Basic Approach Types
- Tizi
- Apr 18
- 16 min read
Updated: Jul 7
"I shall follow the glideslope to the DA" - IFR student shooting a VOR approach.
Table of Contents
To simplify the scrolling back and forth, this is the table of contents for this article.
Ground-Based Navigation Approaches
Very High Frequency Omnidirectional Range (VOR)
Minimum Operational Network (MON)
Stabilized Descents
Dive and Drive
Localizer (LOC)
Instrument Landing System (ILS)
CAT I, CAT II, CAT III a/b/c
RNAV (GPS) - Based Navigation Approaches
Lateral Navigation (LNAV)
Stabilized Descents
Dive and Drive
Localizer Performance (LP)
Lateral and Vertical Navigation (LNAV/VNAV)
Localizer Performance with Vertical Guidance (LPV)
Widea Area Augmentation System (WAAS)
Advisory Vertical Guidance (+V)
GRAPHIC - Summary of Most Common Approaches
Ground-Based Navigation Approaches
The most traditional forms of approach use ground-based navigation sources. These are physical systems located on the ground across the United States: Very high frequency Omnidirectional Range (VOR), Localizers (LOC), and Instrument Landing Systems (ILS). There are a few more like NDB and various localizer types.
In Garmin-world, we describe these as "green needle" approaches, because the CDI changes color to green when choosing 'VLOC' (VOR / LOC) as a navigation source.
These are physical stations located on the ground. The aircraft must be equipped with appropriate antennas and interfaces to tune into the station and receive information. For example, to tune to a VOR, you must have a VOR receiver antenna and a navigator to select the desired frequency.
With the advent of GPS, ground-based navigation equipment has been coming relatively obsolete. That doesn't mean it's not relevant (far from that, actually). However, since they are physical equipment, they require a lot of maintenance. If they brake, staff needs to be dispatched. If someone drives into the station with their car, staff needs to be dispatched to fix it. Get it? As such, more and more of these have been decommissioned. Nevertheless, the United States Government recognizes that relying solely on GPS-based navigation is risky, as an incapacitation of the satellite system would result in aircraft not being able to navigate or land safely. As such, the FAA has established the Minimum Operational Network (MON), a series of ground-based navigation sources that will always be maintained. Airports that are served by the MON are labeled above the airport name on the IFR chart, as shown below.
Very High Frequency Omnidirectional Range (VOR)
The most basic approach using ground-based navigation sources is the VOR. I won't cover how VORs work in this article (read the article "VOR for Idiots" instead). VOR approaches are non-precision approaches with a Minimum Descent Altitude (MDA). This is an altitude below which an aircraft may not descend without having visual of the runway environment. The approach can be flown to the missed approach point (MAP) before the aircraft must execute the missed approach (i.e., the approach failed, the runway cannot be seen).
The VORs must be tuned into your navigation radio "NAV" and properly identified. You tune the various radials based on the specific approach plate.
The VOR approaches provide angular lateral guidance. What I mean by lateral is that it does not offer any vertical guidance. The lateral guidance is not particularly precise, however it is still a form of angular guidance. It is not designed to be like a localizer, but the guidance becomes more sensitive as you get closer to the nature of a VOR. As you get closer to the station, the radials are “closer together” and you therefore need more precision to fly a specific one. See the image below for reference.
They are angular only by virtue of the physical nature of VOR radials being farther apart when away from the station (see yellow and blue markers) and closer together when near the station (see red marker). You should in no way expect the precision of a localizer with a VOR. Remember also from basic private pilot knowledge that a VOR station is horribly imprecise when you’re over it. You have the “cone of confusion” and as such really close guidance is simply not available or unreliable.
VOR approaches could be considered "old school". They are notoriously more difficult to fly, require more workload, and are dependent on a finely tuned VOR station (i.e., well maintained). You should also know that VORs can be imprecise and a bit iffy. Not the big ones like SAV, but the smaller ones that rely on local maintenance. To this effect, the FAA allows pilots to fly VOR approaches using GPS, as long as you are monitoring the VOR in some fashion (a secondary CDI or a bearing pointer will do just fine... I prefer the bearing pointer...).
