12.2 Climb Segments & Obstacle Clearance

Climb Segments & Obstacle Clearance

Core Airline Transport Principle: In the event of an engine failure at $V_{EF}$, transport category aircraft do not merely remain airborne; their climb profile is certified under 14 CFR 25.111 and 25.121 to guarantee minimum positive climb gradients across four distinct geometric segments. Obstacle clearance calculations are based strictly on the degraded Net Takeoff Flight Path, ensuring a minimum 35-foot vertical clearance over every terrain feature and man-made obstacle in the departure corridor.

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1. 14 CFR Part 25 Takeoff Flight Path Architecture

The certified takeoff flight path begins at the end of the takeoff distance (at a screen height of 35 feet AGL on a dry runway, or 15 feet AGL on a wet runway) and extends to a minimum altitude of 1,500 feet AGL (or the altitude where the transition to the en route configuration is completed).

+---------------------------------------------------------------------------------------------------+
|                               14 CFR 25 TAKEOFF CLIMB SEGMENTS PROFILE                            |
|                                                                                                   |
|  Altitude                                                                                         |
|    ^                                                                           [ 4th / FINAL ]    |
|    |                                                                           Clean Config       |
|    |                                                      [ 3rd SEGMENT ]      MCT Thrust         |
|    |                                                    Acceleration / Flap    Speed: VFTO        |
|    |                                                      Retraction Level     Gradient: >= 1.2%  |
|    |                                                    (Min 400 - 1500 ft)    (to 1,500 ft AGL)  |
|    |                                  [ 2nd SEGMENT ]  ---------------------->                    |
|    |                                  Takeoff Flaps,   /                                          |
|    |                                  Gear UP, TOGA    /                                          |
|    |                                  Speed: V2        /                                          |
|    |                 [ 1st SEGMENT ]  Gradient: >= 2.4%/                                          |
|    |                 Gear Retraction  (35 to 400+ ft) /                                           |
|    |                 Speed: V2       /                                                            |
|    |   [ RUNWAY ]    Positive Gradient                                                            |
|    +-------+--------------+---------------------------------------------------------------------> |
|    0      VEF/V1         35 ft                                                              Distance  |
+---------------------------------------------------------------------------------------------------+

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2. Detailed Analysis of the Four Takeoff Climb Segments

Each segment reflects a specific mechanical and aerodynamic configuration designed to manage drag, accelerate the aircraft, clean up high-lift devices, and establish maximum continuous climb power.

1st Segment (Liftoff to Gear Retraction)

• Start Point: 35 feet AGL at the end of the takeoff distance ($V_{LOF}$ reaching $V_2$).
• End Point: Complete retraction and locking of the landing gear doors.
• Configuration: Takeoff flaps/slats, landing gear extending/in transit (producing massive parasite drag), Takeoff Thrust (TOGA), speed stabilized at $V_2$.
• Minimum OEI Climb Gradients (14 CFR 25.121(a)):
  • 2-Engine Aircraft: Positive gradient ($> 0.0%$)
  • 3-Engine Aircraft: $0.3%$
  • 4-Engine Aircraft: $0.5%$

2nd Segment (Gear Up to Acceleration Height) — The Most Critical Segment

• Start Point: Landing gear fully retracted (minimum 35 ft AGL, typically 35 to 400+ ft AGL).
• End Point: Minimum acceleration height (400 feet AGL minimum per regulation; airline Standard Operating Procedures typically use 800 ft to 1,500 ft AGL depending on local obstacle profiles).
• Configuration: Takeoff flaps/slats deployed, landing gear fully UP, Takeoff Thrust (TOGA), airspeed maintained precisely at $V_2$.
• Minimum OEI Climb Gradients (14 CFR 25.121(b)):
  • 2-Engine Aircraft: $2.4%$
  • 3-Engine Aircraft: $2.7%$
  • 4-Engine Aircraft: $3.0%$
• Operational Importance: The 2nd Segment is almost universally the climb-limiting segment for Maximum Takeoff Weight (MTOW). Because the aircraft is climbing in a high-drag flap configuration at maximum thrust with one engine failed, high ambient temperatures or high airport pressure altitudes severely degrade 2nd segment performance.

