Wednesday, June 24, 2009

The VOR Approach

Hi there. In this post I'll explain a VOR approach. When flying IFR, you'll have to fly different approaches and departures. The first thing you have to check is the active runway of the airport you're arriving at. In this example I have chosen the VOR Rwy 08 of Mazatlán Airport:

First, you must confirm if it is the correct chart. Sometime pilots take an incorrect chart. It's strange, but it happens. At the top left you can see the ICAO and IATA code of Mazatlán. Below from it, the name of the airport. Then, you must check the Effective Date at the top of the chart: 7 May 2004. It will be effective as long as they don't make a change.

The approach chart is divided in four sections:

Header (Where the airport frequencies, missed approach procedure description, Minimum Safe Altitude and other important data is located).
Plan (A map-like drawing of the approach showing the courses, VORs, terrain and obstructions).
Profile (A vertical-view and preliminary minimum altitudes you'll use as you descend).
Minimums (The Minimum Descent Altitude for the final approach segment and the visibility minimums divided into 4 airplane categories: A is the lightest and D is the heaviest).

The MZT VOR frequency is 114.9 Mhz, Final approach course is 087º (Always magnetic), No FAF meaning No Final Approach Fix, Minimim Descent Altitude is 860 feet (Height 841ft) and the airport elevation is 38ft above sea level.
The circle at the right side of the Header is the Minimum Safe Altitude from the MZT VOR. At those altitudes you won't hit any mountains, towers, cows, etc. There are four different MSAs, because at those cuadrants there are different obstructions at different heights. Between the 180 and the 270 Radial the MSA is 2000ft. There's only water in there, so 2000ft is a safe altitude. Why not 1400ft? because 2000ft is the minimum altitude you must fly above terrain or water or a mountain.

The missed approach procedure (A procedure you have to follow if you don't see the runway at the MAP=Minimum Approach Point or you have to go around for some reason) is to climb outbound on the 102 radial of the MZT VOR, make a right teadrop turn to the VOR within 10NM to the Minimum Holding Altitude (The racetrack-type thing is the holding pattern and shows 4000ft).

On the Plan section of the chart, the approach procedure is drawn with a thick line. The VOR is at the center and the missed approach path is drawn with a dashed line. You can see the obstructions in the area with their heights in feet. The highest obstruction in the area is 2359ft high and it is marked with an arrow.

Now lets shoot the approach: Suppose that we are flying from the Southeast in a slow Cessna 172 doing 80kts groudspeed. You must fly direct to the MZT VOR and then intercept the 267º Radial. You must fly at 4000ft before crossing the VOR. When you crossed the VOR and intercepted the radial, you must descened to 2000ft as in the Profile section is shown and count 6 minutes*. After 6 minutes have passed, you turn left 45º to a heading of 222º and count 1 minute. The minute has passed, so you turn 180º to the right to a heading of 042º.
Then you intercept the 087º course to the VOR and descend to the Minimum Descent Altitude. What is our MDA? Checking the Minimums section, we see that it is an altitude of 860ft. We must maintain 860ft before we have a visual on the runway. The minimum visibility for our type A Category airplane is 1 mile.
If we don't see the runway and cross the MAP=Missed Approach Point (the small M above the VOR in the Profile section), we execute the missed approach procedure to try again.

*Why 6 minutes? There is a table on the Header section: MAX TIME TO TURN is the time you must complete to start the procedure turn. If you have 80 knots ground speed, the maximum time on the 267º course is 6 minutes. With 80kts you cover 8NM in 6 minutes. This is because you must remain inside 10NM from the airport/VOR.

What happens if we arrive from the West? We can't just land straight in. We must cross the IAF=Initial Approach Fix which is the VOR in this approach, so we cross the VOR from the heading we are flying to and after one minute, we fly left or right to the VOR and then make the approach.

What is the CIRCLE-TO-LAND section? When you're on the approach but don't want to land in that runway, because of the winds or the other runway is more comfortable for taxi, you circle to land. If the weather conditions are good, you can do a circling landing. The minimums for this approach on the A category are 860 feet. So when you're maintaining those 860ft, you fly close enough to the other runway so that you keep an eye on it and land. Just like a normal visual pattern. It mixes instrument and visual procedures.

Sounds fun, doesn't it? I've covered most of what the chart shows. The next approach explanations will be shorter, because I won't have to explain the header, minumums, etc.

Stand-by for more :)

1 comment: