoints necessary for the approach be programmed into the receiver's database and that the receiver lead the pilot from waypoint to waypoint right to the missed approach waypoint. But that means the receiver might get confused if a waypoint is passed more than once in a procedure such as crossing a fix outbound for a procedure turn and then crossing it again inbound. The receiver would attempt to steer the pilot directly to the next waypoint in the database after crossing the fix the first time. To prevent that, the pilot must put the receiver on "hold" before crossing the waypoint the first time to prevent it from sequencing.

With that in mind, we hit the Hold annunciator button as we approached the Topeka VOR waypoint. When we crossed the point, the GPS appropriately did not sequence and attempt to immediately lead us to the next waypoint in the approach database. After our procedure turn and when established inbound to the VOR again, we took the GPS out of the hold mode by hitting the annunciator again, causing the normal "Auto" light to come on.

When the GPS is in the auto sequence mode, adjusting the omni- bearing selector (OBS) has no effect on the needle deflection, much like on a localizer procedure. The receiver knows the course from one fix to the next, and if you stray from it, the pointer moves right or left to direct you back to the proper course, regardless of where the OBS is set. As with a localizer or ILS approach, most pilots flying a GPS approach will elect to set the OBS to the inbound course as a reminder. When the Garmin 155 is in the "Hold" mode, the pilot is able to set a desired course by changing the OBS, making it possible to track the 25-degree radial inbound (205-degree bearing) to the TOP waypoint on the procedure turn, for example.

Crossing the VOR inbound, the GPS sequenced to the next waypoint, which it showed as ff21, for final approach fix for the Runway 21 approach. The ff21 fix is co-located with the TOP 7 DME point on the VOR/DME approach. As we came within 2 miles of ff21, the receiver automatically switched from the "Arm" mode to the "Active" mode, as designated by the annunciator panel. At that point, the CDI sensitivity ramps further down to just .3 miles either side of center. The TSO requires that the change in sensitivity be gradual so as not to cause the pointer to immediately slam against the stops if a pilot is a dot or two off on his procedure.

After crossing ff21, the GPS sequenced to the rw21 waypoint, which is the runway endpoint. From there, it was just a matter of following the pointer and watching the distance and time readouts march down to zero. Had we needed it, the direction and distance to the first waypoint in the missed approach procedure were available at the touch of a button.

Flying the traditional VOR/DME approach to Runway 21 would require less manipulation by the pilot. Tune the station and identify it. Fly to it, cross the station, and set the OBS to the inbound. The OBS would have to be reset again in this case from 205 degrees to 200 degrees. On this approach, you would have the DME readout to help maintain situational awareness and to identify the missed approach point (MAP), but on a simple VOR or NDB approach, that's not the case. With a GPS, both distance and time to the waypoint are always available.

The GPS signal is smooth and easily tracked; the pointer movements always steady. The accuracy is astounding. VOR signals often scallop and can be difficult to track.

Flying a GPS approach is a matter of making a number of selections offered by the receiver — which approach and which IAF, for example. The only thing entered into the receiver is the altimeter setting. A conventional RNAV approach requires the pilot to enter the radial and distance of at least two waypoints and sometimes three, offering plenty of opportunity to misdial an input.

And of course, with a GPS there is none of the mental gymnastics required of those flying an NDB approach.

With conventional approaches, the pilot is at the mercy of one transmitter. If the VOR or NDB goes off the air whether because of equipment failure or maintenance, you'll be shopping for another place to land. If a satellite fails, the GPS receiver usually can continue to compute a precise fix based on other available signals, but that is an advantage that we cannot yet fully realize.

At this point in the GPS program's young life, the aircraft must still be equipped to receive the signals from the ground station, such as VOR or NDB, in order to shoot some of the GPS approaches. And in those cases, the ground stations must be operational, but there is no requirement to monitor the ground stations. At the end of April, the FAA declared GPS to be in the final Phase III for some of the approaches. In those cases, the approach now shows up on the approach plates with GPS as part of its name, "VOR/DME RWY 21 or GPS RWY 21," for example. But only about 300 approaches were charted that way in the recent charting period. Every new 56-day charting cycle, the FAA plans to rename an additional 500 approaches for Phase III. When flying those approaches designated under Phase III, you need not have receivers for the ground stations even on the airplane. But if you need to fly to an alternate airport, you do need the ground-based receivers, so it will be awhile before we can fully cut the apron strings to the ground-based system.

Much has been done to assure that IFR GPSs provide precise position data. Before it will allow you to enter the approach mode, the receiver must know that the satellite coverage and geometry will be satisfactory to compute an accurate position at the time of your predicted arrival at the destination. In the unlikely event that the coverage is not adequate when you arrive at the destination, you can elect to hold until the geometry improves, fly a different type of approach, or go to another airport where the accuracy can be assured.

The receiver knows the paths of the satellites, and it looks ahead to your computed time of arrival and determines if adequate coverage is likely to be available to complete the approach. According to Garmin data, the proper satellite geometry should be available more than 96 percent of the time, considering 21 operational satellites. If the three additional operational spare satellites are thrown into the equation, the percentage is significantly increased. This "predictive RAIM" function can be accessed at any time in flight. RAIM stands for receiver autonomous integrity monitoring.

RAIM is a way for the receiver to ensure that all of the satellite signals it is receiving are accurate. The receiver computes a three- dimensional position using four satellites. It then must add in signals from additional satellites and recompute the position. If, in its comparisons of positions, the receiver detects a discrepancy beyond tolerances specified in the TSO, it must alert the pilot. In that case, the pilot may continue to fly the approach, but the conventional signals must also be monitored.

What's the price for taking advantage of this technology? List price for the panel-mount Garmin 155 is $4,995; the Garmin 165, a Dzus- mount version that also has been approved for approaches, lists for $5,995. The annunciators, available from Garmin and other sources, can add $500 to $600. By comparison, the similarly featured but VFR-only Garmin 150 lists for $2,995. In order for the 155 or 165 to be legal for approaches, it must be equipped with a current database from Jeppesen. List prices had not been set at press time, but they are estimated to run about $600 for an annual database subscription with the required 56-day updates.

Early on, expect to pay between $2,000 and $3,000 to have an IFR GPS installed, according to estimates we received from several avionics dealers. The FAA is just now beginning to feel comfortable approving the installation of GPSs for IFR oceanic, enroute, and terminal use. In some FAA regions, the bureaucrats have been frustratingly slow to approve the installations, apparently because of little direction from the FAA's Washington, D.C., headquarters and the regional headquarters. No one is quite sure just what the FAA may demand of dealers installing approach receivers.

According to an FAA official in Washington, draft bulletins and advisory circulars clarifying the installation procedures had been completed by mid-April and sent to the general counsel's office for approval. The official expected the documents to be released for distribution before the end of May. Meanwhile, he and other headquarters staffers claimed to be trying to clarify issues one on one with FAA avionics inspectors in the field to try to open up the installation bottleneck that had left dozens of airplane owners flying around with IFR- capable receivers approved only for VFR operations.

Probably by this fall, we'll see more GPS manufacturers with IFR approvals and a smoothed-out installation process that will allow more pilots to take advantage of this latest advance in avionics.

For more information on the Garmin 155, contact Garmin International, 9875 Widmer, Lenexa, Kansas 66215; telephone 913/599-1515.