Thank you for your letter of 8 July about flight capping in UK airspace. I regret that statistics are not collected in the form requested in your letter. However, I hope it may assist the Committee in its deliberations if I explain how and why flight level capping is operated in European airspace and the statistics that are available from the Central Flow Management Unit (CFMU).
Flight level capping is the name given to the Air Traffic Flow Management (ATFM) procedure whereby a flight has a limit applied to the altitude/flight level at which it will be allowed to operate. This is usually applied to restrict the amount of air traffic entering a particular vertical sector of airspace in order to balance demand and capacity.
The introduction of reduced vertical separation minima (RVSM) created more useable flight levels above FL290. In order to optimise the use of these levels many Air Navigation Service Providers (ANSPs) redesigned the vertical sectorisation of their airspace. This created more capacity but this could only be realised if the demand could be shared between the different vertical sectors. Flight level capping is the method most employed to achieve this.
It is true that there is a cost to Aircraft Operators (AOs) in pursuing such a strategy since flights may be prevented from operating at their optimum flight level. Conversely there is an advantage in that such procedures usually reduce overall delay within the ATFM system.
Of course the imposition of a flight level capping procedure does not always result in aircraft not achieving their optimum flight level. At the moment there is no procedure in place to ensure that air traffic controllers do not change the flight level of level capped flights. Therefore the mere fact that a flight is captured in a flight level capping regulation does not mean that the regulation will be fully observed by all players. The result is that some traffic may still operate at their planned flight levels not their capped levels.
The data that we have in the CFMU pertaining to level capping is attached but I should perhaps give some introductory clarification. This data is based on the codes used to identify ATFM measures and it is true to say that until quite recently this coding has not been structured in such a way as to identify readily the causes for the application of a measure.
RVSM was introduced in the UK in April 2001, a year ahead of Europe as a whole in order to de-conflict such a major change from the opening of Swanwick in 2002.
Prior to RVSM there was very little level capping within the UK which is not surprising since the airspace structure was such that there would have been little capacity increase in so doing. I would like to now address some of your specific points.
- For the above reasons it is extremely difficult, if not impossible, to give an accurate and detailed picture of height capping activity in the UK in the three years prior to the opening of Swanwick. Similar difficulties would also occur in quantifying the height capping activity in the rest of Europe. The reliable data for the UK covers the period from June 2002 and this will give you some indication of how the situation has developed in the UK post the Swanwick introduction. I believe it is safe to say that prior to this flight level capping was only used exceptionally if at all.
- In the attached data table it is necessary to explain the data in each column.
- The first column is the week in which the flight level capping (known as a scenario) was applied.
- The second column shows the number of flights that could have been affected based on the AO's flight planned intentions.
- The third column shows the duration of such scenarios in minutes.
When a scenario is applied it can be one of three types
- Institutionalised flight level capping between city pairs or between geographical areas. These are permanently applied and an example in the UK is between Manchester/Liverpool and the London Terminal Area where traffic is restricted to "not above FL190". Such restrictions are much more
prevalent in Europe. In the UK there are only three such restrictions.
- A blanket restriction, known as a "zero rate" that is applied to a particular route on a tactical basis. For example in Swanwick's airspace the most common restriction would be to flight level cap aircraft flying between the South East of England and Scotland. Such a restriction would "free" extra capacity at higher levels for the traffic transiting the UK for entry onto the North Atlantic whilst possibly forcing the UK traffic to fly at less than optimum flight levels. This is a mandatory restriction and AO's are expected to adhere to it.
- Another form of scenario is the advisory one. Here AO's are advised to re-file at a different flight level in order to avoid delay caused by over demand. The AO is given the choice between delay or a less than optimum flight level. A criticism of this scenario is that AO's may not re-file and therefore the capacity available is not optimised and the delay attribution is increased.
Flight level capping is a common procedure within European airspace to optimise the use of available capacity in order to minimise attributed delay. Of course there is a cost to operators and the question is whether such costs are off set by delay reduction.
It is true to say that in the UK there had been a marked increase in flight level capping restrictions since July 2002 and that this has shown a marked reduction since the end of April 2003. At its peak there was in excess of 13,000 minutes of such restrictions per week, that is an average in excess of 30 hours of restriction per day. These restrictions impacted on average some 350 to 430 aircraft per day. Not all of these aircraft would have been subjected to flight level capping; some would have benefited from the optimisation of available capacity.
The above is my initial response to your letter and should you require a more detailed analysis please contact me. I hope I have given you some understanding of this complex area and that it will be beneficial to your Committee.
John Byrom
Head of Flow Management Division
Eurocontrol
29 July 2003