FlightLog: Adding ILS to Malacca Airport (WMKM) in X-Plane

I discovered that (in real-life) Malacca Airport (WMKM) has an Instrument Landing System (ILS) on runway 03, but this is not reflected in the current X-Plane flight simulator (10.36). So I did some research on how to fix this problem.

Background

An ILS system consists of of two transmitters: a Localiser (LOC) and a Glideslope (GS). In each, two signals are transmitted from directional antennas towards the approach path of landing aircraft. One signal is modulated at 90Hz and the other at 150Hz, and they are transmitted in a parallel beam, slightly apart. Aircraft can tune in to these signals, and determine how close they are to the center of the beams by comparing the relative strength of the signals. By piloting towards the center of both beams, an aircraft can ensure it is on the correct approach path.

The Localizer transmitter handles lateral (left-right) navigation whereas the glidescope handles vertical (up-down) navigation. The transmitter for the localizer is usually located at the far end of the runway the aircraft is landing into, whereas the glidescope transmitter is located by side of the runway, near the touch down zones.

The ILS is part of an Instrument/Precision approach, which allows you to land the aircraft even in low visibility, where you can begin your final approach even if the runway is not in sight. ILS systems usually can be coupled with the aircraft's autopilot system, allowing the aircraft to fly itself during the final approach.

Can you just leave it to the ILS and autopilot to complete the landing? Only if you want to hit the ground really hard! The ILS descent angle is usually a few hundred feet per minute (depending on your airspeed) and you will need to perform a landing flare (http://flighttraining.aopa.org/magazine/2011/June/Cover_landings.html) just shortly before touchdown to effect a gentle landing.


NAVAID Details

NAVAIDs in X-Plane are not stored as part of the airport's scenery (since they consists of other things such as VORs and NDBs which may not "belong" to an airport). Instead they are stored in a separate NAVAID database. This means you can't issue an update airport scenery to fix a NAVAID problem, but rather it needs an update typically provided in a new version of X-Plane. However, X-Plane's local map features a simple editor to manage this NAVAID database, so with some editing, I was able to add the ILS facility to WMKM.

First of all, you have to know the exact placement of the localizer and glidescope. Since these are the origins of the guiding beams, having these in the wrong position will result in the aircraft "aiming" towards the wrong place. Fortunately, the position of these transmitters are usually documented in the respective Aerodrome section of the Aeronautical Information Publication (AIP), a set of documents each country is required to publish (in some countries these are restricted or a fee is required, in Malaysia the Department of Civil Aviation makes them freely available). WMKM's aeredrome information document can be found here:


Here's the relevant section:

So there you can find the coordinates of the Localiser (LOC) and the Glideslope (GP) transmitter. It took a bit of fiddling with google earth before I figured out that the coordinates are in decimal seconds, i.e. the localizer latitude of 021633.74 is to be read as 2 degrees 16 minutes and 33.74 seconds (Why can't every just stick to decimal degrees and make life easy?). I confirmed it with google earth:




Configuring X-Plane

The next step was to register these NAVAIDs in X-Plane. I started the sim at WMKM, and opened X-Plane's Location->Local Map, pressed the Edit button, and then clicked on add "ILS" (there's also an add LOC option, I suspect this is to be used if you have a localizer only, with no glideslope transmitter). X-Plane needs coordinates in Decimal Minutes (Really, why can't everybody just stick to decimal minutes and make life easy?) but I found a useful online converter here (http://www.directionsmag.com/site/latlong-converter/).

For the localizer, I used the following parameters:

  • Name: WMKM 03 ILS-cat-I
  • ID: IKKM
  • Lon: 102 15.528
  • Lat: 02 16.562
  • Elevation: 73ft (
  • Heading: 030.00
  • Frequency: 111.70
  • Range: 18nm
Then I pressed "add GLS" and entered the following:

  • Name: WMKM 03 GS
  • ID: IKKM
  • Lon: 102 14.910
  • Lat: 02 15.619
  • Elevation: 26ft
  • Heading: 030.00
  • Frequency: 111.70
  • Range: 10nm

Technically, the localizer frequency is in the VHF band, whereas the glideslope frequency is in the UHF band. However, they are paired according to a standard so that pilots only have to set the localizer frequency, and the glideslope frequency is automatically determined. X-Plane seems to want the localizer frequency (111.70 in this case) to be keyed in for both the localizer and glideslope.

I didn't have any figures for the range of the signals, which depends on the strength of the transmitter, so I copied the figures from Subang's ILS system: 18nm for the localizer and 10nm for the glideslope. The Heading to be keyed in is the aircraft and runway heading, although in actual fact the beams are transmitted in the opposite direction, i.e. facing towards the aircraft).

Approach Charts

The AIP also includes approach charts that describe how you use the ILS. These start you off at a nearby NAVAID that you would have used to reach WMKM:

Here's the VOR approach:
The top part of the chart describes the lateral navigation and the lower half describes the vertical navigation. You would need to arrive at the VMK VOR at minimum height of 2500 ft, and then fly out on the 210 radial, while beginning descent to 1600ft. At 6nm away from VMK, turn right to heading 255. then 10nm from VMK, do a left 180 degree turn, and head back towards the airport on heading 030 (the localizer will help you find this direction by that point). When you are 5.2nm from VMK, you will cross the glidslope beam and can start tracking your descent.

Flight Testing

So now for the fun part, flight testing!

I picked the Bonanza A36 for this, a nimble yet straightforward aircraft to fly. I didn't fly the full approach, but started with the aircraft about 20nm out past the runway, heading towards the airport (but slightly off). I set the autopilot to descend and maintain a height of 1600ft, and the NAV1 radio was set to a frequency of 111.70, which is the frequency of WMKM's ILS. The A36 has no DME, but I set the GPS waypoint to that of the airport so I could monitor the distance. The CDI setting was set to VLOC to ensure that the aircraft's autopilot uses the ILS and not GPS to reach the airport.

Nothing happens at first as the ILS is out of range, and the aircraft is just flying the set heading in the autopilot.

Once the ILS signal is picked up, two green dots appear at the side and bottom of the Primary Flight Display (part of the "Glass Cockpit" of this A36):
The green dots indicate your deviation from the ILS beam. If you're flying manually, you will need to fly to the left and up in this case, so that the green dots reach the center of the bars at the bottom and right. If you've got the autopilot set to approach mode (APR) then just sit back and the autopilot will do the work for you.

While the autopilot will find its way to the center of the localizer beam by itself, it will not "hunt" for the glideslope beam. Instead, you're expected to fly until you "cross" them glideslope beam, at which point the autopilot will begin to track it. In WMKM's case, The vertical dot will be centered when the aircraft is 5.2nm away from the airport at a height of 1600ft, this is known as the "FAP". So you will usually want to make sure you are at 1600ft before you cross the FAP. You'll usually hear a beep and the aircraft will start to descend when you reach that point.

Now if you look out the window (this is not strictly necessary, nor may it be possible under low visibility conditions) you will see the airport runway lined up and your descent targeted at the touchdown point.

You can stay on the approach until you reach the Decision Height (DA, 300ft for WMKM) where you have to execute a missed approach (i.e. abort landing and do a go-around) if the runway is not in sight yet.