Increasing Range of Speeds for Same Aircraft Type
Increasing Range of Speeds for Same Aircraft Type
- Aviation safety professionals, particularly Aircraft Operators and Air Navigation Service Providers, were invited to share experience and good practices regarding the issue of increasing range of speeds for same aircraft type.
- One ANSP reported that they are currently experiencing an increasing range of speeds during climb and descent between different Aircraft Operators of the same aircraft type.
- ANSPs providing ACC Service have no basis to predict aircraft climb and descent speeds except their general past experience of the way Operators in their airspace fly the aircraft type.
- Historically, variation in Operator Policy on speeds for the same aircraft type has mainly been in cruising speeds, which are required to be filed in the Flight Plan and are therefore apparent for ATC tactical planning purposes (speed changes of more than 5% from that indicated in the FPL shall be reported to ATC).
- Since the reported diversification of flight parameters is believed to be a consequence of increased fuel prices and a varying focus on cost control between Operators, it seems likely that this problem will continue.
- This is an issue that may have possible safety implications. It changes the operational environment by requiring ATC not to assume a speed profile.
- It was reported that cost Index flying is becoming a concern, especially for ATC in oceanic environment, and developments are being monitored.
- All ANSPs are therefore alerted to the additional difficulty which this situation may present in achieving safe traffic separation and are specifically recommended to observe, and if necessary directly establish, the climb and descent speeds being flown.
- In this context, it may be worth noting that, between FL200 and FL250, a transition between IAS and Mach number speed control will often occur and this might have the effect of altering the initially observed or established climb and descent airspeeds.
The full content of the responses.
The views of the representatives of 22 Operators as well as views from 3 ANSPs and two other stakeholders were provided. Four of the Operator responses were provided through the Association of Asia Pacific Airlines (AAPA) and the comments on this subject from 8 of the Operators were provided on their behalf by the European Regions Airlines Association (ERAA) which conducted a specific survey to this end. Those 8 responses follow the other 19 in survey format. It is evident from the responses that there is general awareness of the potential difficulties for ANSPs arising from the increasing range of climb and descent speeds currently being used as an increased focus on the cost control of aircraft operation. The recent rapid rises in the cost of aviation fuel seems to have increased both the number of Operators looking more closely at the relationship between aircraft operating costs and the way their aircraft are flown and the degree of attention being given to this issue by Operators who have already begun the process of embracing it.
- There has been a recognition by ANSPs that the range of speeds being used by a given aircraft type during climb and descent has recently increased.
- Some Operators believe that some ANSPs may be unfamiliar with the rapidly spreading practice of cost index based flight planning. Many respondents noted that the recent rise of fuel price had resulted in significant changes to cost index based flight planning in general and to actual climb and descent speeds in particular.
- Some ANSPs and Operators accepted speed variation as a safety issue which might prejudice the tactical planning for safe separation by ATM whilst others were inclined to the view that it is just an operational reality for ATC which can be managed without serious prejudice to safety.
- The current absence of ATM controller training simulators with sufficient sophistication to represent realistic traffic performance with respect to varying forward and vertical airspeeds during climb and descent was noted.
- It appears that a complete transition to 250KIAS below FL100 would be helpful to ATM operational safety in those terminal airspace areas which do not already require it and that few operators would have any difficulty with this.
- ATM speed control of climb above FL100 does not currently appear to be very prevalent but Operators seem willing to accept general speed control if it is considered necessary to maintain safe separation. ATM speed control in descent seems more widespread but entirely tactical and Operators express the wish for timely pre-advice of ATC intentions so that they can plan an optimum descent within such constraints
- It was pointed out that an increasing range of forward airspeeds in climb and descent had also been accompanied by a similar increase in the corresponding range of vertical speeds, which could represent a similarly important tactical consideration for ATM operational safety.
- Operators note that optimum climb and descent airspeeds generally depend mainly upon aircraft weight but that the prevailing head/tail wind component remains a lesser factor. The extent to which minimum-fuel vertical profiles are preferred will often be affected by actual versus schedule performance of a flight on a particular day. One Operator also noted that variation in forward and vertical speeds for short periods could sometimes be associated with the minimisation of air turbulence and the rules relating to cabin service which may be restricted until above cloud. Another noted that alternative power plants on the same aircraft type can result in different optimum speeds under otherwise similar circumstances.
- Some Operators of long haul aircraft note that variable climb and descent speeds have long been a feature of their of operations. However, it is apparent that many more short haul operators are now becoming cost-conscious and that this has increased the extent of the issue for some ANSPs, particularly those managing periods of high traffic density.
- It appears that the habit of operating on an RPL which gives a particular cruise speed which on a particular day is then planned to vary from this quite considerably has become a widespread practice tolerated by ANSPs except in the NAT region where higher traffic densities have required a greater degree of both ATM control and Operator compliance. Acceptance of this by ANSPs because it is of little actual concern may have indirectly encouraged Operators to use wide tactical variation in climb and descent speeds which can become much more of a issue in busier airspace.
- Some Operators report that the ICAO requirement to advise ATC of any variation of more than 5% from the declared FPL cruise speed is widely ignored and that this is mainly because ANSPs have shown no interest or concern at the increasing prevalence of variation outside the 5% ‘allowance’.
- In appreciating that the trend to widely varying forward speeds might cause difficulties in busy ATM environments, Operators are prepared to compromise on optimum cost vertical profiles to the extent necessary to maintain ATM operational safety
- As the proportion of aircraft with more sophisticated FMS capabilities increases, operators are more easily able to systemise their tactical flight planning and remove some of the decision making from individual crews.
- It is noted that flight crew are not always able to provide accurate responses to ATC queries on whether they will be able to achieve certain fight levels by given positions. Whether this difficulty can be generally attributable only to aircraft with legacy navigation system capability or whether this is not a factor is not reported.
- One respondent noted that the prospect of increasing numbers of VLJs could well complicate tactical forward speed variation.
- Whilst many airlines are now using cost-index based tactical flight planning, one respondent noted that business jet operators are, subject only to meeting landing slots, tending towards fuel efficiency alone as a driver.
- It appears that cost-index based flight planning using generic tables is beginning to be superseded by ‘dynamic cost index’ planning which significantly increases the variation in desired climb and descent speeds for any particular aircraft type and operator on a day-to-day basis.
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