22ND ANNUAL CONFERENCE, Split, Yugoslavia, 21-25 March 1983
WP No. 74
Optimisation of Climb and Descent Profiles and Fuel Economy
This subject was first raised by the German Association at the 21st Annual Conference when they introduced a paper at Amsterdam highlighting some of the problems that are being caused by the introduction of the latest versions of jet aircraft and the recently increasing requirements of aircraft operators to conform with flight profiles which maximise fuel economy.
The following policy statement was adopted at the Amsterdam Conference: “ Aircraft Operations using optimum flight management system profiles shall not be permitted to cause other aircraft to operate uneconomical. Resolution B15 at Amsterdam required SC 1 to review procedures whereby ATC requirements were to conflict with flight management system profiles.
Current ATC systems have evolved from those originally set up to cope with the low performance transports of the 50’s and 60’s. Subsequently airspace changes have been made to cater for jet traffic and its higher cruising levels and better performance. However, throughout this evolutionary process it has been necessary to cater for not only high performance jet traffic but also for slower piston traffic. Because, until recent years, the density of traffic has been relatively light, for most of this time there has been sufficient flexibility to deal with the variable performance of aircraft. The 1970’s saw large increases not only in size but in numbers. Extensive delays started to occur due to the limitations of both airport capacity and available airspace. Secondary Surveillance Radar (SSR), scheduling committees, flow control, one way airways and more modern ATC techniques all combined to ease the congestion.
It has become apparent in the last few years that modern technology has enabled the airline companies to introduce aircraft whose performance far exceeds anything previously experienced by ATC. Coupled with this improvement in performance, provided that aircraft are allowed to operate in an optimum manner, a dramatic reduction in fuel consumption has become possible. This is achieved by using sophisticated flight management systems deriving information from 2 or 3 dimensional airborne navigational equipment.
The ultimate goal may well be the use of four dimensional R-NAV equipment in all air transport aircraft. This would include horizontal position, altitude and time factors, and might allow controllers to assign an aircraft a particular point and altitude to reach at a particular time, leaving the optimum flight profile to be calculated by an onboard flight management system.
Implementation of this sort of system would however require an improved datalink between the aircraft and the ground, such as Mode S, since the system would probably involve some form of “negotiation” between the on-board computer and the ground controller to agree a flight trajectory both fuel efficient and meeting air traffic control planning needs. In addition to the introduction of high performance aircraft, fuel costs have escalated to such an extent that they have become a highly significant factor in the economy of airline operations.
As a consequence of the above main factors, there is an increasing pressure on the air traffic services to provide a system that will cope with not only “normal” air traffic climb and descent profiles but also the requirements to optimise the flight profiles of the higher performance traffic now rapidly becoming familiar on busy routes. Significantly, at the same time a large increase of so called “commuter traffic” provided by third level operators has occurred throughout the world. Unfortunately the performance of this traffic appears to embrace all aspects of the performance envelope ranging from the small jet to the modern unpressurised workhorse. There is , of course, a finite limit on the amount of airspace available for air transport operations and it is becoming apparent that this limit is being rapidly reached. As a consequence the flight profile requirement of both low and high performance aircraft have to be catered for within existing airspace constraints.
The problem is aggravated by the fact that traffic growth is currently contained by the introduction of new aircraft (often of increased size) capable of higher rates of climb and descent than the traffic which ATC has generally been accustomed to handle previously, while, at the same time there is also a significant increase in commuter operations with unpressurised aircraft capable of barely 500’ per minute level changes.
The real problem facing us is the impact of the performance of modern jet aircraft on ATC systems designed to deal with the “worst case” climb/descent characteristics of lower performance aircraft. The situation is being aggravated by continuous requests by pilots of higher performance aircraft on already busy frequencies for higher levels or delayed descents. Unfortunately there is no easy solutions; the need for ATC separation is basically a safety requirement while optimum flight profile operation is an economic requirement. It is apparent that every effort must be made to ensure that the ATC system is flexible enough to accommodate the varying requirements of the present generation of aircraft.
Because , as a general rule , the same Standard Instrument Departures (SID’s) and Standard Arrival Routes (STAR’s) have to be used by both low and high performance aircraft they are restrictive in terms of levels to be reached and/or maintained and do not adequately cater for the optimum flight profile requirements of individual aircraft. Unless discrete SID’s and STAR’s are introduced for low and high performance aircraft ATC should have the ability to intervene tactically to permit restrictions to be lifted as soon as possible. Airways should be wide enough to accommodate more than one track, so that ATC has overtaking capability and the pilot need not be constrained to operate at a reduced Mach Number. Where this cannot be achieved, the introduction of one-way airways also allows parallel track implementation.
Operators should ensure that their flight management programme do not contain flight profiles that embarrass ATC – for example automatic levelling off to effect speed reduction (the Lockheed Tristar was a classic example – one major operator is known in fact to have changed the programme to avoid this particular embarrassment to ATC – it can be done!). Operators should ensure that their crews understand that ATC do not apply restrictions unnecessarily – indeed, by their very nature, all ATC restrictions applied to flights involve additional work for the controller. Planners must ensure that ATC system design requires as few ATC imposed flight profile restrictions as possible and should endeavour, if at all possible, to provide discrete routeings for aircraft of varying performance.
The flight profile requirements of second generation jets particularly in respect of fuel economy are posing significant problems for ATC. As long as ATC systems are designed basically for “worst – case” performance aircraft there will continue to be discrepancy between the requirements of high performance aircraft and the system. Systems must be designed for maximum flexibility and, if possible, provide discrete routeings for aircraft with differing performances. Where ATC and optimum profile requirements are incompatible the ATC requirement must take precedence because of the safety element.
Both controllers and pilots must show understanding and tolerance of each others needs.
It is recommended that:
The following additional policy statements be adopted on the optimisation of climb and descent profiles for fuel economy :
1. ATC system and associated airspace planning should ensure that the flight profile requirements of both high and low performance aircraft are met.
2. The provision of discrete SID’s and STARS’s for high and low performance aircraft is desirable.
3. When there is a conflict of interest regarding flight profile requirements, the safety requirements of ATC must take priority over the requirements of aircraft for economy of operation.
4. Controller training should include instruction concerning the performance requirements of modern aircraft and their flight management systems in order that ATC can, so far as possible, take account of operators’ flight management profile requirements.
5. Where airborne computerised management systems are used they should be programmed in such a manner that they do not create additional ATC problems (e.g. levelling out for speed reduction/acceleration).
Last Update: September 20, 2020