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Icing up on the ground in the tropics

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Icing up on the ground in the tropics

Old 17th Apr 2019, 20:37
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Icing up on the ground in the tropics

Today I experienced something new as a passenger. After a ten hour flight the aircraft landed for its routine two hour refuel stop in the tropics. After re-boarding the aircraft we learnt that there was ice on the wings caused by the very cold fuel on board. After taking on more warm fuel and spending over half an hour circulating the fuel to warm the wing the engines were started up to aid the warming process. Unfortunately, due to its tropical location, there are no need to have de-icing facilities at this airport. After a short taxi to the holding point, another half hour or more was spent hoping the ice would melt it was decided to return to the apron to shut down........and attack the ice with a broom, something I have done in the past in a light twin, but would have thought this would be a bit hazardous on top of a large twin jet! The return to the apron moved the wings enough to free the ice and we were finally able to depart.

My question to those with the experience of large jet ops is whether or not this is a common occurrence? My thoughts were drawn to the Heathrow 777 of a few years back which (I believe) had fuel icing issues. could this have had the potential to be similar?
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Old 17th Apr 2019, 21:38
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Sounds like the aircraft landed with a lot of cold fuel on board. Normally taking on a load of warm fuel will bring up the fuel temp to above the dew point and moisture/ice will not form on the wing. It seems on this occasion, it did not.
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Old 17th Apr 2019, 21:39
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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.
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Old 17th Apr 2019, 22:19
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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...
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Old 17th Apr 2019, 23:35
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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..
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Old 18th Apr 2019, 09:46
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Interesting, thanks for the replies ��
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Old 18th Apr 2019, 10:21
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Originally Posted by MarkerInbound
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.
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Old 18th Apr 2019, 10:35
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Originally Posted by Wizofoz
"Half full' can still mean near full wing-tanks but an empty center tank.
Yep, I’ve seen this on a T7.

It had arrived at the warm, maritime very humid tropical destination “tankering” a lot of extra fuel...I can’t remember the exact numbers but there was a significant amount left in the wings. By the time we arrived at the aircraft to take it home there was a lovely layer of ice on the top of the wing (inboard section)...which took quite some time to clear.

The company reviewed it’s tankering policy to that destination after that.
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Old 18th Apr 2019, 13:56
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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.
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Old 18th Apr 2019, 14:28
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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?
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Old 18th Apr 2019, 14:48
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Originally Posted by lomapaseo
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?


Not sure anyone was making a big deal of under-wing icing ..but I’m sure there has been at least one mention of upper wing skin icing due to the lack of ulage clearance and the problems that caused..
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Old 18th Apr 2019, 15:11
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Originally Posted by lomapaseo
So I am perplexed what the big deal is with under-wing frost in the tropics?
Depends ...
Originally Posted by the original poster
attack the ice ..... on top of a large twin jet
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Old 18th Apr 2019, 17:59
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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.
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