Updated: Nov 20
Acronyms like LNAV, LPV, LNAV+V and LP can be found splashed across the minimums section on some instrument approach plates. These can add a good deal of confusion to instrument pilots. Let’s examine what each of them means, as we cover in depth as part of our IFR Ground School.
Figuring out your minimums on a basic instrument approach like the VOR into Oshkosh Wisconsin, pictured below, isn’t too hard. There are two options: either the straight in approach to runway 27, or the circling approach. In both cases, the minimums for a Category A aircraft like a Cessna 172 are 1,280 foot ceilings with 1 statute mile of visibility.
Many airports, including smaller ones, now also have GPS approaches. Also pictured below is the GPS approach into the same runway at Oshkosh. The minimums section gets a bit more complicated, and we have an alphabet soup of different types of approaches. Remembering what these are and what they mean gives many instrument students trouble. Let’s break down each one.
LPV? LNAV? LNAV+V? What are these?!
The approach minimums, there are four on this approach, are organized from the lowest minimum at the top, to the highest minimum at the bottom. The highest of all is the circling approach. Circling requires maneuvering in the traffic pattern to a runway the approach didn’t necessarily line you up for, all while maintaining visual contact with the field, so the weather has to be a bit more favorable with higher minimums.
Next up from there is an LNAV minimum, which stands for Lateral Navigation. This is the most basic type of GPS approach. As the name suggests, it provides only lateral guidance, much like a VOR approach or a localizer approach. It does not provide vertical guidance like a precision ILS approach.
Just like a VOR or localizer approach then, a GPS approach with LNAV minimums is a non-precision approach. Also, like any non precision approach, it has a Minimum Descent Altitude, an MDA. This one is 1,140 feet for all category aircraft.
GPS Course Sensitivity
A VOR or localizer signal gets more sensitive the closer you are to the station. But GPS works differently, it must artificially create the course sensitivity. When you’re flying inbound to an airport, along the extended centerline, the course starts out being 2 nautical miles to either side. It stays that way until 30 miles from the airport, and then it shrinks down to 1 mile. Close to the final approach fix, the GPS goes into approach mode, and the sensitivity goes to .3 nautical miles to either side of the extended centerline, and it stays that way. So unlike a localizer course, we’ll be flying an LNAV approach from the FAF all the way in with the same sensitivity, .3 miles.
Like we said, the LNAV approach has only lateral guidance, no vertical guidance, so like other approaches that are lateral only like a VOR or localizer, we use an MDA, we level off at 1,140, or ideally a bit above that so as not to dip below and stay there until either we get sight of the runway or go missed.
Having vertical guidance, the way we would on an ILS approach, allows us to fly a bit differently. The next approach minimum up from LNAV, is something called LNAV/VNAV, Lateral and Vertical Navigation. The vertical guidance in this case comes from something external to the GPS, usually a sensitive altimeter, which is why this is sometimes known as a baro-assisted approach. With vertical guidance like this, the approach can be flown the way we fly a precision approach, meaning we follow a glidepath down to a decision altitude, which can be a bit lower than the MDA of the LNAV. For this approach, we can go down to a decision altitude of 1,078.
You don’t see many aircraft equipped for these baro-assisted LNAV/VNAV approaches anymore. In their place, newer GPS units are able to do approaches to what are called Localizer Performance with Vertical Guidance (LPV).
The LPV Approach – A precise approach, but not a precision approach!
The localizer performance in the name refers to the fact that unlike on the LNAV approach, where the course sensitivity stays the same along the entire final segment, the LPV gets more sensitive as we fly closer to the runway, just like on a traditional localizer. The course is only 350 feet wide on either side of the centerline when we are at the runway threshold. It’s a much greater level of precision.
In addition, the vertical guidance provided by the GPS is accurate enough to follow a glidepath down to a lower decision altitude, on this approach it’s 1,036 feet, the lowest minimum of all. Not all GPS units can fly approaches to LPV minimums. The unit must be what’s called WAAS enabled.
WAAS, or Wide Area Augmentation System, is a way for correction signals to be sent to a GPS receiver by ground stations, so that small position errors can be ignored and replaced, making the fixes more precise. GPS units that aren’t WAAS equipped won’t be able to fly to LPV minimums, and so will need to fly the approach as an LNAV, and without any other form of vertical navigation, will have to use the higher minimums and treat them as an MDA, rather than as a decision altitude.
The rest: APV, LP, LNAV+V
Here are a few other acronyms you will see thrown around when talking about GPS approaches. The approaches with minimums we use as decision altitudes, thanks to the vertical guidance, are appropriately called Approaches with Vertical Guidance or APV. It’s tempting to call these precision approaches because they are flown very much the same way as other precision approaches like an ILS, but the FAA doesn’t define them as such, requiring for instance that we use non precision alternate approach minimums when using them in our flight planning.
You’ll also see an acronym LNAV+V, Lateral Navigation plus Vertical guidance. You won’t see this acronym on any FAA or Jeppesen approach plate because it’s not an official type of GPS approach. It means that the GPS unit you’re using is able to simulate a glidepath for advisory purposes. Many airports have GPS approaches that don’t use lower LPV minimums. Your unit may be WAAS equipped meaning its able to fly an LPV, but if the approach doesn’t offer LPV minimums, it won’t be available. Instead, the unit will compute a glidepath anyways, and you can reference it for a stable, continuous descent down to minimums. You’re still flying an LNAV approach though, and have to respect the higher LNAV minimums, 1,140 here, treating it as an MDA. Going below the MDA without the required visual runway cues, even if you’re following the advisory glidepath, won’t protect you from obstacles and is against the rules.
Another approach minimum that is rarer and not shown on our plate here is an LP. This is a Localizer Performance approach, but unlike the LPV above, doesn’t include vertical guidance, usually due to terrain considerations. It provides that same super precise sensitivity on final down to 350 feet on either side of centerline but doesn’t include a glidepath to follow. In that sense then, LP is the GPS equivalent of a localizer only approach, and so is flown the same as a non precision, using an MDA, rather than a decision altitude. Again, only WAAS enabled units can fly it. Without that receiver, or if there was a WAAS system outage, we’d revert to the LNAV minimums.
So here below are the more common GPS acronyms that get thrown around in training and IFR flying. Go ahead copy this to your own device so you have an at a glance reference of these. The acronyms are very similar, and so can be hard to distinguish, but hopefully this quick breakdown clears up much confusion.
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