Every time you study a Departure Procedure (DP) on a chart and notice a note like "Climb via runway heading to 2,400 before turning" or "400 ft/NM climb gradient required," you are reading the direct output of a process called TERPS — Terminal Instrument Procedures. TERPS is the FAA's engineering standard that defines how instrument procedures are designed so that aircraft following them will remain safely separated from terrain and obstacles. For instrument-rated pilots, understanding TERPS is not just academic: it determines whether you can legally depart under IFR from a given runway, and it can be the difference between a manageable emergency and a controlled flight into terrain.
This article focuses specifically on how TERPS governs departure procedure obstacle clearance — covering what the standard requires, how climb gradients are derived, why certain runways get Obstacle Departure Procedures (ODPs) or Standard Instrument Departures (SIDs), and what you must do when the standard climb gradient cannot be met.
What TERPS Is and Where It Comes From
TERPS stands for the United States Standard for Terminal Instrument Procedures, codified in FAA Order 8260.3 (and its successor editions). It is the engineering rulebook that FAA procedure designers use to create every instrument approach, departure, and arrival procedure in the National Airspace System. TERPS defines obstacle identification surfaces (OIS) — imaginary sloped planes around an airport and along a flight path — and requires that procedures be constructed so aircraft clear all obstacles that penetrate those surfaces by specified margins. For departure procedures specifically, the foundational surface is a slope beginning at the departure end of the runway (DER).
It is important to note that TERPS is an engineering standard, not a flight crew regulation per se. However, the procedures it produces are reflected in 14 CFR Part 97, which makes the published procedures regulatory. When you file and fly a DP, you are implicitly agreeing to fly within the TERPS-derived parameters that make the procedure safe.
The Obstacle Identification Surface: How Departure Clearance Is Calculated
The core of TERPS departure design is the Obstacle Identification Surface (OIS), sometimes called the departure obstacle clearance surface. Starting at the DER, this surface climbs at a slope of 152 feet per nautical mile — a 40:1 slope, equivalent to approximately a 2.5% gradient. This 152 ft/NM surface is a distinct value from the 200 ft/NM standard climb gradient that aircraft are required to achieve; the two numbers serve different purposes in the TERPS design. If every obstacle in the departure corridor stays below the rising 152 ft/NM obstacle identification surface, the procedure can be published with the standard required climb gradient of 200 feet per nautical mile (ft/NM). The 200 ft/NM standard is the baseline that every aircraft is assumed capable of achieving, and the difference between the 200 ft/NM required climb gradient and the 152 ft/NM obstacle identification surface provides a 48-foot-per-nautical-mile obstacle clearance margin that builds as the aircraft climbs away from the DER.
If an obstacle penetrates the obstacle identification surface, the procedure designer has two options: raise the required climb gradient until the obstacle is cleared, or define a specific flight track (turn, heading, fix) that avoids the obstacle laterally. This is why some DPs require steep climb gradients like 340 ft/NM or specify precise headings — a hill, tower, or building pokes through the standard surface and forces the aircraft onto a steeper or different path.
Obstacle Departure Procedures (ODPs) vs. SIDs
TERPS-derived departure procedures come in two forms. An Obstacle Departure Procedure (ODP) is the minimum required procedure to provide obstacle clearance. ODPs may be text-only (published in the front of the Terminal Procedures Publication or the ATC facility's A/FD remarks) or charted. Flying an ODP is generally not mandatory unless ATC assigns it or the pilot elects it, but 14 CFR 91.175 and the Instrument Flying Handbook make clear that pilots operating IFR are responsible for their own obstacle clearance until assigned a route by ATC. If no DP is specified by ATC, pilots should fly the ODP or coordinate with ATC for an alternate clearance.
A Standard Instrument Departure (SID) is an ATC-assigned procedure that also satisfies TERPS obstacle clearance requirements but is primarily designed for traffic flow efficiency and communication workload reduction. SIDs are charted, and pilots should not accept a SID clearance unless they have the chart and can comply with all gradient and equipment requirements.
Climb Gradients: What They Mean in the Cockpit
When a DP publishes a climb gradient above the standard 200 ft/NM, you must verify your aircraft can actually achieve it. The climb gradient (expressed in ft/NM) relates to your aircraft's rate of climb and groundspeed through this formula:
Required Rate of Climb (ft/min) = Climb Gradient (ft/NM) × Groundspeed (knots) ÷ 60
For example, if a departure requires 340 ft/NM and you are departing at 90 knots groundspeed: 340 × 90 ÷ 60 = 510 ft/min. You must confirm your aircraft's climb performance under the actual conditions — density altitude, weight, temperature — exceeds that number. The Airplane Flying Handbook emphasizes using the most conservative performance data (hot day, max weight, engine aging) rather than book-perfect numbers.