You may notice that the VOR will often be the Initial Approach Fix (IAF) to the approach. That is because they belong to the traditional network of VORs and Victor airways. If you only had VOR navigation, you need a way to commence the approach from the enroute world (composed of VORs and Victor airways). For example, here is Savannah International (KSAV) VOR/DME-A approach (circling only).
Note how the SAV VORTAC is at the center of a vast network of airways (left): V3, V437, V441, V37, etc.). On the plate, the VOR is labeled 'IAF'. In this case, the IAF leads you to a simple procedure turn to capture the Final Approach Course (FAC) of 024 degrees.
Because there is no vertical guidance, it is the pilot's responsibility to descend respecting all the stepdown altitudes. Depending on the complexity of the approach, there could be few or many. There are two philosophies on descending to minimums on non-precision approaches without vertical guidance:
Stabilized Descent. This procedure uncourageous pilots to establish a stable continuous descent, verifying that all altitudes are respected. A drawback is that you may not break out of the clouds as soon as you may want.
Dive and Drive. This procedure involves descending as quickly as possible to the stepdown altitudes - dive - and leveling off until the next one is sequenced - drive. The point is to go as low as you can as quickly as you can in hopes of going visual. I need to emphasize that certain approaches require you to "dive and drive" to offer the pilot the opportunity to see the runway environment prior to the MAP and be in a position to land.
VOR approaches may require an extensive manipulation of courses, increasing pilot workload. Just like any other approach, they can also be flown vectors-to-final (VTF) with Air Traffic Control (ATC) support. Because of their imprecise nature - like the linear guidance - you can expect the MDA to be relatively high.
Localizer (LOC)
Localizer approaches are non-precision approaches with an MDA. Just like VOR approaches, the localizers are for lateral guidance only. The localizer antenna is traditionally located at the very end of the runway (departure end) and you need to tune the localizer frequency into your NAV and tune the appropriate course (radial). However, differently from VOR approaches, the guidance is very precise and gets better as you fly closer to the antenna. We call it angular guidance. The precision of the guidance changes as you get closer to the runway. What do I mean by this? Imagine you are 5 NM out on a VOR course towards the runway. If your course deviation indicator (CDI) is fully deflected to one side, you are off-course by X feet. If you maintain that deviation all the way to the missed approach point, you will be a lot less than X feet off the course. See the image below for reference. The two dots on the approach path represent the CDI deviation dots.
Localizers also are relatively workload intensive. Depending on how you approach the IAF, you may need to transition from GPS to LOC (change navigation source) at the right time. Because localizers are a component of the instrument landing system (ILS) (see section below), LOC approaches are often combined with ILS plates: 'ILS or LOC RWY 10'. There are however cases where these are standalone procedures. The most famous is probably the one in Aspen, CO (KASE), where both the approach and the missed approach are composed of localizers (two, in fact!):
Just like the VOR approaches, localizers are flown vertically at the user's discretion, as long as the stepdown altitudes are respected. In the case of this approach, there are plenty of stepdown fixes and altitudes:
Instrument Landing System (ILS)
The Instrument Landing System (ILS) approach is the only ICAO recognized precision approach with a Decision Altitude (DA). A decision altitude, or "DA", is an altitude at which, on your descent, you must decide whether to continue to land or execute the missed approach. This DA has a safety margin allowing you to descent slightly below it while you make the decision. Note that the DA becomes your missed approach point, since you are following vertical guidance down to the runway.
The ILS provides angular guidance both laterally and vertically by virtue of two separate antennas: the localizer (laterally) and the glide slope (vertically). The localizer is the same localizer used for the "localizer approaches" (see previous header).
Because the ILS is composed of two antennas, you will very often see the ILS approach also provides localizer only minimums. The title may in fact say "ILS or LOC". This article talks about regulations concerning inoperative glideslopes, and how an ILS approach becomes a LOC approach.
ILS approaches are considered the most precise approaches in existence, and it’s a good bet to assume that you will have the lowest available minimums. When departing a field that has an ILS under IFR, it may be advisable to tune the ILS on the standby NAV frequency so that you can quickly get to it in case of emergency.
