What is the minimum second-segment climb gradient for a two-engine transport-category airplane with one engine out?

Under 14 CFR 25.121(b), a two-engine transport-category airplane must achieve a minimum climb gradient of 2.4 percent during the second segment with one engine inoperative. This segment runs from gear-fully-retracted to 400 feet AGL with flaps in the takeoff position and remaining engines at takeoff thrust. It is typically the most limiting performance requirement for takeoff weight calculations.

What is the difference between the gross flight path and the net flight path in engine-out performance?

The gross flight path is the actual average climb performance a transport-category airplane is expected to achieve with one engine inoperative under specified conditions. The net flight path is the gross flight path reduced by a fixed gradient — 0.8 percent for turbine-powered airplanes — to account for real-world variables like technique and atmospheric irregularities. Obstacle clearance requirements must be met using the more conservative net flight path, not the gross.

At what altitude does the third segment of the engine-out takeoff flight path begin?

The third segment begins after the airplane reaches a minimum of 400 feet above the takeoff surface, which is the point where the second segment ends. During the third segment the airplane levels off or maintains a very shallow climb while accelerating and retracting flaps to the clean configuration. No minimum climb gradient is specified for this acceleration phase, but the airplane must not descend below the minimum altitude while clearing obstacles by the required net margin.

Engine-Out Climb Performance and the Net Flight Path

When a transport-category airplane loses an engine during or just after takeoff, the crew cannot simply fly the same profile as if all engines are operating. Performance degrades significantly, and regulatory standards must ensure the airplane can still clear terrain and obstacles with an acceptable margin of safety. The concept of the net flight path bridges the gap between what the airplane can actually do (the gross flight path) and what the regulations require as a minimum, applying a deliberate reduction to account for real-world variables.

14 CFR 25.121 is the heart of this topic for transport-category airworthiness. It establishes minimum climb gradient requirements for each segment of the engine-out takeoff flight path, specifying the exact airplane configuration, flap setting, gear position, and power setting that apply at each point. Understanding this regulation is essential not only for the Flight Engineer written and practical tests, but for safe day-to-day operations on every departure.

The Gross Flight Path versus the Net Flight Path

The gross flight path represents the average performance a properly maintained airplane is expected to achieve under the conditions specified — no shortcuts, no degraded technique, everything precisely flown. It is, in a sense, the best-case documented outcome.

The net flight path is the gross flight path diminished by a specific performance reduction. For turbine-powered transport-category airplanes, the net flight path is the gross flight path reduced by a gradient of 0.8 percent (8/10 of one percent of climb gradient). For reciprocating-engine airplanes the reduction is 1.1 percent. This built-in penalty accounts for airspeed deviations, technique variations, atmospheric turbulence, and minor system differences between individual aircraft. When operators use the net flight path to plan obstacle clearance, they are working with a conservatively degraded performance number — the obstacle must be cleared by the required margin using this reduced gradient, not the gross gradient.

The practical implication is powerful: if the net flight path cannot clear all obstacles in the departure corridor with the required vertical margin (35 feet for a straight-out departure, or 35 feet with a 300-foot horizontal buffer for each side of the path), the airplane is too heavy, the weather is too hot, the field is too high, or some combination — and the departure cannot legally be made at that weight.

The Four Segments Defined by 14 CFR 25.121

The engine-out takeoff flight path is divided into four distinct segments, each characterized by a specific configuration and minimum climb gradient. The transition between segments is defined by configuration changes — primarily gear retraction and flap retraction — rather than by elapsed time or altitude.

First Segment

The first segment begins at the point of lift-off (VLOF) and ends when the landing gear is fully retracted. The airplane is in the takeoff flap configuration, gear is transitioning to up, and the operative engines are at takeoff (maximum) thrust. Under 14 CFR 25.121(a), a two-engine airplane must achieve a positive gradient (greater than zero), a three-engine airplane must achieve at least 0.3 percent, and a four-engine airplane must achieve at least 0.5 percent. These are deliberately modest requirements because gear retraction drag is a major penalty during this brief phase.

