Beyond the Flat Earth

Turning IFR paper into a 3-D world

October 1, 2003

Part of the wonder of flight is exploring the world and its mountains, forests, deserts, and cities from the air. But flying while enveloped in a damp towel of gray prevents you from seeing that world — then the wonder becomes finding your way to the destination merely from a combination of paper charts and six dials on a flat panel.

The instrument approach procedures (IAPs) contain the facts, a set of raw numbers that you learn during your instrument training to turn into action items. Nav frequency and course: Tune in this radial and fly in this direction. Glideslope intercept altitude: Descend to this altitude before this point — but not below! — so that you capture the slide down with the needle centered. Missed approach instructions: If you can't see the screws on the cowl in front of you at decision height, climb to this altitude and fly to this fix to regroup.

But flying an approach by rote is only a small part of being a successful instrument pilot. To be safe, you need to be situationally aware.

There are clues on the chart that aid your awareness. If you go beyond the approach briefing (nav frequency and course, intercept altitude, minimums, and missed), you can ferret out the details that make the chart worth far more than the tissue paper on which it's printed. And if you go an extra step and combine what the sectional charts and plan and profile views on the terminal charts add to the story, you can visualize that world below in your mind. This picture easily buys a thousand words.

Put it together

The approach procedure for the VOR or GPS-A into Bishop, California, contains a lot of guidance for maneuvering the airplane to a safe altitude and position from which to begin the descent into the Owens Valley. This IAP is an excellent example of an IAP enhanced with contour shading to represent the surrounding terrain. If obstructions in the area covered by the plan view of the chart exceed 4,000 feet above the airport elevation, or if an obstacle more than 2,000 feet higher than the airport lies within 6 nm, the approach chart gets contours.

If you lay the San Francisco sectional next to the approach chart, you see this shading take on greater relief. You can note the concessions charted on the sectional for the approach — primarily a corridor of Class E airspace to 700 feet agl along Victor 361 to NICOL Intersection to protect an extension of the outbound course.

The sectional gives a wider view of the valley, and makes it clear why the instruction exists to hold over the VOR until the descent is made to 12,000 feet — the steepness of the terrain would not otherwise allow for a safe transition from the feeder route altitude to the initial approach altitude of 8,900 feet. The sharp drop from 14,000-foot mountains to a valley floor at just over 4,000 feet predicts turbulence, at times severe during periods of high westerly winds.

Other IAP chart contingencies also make sense when looking at the sectional, such as the direction of the procedure turn, and the absolute requirement to stay within 10 nm of the Bishop VOR while executing the procedure turn.

While terrain is depicted on the IAP for the VOR/DME or GPS-A approach into Erie Municipal Airport in Erie, Colorado, it doesn't take center stage like it does on the Bishop chart. Instead, the plan view shows the approach in the "flatlands" east of the foothills in eastern Colorado. DME (or the GPS equivalent) is required for this approach, and for a very good reason. A close look near the inbound approach course reveals a tower located just to the left of the course line, and just prior to the 9.5 DME fix off of the Jeffco VOR. Dropping to the profile view, you note the step-down fix has a minimum altitude of 6,560 feet.

But wait — the plan view gives the tower elevation at 6,165 feet msl. A check of the Cheyenne Sectional concurs — the tower is lighted, and juts up nearly 1,000 feet agl. Those who have flown the approach in visual conditions attest that even with the needle centered, the top of the tower comes hauntingly close to the course. Here's where a review of the charts may cause you to add a little cushion to a minimum altitude, or to brace yourself for the sight of that strobe piercing at you through the muck.

Speaking of muck, pilots get instrument-rated so that lingering muck doesn't faze them. After studying an IAP, pursuing every last detail so that you can get safely to minimums, you think only frontal weather or Hurricane Next could stymie your plans. Again, the sectionals illuminate the futility of clinging to this belief.

Take the IAP for the ILS 24 approach into Elmira/Corning Regional Airport in Elmira, New York. If you have an ADF or DME (or an appropriate GPS substitute), this ILS promises to take you safely below the 1,000-foot ridges that mark the north and south sides of the valley in which the airport sits — to a decision height of 250 feet, and a corresponding 2,400-foot RVR (which equates to one-half statute mile visibility). No problem on a clear, calm morning, right?

