Understanding Glidepath Differences

• Why are there Differences Between Glideslopes, Glidepaths, and Visual Glide Slope Indicators?

ILS and LPV have become almost synonymous with how operators perceive them. From an operational perspective, we load the approach, press the approach button, and watch the magic happen. Behind the scenes, there is vastly different technology at work to make it all appear the same on the surface. This article will explain why an LPV glidepath may differ from an ILS glideslope, especially when descending below the DA.

By Definition

Despite the operational consistency, these approach types are very different, beginning with their definitions. An ILS is a precision approach by definition, whereas an approach with LPV minimums is an approach with vertical guidance. According to the AIM:

“a precision approach is an instrument approach based on a navigation system that provides course guidance and glidepath deviation meeting the precision standards of ICAO Annex 10. For example, PAR, ILS and GLS…

Approach with vertical guidance (APV) is an instrument approach based on a navigation system that is not required to meet the precision approach standards of ICAO Annex 10 but provides course and glidepath deviation information. For example, LNAV/VNAV and LPV are APV approaches.”

For pilots that may not be familiar with ICAO Annex 10, it’s a long list of requirements to which an ILS must conform. This is one reason an LPV is not a precision approach by definition.

An ILS Glideslope and an LPV Glidepath are similar in that they both provide an obstacle-free path to the runway. The big difference: how they are constructed. Visual Glide Slope Indicators (VGSIs) have their own variables that can come into play causing operators to question – shouldn’t they all agree? The answer is sort of. We live in an age where we like black and white answers. Unfortunately, with various technologies, there are still many shades of gray. Let’s look at each approach type and explore where differences can occur and what to do if there’s a discrepancy.

It's been around 75 years for a reason…

First, we’ll look at ILS; considered the gold standard of approaches because it has a fixed path in space that isn’t susceptible to temperature or pressure variations. Despite being more than 75 years old, it was probably ahead of its time. Without going into too much detail of the localizer and glideslope lobes, the ILS accurately defines a path in space that safely and reliably guides the aircraft to the runway. The glideslope transmitter is calibrated to deliver the aircraft down a vertical path, usually 3°, cross the runway threshold at 50 feet and touchdown approximately 1000 feet down the runway. The glideslope antenna on most aircraft is near the nose of the airplane to give maximum reliability for tracking the signal.

A Glideslope Transmitter next to the Runway

Glideslope Antenna in the Radome of the Aircraft

Enter the New Kid on the Block

Next, let’s look at LPV. Localizer Performance with Vertical guidance was introduced about 15 years ago. Unlike LNAV/VNAV, using linear deviation, LPV is designed to use angular deviations (like an ILS), meaning they get more sensitive closer to the runway. In fact, the same control laws are used for both ILS and LPV. LPV was purposely developed to look and fly like an ILS. Despite their similar appearance from the cockpit, internally, they are completely different animals. Where an ILS is projected by two overlapping lobes of energy, LPV is a result of increased accuracy from a Satellite-Based Augmentation System (SBAS). Unlike most traditional RNAV approaches, the lateral and vertical deviations for LPV come directly from the GPS receiver and are not part of the FMS solution. LPV minimums are a subset of an RNAV (GPS) approach and, with the increased accuracy, can provide minimums that are comparable to a category 1 ILS using GPS navigation. 

To make an LPV mimic an ILS’s behavior, LPV relies on programmed coordinates and instructions contained in a Final Approach Segment (FAS) data block. The FAS data block contains instructions for the approach, including coordinates for the runway, threshold crossing height, elevation, glidepath angle, and horizontal and vertical alert limits. The GPS receiver computes both linear and angular deviation but, as previously mentioned, only angular is displayed. They can be thought of as instructions to provide a pseudo localizer and glideslope. 

Example of a FAS Datablock

An LPV glidepath is calculated entirely using SBAS altitude. That is, the GPS constructs the glidepath using GPS altitude and height above earth ellipsoid to produce a path that unlike baro-VNAV, is not susceptible to pressure and temperature variations. This means that like an ILS, an LPV glidepath is essentially a fixed path in space being tracked using GPS altitude to a DA that the pilot reads from a baro-altimeter. This is a component to how an LPV can achieve minimums comparable to an ILS. 

Final Approach Segment Architecture

Two Points of View?

With our knowledge of how the systems operate, let’s look at the differences in the ILS/PAPI and LPV glidepath, which can have a cumulative effect due to glidepath intercept point, runway slope, and antenna location.

