For larger (long range) aircraft the production flight tests could be undertaken during one flight or even amalgamated with delivery to a cabin completion centre. In my experience in the regional market a minimum of two flights were flown and a third snag clearance flight always planned before the aircraft was offered to a customer. Few if any major issues were found during flight test; although I do recall a complete air-system failure, no anti icing, and stuck flaps on one first flight. Generally the performance tests confirmed the certification requirements and compared with the type database which validated the build quality. The snag clearing flight was always a point of debate between production, ground and flights test, and the bean counters. For late production run aircraft (after the first 50) the production tests could easily be completed in two flights, providing the aircraft was complete. However in order to meet late customer requests, the advent of ‘just in time’ (always late) build policy, and need to meet set dates (keep the shareholders happy), the first flight was often undertaken with acceptable discrepancies (mainly in the cabin).
On rare occasions (large powerful customers) operator pilots participated in the production tests; the aim was to satisfy the customer and speed the acceptance process. This policy was fraught with problems; most line pilots had not experienced a flight test environment before, you cannot test fly an aircraft within airline SOPs. Furthermore the experience of operator pilots was variable; the presence of some on the flight deck was more hazardous than the test it’s self i.e. those who had not conducted a full stick push stall, or real engine out climb before. It was somewhat surprising to find so called experience pilots from leading aviation countries who had not stalled a large aircraft, flown with one engine shut down, or even exceeded 45 deg bank; and then there were those who though that the aircraft would disintegrate at VMO +5 !!!
Latterly common sense prevailed and customers were given up to 5 hours flight time for their acceptance against their own (usually very restricted) test plan. During this time flying with operator pilots enabled a positive transfer of knowledge about the aircraft, particularly where the manufacturer used training captains as production test pilots.