When an instrument approach procedure (IAP) is published in the United States, it is not drawn by hand or guesswork. The Federal Aviation Administration applies a precise engineering standard called the United States Standard for Terminal Instrument Procedures, universally abbreviated TERPS (FAA Order 8260.3, which references and is grounded in the instrument procedures design criteria). TERPS defines exactly how much vertical and horizontal clearance must exist between a descending aircraft and every obstacle in the vicinity of the runway. The result of that calculation — the lowest altitude at which an aircraft can safely fly a given segment while remaining clear of all obstacles — ultimately becomes the minimums you read on the approach chart: the Minimum Descent Altitude (MDA) on a non-precision approach or the Decision Altitude (DA) on a precision or LPV approach.
For instrument students, TERPS is often treated as an abstract behind-the-scenes process. In reality, a basic understanding of its geometry demystifies why two airports that appear similar can have dramatically different approach minimums, and it gives you critical insight into how to use approach chart data safely. This article explains the key obstacle clearance surfaces, slope ratios, and practical implications that every instrument pilot and instrument-rating applicant should understand.
The Core Concept: Obstacle Clearance Surfaces
TERPS works by projecting imaginary three-dimensional surfaces outward from the runway environment. Any obstacle — a tower, tree, building, or terrain feature — that penetrates one of these surfaces forces the procedure designer to raise the applicable minimum altitude until the required clearance is restored. These surfaces are called Obstacle Clearance Surfaces (OCS).
Think of the OCS as an invisible ramp or funnel that slopes upward as you move away from the runway threshold. A perfectly flat airport with no obstacles nearby will have an OCS that nothing pierces, allowing very low minimums. An airport surrounded by rising terrain or tall structures will have penetrations that push minimums upward. Every foot of unnecessary terrain or obstacle height can cost you feet of approach altitude — and sometimes the difference between a usable approach and an approach that is simply not published at all.
Each segment of an instrument approach procedure has its own associated OCS with its own slope and width requirements. The primary segments are: the initial approach segment, the intermediate approach segment, final approach segment, and the missed approach segment. The final approach segment and missed approach segment are the most consequential for the minimums printed on the chart.
Final Approach Segment OCS and Slope Ratios
The slope of the OCS in the final approach segment is the critical number that determines how aggressively a procedure can be designed. TERPS expresses slopes as ratios, and these ratios differ depending on the type of approach.
For non-precision approaches (VOR, NDB, LNAV), the final approach OCS uses a slope ratio of 34:1 in the primary area. This means that for every 34 feet traveled horizontally along the approach path, the OCS rises one foot vertically. Stated differently, the surface is relatively shallow — a 34:1 slope is about 1.7 degrees above horizontal. The required obstacle clearance above the OCS in the primary area of the final approach segment for non-precision approaches is 250 feet. This 250-foot buffer ensures that even at MDA, you have meaningful separation from anything that just barely touches that imaginary surface.
For precision approaches and approaches with vertical guidance (ILS, GLS, LPV, LNAV/VNAV), TERPS uses what is called the Obstacle Clearance Surface slope tied to the glide path angle, typically 3 degrees for a standard ILS. The geometry here is different: instead of a flat surface with a fixed buffer, the OCS rises along the glidepath itself, and the required clearance from obstacles is evaluated relative to the glidepath angle and the decision altitude. The standard ILS OCS slope for Category I operations results in a DA typically no lower than 200 feet above the touchdown zone elevation (TDZE) when the equipment and obstacle environment permit it. When obstacles penetrate the approach surface beneath a normal 3-degree path, the DA must be raised.
The visual descent point (VDP) concept on non-precision approaches is also a TERPS-derived value. It marks the point on final approach from which a normal, stabilized visual descent to the runway can be made — essentially identifying where the 3-degree visual slope intersects with the MDA. If no VDP is published, it usually means an obstacle prevents a normal descent from the MDA, a significant safety cue.
Primary and Secondary Areas
The OCS is not uniform across its full width. TERPS divides the area on either side of the final approach course into a primary area and a secondary area. The primary area extends laterally from the course centerline and receives the full obstacle clearance buffer — 250 feet for non-precision approaches. The secondary area begins where the primary area ends and extends further outward; within the secondary area, the required obstacle clearance decreases linearly from 250 feet at the primary/secondary boundary to zero feet at the outer edge of the secondary area.
