GPS Errors

Getting the most from the magic in your panel without losing the "flick".


Theres no question ours is an era of great advances in safety and position awareness. Its the rare IFR airplane that isnt equipped with at least a portable, moving-map GPS; theres not an IFR-certified airplane in production that doesnt include a GPS-driven “glass cockpit” as at least an option-one thats almost always added.

The capability of GPS comes with great complexity. There are very different operating interfaces with units from competing manufacturers. Its a little surprising, then, that pilots seem to make the same errors and omissions pretty much regardless of the unit involved. What are these common GPS errors? What


can we do to avoid them?

Phase of Flight

David Zitt is the Flight School Manager of Sportys Academy, the flight instruction arm of Sportys Pilot Shop in Batavia, Ohio. He and his instructor staff work exclusively in GPS-equipped airplanes, some with full “glass cockpit” panels but most conventional round-gauge airplanes meeting the definition of Technologically Advanced Aircraft (TAA) through the installation of moving-map GPS. Zitt notes that “each [GPS] unit has its own pitfalls,” but finds common pilot mistakes regardless of the type of GPS installed. Prime among them is “instrument fixation” during the transition to TAA flight, a focused stare and excessive concentration on which button to push when the pilot is not completely familiar with the GPS used. Beyond this fixation, Zitt sees these typical mistakes in various phases of flight:

Preflight: The most common GPS error before flight is failure to use the entirety of the flight plan page. In many cases the pilot knows (or can reasonably expect) hell/shell be cleared to a specific fix or along a STAR, but very often he/she doesnt load in the data. This greatly increases workload right after takeoff, when the pilot must either enter the data in the air or devise some work-around that negates the safety and capability of the GPS navigator.

Taxi: Pilots rarely use the GPS features to their advantage, according to Zitt. Before and during taxi they fail to access frequencies and runway data from the GPS database; many dont use taxi diagrams or other situational awareness tools.

Takeoff and climb: Aside from scrambling to input departure fixes or procedures, the biggest problem during takeoff and climb is fixation on the moving map. As another instructor put it to me, you can tell the GPS checkout is complete when the pilot can use it and still scan outside for traffic.

Cruise: Zitts “biggest GPS peeve” is pilots who dont know how to activate legs of a flight plan, or to change a flight plan to continue precise, safe and legal flight as the route changes. Other errors of omission: failure to fully utilize the calculation and database features of the GPS, including winds aloft, true airspeed calculations and runway information for the destination airport. Pilots rarely use the vertical navigation (VNAV) function for descent planning-“Most pilots dont even know how to use them,” says Zitt.

Approach: The greatest potential for critical GPS errors comes during an instrument approach. Common mistakes include:

Entering approach procedures improperly. One manufacturers logic differs from anothers; its easy to load the approach in the wrong place in a flight plan, causing it to send you in the wrong direction or making the procedure unavailable when you need it.

Activating the approach at the wrong time, or not activating it at all.

Assuming all GPSs will automatically sequence through a holding pattern or procedure turn.

Missed approach: Close to the ground but still in the clouds is no time to be unfamiliar with your GPS. Here, too, common errors occur. At the missed approach point many units require the pilot to manually sequence the GPS by selecting the “suspend” or “OBS” mode. Heres the logic: the GPS navigator knows where youre headed in the missed approach, but it does not know how long itll take a given airplane in given winds to climb high enough to safely turn toward the fix. Consequently the pilot has to tell the unit to “wait” by hitting OBS or Suspend, then go back into GPS Nav mode or reactivate the flight plan once at that safe altitude.

Instead, Zitt notices many pilots using the GPS moving map as a crutch. In place of properly sequencing the unit, he/she looks at the map display to see the holding fix it “over that way” and steers in the general direction of the fix, instead of safely navigating along the prescribed route. Its a controlled flight into terrain accident waiting to happen.

The Button of Death

This focus of this error is so common it has a name among instructor pilots: misusing the “button of death.” The button in question is labeled differently by different manufacturers, but it has a similar function-selecting whether GPS or VOR/localizer functions are displayed on the primary navigation display.

The potentially fatal mistake with the GPS/VLOC switch happens when the pilot is navigating direct to a VOR on or near the airport and does not change over to GPS for an approach. More frequently it occurs when using GPS to navigate to a destination on a heading that is very nearly aligned with an ILS or VOR final approach course. If you make either of these mistakes you may think youre navigating properly to the runway, when in fact youre descending along a path without a guarantee of terrain clearance. Thats why most GPS installations require a GPS/VLOC annunciator light in the pilots primary scan if the unit is to be certified for instrument approaches.

Early in the “direct-to” era, with approach-certified LORAN, many installations had a safety feature that automatically kicked the primary navigation display into VLOC mode any time a localizer frequency was dialed into the primary nav radio…whether it was receiving a signal at the time or not. The hope was this would prevent the “button of death” type of controlled flight into terrain. It worked, but it prevented a pilot from setting up for an ILS approach until he/she was ready to switch over from en route navigation. By the time approach-certified GPS made its appearance, installations again gave the pilot the ability to set up for an ILS before actually flying the approach-reintroducing the manual GPS/VLOC selection, albeit creating the potential for this common GPS error.

