The invention relates to a seat which can be converted into a couchette with a deformable armrest, and more particularly such a seat for a vehicle such as an aircraft comprising a squab structure and a backrest structure mounted so as to pivot with respect to the squab structure between a raised position and an elongated position, and a deformable armrest structure comprising a front upright and a support member, the upright being pivotally connected at one of its ends to the front part of the seat structure and at its other end to the front part of the support member so that the deformable structure forms an armrest when the backrest structure is in the raised position and so that the upright and the support member come into line with each other at the level of the squab structure and backrest structure when the latter is in the elongated position.
Such a seat is already known, for example through the document EP-A 0 982 226.
In this known seat, the rear part of the support member is articulated on the backrest structure.
When the backrest is in the raised position, the support member is in the high position held up by the upright at its front end and by the backrest at its rear end. The other end of the front upright is connected to the squab structure.
When the backrest is in the elongated position, the support member is in the low position and thus comes to locally widen the couchette formed by the squab and the backrest of the seat.
This arrangement assumes however that the support member, the upright, the squab structure and the backrest structure form a deformable parallelogram.
This has a number of drawbacks for the comfort of the occupant of the seat, in particular in the intermediate positions between the raised position and the elongated position.