This tapering of the OCS in the secondary area is the engineering recognition that as you get farther from the centerline, the probability of being there decreases — but it is also a warning to pilots. Flying significantly off-course on an approach, even while technically still within the protected airspace, may erode your true obstacle clearance margin far more rapidly than most pilots appreciate.
Missed Approach Segment Obstacle Clearance
Once you cross the missed approach point (MAP) or reach the DA and initiate a climb, you enter the missed approach segment. TERPS requires a standard climb gradient of at least 200 feet per nautical mile in the missed approach primary area — a standard of roughly 400 feet per minute at typical missed approach speeds. This standard climb gradient is built on a 40:1 obstacle clearance surface (152 feet per nautical mile) with a required obstacle clearance margin of 48 feet per nautical mile above that surface (200 minus 152). The 48 feet per nautical mile is the clearance provided above the OCS, not a separate climb requirement — the gradient you must actually fly is 200 feet per nautical mile. When obstacles in the missed approach area are particularly challenging, the procedure will publish a non-standard climb gradient, such as 300 or 400 feet per nautical mile, printed in the notes section of the approach chart.
This is critical operational knowledge: if your aircraft cannot achieve the published non-standard climb gradient — due to weight, density altitude, or engine performance — you cannot safely fly that approach, regardless of whether you can reach the MDA or DA. The missed approach is not a secondary concern; TERPS treats it as a primary design constraint.
Why TERPS Minimums Vary Between Airports
Students often wonder why two airports that appear nearly identical have very different approach minimums. The answer lies in local obstacle environments and the geometry of the OCS. A tower located 1 nautical mile from the runway threshold and 300 feet tall might sit squarely within the primary area of the final approach OCS and require the MDA to be raised by several hundred feet. The same tower 3 miles out and slightly offset might fall in the secondary area or just outside the protected area entirely and have no effect at all. Runway length, the location of the MAP relative to the threshold, and the glide path angle all interact with the OCS geometry to produce the final minimums.
The Instrument Procedures Handbook (FAA-H-8083-16) emphasizes that pilots should not assume minimums are interchangeable between airports or even between different approaches to the same runway. Always brief the specific minimums for the specific procedure you are flying.
Key Numbers and Rules
- Non-precision final approach OCS slope: 34:1 (approximately 1.7 degrees)
- Required obstacle clearance, primary area, non-precision: 250 feet above the OCS
- Required obstacle clearance, secondary area: Tapers from 250 feet to 0 feet at the outer edge
- Standard ILS glide path: 3 degrees; Category I DA typically not lower than 200 feet HAT when the environment permits
- Missed approach minimum climb gradient (standard): 200 feet per nautical mile, built on a 40:1 OCS (152 ft/NM) with a 48 ft/NM clearance margin above it; non-standard gradients are published in approach notes when obstacles require more
- Visual Descent Point (VDP): Derived from a 3-degree descent angle from MDA; absence of a VDP implies obstacle or procedural constraints on normal descent
- Approach categories (A through E): Based on 1.3 times Vso; higher-speed aircraft require wider OCS protection areas, which can result in higher minimums
Common Test Traps
- Confusing MDA with DA: MDA applies to non-precision approaches; you level off and may not descend below it unless you have the required visual references. DA is a decision point on precision or APV approaches — you act at DA, not below it. TERPS calculates both using OCS geometry, but they are operationally distinct.
- Ignoring non-standard climb gradients: Many students focus entirely on the descent minimums and overlook a published missed approach climb gradient of 300 or 400 feet per nautical mile. If performance cannot support that gradient, the approach is not authorized for your aircraft on that flight.
- Assuming secondary-area obstacle clearance equals primary-area clearance: It does not. The 250-foot buffer exists only in the primary area. Off-centerline deviations on final progressively erode your actual margin.
- Misreading the VDP: The absence of a VDP is a warning, not a trivia item. It tells you that a normal 3-degree descent from MDA is obstructed or otherwise not protected, so a dive-and-drive technique below MDA is especially hazardous at that airport.
- Assuming all 3-degree ILS approaches have the same DA: The 3-degree slope is standard, but local obstacles can force a higher DA even on a precision approach. Always check the specific procedure's minimums, not a generic assumption.