A runway may be advertised as 3,000 metres long, yet the takeoff or landing distance available to a particular aircraft may not be 3,000 metres. Aviation therefore uses four declared distances: TORA, TODA, ASDA and LDA. They describe the portions of runway, clearway and stopway that an airport has formally made available for different parts of takeoff and landing performance. Understanding them explains why two aircraft using the same strip of pavement can work with different performance numbers.[1][2]
The four declared distances
TORA is Take-Off Run Available. TODA is Take-Off Distance Available. ASDA is Accelerate-Stop Distance Available. LDA is Landing Distance Available. They are not interchangeable names for runway length. Each corresponds to a different certified performance problem, and an airport can publish different values for opposite directions on the same physical runway.[1]
TORA: the ground run
TORA is the length of runway declared available and suitable for the ground run of an aircraft taking off. In simple terms, it is the runway surface available for the takeoff roll. It does not automatically include every paved area visible beyond the markings. Blast pads, stopways and other pavement may have different declared purposes.[1]
TODA: takeoff distance including clearway
TODA is TORA plus any clearway that can legitimately be included under the applicable standards. A clearway is an area beyond the runway over which an aircraft may make part of its initial climb. Because the aircraft is airborne while using this portion of takeoff distance, the area does not have to function as ordinary load-bearing runway pavement.[1][2]
Why TODA can exceed the paved runway
This is one of the most counter-intuitive features of declared distances. A performance calculation can use a takeoff distance greater than the takeoff run because part of the certified takeoff path occurs after liftoff. That does not mean the aircraft is allowed to roll beyond the runway. It means an approved clearway contributes to the airborne portion of the performance calculation.
ASDA: the rejected-takeoff problem
ASDA is the length available for an aircraft to accelerate and then stop if the takeoff is rejected. It normally consists of TORA plus any declared stopway. This distance is relevant because takeoff certification considers both continuing after a critical failure and stopping after a decision to reject, under the assumptions applicable to the aircraft.[1]
What a stopway is
A stopway is a defined rectangular area beyond the end of TORA prepared so an aircraft can be stopped in the case of an abandoned takeoff. It is not simply “extra runway.” Its design purpose is different, and it is not normally counted as ordinary takeoff-run or landing distance unless separately declared and marked for those uses.[1]
LDA: the landing distance
LDA is the length declared available and suitable for the ground run of an aircraft landing. It can be shorter than the physical runway because of a displaced threshold, obstacles, pavement limitations or other operational factors. Pilots and dispatch systems therefore use published LDA rather than assuming the full pavement length is available after touchdown.[1][2]
A displaced threshold changes the picture
A displaced threshold moves the beginning of the landing portion farther down the runway. Pavement before the threshold may still be usable for taxi or for takeoff in one or both directions depending on the airport design and markings. For landing in the direction of displacement, however, touchdown must occur beyond the threshold and LDA is correspondingly reduced.
Why thresholds are displaced
Reasons can include obstacle-clearance requirements on the approach path, pavement strength or environmental and operational constraints. The displacement is an engineered way of changing where landing distance begins without necessarily abandoning the entire piece of pavement. The airport’s published aeronautical information defines the usable distances.
The same runway has two directions
Runway 09 and Runway 27 are the same strip of pavement used in opposite directions, but their declared distances need not match. A displaced threshold at one end, terrain beyond one departure end or a clearway available in only one direction can produce different TORA, TODA, ASDA and LDA values. Performance planning is therefore directional.
A simple 3,000-metre example
Imagine a runway with 3,000 metres of takeoff run, a 300-metre clearway and a 150-metre stopway. TORA could be 3,000 m, TODA 3,300 m and ASDA 3,150 m. If the landing threshold were displaced 250 m, LDA in that direction might be 2,750 m. The physical site has not changed between calculations; the declared purpose of each area has.
Why pilots cannot simply use whichever number is largest
Each number belongs to a specific performance requirement. TODA cannot be substituted for ASDA because an airborne clearway cannot help an aircraft stop on the ground. A stopway contributing to ASDA does not automatically increase LDA. Performance software pairs each aircraft requirement with the correct declared distance.
Takeoff performance is a balanced problem
For transport-category aircraft, takeoff planning considers acceleration, engine failure assumptions, continued takeoff, rejected takeoff, obstacle clearance and climb requirements. Available runway distances are only one side of the equation. Aircraft weight, thrust, flap setting, wind, temperature, pressure altitude, runway slope and surface condition all influence the required distances.
