Tank frost margins
Just how closely the under-wing frost defines the ‘wet’ part of a wing tank is a matter for some conjecture. I don’t disagree with anything that’s been said; nor amend my suggestions above.
Swedish Steve is our most experienced B777 witness on the ramp, and has stated that the inboard edge of the tank is clearly discernable. This is, of course, where the fuel is deepest; and the massive rib that forms the end of the tank has itself been refrigerated by the fuel for many hours. At the opposite end of the tank (the end we are considering), the shallow fuel is lapping the shore, so to speak; and not contained by a rib in the presumed fuel state.
CONF iture has reminded us of the centre-section (belly) fairing, which on most types is deepest at the forward end. My dial-up connection discourages me from downloading snowfalcon2’s excellent photos of the B777. But other types have enough depth to accommodate the air-con packs at the front. Further aft, the forward base of the centre tank is also well within the fairing. For example, tank-pump canister-plates are only accessible via an access panel on the fairing; likewise the magnetic fuel-level-indicator sticks for the centre tank.
snowfalcon2, could you tell us the extent of the B777’s belly fairing?
Subject to the above, I don’t think absence of frost on the belly fairing would tell us anything about centre-tank contents. As for the wing tank, see my post above. To paraphrase what I said: presence of frost indicates presence of fuel; conversely, absence of frost does not necessarily define precisely where the fuel is absent.