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How to taxi in IMC: before you test your nerves of steel departing into OVC010, you may need to review your SMGCS just to find the damn runway.

No, SMGCS isn't some cute little fuzzy critter in cartoons. It's an acronym for Surface Movement Guidance and Control System and is actually pronounced, "SMGGS." Think of SMGCS as some procedures and a collection of lights, marks, and signs that will exist at airports supporting runway operations in less than 1200 RVR.

Even if you don't normally putt around with visibility that low, you should know what all theses items mean before you encounter them on a cruddy day.

Who Cares?

Good question. Pilots operating under Part 91 of the FARs can take off in any weather, and you may choose to do so at your home 'drome that perhaps only has a Cat I ILS and isn't SMGCS equipped. However, pilots operating under Parts 135 and 121 have different rules. Their visibility requirement is a mile for takeoff, but their operations specifications may permit them to take off in lower conditions. Plus, there are Category II and III ILS approaches that may be present and permit landing in visibility under 1200 RVR.

SMGCS-equipped airports are typically served by at least one Part 121 carrier but that includes many primarily GA airports. When you share the tarmac at these airports, you should know what the new and different lights and marks mean.

One of the FAA's hottest issues over the last few years has been run way incursion incidents and accidents. Although our total accident rate is still low, improper ground operations remain at the top of the list of potential plane-on-plane collisions and are entirely preventable. The horrific collision of two 747s on Tenerife Island in 1977 remains the poster child for these preventable accidents.

There are SMGCS terms that many of us may have heard and even used without fully knowing what they mean. There are also things we may never have encountered, so let's take a look at some of the biggies.

A clearance bar is a group of three steady yellow lights embedded in the pavement. Along with appropriate signs and markings, these lights designate a taxiway hold-short point. These are convenient points typically located at intersections where ATC may instruct aircraft to hold during ground operations.

Geographic position markings are circular pink spots on the pavement with black and white concentric borders that contain a black number or a letter (or both) and are used to designate a specific place on the airport surface. For example, you may be asked to "taxi to Spot Four and contact Ground on 121.85." These spots may be used as holding points and, if so, are combined with clearance bars. However, if you are given instructions to taxi to a spot and contact another frequency, you should not proceed beyond that spot until given further taxi instructions. It's common practice to call the next frequency for further instructions as you are approaching the spot so you don't have to stop.

Runway guard lights can be both elevated (off to the side) and embedded in the pavement. The elevated lights are a pair of flashing yellow lights just off the pavement on either side of the runway hold-short line. The in-pavement versions are embedded flashing yellow lights all along the hold-short line. They both flash all the time and simply serve to draw attention to the hold-short line, hopefully, getting you to think one last time before blundering across.

Runway Visual Range (RVR) is an old friend. RVR is measured by sensors just beside the runway. While the tower (or ASOS/AWOS) reports prevailing visibility for the airport, SMGCS-equipped airports measure the actual visibility at one or more places along one or more runways.

Commonly there are up to three RVR sensors located to mea sure the touchdown zone, runway mid-point, and runway-end visibility from a few hundred feet out to 6000. Many installations have fewer than three. The number and placement of the sensors depends on the visibility minimum in which the airport wants to be able to operate and the likelihood of runway use. Logically, the lowest visibility operations require the most sensors. For instance, an air carrier maybe able to take off at 1600 RVR if there's only a touchdown zone sensor, but may be able to go down to 600 RVR or less if there are all three. On the very longest runways, you may see a fourth sensor called "The Far End."

Stop bar lights are similar to Runway Guard lights, with elevated lights off to the side and a row of lights embedded in the pavement crosswise to the centerline. The biggest difference is that stop bar lights are red and typically are controllable by the tower. Additionally, they may be located at runway hold short lines and/or at ILS critical-area hold lines. They're accompanied by a string of green taxiway centerline lead-on lights at runway crossing locations. Think of them as a version of the standard traffic light on streets. When the red stop bar lights are lit, stop. When ATC gives you a clearance to enter the runway, the stop bar lights will go out and the green taxiway centerline lights will light up. Go.

