Introduction to ILS Instrument Approaches

 

Introduction to ILS Instrument Approaches

by Dave Klain

Purpose
To explain the basic concepts of ILS instrument approaches 

Prerequisites
NavigationAids

Definitions:

  • A localizer is an electronic beam which transmits a specific signal and a VOR receiver can determine left/right (horizontal) reference to the beam. Think of it as a single radial off a VOR. It is typically (but not always) aligned with the runway centerline.
  • A glideslope is an electronic beam which is vertically as opposed to horizontally oriented. As a result, a glideslope receiver can determine above/below (vertical) reference. A glideslope signal is typically aligned with a localizer beam.
  • An ILS (Instrument Landing System) is a type of instrument approach consisting of a localizer, a glideslope and specific approach and runway lighting. It is one of many kinds of instrument approaches which enable an airplane to safely get from the enroute environment down to a position where it can see the runway and make a landing. There are three kinds of ILS approach with the CAT-III being the one with the lowest minimums...typically allowing the approach to be flown even with no visibility.


Discussion

Now let's start with the basics. As mentioned above, the ILS is one kind of approach and on the chart it is typically labeled with ILS and the runway number (i.e., "ILS31"). The chart is made up of several areas...generally with communications information at the top, a plan (horizontal) view in the middle, a vertical profile below that and the minimums either below or next to the vertical profile. On the horizontal view you will see a number of fixes which can be flown to via the enroute system. Upon arriving at that point, you would begin flying the approach if cleared to do so by ATC. There are many different routings and steps, but generally they all end up with you on the extended runway centerline aligned with the localizer beam. It is at this point that you would start flying the localizer beam for horizontal guidance and you have a specific altitude to fly with a minimum altitude that will guarantee obstacle clearance. This is depicted on the vertical profile by the altitude with a line under it (meaning it is a floor).

The reason you typically pick up the localizer before the glideslope has nothing to do with the strength of the signals but because the approaches are desiged that way (for technical reasons having to do with false glideslopes too complex to discuss here). So the approach has you at a (relatively low) altitude, flying in on the localizer beam. Eventually you will reach the point where the glideslope crosses that altitude...it is depicted on the vertical profile and is the final approach fix. At this point you would leave the altitude and start following the glideslope for vertical guidance while still following the localizer for horizontal guidance and it will lead you down to the runway while ensuring obstacle clearance. If you do not see the runway environment (defined in the FARS) by the time you reach Decision Hieght (DH - listed in the minimums section of the chart), you must execute the missed approach. If you see the runway environment, you may continue descending and land. Most ILS systems have a decision height of 200 feet above ground level. CAT II and III ILSs typically have a lower DH, however there are additional requirements (equipment in the plane and training for the crew) before you can be certified to fly a CAT II or III ILS. Any instrument rated pilot with a localizer and glideslope receiver can fly a CAT I ILS...

OK now, let's talk about a localizer approach (typically labeled with LOC and the runway number, i.e., "LOC18"). A localizer approach is nothing more than the localizer portion of an ILS approach. The difference is that there is NO vertical guidance provided. Instead you have some other way to determine how far away from the runway you are (might be DME distance, might be a VOR crossing radial, might be an NDB). At the point where you cross that point (again, called the Final Approach Fix) you are cleared to descend down to your final altitude. Some pilots chose to do an immediate steep descent down to that altitude, others choose to do a slower descent at a rate which will get them there by the time they reach the missed approach point (MAP). The MAP may be defined several ways...again it might be a DME distance, might be a marker beacon, might be based on elapsed time since leaving the Final Approach Fix)...it will be defined on the chart. When you reach the MAP, if you don't have the runway environment in sight and are in a position to land, you must execute a missed approach.

Now all localizer beams actually transmit two ways. The primary beam is the one flown for the ILS or LOC approach, but it also has a beam that transmits 180 degrees out the other way (i.e., down the runway and beyond). In some places, an instrument approach is constructed off of this beam as well. It is called a Back Course (BC) and is flown identically to a LOC approach with one exception. Since the beam is 180 degrees out, you get "reversed sensing" on your localizer/VOR receiver. What this means is that if you are to the left of the beam (need to turn right to reintercept), the needle will be to the left. On an ILS, VOR or LOC if you were to the left of the beam, the needle would be deflected to the right... Other than this reverse sensing, the approach is the same as any LOC.

Now let's talk about having the plane fly the approach. Any plane that has the ability to track a VOR (typically called "NAV" mode) ....including the default MSFS plane models, has the ability to fly a "coupled" approach. This means that the autopilot is flying the plane (or at least one axis of flight) during the approach. Think of it this way...there is NO difference between being at altitude and having your Autopilot in NAV mode tracking an VOR and having the autopilot in NAV mode track the Localizer beam during an approach.

Now because a localizer beam has increased sensitivity compared to a VOR radial, most autopilots (including in MSFS) have a mode called "LOC"...this tells the autopilot that it is tracking a localizer beam. If your autopilot has this mode, it would track the "left/right" of the localizer beam but you would still be responsible for altitude and descent control whether flying an ILS, LOC, or BC approach... Most sophisticated autopilots also have a mode called "BC" which tells the autopilot it is flying a Backcourse and allows it to compensate for the reverse sensing. This is not modeled on all MSFS planes...

Some autopilots also have the ability to track the glideslope for vertical axis control. They will have a mode called "APP" which tells the autopilot it is flying and receiving guidance from both a localizer and a glideslope and the autopilot will fly the beam all the way down...in theory to the runway...however in the real world the beam may have some distortions so unless flying a CAT II or III approach, you typically disengage the autopilot at or before DH (200 feet) and then manually fly the plane the rest of the way down.

All the the capabilities spoken about so far are common to most modern autopilots (by modern I mean mid 1970's and beyond). There are many Cessnas, Pipers and Mooneys capable of flying coupled approaches. I do it all the time in my Mooney!

Now some planes (and these are typically planes where the ability to land in bad weather is critical for operational reasons (read airlines making flights happen, corporate jets, some other special types because the equipment is expensive) have the ability to autoland. This means that the autopilot will fly the airplane all the way down to the runway and, after landing will keep the airplane aligned with the runway. Simultaneously with this, the autothrottles will control the engines and (after landing) apply reverse thrust to slow the plane until below a certain airspeed. The autobrake system will also apply pressure to the brakes to slow the plane and (ultimately) stop it on the runway. Autoland is modeled in some planes you can fly on MSFS including most of the better payware models that simulate aircraft really capable of autoland.