GM1 ATS.TR.130(b) Determination of the transition level

EXPLANATION FOR THE CONSISTENT NEED FOR THE TERM ‘NOMINAL’ IN EU REGULATORY MATERIAL

(a)       Introduction

             ICAO Doc 4444 ‘PANS ATM’ Section 5.3.2 stipulates that the ‘vertical separation minimum shall be a ‘nominal’ 300 m (1 000 ft) below Flight Level 290’. However, the term ‘nominal’ is used inconsistently in ICAO provisions which relate to the vertical separation minimum. An example of such inconsistency may be found in ICAO Doc 7030 ‘EUR Regional Supplementary Procedures’ Chapter 6.3.1.2 (transposed with some modifications as point (b) of ATS.TR.130) which states that ‘the transition level shall be located at least 300 m (1 000 ft) above the transition altitude to permit the transition altitude and the transition level to be used concurrently in cruising flight, with vertical separation ensured’.

             In transposing ICAO provisions into the EU regulatory framework, it is considered that consistent descriptions should be used in relation to the determination of the transition level, in order to ensure that the flexibility permitted by ICAO through the use of the term ‘nominal’ is maintained.

(b)       History of the vertical separation minimum

             The advent in the early 1950s of commercial turbo jet aircraft operating at high levels necessitated a re-evaluation of the vertical separation minimum and thus, in June 1954, ICAO established the Vertical Separation Minima Panel. Based on the work of this Panel, the use of 1 000 ft vertical separation minimum between IFR traffic below 29 000 ft was agreed by ICAO at the 1958 RAC/SAR Divisional Meeting and incorporated within PANS ATM Section 5.3.2 as highlighted above.

             Although ICAO does not define ‘nominal’, when transposing ICAO provisions into EU legislation, it is necessary to have clear and consistent understanding of the terms in relation to the establishment of a transition level. Accordingly, the Agency notes the following factors relating to the use of the term ‘nominal’:

(1)       300 m is equal to 984.3 ft, whilst 1 000ft is equal to 304.8 m;

(2)       the vertical distance between flights at two altitudes or two flight levels, for example FL 090 and FL 100, will only be 300 m (1 000 ft) under conditions where the ICAO Standard Atmosphere (ISA) — ICAO Doc 7488 ‘Manual of the ICAO Standard Atmosphere’ prevails. When conditions in the atmosphere differ from the ICAO ISA, the vertical distance will be greater/less than 300 m (1 000 ft) respectively.

             Moreover, in addition to the equivalence between 300 m and 1 000 ft, other factors including variances between aircraft altimeter settings, aircraft total vertical error (TVE) and the dimensions of an aircraft above and below its static pressure source are encompassed within the term ‘nominal’.

(c)       Determination of the transition level

             The transition level is a function of the transition altitude of the aerodrome concerned and the difference between the aerodrome QNH altimeter setting value and the standard pressure setting. Regarding the relationship between pressure and height, the following is to be noted:

(1)       The relationship between height and pressure is non-linear. The specification of the ISA is set out in ICAO Doc 7488 ‘Manual of the ICAO Standard Atmosphere’.

(2)       The real atmosphere is rarely consistent with the ISA. Consequently, variations in the conditions specified in the ISA generate differences in the vertical distances between surfaces of equal atmospheric pressure, dependent upon an aircraft’s level within a particular column of air.

(3)       Altimeters are calibrated against the ISA as defined in ICAO Doc 7488.

(4)       The standard pressure setting is 1013.25 hPa as defined in ICAO Doc 7488.

             In those instances where an aerodrome QNH of 1 013 hPa  exists, a vertical difference of 6.8 ft exists between 1 013 hPa and 1 013.25 hPa (equivalent to a vertical distance of 27.3 ft per 1 hPa at mean sea level in accordance with the ISA). Where a transition altitude of 6 000 ft exists, this would result in a transition level of FL 75 as detailed below:

(1)       Transition altitude 6 000 ft + 1 000 ft (ICAO Doc 7030 EUR Chapter 6.3.1.2) = 7 000 ft

(2)       (1 013.25 hPa – 1 013 hPa) x 27.3 ft = 6.825 ft = 7 ft

(3)       Transition level = 7 000 ft + 7 ft rounded up to nearest 500 ft increment = FL 75

             Consequently, in those instances when the reported aerodrome QNH is 1 013 hPa, a flight level is ‘lost’ through the need to ‘round up’ by 493 ft; thus providing a vertical separation of 1 493 ft between aircraft cruising concurrently at the transition altitude and the transition level. In high-density/high-complexity airspace, the loss of airspace capacity that this represents, coupled with the second order effect of increased controller workload, is considered to have a significant impact by industry.

             Through the application of a vertical separation minimum of 300 m and considering the ‘nominal’ equivalence between 300 m and 1 000 ft, ICAO implicitly endorses a vertical separation of only 984.3 ft. Continuing the example given above, the inclusion of the term ‘nominal’ would permit the transition level to be FL 70, resulting in a vertical separation of 993 ft between aircraft cruising concurrently at the transition altitude and the transition level; a value which remains within the 300 m/984.3 ft equivalence. In this instance, the practical safety effect of a 7 ft reduction in the vertical distance between aircraft is considered to be operationally insignificant.