.
Spring from 21/3 to 20/06.

.
Summer from 21/06 to 20/09.

.
Autumn from 21/9 to 20/12.

This subdivision is globally accepted and was preferred to a traffic importance division that can fluctuate from country to country.

3.2.2. Day variation
Traffic importance varies between weekdays and weekend; traffic generally decreasing on Saturdays and Sundays. To overcome this point, data were collected for full week periods.

3.2.3. Country variation
The number of flights operating between countries in the world varies greatly e.g. Back Aviation retrieves 27139 flights for a day originating from the USA and only 353 flights from South Africa for the same day. Specific studies for each country could not be accommodated in the timeframe allocated to the project. For this reason, countries were grouped into the seven ICAO regions:

.
Asia and Pacific

.
Eastern and Southern Africa

.
Western and Central Africa

.
Middle East

.
North American, Central American and Caribbean

.
South American

.
European and North Atlantic

The classification of the countries in each region is shown in Table 71 in the Appendix.
3.3. Material

Back Aviation database is the essential freight data source to derive regional and world trends. Seven files, one per region (in respect of ICAO classification) were generated as Excel files. As for retrieving schedule data (Phase 1 Paragraph 4), similar constraints have to be set up (Table 44). The fields extracted from Back Aviation and displayed are shown in Table 45.
Table 44: Constraints linked to selected fields.

SELECTED FIELDS  CONSTRAINTS 
Number of stops  EQUAL TO 0 
Flight record type  EQUAL TO U 
Restriction code  EQUAL TO PUB  
Origin  EQUAL TO .CARIB .SAMER 
Equipment  NOT EQUAL TO .R .A 
Service-fare class   NOT EQUAL TO 2 3 4 

Table 45: Sample of a file extracted from Back Aviation database showing the fields displayed for 2002.
  BACK AVIATION SOLUTIONS OAG DATABASE 
VERSION: CD-ROM 1.4 - RELEASE: APRIL/2002(EXPIRES: MAY. 31/2002)   LICENSED TO: Sophie Michot - Eurocontrol   PUBLISHED NON-STOP PASSENGER DEPARTURES  
 NUMBER OF OPERATIONS BY: ORIGIN
 FLIGHT RECORD TYPE EQ U  RESTRICTION CODE EQ PUB   ORIGIN EQ 497 493 488 486 484 483 482 481 479 475 473 469 +...  EQUIPMENT NE .R .A SERVICE-FARE CLASS NE 2 3 4
 1 2 3 4 5 6 7 DAILY DATED 01 01 2002TO31 07 2002 REPORT DATED AT: APRIL 4-2002. TIME: 15:41
ORIGIN 20020101 20020102 20020103 20020104 20020105 20020106
AAE  8  11  10  7  9  11 
AAL  6  16  16  16  8  10 
AAQ  0  1  0  1  0  3 
AAR  5  11  11  10  4  7 
ABZ  23  61  70  71  25  36 
ACE  44  25  35  24  29  51 
ACH  0  3  3  3  3  2 
ACI  0  8  9  9  9  7 

3.4. Methods
From the worldwide collection of data, 42 merged inventories (6 weeks, 7 days per week) are created. From these inventories, the number of flights per region and for each representative day is calculated.
From Back Aviation database, the number of flights per region for each day of the year 2002 is retrieved.
Because Back Aviation database contains only commercial scheduled flights, it is necessary to estimate how many flights should be added to the number of flights obtained from Back Aviation. This is done by combining the two sources of information and calculating a ratio. The ratio is calculated for each representative day of each representative week such as:
　
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