3rd Segment (Acceleration and Flap Retraction)

• Start Point: Acceleration height (minimum 400 ft AGL).
• End Point: Aircraft accelerated to final takeoff climb speed ($V_{\text{FTO}} / V_{\text{clean}}$) with all flaps and slats fully retracted.
• Configuration: Aircraft flies level (or at a reduced climb gradient) while accelerating from $V_2$ through intermediate flap retraction speeds, flaps/slats retracting on schedule, Takeoff Thrust transitioning to Max Continuous Thrust (MCT).
• Gradient Requirement (14 CFR 25.121(c)): The available excess thrust must provide sufficient acceleration to retract flaps while maintaining a net climb capability of at least $1.2%$ (2-engine), $1.5%$ (3-engine), or $1.7%$ (4-engine).

4th / Final Takeoff Segment (En Route Transition)

• Start Point: Flaps and slats fully retracted, aircraft established at final clean climb speed ($V_{\text{FTO}}$).
• End Point: Minimum 1,500 feet AGL (or higher obstacle clearance altitude).
• Configuration: Clean aircraft (gear up, flaps up), Maximum Continuous Thrust (MCT - unrestricted time limit), speed at $V_{\text{FTO}}$.
• Minimum OEI Climb Gradients (14 CFR 25.121(c)):
  • 2-Engine Aircraft: $1.2%$
  • 3-Engine Aircraft: $1.5%$
  • 4-Engine Aircraft: $1.7%$

+-----------------------------------------------------------------------------------------------------+
|                   SUMMARY OF 14 CFR PART 25 TAKEOFF CLIMB SEGMENT SPECIFICATIONS                    |
|                                                                                                     |
|   Segment     Start -> End       Thrust   Flaps    Gear   Speed    2-Eng Min   3-Eng Min  4-Eng Min |
|   ---------   ----------------   ------   ------   ----   -----    ---------   ---------  --------- |
|   1st         35 ft -> Gear Up   TOGA     Takeoff  DOWN   V2       Positive    0.3%       0.5%      |
|   2nd         Gear Up -> 400'+   TOGA     Takeoff  UP     V2       2.4%        2.7%       3.0%      |
|   3rd         Acc Height -> FTO  TOGA/MCT Retract  UP     V2->VFTO Accel Cap   Accel Cap  Accel Cap |
|   4th/Final   Flaps Up -> 1500'  MCT      UP       UP     VFTO     1.2%        1.5%       1.7%      |
+-----------------------------------------------------------------------------------------------------+

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3. Gross vs. Net Takeoff Flight Path & The 35-Foot Obstacle Rule

In airline dispatch and performance engineering, obstacle clearance is never calculated using raw test-flight aircraft performance. Certification rules mandate a strict mathematical reduction from Gross to Net flight path.

+-----------------------------------------------------------------------------+
|                   GROSS VS. NET TAKEOFF FLIGHT PATH PROFILE                 |
|                                                                             |
|   Altitude                                                                  |
|     ^                                                GROSS Flight Path      |
|     |                                              . ' (Actual Aircraft)    |
|     |                                          . '                          |
|     |                                      . '                              |
|     |                                  . '  <-- 0.8% Penalty (Twin-Engine)  |
|     |                              . '                                      |
|     |                          . '  ================ NET Flight Path        |
|     |                      . '                   . ' (Dispatch Planning)    |
|     |                  . '                   . '                            |
|     |              . '                   . ' |                              |
|     |          . '                   . '     |                              |
|     |      . '                   . '         | >= 35 ft CLEARANCE           |
|     |  . '                   . '             |                              |
|     +--------------------+--+----------------[OBSTACLE]-------------------> |
|    35 ft                VEF V1                                     Distance |
+-----------------------------------------------------------------------------+

Mathematical Gradient Deduction (14 CFR 25.115)

To account for pilot technique variations, atmospheric turbulence, engine degradation with service life, and airframe rigging differences, the Net Flight Path is derived by subtracting a fixed regulatory gradient penalty from the Gross Flight Path: $\text{Net Gradient} = \text{Gross Gradient} - \Delta_{\text{regulatory}}$

Where the mandatory regulatory penalties are:

• Two-Engine Aircraft: Subtract $0.8%$ from the gross climb gradient.
• Three-Engine Aircraft: Subtract $0.9%$ from the gross climb gradient.
• Four-Engine Aircraft: Subtract $1.0%$ from the gross climb gradient.

