Cranfield
College of Aeronautics Report No. 8614February 1986
2MEn986
Pressure Based Separation in the Upper Airspace
by M. E. Eshelby
TECHNISCHE «^n?:^rHOOL DELR
LUCHTVAART- L VAARTTECHNIEK
BIBLtOfHEEK
Kluyverweg 1 - DELFT
College of Aeronautics Cranfield Institute of Technology Cranfield, Bedford MK43 OAL, England
Cranfield
College of Aeronautics Report No. 8614February 1986
Pressure Based Separation in the Upper Airspace
by M. E. Eshelby
College of Aeronautics Cranfield Institute of Technology Cranfield, Bedford MK43 OAL, England
ISBN 0 947767 39 8
£7.50
"The views expressed herein are those of the authors alone and do not necessarily represent those of the Institute. "
"The views expressed herein are those of the author alone and do not necessarily represent those of the Institute"
Abstract
The vertical separation between aircraft above FL290 is 2000 ft which imposes a severe restriction on operations in the upper airspace. The possibility of reducing separations to 1000 ft has been discussed for many years but does not appear to be possible if current altimetry methods are used. This note proposes the use of atmospheric pressure as the means of providing upper airspace separation. Current technology in altimetry would be adequate to enable separations based on pressure differentials to be used which would allow a 50% increase in airspace utilisation between FL290 and FL350. The implementation cost of such a system is minimal.
Notation and Abbreviations
Abbreviations
ATC Air Traffic Control CRT Cathode Ray Tube ft foot, feet
PL Flight Level
ICAO International Civil Aviation Organisation ISA International Standard Atmosphere
m metre mb millibar
MNPS Minimum Navigation Performance Specification pa, kpa pascal (pressure N / m ^ ) , kilo pascal
Notation
g Acceleration due to gravity, m/s^ H Geopotential Height, ft
L Lapse rate, K/m
p Atmospheric static pressure, pa R Gas constant Nm/kgK
T Temperature, K
Suffices
0 at sea level, ISA 11 at 36089 ft or 11 km
1 -1. Introduction
The control of air traffic is based on a set of rules which are intended to ensure safe separation of one aircraft from another; these rules are known as separation standards and are internationally agreed by ICAO for world wide application. Both vertical and horizontal separation standards are defined but only vertical standards will be considered in this note.
Vertical separation must be applied when horizontal separation cannot be maintained and the current ICAO rules for vertical separation
are:-1000 ft upto flight level 290 2000 ft above flight level 290
The increase in vertical separation above FL290 is due to the inherent limitations of altimetres to discriminate pressure changes at such heights.
The 2000 ft separation represents a considerable restriction on the efficient use of the available airspace above FL290 and studies to reduce separations in that region to 1000 ft will require a major improvement in altimetry, or perhaps a completely new means of measuring vertical position or distance, both of which are likely to be costly. This note considers an alternative means of providing safe separation based on current technology; the use of a pressure differential, rather than pressure derived height differential, as the indication of separation.
2
-2. The Pressure - Height Relationship
The barometric altimeter senses local atmospheric static pressure and converts the pressure into height in units of length. The conversion of pressure into height is made by the altimeter calibration equations
(eqns 1 & la) and is based on the characteristics of the international standard atmosphere which is a simplified, linear atmosphere model. As height increases so pressure decreases and the change in pressure over a given vertical distance becomes smaller. The rate of change of pressure with height is given in eqns 2 & 2a.
-RL. ^ ^ 0 {^) 9o - 1 1) ^0 and g
-(nf+l)
P g r L H , '^ 0dH RT„ l^ ^ T„ 1 '^'
These relationships apply below the topopause at 36089 ft, above this height RT,, H = 36089 - — ^ I n ( i ^ ) la) and d£ Pll^o , Rll^(H - 36089) dH " " RT^^ ^ 23)
Figs 1 and 2 illustrate the change of pressure with height, and rate of change of pressure with height respectively.
The rate of change of pressure with height in the upper airspace varies from - 1641 pa/1000 ft at FL250 to -709 pa/1000 ft at FL450 in the standard atmosphere. The ability of the altimeter system to resolve pressure
differentials into heights will be proportional to the rate of change of pressure with height and a height will exist above which the altimeter
system performance will be unacceptable. The current regulations imply that FL290, which corresponds to a rate of change of pressure with height of -1421 pa/1000 ft, indicates the limit of acceptability of altimeter systems based on current technology.
The question needs to be asked, "What value is an indication of height in the upper airspace?" To answer this question the indicated height needs to be considered. The altimeter pressure setting in the upper airspace is 1013 mb regardless of the sea-level barometric pressure which may vary between
typically 980 mb and 1050 mb; an error in datum height relative to mean sea-level of about +1000 ft may therefore exist. If the atmosphere is non-standard either in respect to the datum temperature of the lapse rate (and this is
usually the case since the I.S.A. is only a simplified model) then the pressure height indicated by the altimeter will not correspond to the geopotential height at FL250. The lapse rate will similarly affect the pressure height. Geopotential height is based on the assumption of a constant value of the gravitational
acceleration at all latitudes and heights.