The more intellectual (NERD ALERT) pilot will want to know that ILS minimums are predicated on visibility requirements, and not just minimums. That is because ILS references Required Visual Range (RVR) values and not traditional "statute miles of visibility". Check out the line of minima for the ILS 10 at KSAV:
The above approach allows the pilot to descend to 230 ft MSL (that is barometric altitude) given at least 1800 ft of RVR (yes, that is the 230/18). The RVR is provided by ATC given sensors on the runway that determine what the visibility is. On a normal approach, the visibility is to be determined at the pilot's discretion. On the ILS, there is a provided visual value that can help you determine whether or not you will be able to land from the approach. Now that we know that ILS minimums are predicated by visibility requirements, let's take a look at the various ILS categories (yes, there are categories. You didn't expect that to be that easy, right?).
There are different types of ILS approaches, also known as “categories”. In small general aviation aircraft, you will most likely fly a CAT I (“one”). Larger aircraft equipped with radio altimeters, which provide height above the ground, may be certified for CAT II, and only large airliners have the weight and volume necessary to enable CAT III approaches. Here is a brief description of each.
CAT I - This is your generic ILS approach. The required visibility is typically greater or equal to 2400 ft RVR. Exceptions of course exist, but that is an average. The approach can be flown with or without autopilot and you are authorized to execute it with an instrument rating. To fly it, you need to have at least one RVR unit functioning on the field for your intended runway. But don't worry, ATC will let you know if the ILS is open or closed. Yeah, they may close it if the RVR units are inoperative.
CAT II - This ILS requires authorization. The required visibility is typically around 1200 ft RVR. As such, the CAT II can get you as low as 100 ft over the ground. We can think of these as "big boy" ILSs, where you need two trained crew members (yes, special training), two ILS receivers (redundancy) and dual autopilot systems (so both pilots have their own displays and systems). There are rare exceptions for non revenue operations - see FAR 91.193 if you're that bored). If you re-read the third sentence, you might have noticed that I told you 100 ft over the ground. I meant that on purpose, becuase the CAT II is based on Radio Altimetry, which provides you height over the ground. The minimums are called "Decision Altitude" just like a CAT I. Here is an example of a CAT II at KGSO (Greensboro, NC). Note the CAT II in the title and the minimums with the letters 'RA' (Radio Altimeter) and the special authorization note. What the note refers to is that the crew needs to be certified (trained), the aicraft needs to be certified with that capability (with the FAA), and the operator needs to be authorized since a specific operator manual is required.
CAT III – This is often referred to as Autoland. The aircraft is indeed able to land itself when the visibility is so bad that CAT I and CAT II don't work for you. Only large airliners have CAT III Autoland due to the regulatory requirement to have triple autopilot redundancy. That’s right, you need three (3) autopilot systems that continuously cross check each other. Most mid-sized cabin aircraft and private jets have only two, and of course a small Piper will barely have one. Ok. But now it gets a little more complicated. There are three types of CAT III... and can go as low as RVR 300... or RVR 0 (but that's almost always not authorized) - CAT IIIa: The typical minimum RVR is 700 ft. With the 'a' CAT III the aircraft is able to land itself and use autothrottles. - CAT IIIb: The typical minimum RVR is also 700 ft. However, not only does it use authothrottles, it is also capable of stopping the aircraft on the runway centerline. I know right!! That's awesome. However, it's up to you to figure out how to get to park. HA! - CAT IIIc: This approach allows the airplane to do the 'a' and 'b' stuff, plus also taxi the airplane to park. This is required if the visibility is... well, zero. However, these are mostly Not Authorized (NA) in the United States. Please reference the image below. Oh, and the decision altitude doesn't exist. It is replaced by "Alert Altitudes" that need to be announced as you execute the approach. Definitely, there is a lot of Crew Resource Management (CRM) in CAT II and CAT III approaches.
RNAV (GPS)-Based Navigation Approaches
With the advent of GPS, ground-based navigation equipment has becoming relatively obsolete. That doesn't mean it's not relevant (far from that, actually, did you read about the MON, above?).
