An intersection take off is a departure initiated from an intersection line-up position other than the runway threshold without using the entire runway length. This results in an actual Take Off Distance Available (TODA) that is less than that published for the runway in use. An intersection take off may present certain operational advantages, but without appropriate due diligence and careful planning, an intersection take off has the potential to erode, or even negate, safety criteria, obstacle clearance capability and noise abatement requirements.
An intersection take off may be requested by the pilot or offered by air traffic control (ATC). In all cases, the ultimate decision to request, accept, or decline, an intersection take off rests with the pilot. As there are many potential pitfalls associated with an intersection takeoff, it is not something that should be undertaken unless appropriate performance planning and pre-consideration of other factors has been carried out.
The layout of most airports provides a point of runway access at, or near, the runway threshold from which take off is normally initiated. At many locations, however, multiple runway "threshold' access points, often on both sides of the runway, are used tactically by ATC to control aircraft departure sequence and make best use of runway capacity. These access points can be closely spaced or be many metres apart and, obviously, will not all provide the same length of runway for the take off run. At other aerodromes, the main taxiway intersects the runway at a point well before the actual runway end. In these cases, backtrack of the runway must be undertaken to achieve full runway length. Most major airports also have multiple taxiways that join the runway at various points along its length which are available for runway exit or, potentially, for runway entrance to facilitate an intersection take off. Not all runway access points or intersections are available for take off at all times. Potential restrictions may be imposed based on:
- Time of day - intersection take offs may only be permitted during hours of daylight
- Visibility - intersection take offs are generally prohibited when Low Visibility Procedures (LVP) are in use and may be subject to an even higher minimum visibility requirement
- Location - intersection take offs may be restricted to certain positions on the runway
There are some advantages to an intersection take off but they are relatively few. Primarily, an intersection take off has the potential to reduce taxy time by reducing the total taxy distance and potentially eliminate delays waiting for clearance to cross active runways. This, in turn, will reduce the amount of fuel burned during the taxy, reduce the noise signature of the aircraft whilst on the ground and reduce engine emissions during the ground period. An intersection departure might also provide a tactical advantage to the aircraft, especially when suggested by ATC. Depending upon the destination speed, route and altitude of the aircraft, an intersection take off can potentially allow ATC to change the aircraft departure sequence to help make maximum use of runway capacity. Note that this advantage may hinge upon the aircraft being fully ready for departure upon reaching the runway holding point for the intersection in question.
Whilst there can be advantages associated with an intersection take off, there are also potential pitfalls and liabilities which should be considered before requesting or accepting an intersection take off. These include, but are not limited to, performance criteria, obstacle clearance profile and adherence to noise abatement requirements. Historically, failure to consider these factors has resulted in accidents or incidents involving runway excursion, collision with obstructions in the departure path and noise violations.
For any aircraft, factors such as weight, configuration, pressure altitude, temperature and wind will determine the minimum ground roll required to achieve take off speed and become airborne. This ground roll will increase, sometimes substantially, when engine thrust is intentionally reduced by derate or assumed temperature (FLEX) protocols. Calculation of thrust requirements, and the corresponding Flight Management System entries, are generally made prior to engine start. In calculating the thrust requirements, the FMS (or paper chart) assumes a specific runway length, either published or extracted from the database. Initiating the take off from an intersection reduces the runway length available, thus invalidating the calculated performance. The invalid performance data can result in failure to become airborne prior to the end of the runway, a substantially reduced height crossing the runway end or inability to stop within the confines of the runway in the event of a Rejected Take Off.
Initiating the take off roll from a position other than the runway threshold effectively increases the risk that close-in obstacles such as lights or antennae might present. In combination with performance data that has not been corrected for the reduced runway length, this can (and has) result in collision with obstacles in the departure path.
In many locations, noise sensors have been installed along the published runway departure path(s) in the area immediately adjacent to the aerodrome. Noise levels exceeding a certain threshold will result in a violation against the aircraft which can lead to a fine, or to an operating restriction or prohibition. Initiating the takeoff from an intersection has the potential for an aircraft to overfly a noise sensor at a lower altitude (and, potentially, at a higher power setting) than would be the case had full runway length been utilised.