The combined effect of latitude and local atmosphere structure means that a pressure height of 25,000 ft referenced to 1013 mb could correspond to a true height above mean sea-level of anything between typically 23,000 ft and 27, 000 ft; thus the altimeter indications do not correspond to topographical heights and
are of no direct use in terms of terrain clearance for example. The value of a height indication is questionable and it could be described as a comforting guide to absolute vertical distance.
Relative distances, or separations, between aircraft flying in the same atmosphere at the same time and place, and measured by reference to the same atmospheric pressure are however acceptable since all altimeter systems use
the same calibration equation and sense the same parameter, local static pressure, and are referenced to the same datum, 1013 mb, or should be! Airworthiness
requirements limit the individual instrument error and system pressure error so that the overall error of an individual aircraft system is within
acceptable limits. The separation now becomes primarily a function of the ability of the altimeter system to resolve changes in pressure into heights.
4 -3. Upper Airspace Separation Criteria
In the upper airspace between FL250 and FL290 1000 ft separations are permitted which implies that, using current barometric height measurement technology, pressure rates of change of -1421 pa/1000 ft can be accepted as the separation standard. The arbitrary change to 2000 ft separations above FL290 arises from the calibration of the altimeters used in civil aircraft. The calibration in imperial units (the foot) leads to the logical conclusion that the altimeter pointer makes one revolution of the dial for each 1000 ft; and even in the case of altimeters with digital readout the analogue pointer has been retained to give an indication of rate of climb and descent. (It is also interesting to note that the same form of readout is found in CRT based systems which shows the hardened attitude towards altimeter displays). The 12 O'clock position of the pointer is used as the indication of each integral 1000 ft; pointer positions of other than 12 O'clock do not appear to be
acceptable on grounds of ambiguity so that separations of 1500 ft are not acceptable and the next logical separation interval after 1000 ft is 2000 ft. The problem therefore is not only one of instrument resolution but also one of instrument display design.
It should however be noted that separations of 500 m (1640 ft) are
permitted in the USSR in the 8,100 m and 12,100 m range of heights (26,575 ft to 39,698 f t ) , and that metric altimeters are also permitted in Soviet airspace.
If current thinking on height scales and altimeter display presentation is to continue then the prospect of reducing vertical separations to a half of their present value will require either a major improvement of barometric altimetry or an alternative means of assessment of height, both of which would be costly solutions.
If however a pressure differential could be used as the separation
parameter, rather than a distance derived from the pressure differential, then upper airspace utilisation could be increased substantially without recourse to improvements in barometric altimetry standards. Separation of aircraft by flying with reference to an indicated static pressure level, rather than a flight level, referenced to 1013 mb would lead to a progressive increase in vertical separation distance as height increased. Fig. 1 shows the operating levels which would be available if a pressure level standard was used compared with the current flight level standard. The pressure levels are at 1500 pa
intervals which in fact represents a small relaxation in the current separation standard of 1421 pa at FL290.
5
-4. Cost-Benefit Considerations of Pressure Based Separations
The benefits of using the pressure level system are summarised by fig. 1 and amount to an increase in airspace utilisation of 50% between FL290 and FL350 or 33% between FL290 and FL450. These benefits accrue from the graduated separations made possible by reference to pressure rather than the arbitrary separation based on the height scale. Table A shows that in the lower region of the upper airspace, between FL250 and FL290 the difference between the pressure level system and flight level system is minimal which suggests that it would be sensible to apply the pressure based separation to all aircraft in the upper airspace; such a proposal could probably come within the concept of MNPS upper airspace.
pressure level height flight level Difference Separation kpa (1013mb) AH ft ft 37.5 25061 250 36.0 25992 260 34.5 36956 270 33.0 27954 280 31.5 28989 290 30.0 30065 300
Table A Comparison of Pressure Levels and Flight Levels in Upper Airspace
A further benefit lies in the use of pressure level units in kilopascals rather than feet or metres to define the operating levels. This unit is
common to both metric and imperial height measurement and would enable
aircraft equipped with either system to share common upper airspace. The use of pressure levels and flight levels should not cause any ambiguity since they are different by an order of magnitude.
The costs of implementation of are small and possibly only software changes would be needed to flight management systems with CRT displays. A "pressure level" indicator would be included in the pilots display, along with the conventional altimeter for lower level operational needs, and to act as a "comfortor" in the upper airspace. ATC services similarly would need only software changes to accept the new operating procedures.
+61 - 8 -44 -46 -11 +65 931 964 998 1035 1076
6 -5. Conclusions
The ICAO Review of General Concepts of Separation Panel report (RGCSP/5) looks towards reductions in separations between FL290 and FL450 from 2000 ft to 1000 ft which calls for major improvements in altimetry or alternative techniques for height measurement. The suggestion proposed here is based on current technology and could be introduced quickly with only minor alterations to equipment or procedures; aircraft using the upper airspace would use
"pressure levels" rather than "flight levels".
The assessment of vertical separation would continue to be based on the atmosphere and be independently measured by each aircraft rather than depend on any external system for reference; satellites or radio altimeter and digital map for example. The current well known and proven techniques would be
retained.
The most severe objection to the proposal is likely to come from the human element which has been conditioned to read and believe the altimeter. The fact that the vertical separations used now are based on pressure and do
