Subscriber question:
"I’m looking at buying a share of an airplane I plan to use for my instrument rating. It has a Garmin 430 that has not been upgraded to WAAS. What’s the difference between WAAS and non-WAAS, and can I do my instrument training without WAAS?" — Peter W.
Jeff:
“There are really three parts to the answer: What’s the technical difference, what’s the practical difference, and is the gain worth the expense?
First the technical part: Non-WAAS GPS relies solely on the transmissions from the GPS satellites for its position. The accuracy of that position varies from day to day but your IFR non-WAAS GPS should be accurate to within 30 meters most of the time. WAAS is one form of Differential GPS, which means the GPS receiver uses the satellites and then applies a correction appropriate for that calculated location. These corrections are continuously updated by ground stations and relayed through the GPS system. That correction might increase accuracy 10-fold, so it becomes location plus or minus about 3 meters.
Practically speaking, 30 meters of accuracy is more than enough to fly from Airport A to Airport B even in the clouds. It’s accurate enough for a non-precision GPS approach. So if your non-WAAS GPS is certified for IFR to the approach level—and a Garmin 430 is—you can use it for IFR and for training. That’s presuming it’s installed correctly and up-to-date.
However, an IFR-approved WAAS GPS is required for vertical approach guidance. That could be advisory vertical guidance to help fly a stable approach all the way to LPV approaches that are arguably more reliable than an ILS. If the airport only has RNAV (GPS) approaches, having WAAS might be the difference between landing and going elsewhere. WAAS GPS gives you more options for planning alternate airports. WAAS GPS units update their position more rapidly, usually have more sophisticated terrain warning systems, and can offer more options. For example, Garmin’s GTN series offers advisory guidance for visual approaches at night to many runways that have no instrument approach.
This brings us to whether the gains are worth the cost. I can’t answer that for you, but I can tell you what I’d do if it were my wallet on the line. I’d be OK renting an airplane for IFR training without WAAS. If I were buying into an airplane, as you’re suggesting, I would require it had WAAS if I planned to fly IFR.
I know lots of people who have complained about the cost of upgrading their airplanes to WAAS GPS—but not a single one of them has said they regret it.”
A visual approach to an airport carries extra risk. See how the Garmin GTN visual approach works in flight to make them safer, as well as some of the feature’s limitations.
Installing a late-model GPS is an expensive, but common, upgrade. If you've upgraded to a modern WAAS GPS, did you feel the upgrade was worth it? Share why or why not in the comments.