Ask Captain Lim - Some questions on the various Landing Systems.

I have some general questions regarding the Landing System of an airplane. Many of the new types of landing systems are getting installed on modern aircraft like the A320/A340/B777. The traditional one being the ILS (Instrument Landing system). We have now MLS (Microwave Landing System) and GLS (GPS Landing System).

1. Can you use more than one system to land the aircraft at a time?

2. Suppose you are preparing for an approach, which one of the system do you select among the above to land? What is the criteria of selection?

3. I know that you tune the ILS and the course in the "RAD NAV" Page of the FMS and press the "APPR" button to fly to the center of localizer and G/S. Which one of the above systems (ILS/MLS/GPS) would land the plane?

Thanks and warmest regards,

Srihari J

Hi Srihari,

Let me touch on the Landing System briefly before I answer your questions.

The Microwave Landing System (MLS) was adopted by ICAO some years back as
the standard precision approach system to replace the Instrument Landing System (ILS). As far as the presentation of the MLS in the cockpit is concerned, it is similar to the ILS.

For the last forty years or so, the ILS has served as the standard precision approach and landing aid. During this time, it has performed well and has undergone a number of improvements but in relation to future aviation requirements, the ILS has a number of basic limitations. MLS not only eliminates these problems; but also offers many advantages over ILS. However, the implementation of the MLS has been rather slow. As a result, the Global Positioning System (GPS) would soon overtake the MLS implementation.

The GPS is a very reliable and highly accurate system. However it has its inherent problem of integrity. A GPS error can take minutes to report and correct. Hence, FAA has now sponsored a Wide Area Augmentation System (WAAS), which is under development and is intended to fix this problem in the early 2000's. So far, however, none has proven its ability to provide an adequate image in all weather conditions at a reasonable cost. So the ILS continues to be the main Landing System in most major airports today.

Now, to answer your questions:-

1. You use only one system to guide the airplane to land at any one time.

2. Which type of Landing System to use is dictated by the airfield. As far as I know, many major airports have ILS installed and this is the primary landing aid preplanned by the pilots. The Air Traffic Information Service (ATIS) for a particular airfield will normally broadcast the type of landing system when the weather is bad and an ILS Category 3B, when available, will permit an airplane with suitably qualified crew to land with a minimum visibility of 100 meters and 20 feet ceiling.

3. I believe you fly the FS 2002 and is familiar with some of the landing procedures. The Landing System to use is still the ILS and not the MLS or GPS. If you have the proper ILS frequency selected, you don't need to select the course in the RAD NAV page.

Personally, I have not flown an MLS approach in my life yet. So far, I have only tried 2 practiced GPS approaches in good weather in Perth. These GPS approaches are not considered as Precision Landing System yet because they have not been approved by the Authorities.