Another Expert Speaks

Paul Gretschel of Pack Aviation, a Long Island, N.Y.-based specialist in TAA and glass cockpit transition training, lists the common errors he sees made by pilots who have not thoroughly trained in GPS use. Among these:

Using the “direct to” button to excess. Gretschel prefers the flight plan mode when starting a flight (VFR or IFR). It puts your origination waypoint in the flight plan and can always help show you the way back, or how far youve travelled since you left. In some GPS designs a flight plan must be active for the unit to load an instrument approach.

Programming an approach with “vectors to final.” Gretschel always picks a waypoint when initially programming an approach. He will re-program “vectors to final” only when he receives the last vector to intercept the final approach course. Too often, he says, pilots expecting vectors to final are then directed direct to a waypoint along the final approach course that doesnt appear when you use the “vectors to final” option.

Gretschel makes these suggestions for expert GPS use:

Use Nav page #1 on a Garmin 530 for partial panel. It gives track, compass azimuth, moving map and CDI indications. It is especially useful in airplanes with an electric HSI that could fail with a perfectly good attitude indicator.

When programming an IFR flight plan by hand, always double check your routing and waypoints after finishing the flight plan. It is too easy to leave out one waypoint and find yourself off your clearance without realizing it. After completing the flight plan, go over it again with your low altitude en route chart and double-check each VOR and intersection.

Confirm you are in VLOC mode just prior to intercepting the final approach course on ILS, VOR or LOC approaches. If you are in GPS mode, you will not receive the localizer or the glideslope. It is still a mystery to me why some aircraft switch automatically to VLOC sometimes and not others! Never take it for granted that it will switch on its own.

Understand the difference between a procedure turn and a hold. Garmin units call them both “holds,” but treat them very differently. When executing a procedure turn, the Garmin will come out of suspend on its own when you intercept the final approach course inbound. While holding (during a missed approach procedure), the GPS will remain in suspend because it has no sequential waypoint. If you are given holding instructions at an initial approach fix that has a published holding pattern, you will need to manually return the GPS to suspend when you intercept the final approach course each time you complete a turn in the hold and intercept the final approach course.

Specific to the market-dominating Garmin G1000 integrated navigation system, Gretschel observes:

There are many options for display on the multifunction display (MFD) during an approach. It is very cool to display the approach plate and watch your little airplane icon move along the complete approach. Once the approach is loaded and reviewed, however, Gretschel finds it more efficient to display the flight plan with the moving map (not the approach plate). With the WAAS upgrade and the latest software, a G1000 MFD will display courses, distances and minimum altitudes on the flight plan page, while still displaying the moving map.

If you use the paperless approach-plate option, you need to brief yourself on the approach and make notes on your lapboard. An alternative is to carry traditional paper charts in addition to the paperless option on G1000.

Do not put the DA/DH or MDA in your altitude pre-select. It is more useful to put the missed approach altitude in the pre-select. You can fly to DA, then go missed, and the FD/AP will command and climb to the missed procedures altitude. Otherwise, you might linger at DA when you need to be climbing.

And regardless of the type of GPS, do not hit the suspend button on your missed approach until you have safely begun flying the missed approach procedure. Power up, pitch up and clean up. Once you are climbing, then (and only then) return your attention to the GPS and push the suspend button.

Instructional Mode

I fly regularly with clients who are very familiar with their GPS units. In an instructional environment, however, the pilot is sometimes thrust into unfamiliar GPS territory. I, too, see the same GPS errors over and over again. Chief among these are:

Programming the GPS during taxi. Most taxi mishaps go unreported, but Id hazard that the number of taxi accidents has increased dramatically, coincident with the widespread introduction of flight plan-capable GPS (Any insurance claims experts care to comment?). Its far too distracting to be head-down and plugging data into a GPS while moving on the ground; this may be a factor in the reported rise in runway incursions as well. Program your GPS before engine start, before releasing brakes to taxi, or once at a full stop in the run-up area…but not while taxiing.

Forgetting to re-load and reactivate the approach after a “training” missed and when going around for another approach. Unlike traditional non-GPS approaches, you must treat each time around the procedure as a distinct select-load-activate event when making multiple approaches for purposes of training.

All the other common GPS errors noted by Zitt and Gretschel.

Avoiding GPS Errors

It may sound trite, but the way to avoid common GPS errors is to get to know your GPS thoroughly. Just as the best aircraft checkout comes by flying with an instructor expert in type, so too does GPS proficiency come from flying with a model-specific expert. By working with an instructor who not only knows how to use your type of GPS but more importantly knows how to teach it, you can quickly learn its logic and features while avoiding the pitfalls. Be prepared to fly some distance and spend some money if its needed to realize the added safety and utility of your GPS.

Theres a lot to do, and therefore to forget, when flying with GPS. What you dont do can hurt you. Develop a GPS checklist, customized to your own airplanes systems, and use it religiously to confirm youve properly set up and engaged this vital equipment. And if you find yourself making one of the common GPS errors, get some expert instruction before using GPS in IMC.

Tom Turner is a CFII-MEI who frequently writes and lectures on aviation safety.


Please enter your comment!
Please enter your name here