Saturday, November 27, 2010

How To Execute An Actual Instrument Approach

Executing an instrument approach in actual “instrument” weather (ceiling below 1000 ft and visibility less than three miles) is not a game. A simulation done at home or at an FBO (fixed base operator) doesn’t create the same level of anxiety. Why should it? Just turn it off and go about your chores.

In this article I am going to lead you through the thought processes that I deem important to get an airplane and its passengers safely on the ground in really bad weather. OK here we go.

Before arriving at MHT (see my last blog), I tuned in the weather and got the following:
Information foxtrot: Visibility less than ¼ mile in fog and light rain, ceiling indefinite. You can’t land under these conditions unless you have very special equipment and expertise to handle this essentially zero zero weather. The active runway was 35 so I got the instrument approach plate for an ILS to Rwy 35. This plate illustrated everything needed to carry out the approach and landing. It delineates the proper procedure, specifying altitudes and compass headings needed if you were to fly the procedure without being vectored by approach control. All the required radio frequencies for navigation and communication are specified. It is very important to have up to date charts as changes to the above can occur.

So, about 20 miles out from MHT I was handed off to Manchester approach by Boston center. “Navajo 711EB contact Manchester approach on 124.9, good day.” I reply:”roger
Manchester approach 123.9”. Then after changing frequencies:“ Manchester approach Navajo 711EB 6000 with foxtrot”. This lets the controller know my altitude and that I have the latest weather. Approach tells me to descend to 3000 ft. He also advises me to expect a hold until the weather improves. On hearing that, I look at the approach plate and study the hold procedure which is like a racetrack carried out on specific headings with 1 minute legs. The hold area is 8 miles NW of the field, with the starting point, an intersection called Scoop.

I have decided since I have over 4 hours of fuel remaining to try a landing and “Miss” if I have to. Miss is short for a missed approach. A very important thing to clearly have defined in your head before starting any approach, but especially in conditions like this. The controller assigns me a heading of 150 degrees which will place me east of the inbound localizer course for RWY 35. Also I am told to descend to 2000, which is 1766 feet above the runway landing zone (as noted on the chart). Now it’s time to set up my nav receivers for the localizer. After dialing in the proper frequency, I listen for the identifying Morse code signal to make sure I am on the right station. This ILS signal is what can bring us down through the murk. Without it, get the parachute out, or pull the handle if you are in a Cirrus and low on fuel.

About 5 miles beyond the airport I am waiting for the turn in to intercept the localizer. The controller tells me to turn right to a heading of 240 degrees, which will bring me to intercept the localizer. Proceeding further, the course deviation indicator (CDI) is starting to move and I am “cleared for the approach”. Now I have to go through a bit of a check list: Fuel on mains, approach flaps, advance the prop controls to high RPM, mixture controls to full rich, ready for gear down at the Outer Marker (OM). The engines now are set to go to full power in case of the go around. The OM named (DERRY), is normally the start of the approach, and is announced on the instrument panel by a blinking blue light and a specific audio tone heard in my headset. I now descend to 1800 ft and prepare to intercept the all important glide slope, which if followed in concert with the CDI needle (both centered) will place me 200 ft above the runway threshold and on centerline.

Well the OM starts blinking and the audio beeps at me so I watch for the glideslope needle to move. As it comes down, my hand is on the throttles and I prepare to drop the gear, go to full flaps and reduce power to the engines. So now it’s watch, wait and hope for a break in the fog. If at the minimum altitude of 434 ft, I “don’t have the runway” in sight, I will have to execute the missed approach. The point of minimum descent is defined not only by altitude, but also by a DME (distance measuring equipment). As the DME is at the point of minimum descent, if one stays on the glideslope, it is the point of leveling off at 434 ft. There also is a chart of time to the decision point, depending on the ground speed. This is shown on the DME instrument. In our case, at about 100 knots, the time to decide to land or not is 2:31 (Min/Sec). Although I couldn't land on my first approach, after one Miss I manged to get safely on the runway. The details of how I finally landed after a 727 landed ahead of me, were discussed in my former blog.
I hope this has given you insight as to the main things a pilot does and thinks about when landing in bad weather
The main things to glean from this article: plan ahead, follow approved procedures, be current in your flight experience, fly a well maintained airplane and practice, practice and more practice.

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