As said above there are probably too many factors to mention when making an aircraft and engine choice. In brief, the following (in no particular order) would need to be considered in an evaluation:
- Strip lengths and widths
- Passenger demand
- Cargo demand
- Cargo hold loading systems
- Gate size limitations
- Tarmac Strengths
- aircraft speed
- Aircraft economy levels
- maintenance Costs
- Turnaround time
- Fleet commonality
- Financing / Leasing / Ownership type
- Local regulator/authority certifications
- Public perception
- Life expectancy
- Crew availability
- maintenance facilities
- Spares inventory levels
- Political pressure (internally and externally)
- Delivery slots
- Future delivery options
A typical business case for the acquisition of aircraft would combine these factors into various "missions" that the aircraft must be able to complete. The business case would be looked at in stages, as touched on below.
For instance, in the Australian domestic market a benchmark mission for the range component would probably be the Sydney-Perth flight (SYD-PER), as this is the longest regularly flown domestic service with regards to flight time (east to west is into the prevailing high altitude winds - a percentile of maximum winds by month may be used to simulate seasonal fluctuation in range performance).
Over this, the expected total demand and daily demand distribution can be introduced to determine the aircraft sizes needed. As an example, the total determined or forecast daily demand may be for 500 pax per day. Using a 75% load factor (manageable level of demand v' supply) means that you would require roughly 670 seats per day to cater for demand. You will also have high cargo requirements over such a long distance, which will need to be taken into account.
It may be determined that the most demand is for early morning and around midday, with moderate demand for a mid-morning, afternoon and early evening flight. This could be broken into widebody and narrowbody aircraft requirements of around 200 and 100-150 seats respectively. The main players to evaluate would be the B767 and A330/340, and B737/B757/A319/320/321.
Some aircraft you would eliminate straight away. For instance, the A340 is a bit of overkill for this market type, so you would narrow it down to the A330 v' B767 for the widebody. As there are no problems with the range on these aircraft, the evaluation of the widebody would go to the next phase of the analysis (financial evaluation, airport infrastructure, fleet commonality, capacity, etc.).
In this instance the narrow body evaluation would provide more of a challenge. Load restrictions during maximum likely headwinds would be one of the first things looked at. From experience, the A321 and the B739 would both display problems with the range during certain times of year (Sep-Oct in Australia). The A319 and B73G may be determined as too small, leaving the A320, B738 and B757 as the only alternatives for this mission.
The B757 is an untested aircraft in Australia, so the evaluation is likely to stack against it. This is despite it having certain commonalities with the B767, and strong performance data in USA and European operations. So the decision comes down to the A320 and B738. Again, the financial performance (including with different engine types), infrastructure and fleet commonality issues would be evaluated.
If these characteristics are inseparable, then it comes down to manufacturer deals (financial and delivery slot times/options) and political influences (internally and externally). Once the decision is made, it is likely that the airline will use aircraft variants within the same family for other market requirements, capitalising on commonality benefits and/or existing production slots/options from the original order.
This then leads on to same markets operated by different airlines using different aircraft types. The evaluation may determine that the Austrailan based airline use B767's and B737's for its domestic ops, so it is only logical that these aircraft types would be front runners when expanding to international services. A carrier in SE Asia however may have found that they need A330's and A320's for their type of operations, leaving any markets that both carriers fly on being performed by different equipment types.
Fleet acquisition is a long and arduous process, with the above barely scratching the surface of what is required in the evaluation. having said that, it is a hell of a buzz being involved in decisions that may be worth billions of dollars.
P.S. If you read down this far without falling asleep or getting totally lost, you did bloody well