The 35-Foot Obstacle Clearance Standard (14 CFR 121.189)

• Under 14 CFR 121.189, no airplane may take off at a weight exceeding the maximum weight at which the Net Takeoff Flight Path clears all obstacles either by:
  1. At least 35 feet vertically;
  2. At least 200 feet horizontally within the airport boundary, and 300 feet horizontally after passing the boundary.
• Turns in Departure: If a departure routing requires a course change exceeding $15^\circ$, banking the aircraft increases stall speed and reduces the vertical climb component. Under FAA Ops Specs:
  • Turns are strictly prohibited below 50 feet AGL (and typically restricted below 400 feet AGL).
  • Bank angle is limited to $15^\circ$ during the takeoff climb path.
  • When turns exceeding $15^\circ$ are executed, the required vertical obstacle clearance is increased from 35 feet to 50 feet.

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4. Takeoff Thrust Time Limits (5-Minute vs. 10-Minute Limits)

Turbine engines produce extreme turbine gas temperatures (EGT/ITT) and mechanical stress at full takeoff thrust. Engine certification limits how long this power setting may be sustained.

• Normal All-Engine Takeoff: Takeoff thrust is certified for a maximum duration of 5 minutes from brake release.
• One Engine Inoperative (OEI) Emergency: Under FAA Part 25 and engine certification standards (FAR 33), if an engine fails during takeoff, the flight crew may operate the remaining engine(s) at Takeoff Thrust for up to 10 minutes.
  • This extended 10-minute window allows the crew adequate time to fly the second segment, complete the level acceleration and flap cleanup, and establish clean climb speed before retarding thrust levers to the unrestricted Maximum Continuous Thrust (MCT) rating.

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5. Worked Flight Planning Example: Obstacle Clearance Calculation

+-----------------------------------------------------------------------------+
|               WORKED AIRLINE TAKEOFF PERFORMANCE CALCULATION                |
|                                                                             |
|   Scenario: Twin-engine jet departing Runway 09.                            |
|   - Obstacle: 285 ft MSL antenna tower located 12,000 ft from DER.          |
|   - Runway 09 Departure End of Runway (DER) elevation: 45 ft MSL.           |
|   - Screen Height: 35 ft AGL (DER + 35 = 80 ft MSL).                        |
|                                                                             |
|   Step 1: Calculate Obstacle Height relative to Takeoff Screen Point:        |
|           H_obs = 285 ft - 80 ft = 205 ft                                   |
|                                                                             |
|   Step 2: Add 35-foot required Net Obstacle Clearance:                      |
|           H_required = 205 ft + 35 ft = 240 ft                              |
|                                                                             |
|   Step 3: Calculate Required NET Climb Gradient:                            |
|           Net Gradient = (240 ft / 12,000 ft) * 100 = 2.00%                 |
|                                                                             |
|   Step 4: Calculate Minimum GROSS Climb Gradient Required:                  |
|           Gross Gradient = Net Gradient + 0.8% (Twin-engine margin)         |
|           Gross Gradient = 2.00% + 0.80% = 2.80%                            |
|                                                                             |
|   Step 5: Verify 2nd Segment Regulatory Compliance:                         |
|           Gross (2.80%) > Regulatory Minimum (2.40%) -> ALLOWABLE           |
|           The dispatcher must cap MTOW to ensure 2nd segment gross          |
|           climb gradient is at least 2.80% at ambient temp/pressure.        |
+-----------------------------------------------------------------------------+