snowfalcon2, could you tell us the extent of the B777’s belly fairing?
I'm not sure if I understand the question 100%. I believe there are several posters here with a much better knowledge of the B777 tank construction than myself.
What I can add is the regulatory requirement. FAR 25.963 "Fuel tanks: General" states that:
Fuel tanks within the fuselage contour must be able to resist rupture and to retain fuel, under the inertia forces prescribed for the emergency landing conditions in §25.561. In addition, these tanks must be in a protected position so that exposure of the tanks to scraping action with the ground is unlikely.
To me, this means that the centre tank is inside the structure and thus frost formation on the outside of the belly due to cold fuel is highly unlikely. Maybe our Swedish contributor can confirm that.
Subject to the above, I don’t think absence of frost on the belly fairing would tell us anything about centre-tank contents.
Agree, see above. However, the centre tank does reach into the wings, as
this incident report shows, so in theory frost could form on the wing underside portion of the centre tank if it contains fuel.
It is interesting that while underwing frost is commonplace on parked airplanes, judging from the photos it's quite rare to form while the plane is airborne, as it did on BA38. How rare, is an interesting question.