Why heat reduces runway capability
Hot air is less dense. Engines and wings can therefore produce different performance than under cool standard conditions. At a fixed runway, a hot day may reduce the maximum permissible takeoff mass. The declared distances remain physically unchanged, but the aircraft’s required performance changes.
Altitude matters too
High-elevation airports have lower atmospheric pressure and density. True airspeed for a given indicated speed is higher, and propulsion performance can be affected. This is why a long runway at a high, hot airport may not provide the same payload capability as a similar-length runway at sea level.
Wind changes the ground distance
A headwind reduces the groundspeed required to achieve a given airspeed and normally improves takeoff and landing distance. A tailwind does the opposite and is tightly limited. Airlines calculate performance using approved wind rules rather than estimating visually from a windsock.
Runway slope
An upslope can make acceleration harder while assisting stopping; a downslope can have the opposite effects. Certified performance tools account for slope within approved limits. Two runways of equal declared length can therefore offer different practical aircraft capability.
Wet and contaminated runways
Water, snow, slush or ice can reduce braking action and change acceleration and stopping performance. Modern runway-condition reporting provides structured information for crews and performance systems. A 3,000-metre LDA on dry pavement is not operationally equivalent to the same 3,000 metres when braking performance is degraded.
Intersection departures
An aircraft does not always begin its takeoff roll at the runway threshold. ATC may offer, or a crew may request, an intersection departure. In that case the available TORA, TODA and ASDA from the intersection are shorter. The crew must have performance calculated for the actual departure point before accepting it.
Why an intersection can still make sense
A lightly loaded aircraft may need far less runway than is available. Starting from an intersection can save taxi time and reduce runway occupancy without compromising safety when approved performance margins are satisfied. The fact that the aircraft does not use every metre does not mean the unused pavement was unnecessary for heavier or less favourable conditions.
Reduced-thrust takeoffs
When runway and environmental conditions permit, many jet transports can use an approved reduced-thrust method rather than maximum available takeoff thrust. This can reduce engine wear. The calculation still has to satisfy all required takeoff performance criteria using the declared distances; reduced thrust is not chosen casually by the crew.
Obstacle clearance can be more limiting than pavement
A runway can be long enough for the ground roll yet still restrict takeoff mass because terrain or obstacles affect the required climb path. TODA and published obstacle data therefore interact with aircraft climb performance. The limiting weight can come from runway, climb, obstacle, brake-energy or other constraints.
Why runway extensions change several numbers
Adding pavement does not automatically increase every declared distance by the same amount. Airport designers must consider thresholds, obstacle surfaces, stopways, clearways and usable pavement. Aeronautical information is updated only when the relevant areas are formally declared available.
Declared distance is not the same as runway width
Length is only one geometric property. Aircraft also require suitable runway width, shoulders, pavement strength, taxiway geometry and obstacle clearance. A long runway cannot necessarily accommodate every aircraft type merely because its TORA and LDA are generous.
How crews receive the information
Declared distances are published in official aeronautical information and airport data used by flight-planning and performance systems. Crews cross-check runway, intersection and operational information against current charts and notices. Temporary closures can reduce available distance, which is why current operational data matter as much as the permanent airport diagram.
Why the terminology matters to passengers
A passenger may see hundreds of metres of pavement pass beneath the window before takeoff begins, or watch an aircraft turn onto a runway from an intersection. That does not mean the crew has accidentally surrendered safety margin. The usable distance for that specific takeoff has been calculated from the actual entry point and aircraft condition.
The key misconception
“Runway length” sounds like one number because physically there is one strip of pavement. Operationally, however, takeoff roll, airborne takeoff distance, rejected-takeoff stopping and landing rollout are different engineering events. TORA, TODA, ASDA and LDA exist so that each event uses the correct piece of airport infrastructure.
Conclusion
TORA tells an aircraft how much runway is available for its takeoff ground run. TODA can add a clearway for the airborne part of takeoff. ASDA can add a stopway for a rejected takeoff. LDA defines the surface available for landing rollout. Those four values transform a simple-looking runway into the precise performance geometry required by modern airline operations.
Sources / Technical References
- [1] ICAO, Annex 14 — Aerodromes, Volume I, declared-distance definitions and aerodrome standards — https://www.icao.int/
- [2] FAA, Aeronautical Information Manual and airport/runway information — https://www.faa.gov/air_traffic/publications/
- [3] FAA, Advisory Circular 150/5300-13, Airport Design — https://www.faa.gov/airports/resources/advisory_circulars/
Disclaimer: This article is general aviation education. Declared distances, performance methods and operational limitations must be taken from current official aeronautical, aircraft and operator documentation.