GPS approaches are built "in space" based on GPS-altitude (GALT). They are referenced to Barometric altitude, but they are flown following guidance "floating in space". What I mean is that there just isn't any ground equipment emanating radio signals like a VOR, LOC, or GS.
In Garmin-world, we describe these as "magenta needle" approaches, because the CDI changes color to magenta when choosing 'GPS' as a navigation source. Other avionics manufacturers will use similar conventions, sometimes.
There are many lines of minima available for RNAV (GPS) approaches: LPV, LP, LNAV/VNAV, and LNAV. What that means is that you will have one single approach plate for all of these. There will just be multiple lines of minima, as shown below.
The ability to create one of those approach types is based entirely on Terminal Procedures (TERPS) which is an engineering document (not a pilot one) that specifies how to build an approah. Suffice to say that the algorithms might not always work out. Sometimes you cannot have an LPV. And other times you can, but it may have higher minimums than an LNAV approach.
Let's summarize them, below.
Lateral Navigation (LNAV)
The LNAV approaches provide linear lateral guidance. The precision of the guidance does not change as you get closer to the runway. What do I mean by this? Imagine you are 5 NM out on a course towards the runway. If your course deviation indicator (CDI) is fully deflected to one side, you are off-course by 10 degrees (off the set course). If you maintain that deviation all the way to the missed approach point (MAP), you will still be 10 degrees of course. See the image below for reference. The two dots on the approach path represent the CDI deviation dots.
Because there is no vertical guidance, it is the pilot's responsibility to descend respecting all the stepdown altitudes. Depending on the complexity of the approach, there could be few or many. There are two philosophies on descending to minimums on non-precision approaches without vertical guidance:
Stabilized Descent. This procedure uncourageous pilots to establish a stable continuous descent, verifying that all altitudes are respected. A drawback is that you may not break out of the clouds as soon as you may want.
Dive and Drive. This procedure involves descending as quickly as possible to the stepdown altitudes - dive - and leveling off until the next one is sequenced - drive. The point is to go as low as you can as quickly as you can in hopes of going visual. I need to emphasize that certain approaches require you to "dive and drive" to offer the pilot the opportunity to see the runway environment prior to the MAP and be in a position to land.
Localizer Performance (LP)
Imagine you had a localizer approach based on GPS. Yup, that’s right. The LP is just that. It provides lateral angular guidance only, and it provides an MDA (not a DA, since there is no vertical guidance). LP approaches are non-precision approaches with an MDA. Just like LOC approaches, the lateral path is for lateral guidance only and the guidance is very precise and gets better as you fly closer to the runway. Indeed, angular guidance. The precision of the guidance changes as you get closer to the runway.
Note from the image above that the angular guidance is exactly like the localizer. However, it is magenta, because it is GPS based and there is no localizer antenna on the ground. That also means that you will always stay on GPS as your navigation source.
Lateral and Vertical Navigation (LNAV/VNAV)
Ok this one is a little more complicated. The LNAV/VNAV approach is also categorized as an Approach with Vertical Guidance (APV) with a Decision Altitude (DA). Literally, it means Lateral and Vertical Navigation. Just like the ILS, a decision altitude, or "DA", is an altitude at which, on your descent, you must decide whether to continue to land or execute the missed approach. This DA has a safety margin allowing you to descent slightly below it while you make the decision. Note that the DA becomes your missed approach point, since you are following vertical guidance down to the runway.
The LNAV/VNAV approach provides linear guidance both laterally and vertically. It behaves just like an ILS, however the guidance does not get more precise as you approach the runway. And there's more to it. LNAV/VNAV approaches require either Wide Area Augmentation System (WAAS) or a certified barometric vertical navigation (VNAV) system. VNAV is a means by which the aircraft is able to build vertical paths based on barometric altitude (MSL). What does that mean? It means that the vertical path will change based on the barometric pressure. Since the approach path is built form your altimeter, it has to be set to the local barometric pressure (as we always do). But the imprecision of the altimer changes the path. This means that if you put the wrong altimeter setting ("high to low look out below"...) you can actually move the path to the point where you will hit the ground. For extreme temperatures, aircraft can be equipped with temperature compensation, where an appropriate altitude is calculated by the Flight Management System (FMS). This is very typical of integrated flight decks (like Garmin G1000).
