A lease or a joint arrangement?
Five parties jointly buy an aircraft. By contractual agreement, each entity has the right to use the aircraft for a certain number of days each year and shares proportionately in the maintenance costs. They share decision-making regarding the maintenance and disposal of the aircraft, which are the relevant activities for that aircraft. Those decisions require the unanimous agreement of all of the parties. The contractual agreement covers the expected life of the aircraft and can be changed only by unanimous agreement.
Analysis
The agreement is a joint arrangement. Through the contractual agreement, the five parties agreed to share the use and costs of maintaining the aircraft, and decisions require unanimous consent.
Variation If, instead, the five parties entered into an agreement with a separate vehicle that controlled the aircraft this may be a lease. This would be the case if they did not have the ability to direct the relevant activities (e.g. if the management of the separate vehicle made decisions regarding maintenance or disposal).