Ice on Wings

[shon7]

Why does ice not form on the wings/fuselage when airplanes are taking off in the rain and flying to higher altitues? Are the wings heated?

[TURIN]

Sometimes it does!!!!!!!:{ :rolleyes:

[Superpilot]

Speed? Most Jet/Turboprop departures achieve 250KTS very quickly after departure. Ice finds it difficult to stick to a wing after 250KTS due to kinetic heating (i.e. the aircraft skin gains temperature as it goes through the air). Just a layman's explanation.

[Gary Lager]

We have all frequently encountered wing icing at speeds in excess of 250kts.

The wing may be kinetically heated: heating is about 8 degrees per 100kts (I think). Don't forget that over the leading edge of the wing a drop in local static pressure causes a drop in temperature too - all this means than even at high speeds, flight through moisture even at relatively high Total Air Temperature (i.e. above freezing) can still cause ice to form.


I refer you to Turin\'s answer!

[Mad (Flt) Scientist]

The specific question seems to be asking about takeoff in rain and then temperature drop affecting the "wet" wing, rather than icing at altitude.

A common rule is that an aircraft is considered to be in "icing conditions" if any of:

a. on ground, OAT 5 deg C or lower and "visible moisture" - which could be clouds, rain, mist, snow

b. in air, TAT up to 10 deg c, visible moisture as above

c. ice detection system detects.

Under CERTAIN circustances, (c) can override (b) i.e. if (c) says "no ice" and (b) says "maybe ice", then you can assume "no ice". In other cases you have to go with the more conservative, if either says "ice" you assume "ice"

However, all of those are intended to cater for direct ice accretion on the aircraft i.e. you're flying along and start to accrete ice on what may have been a dry wing.

For the case of a "wet" wing, if the takeoff is near freezing, you are at significant risk of ice forming; hence the 5 deg C typical limit up to which you'd assume icing conditions prevailed. If you take off in a summer rainstorm though, the chances are that if you clear the rain before climbing too high, so that you have a "wet and warm" wing, the water will almost all blow off in the airflow; after all, a so called "running wet" anti-icing system leaves water on the protected surfaces, which does exactly that. So for the SPECIFIC case raised, the risk of icing is low to nil.

[hawk37]

Mad Scientist,

Any chance you can comment on the reason Bombardier will use SAT to determine icing conditions and the requirement to use anti icing bleed air (lear 45, between +10 and -40 deg C) while Dassault uses TAT (Falcon 50, at or below +10 deg C). Both use compressor air to warm the wing leading edge

Seems SAT sounds safer, since TAT can often be greater than 10 deg C yet SAT is well below 0 deg C. Yet Dassault can hardly have had any problems in 26+ years of Falcon 50 ops, or they'd have made changes to the AFM.

And with very similar engines, Dassault uses the Honeywell 731-3 engine and TAT, Bombardier uses the newer Honeywell 731-20 engine, and SAT.

Hawk

[arismount]

The "ram rise" or increase in temperature of the air when it goes from free-stream velocity, i.e., True Air Speed to stagnation point may be found by taking the uncorrected Outside Air Temperature (OAT) and computing a "rough" value for TAS, then squaring that value and dividing by the constant 9500.

This will give you the temperature ram rise in degrees C.

This rise may then be subtracted from the uncorrected OAT to obtain actual air temperature, i.e., SAT.

The SAT value thus found can be used to re-calculate TAS and obtain an accurate value for same.

This is the way it used to be done when all we had was an uncorrected temperature probe and a hand-held mechanical computer.

Mathematically:

Ram Rise in Deg. C = TAS sqrd / 9500

Uncorrected OAT - Ram Rise = SAT

[Denti]

There are some weird values out there for anti ice usage, our restriction is on the ground OAT 10° C or lower with visible moisture, in the air 10°C TAT as upper limit, lower limit is -40° SAT though. Thing is we get TAT shown on the panel, SAT only on a progress page in the FMC.

[Old Smokey]

shon7,

Seeing as you mentioned the word "rain", Freezing Rain is considered as potentially the worst meteorological condition conducive to ice formation. Our company's operations prohibit takeoff in Freezing Rain.

Having said that, in conditions other than freezing rain, the temperature range from TAT +10°C to SAT -40°C is generally considered as 'the iceing range'. The upper limit (TAT +10°C) is indicative of skin temperature, and thus the upper temperature limit at which ice can adhere to the aircraft skin. The lower limit (SAT -40°C) is an atmospheric condition, as super-cooled water droplets can exist down to that temperature. At lower temperatures, any atmospheric water droplets will be frozen already (unless someone turns the thermal anti-ice ON, melts them, allowing re-freezing further aft on the wing, it's been done).

Some aircraft have SAT indicators, some TAT, and some both. Both is best, as the upper and lower limits can be accurately ascertained. Aircraft with RAT indicators, or only one of either SAT or TAT, must be examined by the manufacturer to provide information to the pilot of the 'adjusted' criteria pertinent to the aircraft installation and the speed range of the aircraft.

Not so sure of your formula arismount, (Ram Rise in Deg. C = TAS sqrd / 9500), it sounds about right if your speed is in Miles per Hour and you're using a Rosemount probe (Ct = 0.958). I think that you'll get better results if you use -

Rise (°C) = (TAS (Knots) / 87.1)^2,
OR
Rise (°C) = Absolute Temperature (°K) X 0.2 X Mach Number ^2

I agree with you Denti, it would be a good thing to have both TAT and SAT displayed, rather than burying one within the FMC.

Regards,

Old Smokey

[shon7]

Thanks for the replies everyone.