11.1 Standard Instrument Departures (SIDs) & Obstacle Departure Procedures (ODPs)

11.1 Standard Instrument Departures (SIDs) & Obstacle Departure Procedures (ODPs)

In scheduled commercial aviation operating under 14 CFR Part 121, the departure phase represents one of the most operationally demanding regimes of flight. For the certificated aircraft dispatcher, constructing and validating instrument departure profiles requires reconciling two distinct regulatory frameworks: the terminal airspace design standards governed by the United States Standard for Terminal Instrument Procedures (FAA Order 8260.3, TERPS) and the transport category aircraft performance limitations mandated by 14 CFR § 121.189. A failure to recognize how these frameworks diverge—particularly regarding engine-out performance assumptions—can lead to catastrophic terrain clearance failures during initial climb.

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TERPS Departure Design Fundamentals

Terminal Instrument Procedures (TERPS) establish the criteria used by procedure designers to formulate, evaluate, and publish instrument departure procedures. When designing any instrument departure from an airport runway, TERPS assumes specific baseline operational parameters:

The Standard Departure Profile

1. Screen Height at Departure End of Runway (DER): The climb profile is assumed to begin at the Departure End of Runway (DER) at an altitude of 35 feet Above Ground Level (AGL) for civil runways. (Military procedures historically assume crossing the DER at 0 feet AGL).
2. Straight-Ahead Climb to 400 Feet: No turns are prescribed below 400 feet above the DER elevation, except where specifically published for obstacle avoidance or designated visual maneuvers. The aircraft must climb straight ahead on runway heading until reaching 400 feet AGL before initiating any departure turn.
3. Baseline Standard Climb Gradient: The standard TERPS departure climb gradient is 200 feet per nautical mile (ft/NM), which equates to a gradient of 3.29% (commonly rounded to 3.3%).

The 40:1 Obstacle Identification Surface (OIS) and ROC

The standard 200 ft/NM climb gradient is not arbitrary; it is derived from a geometric obstacle evaluation surface combined with a mandatory safety buffer:

• Obstacle Identification Surface (OIS): TERPS evaluates terrain and man-made obstacles using a planar surface sloping upward from the DER at a 40:1 ratio (40 feet horizontal for every 1 foot vertical). Mathematically, because 1 nautical mile equals 6,076.1 feet: $\text{OIS Slope} = \frac{6,076.1 \text{ ft}}{40} = 151.9 \text{ ft/NM} \approx 152 \text{ ft/NM (2.5 percent gradient)}$
• Required Obstacle Clearance (ROC): To protect departing aircraft from objects underlying the flight path, TERPS incorporates a mandatory obstacle clearance buffer of 48 ft/NM (equivalent to 0.8% gradient): $\text{Standard Climb Gradient} = \text{OIS (152 ft/NM)} + \text{ROC (48 ft/NM)} = 200 \text{ ft/NM}$
• ROC Buffer Ratio: The 48 ft/NM ROC represents exactly 24% of the total 200 ft/NM climb gradient ($48 / 200 = 0.24$). As long as an aircraft maintains at least 200 ft/NM, it maintains a 24% clearance buffer above the highest obstacle penetrating the departure corridor.

TERPS Instrument Departure Profile Architecture:

Altitude (ft)
     ^
     |                                          / Standard Climb Gradient: 200 ft/NM (3.3%)
     |                                         /  [Required Obstacle Clearance: 48 ft/NM]
     |                                        /-------------------------------------------
     |                                       /  / Obstacle Identification Surface (OIS):
     |                                      /  /   40:1 Slope = 152 ft/NM (2.5%)
     |                                     /  /-------------------------------------------
     |                   400 ft AGL       /  /
     |               (Earliest Turn)     /  /      ^ Obstacle
     |                      |           /  /      /|\
     |                      v          /  /      / | \
35 ft|----------------------*---------/--/------/--|--\------------------------------------
  DER|_________________________________/
     +-----------------------------------------------------------------------------------> Distance (NM)

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Obstacle Departure Procedures (ODPs)

When terrain, towers, or other natural or man-made obstacles penetrate the 40:1 OIS, the runway does not qualify for a standard, unrestricted instrument departure. The FAA procedure designer must mitigate the penetration by publishing an Obstacle Departure Procedure (ODP).