Beyond those issues previously identified, an intersection take off can result in other potential pitfalls and consequences. For example:
- a light, single engine aircraft using an intersection for departure will have less available runway surface available for re-land in the event that the engine fails immediately after becoming airborne
- the line up for an intersection take off provides fewer visual cues therefore increasing the chance of Runway Misalignment or a taxiway take off
- a Runway Incursion, or even an aircraft/aircraft collision, could occur if the controller becomes distracted and, after clearing an aircraft for a full length take off, clears a second aircraft to line up believing the second aircraft to be at the threshold when, in fact, it is at an intersection
- an aircraft begins to produce wake turbulence from the point at which it rotates. If an intersection departure is conducted, it becomes more likely that wake generated by the previous aircraft, using full runway length, will become a factor. This could lead to a Loss of Control event if departure wake turbulence separation criteria are not strictly applied
An intersection take off has the potential to reduce taxy time and ground fuel burn and, in some situations, may provide some degree of tactical advantage. However, as noted above, there are a number of potential liabilities and pitfalls associated with an intersection take off. Preplanning and careful consideration of those factors should be undertaken before requesting or accepting take off clearance from an intersection. Specifically:
- ensure that the performance calculations are made using the appropriate runway length
- if full runway length is required and a backtrack is required to achieve that length, ensure ATC is aware of that requirement
- at locations where ATC uses multiple departure positions on a tactical basis, base the performance calculations on the position that provides the shortest TODA. If full length is required due performance limitations, convey that requirement to ATC
- do NOT use performance calculations based on full length for an intersection take off unless the data has been validated for the reduced TODA for both runway and climb performance. If performance numbers for the intersection have not been pre-calculated, do NOT guess; take the time to complete the calculations and/or update the FMS, or decline the intersection and utilise full runway length.
Accidents and Incidents
On 18 September 2018, an Airbus A320 crewed by a Training Captain and a trainee Second Officer departing Sharjah was cleared for an intersection takeoff on runway 30 but turned onto the 12 direction and commenced takeoff with less than 1000 metres of runway ahead. On eventually recognising the error the Training Captain took control, set maximum thrust and the aircraft became airborne beyond the end of the runway and completed its international flight. The Investigation attributed the event to the pilots’ total absence of situational awareness noting that after issuing takeoff clearance, the controller did not monitor the aircraft.
On 5 February 2012, an Airbus A340-300 started its takeoff from an intermediate point on the runway for which no regulated takeoff weight information was available and had only become airborne very close to the end of the runway and then climbed only very slowly. The Investigation found that as the full length of the planned departure runway was not temporarily unavailable, ATC had offered either the intersection subsequently used or the full length of the available parallel runway and that despite the absence of valid performance data for the intersection, the intersection had been used.
On 8 December 2011, an Airbus A340-300 did not become airborne until it had passed the end of the takeoff runway at Rio de Janeiro Galeão, which was reduced in length due to maintenance. The crew were unaware of this fact nor the consequent approach lighting, ILS antennae and aircraft damage, and completed their intercontinental flight. The Investigation found that the crew had failed to use the full available runway length despite relevant ATIS and NOTAM information and that even using rated thrust from where they began their takeoff, they would not have become airborne before the end of the runway.
On 30 August 2016, a Boeing 777-300 crew began takeoff from London Heathrow at an intersection one third of the way along the runway using the reduced thrust calculated for a full-length take off instead of the rated thrust calculated for the intersection takeoff. As a result, the aircraft was only just airborne as it crossed the airport boundary and an adjacent public road. The Investigation attributed the data input error to crew failure to respond appropriately on finding that they had provisionally computed performance data based on different assumptions and concluded that the relevant Operator procedures were insufficiently robust.
On 6 October 2014, an A320 crew requested, accepted and commenced an intersection takeoff at Basel using reduced thrust performance data based on the originally anticipated full length takeoff which would have given 65% more TODA. Recognition of the error and application of TOGA allowed the aircraft to get airborne just before the runway end but the Investigation found that a rejected take off from high speed would have resulted in an overrun and noted that despite changes to crew procedures after a similar event involving the same operator a year earlier, the relevant procedures were still conducive to error.
On 1 October 2013, an Airbus A320 took off from a runway intersection at Porto which provided 1900 metres TORA using take off thrust that had been calculated for the full runway length of 3480 metres TORA. It became airborne 350 metres prior to the end of the runway but the subsequent Investigation concluded that it would not have been able to safely reject the take-off or continue it, had an engine failed at high speed. The event was attributed to distraction and the inappropriate formulation of the operating airline's procedures for the pre take-off phase of flight.