Surface signs are typically runway and taxiway identifying and direction signs painted on the pavement and these signs are essentially identical in appearance to the signs you commonly see beside the pavement. You might think that you should at least have enough visibility to see the signs on the side of the taxiway, but that's not necessarily true. In very low visibility you are typically concentrating your attention directly on the centerline in front of you. Although you may be able to see a sign beside the taxiway, you may not look for it and, even if it's lighted in daylight, it may not be sufficiently obvious to draw your attention.

Taxi route, when applied in a SMGCS environment, isn't just any route you follow from the ramp to the runway (or back), it's a specific, designated route marked and equipped for low visibility operations. We're all familiar with the standard yellow taxiway centerline, but the SMGCS version is enhanced with black borders to make it more visible.

According to the SMGCS bible (AC120-57A, which is about as absorbing as aircraft certification standards and the rest of the FARs.), a surface movement surveillance system is (SMSS), "A system which provides positive identification and accurate positional information on all aircraft and vehicles." Many SMGCS-controlled airports do have some system--typically a version of Airport Surface Detection Equipment (ASDE)--that allows the controllers to essentially see aircraft and other vehicles through the fog.

Putting it Together

Each SMGCS implementation is different and not all the SMGCS lights, markings and signs will be present at all airports, or even at all places on a SMGCS airport. Exactly which components are installed depends on the visibility limits desired and other operational requirements of the airport.

Any airport officially supporting operations below 1200 RVR must be SMGCS compliant, but those air ports wishing to support operations below 600 RVR are required to have a much more extensive complement of SMGCS lights, markings and signs.

Let's follow a taxi route in a typical, low-visibility situation at a popular general aviation airport. Take a look at the Low Visibility Taxi Route chart for Eugene, Ore., (KEUG). Notice at the top that it's effective when the RVR for Runway 16R is between 1200 and 600. The next thing you notice is that taxiway A, the parallel taxiway for 16R, is to be used in one direction only, from south to north.

You may ask yourself how you get to Runway 34L. The answer is that you don't. Only Runway 16R is used in low visibility conditions.

Say you're leaving your hangar located northeast of the air carrier terminal and you want to depart. As we work through our virtual route you may wish to refer to the legend. You see the dotted lines on taxiway K and you know that the hangars and associated ramp are in a non-movement area, that is to say that ATC doesn't care (much) what you do there. You call Ground before entering Taxiway K. They tell you to taxi to Runway 16R via K, C and A; report passing Spot 4. So you care fully pick your way along taxiway K to the north.

As you cross taxiway M, you notice the darkening of the taxiway shown on the chart indicating it's now part of the low visibility taxi route and the yellow taxiway center line is now bordered by black stripes. You turn left onto C and in a few minutes of careful, slow taxiing, you see a pink circle with a 4 in it. You call Ground and report passing Spot 4 as you turn right onto taxiway A.

As you taxi up A, this airport may have pavement markings showing the turn to A3 and A1. (These aren't necessarily a required component and we can't tell from the chart if they're present.) Regardless of whether there are pavement markings at these intersections, there will be clear signs at the side.

As you make the left turn at the end of A onto A1, you notice the runway guard (hold position) lights alerting you to the hold-short line. Since Eugene doesn't support operations below 600 RVR, the stop bar lights aren't installed.

Once you're cleared onto the runway, you'll notice the embedded runway centerline lights. In addition to the centerline marks and runway edge lighting that you expect, these lights will really help you hold the centerline as you accelerate for take off and can't see more than a couple centerline stripes.

There are a lot of other chart markings and symbols that you may encounter, especially at bigger airports. But this information is a good starting point. If the visibility is low and the airport has a SMGCS plate, take a few extra moments to study up before you trundle off into the fog.

Frank Bowlin taxies around in low visibility as a Canadair RJ captain.
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Title Annotation:IFR TECHNICAL; Surface Movement Guidance and Control System
Author:Bowlin, Frank
Publication:IFR
Geographic Code:1USA
Date:Oct 1, 2007
Words:1671
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