opening 72 extending from one side of the fuselage 12 to the other across the vertical midplane p of the fuselage the top opening 72 thus opens towards the top and towards the sides of the fuselage moreover the method comprises assembling the elements that make up the box structure 32 of the engine mounting structure 24 notably the spars 26 and 28 the ribs the skins 37 a and 37 b and as appropriate the engine attachment structure and the engines for the sake of clarity the engines and the structure for attaching these engines have not been depicted in fig1 assembly of the engine mounting structure 24 may be performed prior to assembly of the fuselage 12 or after assembly of the fuselage or alternatively in parallel therewith next the method comprises inserting the box structure 32 of the mounting structure 24 into the top opening 72 in the fuselage as symbolized by the arrow 90 in fig1 followed by attachment of the box structure 32 of the mounting structure 24 to the fuselage for example using the mounts 80 82 84 and the load reacting link rods 86 all described hereinabove in parallel with these operations of inserting and attaching the box structure 32 of the mounting structure or alternatively before or after these operations the empennage 16 is also mounted on the fuselage 12 as illustrated by the arrow 92 in fig1 in the first embodiment of the disclosure herein the method next comprises attaching one or more upper cross beams 76 to the forward 66 and aft 68 circumferential frames so that each upper cross beam connects these two circumferential frames together as shown by fig2 in the example illustrated in fig2 a single upper cross beam 76 is depicted and this is centred.