Mitigation Strategies for OIS Penetrations

If an obstacle penetrates the 40:1 surface, procedure designers utilize one or more of the following solutions:

1. Publish a Higher-than-Standard Climb Gradient: Specify a non-standard climb gradient (e.g., "Climb of 340 ft/NM to 5,200 ft MSL") that elevates the flight path and maintains the 24% ROC above the controlling obstacle.
2. Establish Non-Standard Takeoff Minimums: Publish higher ceiling and visibility requirements (e.g., "Ceiling 400 ft, Visibility 2 SM") so flight crews can visually acquire and avoid the obstacle.
3. Design a Specified Turning Departure: Require the aircraft to turn away from the penetrating obstacle prior to reaching it (e.g., "Climb runway heading to 1,200 ft, then turn right direct ABC VOR").
4. Publish a Visual Climb Over Airport (VCOA): Authorize aircraft to climb visually in a circle within a specified nautical mile radius of the airport up to a designated "climb-in-visual-conditions" altitude before proceeding on course. VCOA procedures always mandate a specific published ceiling and flight visibility.

ODP Publication Formats & Clearances

• Textual ODPs: Published in the front matter of the FAA Terminal Procedures Publication (TPP) under the heading "Takeoff Minimums, (Obstacle) Departure Procedures, and Diverse Vector Areas."
• Graphical ODPs: When an ODP is complex, it is charted graphically in the TPP. A graphical ODP is explicitly designated by the word "(OBSTACLE)" in the chart title (e.g., "GUPPY ONE DEPARTURE (OBSTACLE)").
• Regulatory Clearance Rule: Under IFR, an explicit ATC clearance is not required to fly a published ODP unless ATC specifies otherwise. However, in Part 121 air carrier operations, 14 CFR § 91.175(f) dictates that no person may take off under IFR from a civil airport unless compliant with published non-standard takeoff minimums and departure procedures. If ATC issues a radar vector or SID, the assigned clearance supersedes the ODP laterally, but the dispatcher and flight crew remain responsible for terrain clearance.

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Standard Instrument Departures (SIDs)

While ODPs are designed solely for obstacle clearance, Standard Instrument Departures (SIDs) are air traffic control procedures designed primarily for:

• Air Traffic Management & Flow Control: Systematically sequencing departures out of busy terminal areas and metroplexes into the high-altitude en route structure.
• Workload Reduction: Eliminating lengthy, detailed verbal departure clearances over ATC frequencies.
• Noise Abatement: Directing turbojet traffic along corridors designed to minimize noise footprints over populated areas.

Key Operational Differences: ODP vs. SID

Types of SIDs: Conventional vs. RNAV

• Conventional SIDs: Constructed using ground-based navigation facilities, primarily VOR, VORTAC, TACAN, and localizer courses. Aircraft must possess operable dual VOR/DME receivers.
• RNAV SIDs (RNAV 1): Constructed for aircraft equipped with Area Navigation systems (FMS) certified to RNAV 1 / RNP 1 standards. Under FAA Advisory Circular (AC) 90-100A, RNAV 1 requires total system error of less than 1.0 NM for 95% of the flight time, utilizing GPS/GNSS or DME/DME/IRU positioning. RNAV SIDs feature precise waypoints, turn anticipation, and fly-by/fly-over designations.
• SID Anatomy: A SID typically features one or more runway transitions (linking individual runways to a common departure trunk), a common route, and multiple en route transitions (linking the common departure trunk to specific en route airways or fixes).

ATC Phraseology: "Climb Via SID"

When ATC issues the clearance "Climb via SID," the flight crew is legally authorized to:

1. Follow the published lateral flight path of the SID.
2. Climb vertically complying with all published altitude restrictions at each charted waypoint.
3. Comply with all published speed restrictions. Conversely, if ATC states "Climb and maintain FL 230," the altitude restrictions on the SID are cancelled, but published speed restrictions and the lateral track remain mandatory.

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Climb Gradient Conversions and Mathematical Formulas

Aeronautical charts publish climb gradients in feet per nautical mile (ft/NM). However, transport-category flight decks and vertical speed indicators (VSIs) display vertical velocity in feet per minute (fpm). Aircraft dispatchers and flight crews must convert between these units.