Let's make an example of how the baro altimeter setting can affect the path. In the first image below, the correct altimeter setting is 29.92 in Hg. The FAF is at 1,500 ft MSL and my DA (or MAP) is at 300 ft MSL.
Now, I change the altimeter setting to 29.82 inHg (which is incorrect). That results in an error of 100 ft low. Notice how the path has now shifted down to account for the new pressure. Baro-VNAV. Get it? The old setting is shown in grey, while the new resulting altitudes are shown in blue. So by incorrectly setting that altimeter setting, the path is guiding me 100 feet low (which will also mess up your glide path angle).
So what happens if the airport does not have a working weather system and cannot provide the altimeter setting? Ah-ha! The chart will help you with that by offering an alternative location to find the setting, and raise the minimums to be sure you have an extra safety buffer. See the note below.
As I mentioned, you need to have VNAV to execute these approaches. VNAV is typical of integrated flight decks, so don't expect every aircraft to be capable of doing it, especially if it's set up with aftermarket components.
Localizer Performance with Vertical Guidance (LPV)
The LPV approach is the almost identical to the ILS in terms of precision and minimums, however ICAO does not recognize it as a precision approach. Only the ILS is one. It is categorizes as an Approach with Vertical Guidance (APV) with a Decision Altitude (DA). Just like the ILS, a decision altitude, or "DA", is an altitude at which, on your descent, you must decide whether to continue to land or execute the missed approach. This DA has a safety margin allowing you to descent slightly below it while you make the decision. Note that the DA becomes your missed approach point, since you are following vertical guidance down to the runway.
The LPV provides angular guidance both laterally and vertically. It behaves just like an ILS. That's why it's called "Localizer Performance". It gives you indeed the same precision performance as a localizer. And this holds true also for the vertical guidance... just like a glideslope. However, to distinguish the two, we call the lateral guidance "lateral path" instead of "localizer" and "vertical glidepath" instead of "vertical glideslope".
LPV approaches require a high level of precision. To do this, they operate with Wide Area Augmentation System (WAAS). WAAS is a vast network of GPS antennas located all around the nation. They enhance the GPS solution received by the airplane by minimizing position error (adding more data points) and relaying the information to the aircraft via a ground master station. Many GPS units will have an option to disengage WAAS in the system settings (don't do that. Why would you? Well I will, to train you... muahahaha).
Advisory Vertical Guidance (+V)
Remember how I mentioned that there are two philosophies for descents on approaches that do not have vertical guidance? Let me summarize them again, below:
Because there is no vertical guidance, it is the pilot's responsibility to descend respecting all the stepdown altitudes. Depending on the complexity of the approach, there could be few or many. There are two philosophies on descending to minimums on non-precision approaches without vertical guidance:
Stabilized Descent. This procedure uncourageous pilots to establish a stable continuous descent, verifying that all altitudes are respected. A drawback is that you may not break out of the clouds as soon as you may want.
Dive and Drive. This procedure involves descending as quickly as possible to the stepdown altitudes - dive - and leveling off until the next one is sequenced - drive. The point is to go as low as you can as quickly as you can in hopes of going visual. I need to emphasize that certain approaches require you to "dive and drive" to offer the pilot the opportunity to see the runway environment prior to the MAP and be in a position to land.
To help satisfy the stabilized descent which is preferred by the FAA, Garmin created the "+V" navigation solution. That is, it geometrically builds a GPS vertical angular path, just like an LPV and provides it to the user. Building the path is actually very easy. It's certifying it that becomes problematic. Nevertheless, Garmin (and now many other avionics) offer the +V solution automatically. For example, if you load an LNAV approach, Garmin will actually load an LNAV+V and give you a glidepath to follow. There will be a popup note letting you know that it's for advisory guidance only. That means that the pilot cannot trust it blindly but has to verify the stepdown altitudes.
Summary of Most-Common Approaches
Comments