Friday, June 26, 2009

The ILS-DME or LOC Approach

The ILS (Insrument Landing System) approach is the easiest one to make. There are two different ILS approaches: ILS and ILS-DME. So what's the difference? The ILS approach requires a procedure turn to be established on the Localizer (LOC). The ILS-DME approach requires the use of a DME. There are no ILS appraoches in Mexico, only ILS-DME. Example:

The ILS-DME or LOC Rwy 12 is the approach we made to Cancún for our graduation trip. It's special you see :)

Effective Date 16 November 2007. Chart 11-1. ILS Frequency 111.10 Mhz, Identifier: ICUN, Final Approach Course 124º, Glideslope capture at 1300ft (1281' AGL*) 6.0 CUN DME or 4.0 DME ICUN. ILS DA=Descision Altitude is 219' (H=Height is 200') and the airport elevation is 20ft above sea level. Minimum Safe Altitude is 2000ft.
The Missed Approach Procedure is to climb outbound on the 124º radial to 7.0 DME of CUN VOR and then turn right within 10NM to CUN VOR to the minimum holding altitude, which is 2000ft.

There is no ILS-DME1 or 2 for Cancún, because we will be given Radar Vectors to the ILS. Other airports have them, because the procedure starts from an airway, just like the VOR-DME3 of Puerto Vallarta. ATC will clear us for the approach and we will maintain 1300ft and intercept the LOC. At 6.0 DME from the CUN VOR (or 4.0 DME ICUN), we will capture the Glideslope and follow it to our Decision Altitude (or Height). The DA(H) is shown in the minimums section of the chart.

What is DA(H)? It's the altitude (or height) in which you must decide to land or to go around. If we don't have the runway in sight by 219ft, we go around. Checking the minimum section, if we had a C category airplane (A320/B737) we would need at least 1/2 Mile of visibility to land. If the ALS (Approach Lighting System) is out, we would need 3/4 Mile of visibility.

You probably asked yourself why the approach is named ILS-DME or LOC. If the Glideslope is out, we would still have the Localizer. The LOC guides us horizontally to the runway. So, checking in the minimums section, we would have a MDA(H). We would need 3/4 Mile of visibility and if the ALS is out, 1 Mile of visibility.

Look at the table on the profile section. It shows Groundspeeds in knots, the rate of descend with a 3.0º glidepath for the various speeds and the time from the FAF=Final Approach Fix (The small cross at 6.0 DME CUN) to the MAP=Missed Approach Point.
In our A320/737, we would have approximately 140 knots groundspeed. So we would need a rate of descend of 753 feet per minute and the time from the FAF to the MAP would be 1:40 min.

There you go. The next one I will explain is the ILS CATIII approach. Bye

*AGL=Above Ground Level

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