Glidepath Intercept Point

First is what’s referred to as the Glidepath Intercept Point or GPIP. The GPIP is the point where the LPV computed glidepath, based on the approach angle, would intercept the runway. When an ILS is installed, the PAPI should be located at the same distance down the runway as the glideslope transmitter. According to FAA Order 6850.2B:

“When siting PAPI on a runway with an established electronic glide slope, the PAPI visual approach path should coincide, as much as possible, with the one produced electronically. To accomplish this, the PAPI is placed at the same distance from the threshold as the virtual source of the electronic glide slope within a tolerance of ±30 feet (±10 m). The PAPI is aimed at the same angle as the electronic glide slope.”

It goes on to say that the location can be modified for aircraft of a certain height (typically for larger aircraft). This means pilots will usually be on the visual glidepath for the majority of the approach until the transition to the landing flare. Using the example below, at the Amsterdam Schiphol airport, runway 6, the FAS data block for the LPV approach has the glideslope angle of 3° and the threshold crossing height (TCH) of 50’. This means the glidepath intercept point (GPIP) is 954’ from the threshold (50’/TAN(3°) = 954’). However, the PAPI for runway 06 is located 1376’ from the threshold as shown in the illustration. Therefore, the PAPI will indicate below-path while on the LPV glideslope.  

Example of a 3.0° GPIP vs PAPI at EHAM

The following illustration shows the 3°glidepath from the PAPI compared to the path of the LPV. The PAPI would appear “on glidepath” through DA but at some point between DA and the runway threshold, the crew would see the PAPI transition from “On” glidepath to “slightly below” to “below”. 

3 Degree PAPI vs LPV Glidepath at Amsterdam Schiphol Airport

Some other variables that can change the view between the glidepath and the VASI are:

Runways That Have an Upslope

Sloping runways, especially upsloping can cause a difference between the VASI (which is usually installed a couple feet off the ground and the runway slope, which can cause several more feet of error. This can be further compounded based on the size of aircraft and the pilots sight picture and deck angle. Those are beyond the scope of this article but are explained in excellent detail on Code7700.

Effects of an Upsloping Runway on Glidepath

Antenna Location for the Glideslope and the GPS

The glideslope antenna, for most transport category aircraft, is in the nose radome. The GPS antenna is located on the top of the aircraft, where it has the most unobstructed view of satellites. GPS antenna location can contribute to the difference between the glidepath and the VGSI. 

GPS Antennas on the Top for Maximum Satellite Visibility

If we consider the way an approach is flown using a glideslope/glidepath, we can see a difference just from the reference point where we consider the 3° path to be.

Path from the Nose (Glideslope Antenna)

Path from the Top (GPS Antenna)

Which takes priority, the LPV Glidepath or the PAPI?

In these situations, many operators may be left wondering, “Which path do I fly?” This can be confusing because both paths have been flight checked to provide obstacle clearance all the way to touchdown. 14 CFR part 91.129(e)(3), 91.130(a), and 91.131(a) state the following:

§91.129 (e) Minimum altitudes when operating to an airport in Class D airspace. (1) Unless required by the applicable distance-from-cloud criteria, each pilot operating a large or turbine-powered airplane must enter the traffic pattern at an altitude of at least 1,500 feet above the elevation of the airport and maintain at least 1,500 feet until further descent is required for a safe landing. (3) Each pilot operating an airplane approaching to land on a runway served by a visual approach slope indicator must maintain an altitude at or above the glidepath until a lower altitude is necessary for a safe landing.

§91.130   Operations in Class C airspace. (a) General. Unless otherwise authorized by ATC, each aircraft operation in Class C airspace must be conducted in compliance with this section and §91.129.

§91.131   Operations in Class B airspace. (a) Operating rules. No person may operate an aircraft within a Class B airspace area except in compliance with §91.129 and the following rules…

In the United States, approach plates will contain a note when there is disagreement between the Visual Glideslope Indicator (PAPI, VASI) and the LPV Glidepath “VGSI and RNAV glidepath not coincident”. The example below shows a case where the VASI is set to a 3° angle but has a threshold crossing height of 71 feet. The Glidepath for the RNAV approach to LPV minimums is also 3° but has a threshold crossing height of 54 feet. This is a perfect example of the GPIP and the VASI not being co-located and will cause the crew to see a transition to red lights when descending below DA.

Example of the VGSI and the Glidepath not Aligning

As standardized as aviation is, ILS and LPV are still different technologies despite their similar operational procedures and minimums. Although they both provide an obstacle free path to the runway, there are several variables that can cause divergences. Regulations all state the VGSI must be maintained until a lower altitude is necessary for a safe landing. 

Program Pilot David Rogers supports EPIC and NG FMS-equipped Cessna and Gulfstream aircraft for Honeywell Flight Technical Services. He can be reached via email at David.Rogers@honeywell.com.