The Fundamental Climb Gradient Formula

Because groundspeed is expressed in nautical miles per hour (NM/hr), dividing groundspeed by 60 yields nautical miles per minute (NM/min): $\text{NM per Minute} = \frac{\text{Groundspeed (kts)}}{60}$ Multiplying NM per minute by the required climb gradient (ft/NM) yields the required rate of climb in feet per minute (fpm): $\text{Rate of Climb (fpm)} = \text{Groundspeed (kts)} \times \left[ \frac{\text{Climb Gradient (ft/NM)}}{60} \right]$ Alternatively, this can be written as: $\text{Rate of Climb (fpm)} = \frac{\text{Groundspeed (kts)} \times \text{Climb Gradient (ft/NM)}}{60}$

Climb Gradient Percentage Formula

To convert a climb gradient in ft/NM to a percentage gradient (rise over run): $\text{Gradient (percent)} = \frac{\text{Climb Gradient (ft/NM)}}{6,076.1 \text{ ft}} \times 100 \approx \frac{\text{Climb Gradient (ft/NM)}}{60.76}$

Worked Calculation Examples

Example 1: Standard TERPS Climb Gradient

• An air carrier departs runway 28L at a groundspeed of 180 knots.
• Required climb gradient: Standard 200 ft/NM. $\text{Rate of Climb} = 180 \times \left( \frac{200}{60} \right) = 180 \times 3.333 = 600 \text{ fpm}$

Example 2: High-Climb Obstacle Gradient

• An aircraft departs an airport in mountainous terrain where the SID specifies: "Climb gradient of 420 ft/NM to 8,000 ft."
• Groundspeed during initial climb: 150 knots. $\text{Rate of Climb} = 150 \times \left( \frac{420}{60} \right) = 2.5 \text{ NM/min} \times 420 \text{ ft/NM} = 1,050 \text{ fpm}$

Groundspeed vs. Climb Gradient Matrix (Required fpm)

Dispatcher Operational Warning: Higher takeoff groundspeeds (induced by high density altitude or tailwinds) drastically inflate the required vertical speed. A tailwind increases groundspeed, necessitating a substantially higher rate of climb to achieve the same foot-per-nautical-mile gradient across the ground.

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Dispatcher Takeoff Obstacle Analysis vs. TERPS (14 CFR § 121.189)

The single most critical concept an aircraft dispatcher must master regarding terminal instrument departures is the fundamental divergence between TERPS departure design and 14 CFR Part 121 airworthiness operating limits.

The AEO vs. OEI Trap

• TERPS SIDs and ODPs assume ALL ENGINES OPERATING (AEO): Every published SID and ODP climb gradient is engineered under the assumption that the aircraft possesses full rated takeoff thrust on all powerplants. TERPS does not evaluate single-engine climb capabilities.
• 14 CFR § 121.189 mandates ONE ENGINE INOPERATIVE (OEI) Clearance: Under federal air carrier regulations, transport category turbine aircraft must be capable of losing an engine at the most critical point of the takeoff roll ($V_1$) and continuing the takeoff while clearing all obstacles in the takeoff flight path by at least 35 feet vertically (or 200 feet horizontally inside airport boundaries, and 300 feet horizontally outside airport boundaries).

Regulatory Comparison Matrix:

Feature                 TERPS (FAA Order 8260.3)        Part 121 (§ 121.189)
----------------------------------------------------------------------------
Engines Operating       All Engines Operating (AEO)     One Engine Inoperative (OEI at V1)
Screen Height at DER    35 ft AGL                       35 ft AGL (dry) / 15 ft (wet)
Vertical Clearance      48 ft/NM (24% of gradient)      35 ft absolute obstacle clearance
Horizontal Clearance    Trapezoidal TERPS area          200 ft / 300 ft corridor
Purpose                 Airspace procedure design       Air carrier takeoff weight limit

Special Engine-Out Departure Procedures (Runway Analysis)

If a heavy multi-engine transport jet (such as a B737, A321, or B777) loses an engine at $V_1$ at a high-elevation airport like Denver (DEN) or Salt Lake City (SLC), the aircraft's single-engine climb gradient drops from 8–12% down to 2.4% to 3.0%. It is aerodynamically impossible for the aircraft on one engine to satisfy a published TERPS SID gradient requiring 450 ft/NM (7.4%).

To resolve this life-critical gap:

1. Runway Analysis Systems: Dispatchers utilize FAA-approved computerized runway analysis programs (Aeronautical Data Systems / Takeoff and Landing Reports - TLR).
2. Special Engine-Out Procedures (EOPs): For runways with significant obstacle penetrations, the carrier publishes proprietary Special Engine-Out Procedures (also called Emergency Turn Procedures). In the event of an engine failure at or above $V_1$, the flight crew abandons the published SID or ODP routing and executes the specific EOP (e.g., "At 1.2 DME, turn left heading 210° climbing to 6,500 ft, then proceed direct to VOR").
3. Restricted Takeoff Weight (RTOW): The dispatcher limits the maximum allowable takeoff weight so that the aircraft's net takeoff flight path (degraded by 0.8% for two-engine aircraft under Part 25) clears all obstacles along the EOP corridor by at least 35 feet.