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Should You Fly An ILS, LPV, Or LNAV/VNAV Approach?

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Not long ago, you only had one kind of approach with vertical guidance: the ILS. And if you weren't flying an ILS, you were managing step-down altitudes on a non-precision approach.

But all of that has changed. Over the past several years, the FAA has created GPS based LPV and LNAV/VNAV approaches at thousands of airports across the US. Now with GPS, the number of approaches with vertical guidance has tripled, and to an extent, so has the confusion.

So what's the difference between these three approaches, and which one should you be flying? Let's take a look at all three.

ILS: The Only True "Precision" Approach For You

The ILS has been around a long time. In fact, the first time it was used by airlines dates back to 1938.

The ILS is a precision approach because it provides both lateral and vertical guidance. And it does so with two ground-based antennas: a localizer antenna, and a glideslope antenna. The localizer antenna provides lateral guidance, and the glideslope antenna provides vertical guidance.

So how low does the ILS get you? For normal Category 1 minimums, which is the vast majority of single-engine piston operators, a decision height of 200' above touchdown, and 1/2 mile of visibility. If you've ever flown an ILS to minimums, you know that's pretty low.

But the ILS does have some downfalls that come with it. The first problem is interference. Since the ILS is beaming radio signals to your plane from the ground, vehicles and other aircraft can affect the signal. When the weather is low, ATC uses the ILS critical area, which keeps aircraft further from the runway (and your ILS signal) prior to takeoff. But even when that's in effect, large vehicles driving on a road near the antennas can affect the signal.

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There's also the angular sensitivity issue. The closer you get to the antennas, the more sensitive they are. Most ILS approaches are positioned so that the course width of the localizer is 700' wide at the runway threshold. But the closer you get, the more sensitive the signal becomes. So if you're trying to follow the localizer and glideslope all the way to touchdown, it becomes very difficult to follow.

LPV: Localizer Performance With Vertical Guidance

Next up is the LPV approach, which is a WAAS/GPS based approach that is most like an ILS. But there's a difference. Even though LPV approaches have vertical guidance, they're not considered a precision approach. Instead, they're an approach with vertical guidance (APV).

So what's the difference? APV approaches don't meet the ICAO and FAA precision approach definitions, which apply mostly to localizer and glideslope transmitters. The precision approach definition also carries a lot of documentation, definition, and cost with it, so the FAA and ICAO adopted the APV definition so they could build new approaches and not be burdened with the cost and paperwork.

So how do they work? The highly accurate WAAS system (7.6 meters or better accuracy) gives you lateral and vertical guidance down to a decision altitude (DA) like an ILS. And, just like an ILS, an LPV approach's angular guidance gets more sensitive the closer you get to the runway. Keep in mind though, to fly them, you need a WAAS receiver. A baro-aided GPS won't work.

There's definitely an advantage with LPV. Unlike an ILS, which gets more and more sensitive (and difficult to fly near and below DA), the scaling on an LPV approach transitions to linear scaling as you approach the runway, with a total width of 700' (usually) at the runway threshold. That 700' of width at the threshold is the same as an ILS localizer at the threshold, but it doesn't get any tighter than that as you continue to touchdown.

LPV approaches get you low as well. Like an ILS, most LPV approaches will get you down to 200' above touchdown, with 1/2 visibility.

But like most things in aviation, where there's an upside, there's also a downside. Since LPV approaches aren't considered precision approaches, you can't use precision alternate minimums for airports that only have LPV. That means if you're using an airport with LPV only (no ILS) as your alternate airport, you need to use non-precision alternate minimums of 800'/2 miles, as opposed to the precision alternate minimums of 600'/2 miles.

LNAV/VNAV: Lateral Navigation/Vertical Navigation

The second type of GPS based APV approach is LNAV/VNAV. LNAV/VNAV approaches were actually the first type of GPS approach that had vertical guidance. They were originally designed for baro-aided GPS units, but most WAAS receivers can use them today as well.

Unlike LPV approaches, LNAV/VNAV approaches don't have increasing angular guidance as you approach the runway. Instead, they're just like an LNAV only approach, decreasing to 0.3 NM sensitivity when you're within 2 miles of the final approach fix, all the way to the missed approach point.

Because the final approach course is linear the entire way to the runway, the lowest an LNAV/VNAV approach can get you is 250' above touchdown. And because the sensitivity isn't as great as LPV with WAAS, the obstacle trapezoid (the area the FAA draws to make sure you have safe obstacle clearance on an approach) is much larger for an LNAV/VNAV. And because of that, you typically see LNAV/VNAV minimums higher than 250' above touchdown for most approaches.

Which One Should You Fly?

So, with the choice of 3 different types of approaches with vertical guidance, which one should you be flying?

Obviously, ILS and LPV approaches are going to get you the lowest on an IFR day. Beyond that, it depends on what you're more comfortable flying. If you're flying a GPS-based cross country, it's easy to program a GPS based approach into your system, and never have to worry about switching to "green needles" as your reach your destination airport.

If you don't have a WAAS capable GPS, you're going to be limited to ILS or LNAV/VNAV. And with that choice, if you need to get low, you're almost always going to be better off with the ILS.

Either way, you have more choices today on the type of vertically guided approach you fly at almost every IFR airport in the US. And given the increased choices, you have better options to land the direction you want at your destination. And even with the extra approach types you need to know, having more approaches to pick from makes your flying safer and more convenient.

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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