Reading Approach Charts - Distance Symbols That Trip Up IFR Pilots
Approach plates contain multiple types of distance information that look similar but mean different things - a distinction many instrument pilots can't reliably make.
A recent Boldmethod video quiz (catalogued as IFR Short 0247) is exposing a widespread gap in instrument pilot chart literacy: when asked to identify the distance to the runway threshold from a specific fix on an approach plate, many certificated instrument pilots cannot answer correctly on the first try. The problem isn’t carelessness - approach plates contain several types of distance information that look nearly identical but mean fundamentally different things depending on where they appear.
Why Approach Chart Distance Literacy Matters More Than Ever
The IFR system has grown increasingly layered. Legacy approach types sit alongside modern Required Navigation Performance (RNP) procedures, and approach plates have accumulated symbology over decades. Glass panel avionics abstract much of this into a magenta line - efficient, but a trap for pilots who haven’t internalized the underlying approach structure. Trusting a system without understanding it is a practical risk management problem, not a philosophical one.
The Two Categories of Distance on an Approach Plate
Segment distance refers to the mileage between two defined fixes along the final approach or intermediate segment. These numbers describe how far it is from one point to the next - not how far the aircraft is from the runway threshold.
Profile view distance is what the Boldmethod quiz tests. The profile view typically shows a cumulative distance from the final approach fix (FAF) to the missed approach point (MAP). On precision approaches, the MAP is at or near the runway threshold. On non-precision approaches, the MAP may be at a specific fix, a distance from a fix, or defined by elapsed time from the FAF - varying considerably by approach design.
The Stepdown Fix Problem
Some approaches include a stepdown fix between the FAF and the MAP. That fix carries a minimum crossing altitude, and the segment beyond it has a different minimum descent altitude. The profile view will then show two distance values in sequence - and the total distance to the runway is the sum of both segments.
Reading only the second number - the distance from the stepdown fix to the runway - means underestimating remaining distance. In actual IMC with low visibility, that error has real consequences.
Precision vs. Non-Precision: Different Geometry, Different Risks
On a precision approach, the decision altitude is reached before or at the threshold. The pilot descends to a height above the threshold and still has runway remaining when making the land-or-go-missed call. Understanding that geometry is essential for managing the instrument-to-visual transition at low altitude.
On a non-precision approach, the relationship between the MAP and the runway threshold varies considerably. Some missed approach points coincide with the threshold. Others are a mile or more away. When minimums include timing from the FAF to the MAP, that timing is groundspeed-dependent - at higher groundspeeds, the MAP arrives faster, and a pilot may reach minimums with runway still ahead but no approach-defined guidance to continue descending.
DME vs. GPS Distance: Why the Reference Datum Matters
On a localizer approach with DME, profile view distances are measured from the DME antenna - typically near the stop end of the runway but offset from the threshold by a published amount. The published threshold crossing height accounts for this offset, but the raw DME reading is not a direct distance to the threshold.
On a GPS approach, profile view distances are waypoint-to-waypoint. The final waypoint before the runway is typically the landing threshold point (LTP) - a geometric point at runway elevation at the threshold. Distance-to-waypoint on a GPS unit actively navigating to the LTP is a reliable distance-to-runway cross-check.
Modern integrated avionics handle much of this automatically when loading a database approach. But understanding the underlying geometry tells a pilot when to trust a displayed number and when to verify it independently.
The Visual Descent Point: One Symbol, a Lot of Geometry
A Visual Descent Point (VDP) appears on eligible non-precision approach profiles as a small V symbol, with a published distance from the missed approach point. The VDP marks where a normal three-degree descent from the minimum descent altitude (MDA) to the runway touchdown zone can begin - provided the runway environment is in sight.
The VDP is not the MAP and is not a mandatory crossing fix. It does not extend approach authority past the missed approach point. Reaching the MAP without required visual references still requires an immediate missed approach, regardless of VDP position.
Breaking out at the VDP with the runway in sight and the aircraft configured means a normal landing is achievable. Breaking out before the VDP may require a steeper-than-normal descent. Breaking out after the VDP means the aircraft is already inside the standard descent path, and landing demands careful energy management.
Three Questions Every Instrument Approach Pre-Brief Should Answer
Before starting down on any instrument approach, answer these three questions:
1. Where is the final approach fix, and what defines it? Is it a fix overfly, a DME crossing, or a GPS waypoint?
2. Where is the missed approach point relative to the runway threshold? Is it at, before, or beyond the threshold? What is the distance from the FAF to the MAP?
3. Are there any stepdown fixes between the FAF and MAP? If so, what is the distance to each, and what is the cumulative distance to the runway?
Answering these three questions establishes the approach geometry before the descent begins. That mental model is the foundation for every distance cross-check during the approach.
The Gap Between Chart Recognition and Chart Understanding
The Airmen Certification Standards (ACS) require demonstrated competency in flying instrument procedures. They do not always require the depth of understanding needed to explain, from first principles, why a missed approach point is positioned where it is, or what a VDP distance is actually measuring. That deeper understanding is what separates a pilot who is safe in instrument conditions from one who is genuinely proficient.
The FAA publishes approach chart symbology standards through the Aeronautical Information Manual (AIM), the Terminal Procedures Publication, and a dedicated chart symbology document that defines every marking on every instrument approach chart. Pilots who have never read that document have a gap worth closing.
A Practical Exercise for Building Chart Literacy
Pull out a Jeppesen or FAA terminal area chart for an approach flown regularly. Work through the profile view from the initial approach fix to the threshold: label every defined distance and calculate the cumulative distance from each fix to the threshold. Then fly the approach in a simulator and call out distance-to-go at each fix.
Repeat the exercise with four approach types - a precision ILS, a GPS LPV, an LOC DME, and a VOR or NDB if one is accessible at a nearby airport. Chart literacy improves materially with that kind of deliberate, type-varied practice. The chart is not the obstacle. The chart is the tool. Flying it by feel when it should be flown by understanding is where the risk lives.
Key Takeaways
- Approach plates contain multiple types of distance - segment distances between fixes are not the same as cumulative distance to the runway threshold, and conflating them leads to navigation errors.
- Stepdown fixes divide the profile view into multiple segments - the total distance to the runway is the sum of all segments, not just the final one.
- Non-precision MAP location varies by approach design - it may be at, before, or beyond the threshold, with significant implications for how timing-based minimums are applied.
- DME and GPS distance references use different datum points - DME measures from an antenna that may be offset from the threshold; GPS LTP waypoints are geometric points at the physical threshold.
- The VDP marks where a stabilized normal descent to the runway can begin - it does not extend approach authority past the missed approach point.
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