An important cockpit reality: most light aircraft performance charts give climb data in ft/min at various airspeeds. Pilots must do this conversion themselves. There is no automation that alerts you when you cannot meet a published gradient — the obligation is entirely on you to check before takeoff.
Diverse Departure Concept
TERPS also establishes the diverse departure concept: at many airports, there is no published departure procedure, meaning the procedure designer has determined that a straight-ahead climb (or a turn after reaching 400 feet AGL) on any heading clears all obstacles within the standard 200 ft/NM surface. Pilots can depart in any direction without a specific DP and still maintain obstacle clearance — provided the standard 200 ft/NM gradient is met and cloud ceilings allow visual avoidance of obstacles during any initial turn. The Instrument Flying Handbook notes that if an airport has no DP published, it does not automatically mean the departure environment is obstacle-free; it means the diverse departure criteria are satisfied for the standard gradient.
Why It Matters: The Safety Stakes
CFIT — controlled flight into terrain — remains one of the deadliest accident categories in aviation. Departure is one of the most vulnerable phases because pilots are heads-down managing power, airspeed, and communications while in proximity to the ground. TERPS exists precisely to remove terrain-avoidance from the cognitive workload during this phase, replacing it with a simple, pre-engineered contract: follow the procedure and meet the gradient, and you will clear everything. Breaking that contract — by missing a turn, flying the wrong heading, or failing to climb steeply enough — collapses the safety margin that TERPS designed in.
The Risk Management Handbook underscores that IFR departure planning is a critical risk management task. Preflight includes verifying applicable DPs, checking all gradient requirements against actual performance, and having a contingency plan (abort point, emergency return heading) ready before the wheels leave the runway.
Key Numbers and Rules
- Standard climb gradient: 200 ft/NM from the DER to the en route structure (typically the MEA).
- Standard obstacle clearance surface slope: 152 ft/NM (40:1) — the surface the standard 200 ft/NM gradient is measured against.
- Minimum obstacle clearance (MOC) margin: the 200 ft/NM required climb gradient exceeds the 152 ft/NM obstacle identification surface by 48 feet per nautical mile; once above the departure segment, 1,000 feet (or 2,000 feet in mountainous terrain) is required for en route obstacle clearance.
- Turns after departure: TERPS assumes no turns below 400 feet AGL; any charted turn below that altitude is specifically engineered and will be published explicitly on the procedure.
- Non-standard gradient note: Any gradient above 200 ft/NM will be explicitly noted on the DP chart or in the procedure text. If no gradient is noted, 200 ft/NM is assumed required.
- 14 CFR 91.175: Governs IFR operations and reinforces pilot responsibility for obstacle clearance during departure when not under positive radar control.
Memory Aid
A widely used reminder for departure planning is the mnemonic "GRAB" — though not an official FAA acronym, it captures the essential preflight checks for any IFR departure:
- G — Gradient: Can I meet the published climb gradient at today's weight, altitude, and temperature?
- R — Route: Do I have the DP chart and understand the full routing?
- A — Alternate: Is an alternate required, and is one available?
- B — Briefing: Have I briefed the lost comm procedure and emergency turn-back heading?
Use GRAB before every IFR departure, and pay particular attention to the G — it is the most commonly skipped check and the one TERPS depends on you performing.
Common Test Traps
- Assuming no DP means no obstacles: The absence of a published DP means diverse departure criteria are satisfied at standard gradient — it does NOT mean the airport is obstacle-free. Pilots must still meet 200 ft/NM.
- Confusing gradient with rate of climb: The FAA knowledge test often provides a gradient in ft/NM and expects you to convert it to ft/min using groundspeed. Remember: ft/NM × GS ÷ 60 = ft/min.
- Thinking ODPs are always mandatory: ODPs are not automatically required; they become mandatory when ATC assigns them or when you elect to fly them. However, not flying one when no other clearance is provided exposes you to obstacle risk.
- Ignoring the DER as the gradient start point: The 200 ft/NM gradient begins at the departure end of the runway, not the runway threshold. At airports with significant displaced thresholds, this distinction matters for obstacle clearance.
- Overlooking non-standard takeoff minimums: A bold inverted "T" or "A" symbol on a DP chart indicates non-standard takeoff minimums or departure procedures. Many students miss these symbols and plan departures as if standard minimums (1 SM, 300-1) apply when higher minimums are actually required.