Why "de-ice" on a sunny day?

[Eboy]

Waiting to fly out of Spokane, Washington (GEG) the morning of April 19, I was enjoying the sun beaming through the gate windows. It was around 40 deg. F or so, with hardly a cloud in the sky. So, I was surprised to see an America West aircraft (maybe a 737) get what looked to me like the de-icing treatment after pushing off and before entering the taxiway. Is this a precaution in case it rained and dropped below freezing the night before? I also noticed a United Express turboprop that apparently had the same treatment, with puddles underneath and condensing water vapor rolling up from the fuselage for 20 minutes or so. But, it appeared to be parked with no one in it -- I thought de-icing was normally done immediately before takeoff. Thank you for considering my question.

[quid]

de-icing

I'll bet the crew noticed "something" on the upper surface of the wings. De-icing procedures are very conservative these days, and if there's any frost, they'll be de-iced.

The glycol and water mixture for de-icing is usually heated, so it was steam you saw.

If there's no precip, there's no holdover times to consider, so de-icing fluid can be applied anytime prior to takeoff, no need to wait until the last minute.

[Panama Jack]

The only limitation is outside air temperature, and time. I have heard stories of crews arriving at airplanes that had been de-iced by highly motivated ground handlers many hours before they arrived (with sub-freezing temperatures) only to find that their aircraft was a pink popsickel

[mutt]

Why "de-ice" on a sunny day?

In some cases when you "tanker" fuel, the quantity and temperature of the fuel on arrival is sufficient to case a coat of ice on the wing service. De-icing may be required.


Mutt.

[Few Cloudy]

Fuel Ice

Mutt's point is probably closest to the truth. I once landed in Tunis (+14deg) on a slightly misty day after a two hour trip. During a forty minute stop on the ground, quite substantial ice formed on the wing lower surfaces. When we looked, sure enough, there was ice over the fuel tanks as well. The tanks at the wing roots were full of cool fuel, which we had tankered in.

The Fokker 100 has a circulation system to warm up wing tank fuel by sending it inboard and out again. The MD-80 at that time didn't. Why especially mention these two types? Because the ice, if not removed, cracks off at lift-off, when the wing bends and lands in your Tay or Pratt down the back.

In Tunis no deicing could be found and the FO and I ended up overwing, removing the ice with plastic trays.

[A Very Civil Pilot]

737 has the same problem, with cold soaked wings icing on warm days. I believe the -800 is worse, as it has a thinner wing.

[brownstar]

It may be required to de ice an aircraft in a number of situations - factors that would allow the formation of ice include high humidity, cold soaked wing, low fuel temperature ( if the aircraft has been cruising at high altitude the fuel temperatures can easily drop to - 20 to - 30 degrees). If you are on the ground with a lot of cold fuel it can form frost on the upper surface of the wing in 'short's and tee shirt weather'.

As the upper surface must be kept free from frost you may well see aircraft get de iced in all types of weather.

The 737-800 suffers this problem. Does anyone else know of other aircraft that are more prone to this kind of icing.

[Eboy]

Very interesting. I did not know frost was a concern, in addition to freezing precipitation. Thank you.

[beamer]

Pilots often get caught out by tanking fuel - simple remedy is to
limit the tanking in order that the fuel remaining in the wing tanks
on landing is at a level where the fuel is below the level of the top
surface of the tank. On some aircraft it is possible to ground transfer fuel from wing tanks to a centre tank to alleviate the
situation on turn-round - takes time and is a bit of a pain BUT even small amounts of frost on the top-surface inhibit the flow
of the boundary layer thus reducing lift which is not a good plan !

[Faire d'income]

De-icing and anti-icing

I thought de-icing was normally done immediately before takeoff.

There is a difference between de-icing and anti-icing.

De- Icing is the removal of any surface contaminants that have deposited or formed on the surface. The contaminant can remain long after the precipitation, or in the case of frost the conditions causing it, have ceased.

Anti-icing is the treatment of the surfaces to protect against future contamination e.g. during a snowfall. This is done close to take-off. Each type of contamination and each type of de-icing fluid are assessed on the day ( temp. etc. considered ) to give a' hold-over time'. This is the time by which you must get airborne after starting the procedure.

Light snowfall for example give long holdover times. Freezing rain/ drizzle give only minutes and are basically no-go conditions. Hope this helps.

[PlaneTruth]

Brownstar,

The B737NG series (700/800/900) all exhibit icing after cold soak events. This is partially due to the smaller wing tanks in the New Generation series coupled with the higher cruising altitudes they fly at (FL410 vs 370 for the Classic series). With higher (colder) cruising altitudes, longer flights, and less "thermal inertia" due to the smaller wing tanks, the fuel in the tanks upon descent is very cold. As the plane descends through warmer and often more moist air, ice and frost forms on the lower wing surface. Many operators use a heated water pressure washer system to remove the accumulated ice from the lower wing surfaces. It comes right off in the summer. It rarely forms in the winter with lower humidity levels.

PT

[moggie]

I believe the MD 80/83/88 was very prone to icing on top of the wing. My boss tells me of having to de-ice in the middle off summer in Morocco.

It was this that caused the SAS MD crash a few years back - wing flexing after take-off caused the sheets of ice to come loose and they went straight down the intakes for an instant double flame out!