9.2 Takeoff Climb Segments & Obstacle Clearance Gradients

Takeoff Climb Segments and Obstacle Clearance

Engine-inoperative runway analysis checks whether the assigned airplane can meet its approved takeoff path, climb requirements, and operational obstacle limits at the proposed weight. Dispatchers must distinguish takeoff path, takeoff flight path, gross capability, and net performance credited for obstacles. Memorizing a gradient table without these distinctions leads to incorrect releases.

The Path and the Four-Segment Teaching Model

Under § 25.111, the takeoff path starts from a standing start and extends until 1,500 feet above the takeoff surface or completion of the transition to the en route configuration with final takeoff speed reached, whichever point is higher. The critical engine is assumed to fail at the selected engine-failure speed. Under § 25.115, the takeoff flight path begins at 35 feet above the surface and derives from that path. A wet-runway distance determination can use the rule's 15-foot condition under § 25.113; that does not replace the 35-foot origin of the § 25.115 flight path or authorize reducing operational obstacle clearance to 15 feet.

The familiar four segments are a teaching organization of configuration changes. Read the actual regulatory conditions rather than treating each classroom segment name as a separate statutory paragraph.

The § 25.121(a) gear-extended condition runs from liftoff until gear retraction, rather than beginning universally at 35 feet and V2. Section 25.111 requires V2 before reaching 35 feet and a speed as close as practical to, but not below, V2 through 400 feet. Except for permitted gear retraction and automatic propeller feathering, configuration changes and pilot-required power changes are restricted before 400 feet. The approved operating procedure can require a higher acceleration altitude because of obstacles or aircraft requirements.

At each point starting at 400 feet, § 25.111(c)(3) specifies available climb-gradient requirements of 1.2%, 1.5%, or 1.7%. An approved segmental analysis may represent acceleration by the method allowed in § 25.115(c); a dispatcher must not simply draw an unrestricted level acceleration line through obstacles. The final takeoff requirement is § 25.121(c); § 25.121(d) instead governs approach climb. Final takeoff speed is selected under § 25.107(g), including a minimum 1.18 VSR and the specified maneuvering capability. Use the AFM's approved speed, configuration, and thrust limits, not an independently calculated substitute.

Gross and Net Performance

Gross performance represents demonstrated aerodynamic capability under the applicable certification conditions. Net takeoff performance reduces the actual flight path at each point by the gradient decrements in § 25.115:

• Two engines: 0.8 percentage point.
• Three engines: 0.9 percentage point.
• Four engines: 1.0 percentage point.

These are subtractions in gradient, not proportional reductions in the gross value. For a hypothetical twin with 3.2% gross capability, the corresponding net gradient is 3.2% − 0.8% = 2.4%. A twin just meeting the second-segment gross minimum of 2.4% has 1.6% net, not 2.4% net.

To connect gradients with chart units, one nautical mile is approximately 6,076 feet. Thus 1.6% corresponds to about 97.2 ft/NM, and 2.4% to about 145.8 ft/NM. At 150 kt groundspeed, the latter requires approximately 145.8 × 150 / 60 = 364.6 ft/min. Groundspeed matters when converting a geometric climb requirement to vertical speed. These arithmetic examples illustrate units; they are not approved aircraft performance data.

Operational Obstacle Clearance

For airplanes in the modern certification-date branch of § 121.189(d)(2), the approved net takeoff flight path must clear obstacles by at least 35 feet vertically, or at least 200 feet horizontally within airport boundaries and 300 feet horizontally outside. The older certification branch has different vertical wording and must not be silently applied to all airplanes. Corrections include runway, airport elevation, effective runway gradient, ambient temperature, wind component, and applicable wet-runway conditions. The rule also states bank assumptions: no bank before 50 feet and no more than 15 degrees thereafter for this analysis.

An operator's approved obstacle-accountability method determines the area, turns, navigation accuracy, and other assumptions used by its runway analysis. There is no universal 12.5% splay with a 2,000/3,000-foot maximum half-width established by § 121.189 for every departure. Avoid inventing corridor geometry or applying a procedure's obstacle list to a different routing. Turning increases load factor and can reduce climb capability; use the approved turning-flight data and assumptions.

Published Departures and Engine-Failure Routes

A published SID or obstacle departure procedure and an operator's engine-failure procedure answer different planning questions. Published departure gradients often assume normal operation and can exceed available engine-inoperative capability. Conversely, an airplane at a sufficiently low weight may satisfy a particular published gradient with an engine inoperative. Neither “every SID is safe after engine failure” nor “no transport can meet a SID on one engine” is correct.

The dispatcher verifies that the runway analysis and any special engine-failure routing match the runway, configuration, acceleration altitude, obstacles, and aircraft data. Some routes use turns or delayed acceleration to avoid terrain; others require no special turn. The PIC uses the actual approved emergency procedure and emergency authority as necessary, coordinates with ATC as conditions permit, and communicates the situation. Do not describe abandoning every ATC clearance as an automatic universal legal obligation.

Flap Selection and the Controlling Weight

The limiting departure weight is the most restrictive applicable structural, field-length, climb, obstacle, brake-energy, tire-speed, and other aircraft-specific comparison. Second-segment performance is often critical at hot or high airports, but it is not always the limiting condition. A tire-speed or brake limit may control even with substantial climb margin.

Lower takeoff flap settings commonly reduce drag and improve climb capability while increasing liftoff speed and required runway. Higher flap settings commonly shorten the takeoff distance while reducing climb performance. Approved data decide the result. Final-segment configuration is clean, so a lower initial flap setting does not automatically guarantee an improved final-segment gradient under every comparison.

Consider two hypothetical approved-analysis results: configuration A permits 142,000 lb because of climb, and configuration B permits 148,500 lb for climb but only 146,000 lb for available runway. B's allowable weight is 146,000 lb, not 148,500 lb. If the planned weight is 147,000 lb, the dispatcher still needs a compliant loading, runway, configuration, or operating solution. Never select B using only its better climb number.

Practical Verification Before Release

Check the correct tail/configuration, MEL/CDL penalties, current runway and declared distances, surface condition, temperature, wind, obstacles, and effective NOTAMs. Confirm acceleration height and engine-failure routing with the actual analysis. A normal all-engine ATC gradient may need a separate comparison from the engine-inoperative net obstacle analysis. The PIC briefing should identify the route and constraint, not merely announce that the computer produced a green result.

If a runway change, tailwind, anti-ice requirement, or deferred item changes the assumptions, rerun the applicable analysis. A release prepared for one runway cannot be assumed valid for an intersection departure or a different engine-out turn. Preserve the approved data and communicate any revised limitation before concurrence.

Official References

Source check: 2026-10-12. Transport performance certification; Part 121 performance rules. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.