Second Segment

The second segment is the most operationally significant. It begins when the landing gear is fully retracted and ends at a point where the airplane has reached a minimum altitude of 400 feet above the takeoff surface. The flaps remain in the takeoff position, and the engines are still at takeoff thrust. 14 CFR 25.121(b) requires a climb gradient of at least 2.4 percent for two-engine airplanes, 2.7 percent for three-engine airplanes, and 3.0 percent for four-engine airplanes. The second segment gradient is the primary limiting factor for most transport-category takeoff weight calculations. Hot and high conditions crush second-segment climb, which is why maximum takeoff weight on a hot summer day at a mile-high airport can be dramatically lower than the structural limit.

Third Segment (Acceleration Segment)

The third segment is a level or near-level acceleration phase. After the second segment ends, the airplane levels off (or continues a very shallow climb) and accelerates to flap-retraction speed while the flaps are progressively retracted. Takeoff thrust may be maintained during this phase. The minimum altitude entering this segment is 400 feet AGL. No minimum gradient is specified for this level-off segment because the airplane is trading altitude gain for acceleration; however, the airplane must not descend below the minimum altitude. The net flight path is considered to continue along this level path, and obstacles must be avoided by the same 35-foot net clearance requirement.

Fourth Segment (Final Segment)

The fourth segment begins when the flaps have been fully retracted and ends when the airplane is in the final takeoff configuration, operating at maximum continuous thrust (no longer takeoff thrust). The speed is at least VFTO, the final takeoff speed. Under 14 CFR 25.121(c), the required gradient is at least 1.2 percent for two-engine airplanes, 1.5 percent for three-engine airplanes, and 1.7 percent for four-engine airplanes. The lower gradient requirement compared to the second segment reflects both the cleaner configuration and the reduced thrust (maximum continuous rather than takeoff).

Why This Matters Operationally

These regulatory minimums are not academic — airlines and operators translate them into takeoff weight limits published in the Airplane Flight Manual (AFM) and Approved Flight Manual Supplements. Operations specifications (OpSpecs) for Part 121 and Part 135 carriers require that dispatchers and flight crews demonstrate compliance with the net flight path before every departure. An airplane that cannot meet the second-segment gradient at a given weight must either reduce weight, use a longer or different runway, reduce obstacles through a published Obstacle Departure Procedure (ODP), or defer the flight.

The flight engineer's role in this process is central on airplanes requiring an FE station. The FE is responsible for computing or verifying performance data and confirming that the planned departure weight yields a net flight path that clears all obstacles along the departure corridor. Under 14 CFR 121.387, a qualified flight engineer is required at the FE station for the entire flight whenever the airplane's type certificate requires one, and independently for any airplane type certificated before January 2, 1964, with a maximum certificated takeoff weight of more than 80,000 pounds.

Key Numbers and Rules

Net flight path reduction: 0.8% gradient reduction from gross (turbine); 1.1% (reciprocating).
Obstacle clearance: net flight path must clear all obstacles by at least 35 feet (straight departure); 35 feet with 300-foot lateral buffer each side within the corridor.
First segment gradients (§ 25.121(a)): 2-engine > 0%; 3-engine ≥ 0.3%; 4-engine ≥ 0.5%.
Second segment gradients (§ 25.121(b)): 2-engine ≥ 2.4%; 3-engine ≥ 2.7%; 4-engine ≥ 3.0%.
Minimum altitude entering third segment: 400 feet AGL above the takeoff surface.
Fourth (final) segment gradients (§ 25.121(c)): 2-engine ≥ 1.2%; 3-engine ≥ 1.5%; 4-engine ≥ 1.7%.
Thrust setting second segment: takeoff (maximum) thrust, engines operating.
Thrust setting fourth segment: maximum continuous thrust.

Common Test Traps

Gross vs. net confusion: obstacle clearance analysis always uses the net flight path, not the gross. Selecting the gross path will give an overly optimistic (and illegal) result.
Mixing up segment configurations: gear position (not altitude) marks the end of the first segment; flap retraction (not time) marks the end of the third segment. Configuration is the trigger, not the clock.
Second-segment gradient for two-engine airplanes: 2.4%, not 2.0% or 2.5%. This number is heavily tested.
Thrust type in the fourth segment: maximum continuous thrust applies — not takeoff thrust. Takeoff thrust has time limits and is not used beyond the third segment.
Confusing the net flight path gradient reduction with an obstacle clearance margin: the 0.8% reduction and the 35-foot obstacle margin are two separate and independent requirements; both must be satisfied simultaneously.