You would be wrong. Looking at the sectional reveals the fatal flaw of this assumption. The Elmira/Corning airport sits on fine bottom land not far from the Chemung River. Those tree-lined valley walls trap the fog consistently generated by the Chemung so efficiently that while the rest of the Finger Lakes region enjoys a sunny fall morning, Northwest Airlink and Continental Express are scrubbing flights because of quarter-mile — or worse — visibilities and clag below 100 feet agl. Muck indeed. Good news is, once it clears at 9 or 10 in the morning, you won't need the plate at all.

Finally, consider the LOC/DME back-course approach to Runway 8 at Eastern West Virginia Regional/Shepherd Field in Martinsburg, West Virginia. The approach is a favorite with local designated examiners because of its subtle complexity. At first glance, it appears to be a simple localizer back-course approach, with the attendant gotchas of flying a localizer positioned at the near end of the runway versus the far end: heightened sensitivity of the localizer course, and the need to reset the horizontal situation indicator (HSI) so that you fly the tail of the needle, or cope with reverse sensing on a standard VOR display.

However, the plan and profile sections tell tales of several step-down fixes, randomly sprinkled in the most inconvenient spots during the descent. If you must begin the approach from the east and fly a full course reversal at the initial altitude of 3,900 feet, expect to descend quickly to 2,900 feet upon turning inbound — only to nearly level your descent between 16 DME and GERRA Intersection, the final approach fix. Then it's on to losing 1,000 feet in less than three miles between GERRA and BIITO, the back-course marker beacon, before commencing the final descent to the 1,100-foot minimum descent altitude.

There are a couple of ways around the steps. If you approach from the west, you can descend to 3,300 feet inbound from CAPON Intersection. Or perhaps ATC will vector you to ROSSI, at 18.3 DME, where you can proceed inbound at 2,700 feet. While it's clear from the chart that the DME is off the Martinsburg VOR, located about five miles east-southeast of the field, the need for the step-downs is not so obvious. Until you review the sectional.

Is it a tower close to the final approach course? Well, the 1,482-foot tower adjacent to GERRA helps to explain that step-down fix, but not the initial approach altitude of 3,900 feet. That one is based on terrain. Interestingly, the 1,482-foot tower isn't charted on the sectional, and neither is the shorter tower to the northeast. Both carry the symbol showing that the accuracy of the obstacle elevation is doubtful.

However, the highest obstruction depicted on the chart is a 2,844-foot point about five miles south of CAPON. The maximum elevation figure (MEF) on the Washington Sectional for that sector is 3,200 feet, also based on this point.

Once established inbound from CAPON, you can descend close to the MEF, to 3,300 feet. But on the procedure turn, when your position is less defined, and your course takes you straight for the highest ridge in the sector, the extra 600 feet helps keep you well above the West Virginia sandstone.

Find your spot

Seek out an approach procedure for an airport to which you've never been, in some area of the country that interests you. You can download approaches for any U.S. airport from AOPA's Airport Directory online ( www.aopa.org/members/airports/). Your job is to put together a visual plan of the approach using the plan and profile views in conjunction with the sectional chart for the area.

  • Take the sectional and envision it as a 3-D picture in your mind, using terrain features, populated areas, and obstructions noted on the chart.
  • Use the plan and profile views of the approach and create a similar 3-D image of the procedure.
  • Superimpose the approach image over your image of the area.
  • Most important, fly the approach mentally, visualizing where you are over the terrain during each segment of the procedure.
  • As you fly the approach in your mind, point to navaids and intersections you pass, if appropriate. Similarly, call out obstructions as you cross over them on the descent. Can you identify in which direction the missed approach procedure lies? What terrain must you climb over to reach it safely, assuming you execute the missed at the missed approach waypoint or fix?
  • Repeat the procedure for other possible approaches at your proposed destination.

And keep that picture stored in your memory for the time when that gray towel covers the view to the world below.


E-mail the author at julie.boatman@aopa.org.