It all flows from the Mission - define the mission, derive an operating concept, derive operational requirements, which lead to user requirements, which lead to system requirements that lead to system features. The actual aeroplane isn't at the top of the operational hierarchy - it comes a couple of steps down the tree. The following comes from a paper on the subject which started from the premise that:
"Engineering could be described as the field of endeavour associated with devising systems that provide a capability to undertake a mission"
Where “Systems” would be any combination of machines, people, skills, infrastructure and information, a “capability” would be the ability to utilise the system when required for its intended purpose and a “mission” is some activity required to meet a political or economic need, either as discrete events or a continuous activity. From this flows a view that can be visualised as a "mission pyramid":
The problem with trying to jump straight to the systems requirements stage is that you can't establish the root justification for each requirement in terms of how it contributes to (or enables) mission success. They just become rabbits pulled from a hat, and that presupposes that some has actually put some rabbits
into the hat in the first place.
HTH,
PDR