What is the difference between climb-limited and obstacle-limited takeoff weight?

The climb-limited takeoff weight is the maximum weight at which the airplane can meet minimum OEI climb gradient requirements in each segment of the takeoff flight path, driven primarily by temperature and pressure altitude. The obstacle-limited takeoff weight is the maximum weight at which the net OEI flight path clears all obstacles in the departure corridor by at least 35 feet. Both must be calculated before every departure, and the lower value governs.

What gradient deduction is applied when checking obstacle clearance on the net takeoff flight path?

For a two-engine airplane, 0.8% gradient is subtracted from the gross (actual) flight path to produce the net flight path used for obstacle accountability. Three-engine airplanes use a 1.0% deduction and four-engine airplanes use 1.1%. This mandatory reduction, required by 14 CFR Part 25 and reflected in AFM data, builds in a safety buffer against real-world performance variations.

Which segment of the takeoff flight path is usually the most restrictive for climb-limited weight?

The second segment — flown gear up, OEI, flaps in takeoff position, at V2, from 35 feet to 400 feet AGL — imposes the highest minimum climb gradient and is almost always the most weight-limiting. Two-engine airplanes must achieve 2.4% gross gradient in this segment, three-engine airplanes 2.7%, and four-engine airplanes 3.0%, with one engine inoperative.

Climb-Limited and Obstacle-Limited Takeoff Weight Determination

Before a transport-category airplane departs, the operator must determine whether the airplane can safely perform two distinct takeoff challenges: first, can it climb to a safe altitude after losing an engine at the worst possible moment? Second, can it clear all obstacles along the departure path with that same failed engine? These two calculations yield two separate maximum allowable takeoff weights, and federal regulations require the airplane to comply with the more restrictive of the two. Together they are called the climb-limited takeoff weight and the obstacle-limited takeoff weight, and understanding both is essential for Airline Transport Pilot (ATP) applicants and for every air-carrier operation conducted under 14 CFR Part 121.

The governing regulation is 14 CFR 121.189, which prohibits a certificate holder from allowing a turbine-powered transport-category airplane to take off at a weight that exceeds the limits established by the approved Airplane Flight Manual (AFM) for the existing field length, gradient, obstacle environment, and atmospheric conditions. The AFM performance data, in turn, is developed in accordance with the airworthiness standards of 14 CFR Part 25, specifically the climb and obstacle-clearance requirements that define the takeoff flight path. Dispatch and flight crews must work through both limits every time — there are no shortcuts.

The Takeoff Flight Path: A Common Framework

Both limits share the same conceptual runway: the takeoff flight path begins at the start of the takeoff roll and ends when the airplane reaches 1,500 feet above the takeoff surface (or the height at which the transition to en-route climb configuration is complete, whichever is higher). This path is divided into segments, each defined by a specific configuration and minimum climb gradient requirement. The critical engine is assumed to fail at V1 — the takeoff decision speed — which is the worst-case moment the regulations require the airplane to survive. From that point, the airplane must continue to accelerate, rotate at VR, lift off, and climb through each segment while meeting prescribed gradient standards.

The four standard climb segments are:

First segment: Gear retraction in progress; one engine inoperative (OEI); flaps in takeoff position; speed at VLOF (liftoff speed). Positive climb gradient required (gradient varies by number of engines: 0.5% for two-engine, 0.3% for three-engine, 0.5% for four-engine airplanes per Part 25 standards).
Second segment: Gear up; OEI; flaps in takeoff position; speed at V2. This is typically the most demanding segment for climb gradient — 2.4% for two-engine, 2.7% for three-engine, 3.0% for four-engine airplanes. The airplane must maintain this gradient from 35 feet to 400 feet above the runway elevation.
Third segment (acceleration segment): Gear up; OEI; flaps transitioning from takeoff to en-route setting; airplane accelerates from V2 to VENR. This segment is flown at a roughly level attitude to allow acceleration; it is conducted at or above 400 feet AGL.
Fourth segment (final segment): Gear up; OEI; flaps up (en-route setting); speed at VENR. Minimum climb gradient is 1.2% for two-engine, 1.5% for three-engine, 1.7% for four-engine airplanes.

Climb-Limited Takeoff Weight

The climb-limited takeoff weight is the maximum weight at which the airplane can meet every segment's minimum climb gradient requirement with one engine inoperative. It is determined almost entirely by excess thrust — the difference between thrust available from the operating engines and the drag of the airplane in a given configuration. Because thrust and drag are sensitive to atmospheric conditions, the climb limit is primarily driven by:

Temperature: Higher temperatures reduce engine thrust (lower air density), shrinking the excess thrust margin and reducing the allowable weight.
Altitude (pressure altitude): Higher airports have less dense air, further reducing both thrust and aerodynamic lift.
Configuration: Flap setting and gear position determine drag; higher flap angles increase both lift and drag, trading off differently in each segment.

