Icing up on the ground in the tropics
 

Yes, the phenomenon is called Non-Environmental Icing (NEI) or Cold Soaked Fuel Frost (CSFF).
NEI/CSFF forms as a result of cold soaked fuel in contact with the upper surface of the wings cooling the surface towards the fuel temperature. The surrounding air then cools to below its dew-point, freezes, and adheres to the wings. This causes a layer of ice/frost to form. This is similar to how condensation occurs on a glass with ice cubes.

Keep in mind that warm air can also hold more moisture (relatively speaking), so warm humid destinations are at risk. NEI/CSFF has been experienced at temperatures up to +20C.

Remember the aircraft skin has been been flying at an OAT of ~ -60C and the fuel inside the wing been as low as ~ -40C. Both will warm up a little during descent, and adding warm fuel will raise the average temperature in the tanks. The more 'warm' fuel you add, the faster the temperature rises, landing with more fuel in wings aggravates the problem as a result.
But it cannot be emphasised enough the risks that can be encountered when ice formation occurs. Especially if moisture (rain) were to fall on such a cold surface, forming clear ice.

There is no comparable to the BA 777 in LHR however, as this was caused by an unfortunate combination of events and design, which caused fuel filters to get blocked by ice.

A minor nit - it wasn't the fuel filters (which have a bypass if blocked), but the fuel/oil heat exchangers that were obstructed by ice enough to prevent the engines from responding to an increased thrust demand. The heat exchanges were re-designed to prevent a recurrence, and newer designs now incorporate a bypass into the heat exchangers that will bypass fuel around the heat exchanger if it becomes obstructed.

Years ago my boss had me investigated an icing damage incident during a 767 flight test. They had been doing a flight test which required a several hour cold-soak at altitude (39k IIRC). When they finished up, they were right over Everett so they threw up the spoilers and rapidly descended to land at Paine Field. It was a warm, relatively humid summer day and although they were never in 'icing' conditions, large amounts of ice formed on the engine inlet then shed doing minor damage to the fan but ruining the acoustic panels in the fan duct. It was interesting - the pilots swore up and down they'd never been in icing conditions so it couldn't have been engine icing - but I talked to several people in the back who had witness the ice forming on the inlet but failed to inform the flight deck...
Live and learn :rolleyes:

What's odd is the OP says this was after a 10 hour flight. Even if the aircraft was topped off to tanker fuel I can't imagine any airliner more than half full after 10 hours. I've seen frost on the bottom side of the wing many times. Boeing has guidance as to how much is allowed for departure on some types..

"Half full' can still mean near full wing-tanks but an empty center tank.

When I operated the B747-200 we did tanker on occasion using the centre tank (within the max ZFW limitations ) and non standard usage for that very reason. Once on the ground a tank to tank transfer produced the required fuel distribution for departure.

What surprises me is that this discussion makes no distincttion between upper wing skin icing vs underside wing frost. Both may be do to the same cause (cold soaked fuel) but unless you have visible precip below 10C there is little concern for upper skin icing even with cold fuel if enough ulage clearance above the fuel level is present.

So I am perplexed what the big deal is with under-wing frost in the tropics?

Depends ...

Was sent up the front end of a north african operator which had just done three hours
 

Dead heading from geneva to zurich..eco fuel, our engineers had done the external check and not seen the thick layer of clear ice on the wing top. Eagle eyed ex luftwaffe skipper spotted it. Took two full goes with the deicers to clear it. Oat 13 iirc.