I didn't change anything between itinerary and segment from BDM. Only thing i changed were related to Customer which i made as a composite entity. Even i assumed segment as a line item of itinerary.
If a Flight can have many segments AND if a Flight can have only one Equipment associated with it at a time, then are you presuming that all segments in the Flight would be flown using the same equipment? The relationship between a Flight and multiple Segments would exist at the same point in time as the relationship between a Flight and Equipment - the Itinerary is not going to be updated mid-travel to reflect the changes in Equipment.
I think this is more reasonable. BTW, I need to correct the original assumption I made - an Itinerary can have only 1 Departure and 1 optional Return. If you call each of these a "Reservation Line Item", then the association between Itinerary and Segment could be removed. If you compare the Prepare Itinerary and Change Itinerary use cases, you will see that the terms "flights" and "segments" have been used in a conflicting manner. Introducing a "Reservation Line Item" class seems to be the best way to go as far as I can tell.
Additionally, a Segment can indeed have many Flights (going by my earlier assumption of a Flight "qualifying" a Segment by Date, Time and Flight No.) because the same Segment is going to be used in multiple itineraries (by different Customers) for different Flights. The Segment would then have to be aware of all the Flights that are servicing it (obviously for searches to work).