Crews and dispatchers use the AFM climb-limit charts (or performance software) by entering the airport pressure altitude and outside air temperature (OAT). The intersection of these variables yields a maximum weight for each segment; the limiting segment — almost always the second segment — produces the climb-limited takeoff weight. No runway length, slope, or obstacle data enters this calculation. If the actual weight exceeds the climb limit, the only remedies are to reduce payload or fuel, wait for cooler temperatures, or use a higher-thrust engine rating if approved.

Obstacle-Limited Takeoff Weight

The obstacle-limited takeoff weight accounts for the terrain and man-made obstructions that lie along the departure path. Under 14 CFR 121.189 and the associated AFM data derived from Part 25, the net takeoff flight path — which is the actual gross flight path reduced by a 0.8% gradient accountability factor for two-engine airplanes (1.0% for three-engine, 1.1% for four-engine) — must clear all obstacles in the departure corridor by at least 35 feet vertically. Lateral obstacle clearance requires the net flight path to remain within a corridor bounded by 200 feet on each side of the runway centerline, expanding at a specified rate as the airplane climbs.

The net flight path concept is critical: the regulations do not use the gross (actual) flight path for obstacle accountability. The gradient deduction builds in a safety buffer, ensuring that real-world variations in performance, piloting technique, and atmospheric turbulence do not result in an actual obstacle strike when the airplane is flying close to its limit.

To determine the obstacle-limited weight, the operator must:

Identify all obstacles within the departure corridor using Obstacle Departure Procedures (ODPs), Standard Instrument Departures (SIDs), or a certified obstacle analysis conducted under AC 120-91 or the operator's approved OpSpec.
Use AFM obstacle charts or certified performance software to find the weight at which the net flight path — accounting for wind, temperature, pressure altitude, and flap setting — clears all obstacles by 35 feet.
If a specific obstacle is limiting, the dispatcher may coordinate with the crew to use a different departure routing, a reduced flap setting (which trades field length for a steeper climb), or engine-out procedures that avoid the obstacle laterally.

Unlike the climb limit, obstacle analysis is highly airport-specific and route-specific. Two flights from the same ramp to different runways on the same morning may have entirely different obstacle-limited weights because the departure corridor changes completely with runway selection.

Why It Matters: Choosing the Most Restrictive

After completing both analyses, the crew and dispatcher compare results. The lower of the climb-limited weight and the obstacle-limited weight becomes the maximum allowable takeoff weight for that departure. This number is then compared against the field-length-limited takeoff weight (which ensures the airplane can accelerate to V1 and stop, or continue and be airborne within the available runway) and the tire speed limited weight (ensuring V1 does not exceed the rated tire speed). The most restrictive of all these limits governs. Operating above any single limit is an illegal and unsafe act.

The real-world implication: a flight leaving a high-elevation airport on a hot afternoon may be severely climb-limited regardless of runway length. Conversely, a departure from a sea-level airport surrounded by mountains or tall structures may have ample climb performance but a very tight obstacle-limited weight. Operators at challenging airports often develop Engine-Out Escape Routes (EOERs) — specific OEI turning procedures that route the net flight path through gaps in the obstacle environment — allowing a higher obstacle-limited weight than a straight-out departure would permit.

Key Numbers and Rules

Critical engine fails at V1 for all takeoff performance calculations.
Second-segment climb gradient minimums: 2.4% (two-engine), 2.7% (three-engine), 3.0% (four-engine) — usually the most weight-limiting segment.
Net flight path = gross flight path minus 0.8% gradient (two-engine), 1.0% (three-engine), 1.1% (four-engine).
Obstacle clearance requirement along the net flight path: 35 feet vertically.
Governing regulation for Part 121 turbine takeoff performance: 14 CFR 121.189.
Airworthiness climb and obstacle standards developed under 14 CFR Part 25, Subpart B.
The most restrictive of climb, obstacle, field-length, and tire-speed limits governs actual takeoff weight.

Common Test Traps

Confusing gross and net flight path: The exam frequently tests whether candidates know that obstacle clearance uses the net (derated) path, not the gross path. Always subtract the gradient factor before checking obstacle clearance.
Assuming climb limit is always more restrictive: On cool days at sea-level airports, the obstacle limit often governs instead. Never assume — calculate both every time.
Forgetting that temperature drives the climb limit: Students often think field elevation alone sets the climb weight. Temperature affects thrust more directly than elevation at many airports; hot and high together are worst.
Applying the wrong gradient deduction for engine count: The net-path deduction is not the same for all airplane types. Confusing 0.8% (two-engine) with 1.1% (four-engine) will yield wrong answers on performance problems.
Ignoring lateral obstacle accountability: Obstacles to the side of the departure corridor are accounted for; it is not just vertical clearance. Widening the corridor at a rate tied to the departure procedure can expose previously clear paths to new obstacles when runway assignments change.