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Guest ChrisY

Icing temperature

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I've heard that anti-icing should be swicthed on when the temp is between 3*C and -10*C, and clouds/mist etc are in the area.Is this temp SAT or TAT? And do you have to be the same level as the clouds for icing to occur, or is it just if clouds are visible (even if they are a few thousand feet below you?)Also, for large pax aricraft with engine and wing anti-ice, in which conditions are they used, or are they both used during icing conditions.Thanks for any help.-Phil

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Phil,According to my training, even with EFIS instruments, Pilots only have refernce to TAT which is higher than SAT. If I remember correctly approximately TAT = SAT+(TAS/100)^2. In reality, SAT cannot be determined accurately and so no attempt is made hence why TAT is displayed. TAT is always warmer due to friction heat of the air molecules colliding with the temprature probe. Maybe the latest avionics have got around that problem, but that is my understanding. Therefore, icing will be predicted at tempratures above zero. Also, the Standard Operating Procedures (SOPs) for a particular aircraft will relate to that aircrafts cruise speed TAS and so different types may quote different tempratures, the faster the higher I would imagine. The worst icing is between 0 and -10 SAT, with virtually nil icing below -25 SAT.Apart from rain ice (deadly for all aircraft) and hoar frost (not usually an major issue), airframe icing can only form in visible moisture; in fog, mist or cloud. And then some clouds are worse than others, cumuliform clouds being the worst (esp Cumulonimbus (CBs or thunderstorm cloud).>>Also, for large pax aricraft with engine and wing anti-ice, in which >>conditions are they used, or are they both used during icing >>conditions.If large PAX = turbojet airliner then the anti-ice is typically hot bleed air and is usually switched on once in the air and is true anti-ice in the sense that it is used to prevent icing. This is different for chemical, electric and pnuematic wing syststems found on turboprops and pistons that are usually only used once ice is present and so are called de-ice systems. Wing boots for example, one should wait for about 1/2 inch of ice before inflating and breaking the ice up (in reality).I've never experimented with FS sufficiently to work out how realistic it is compared to the theory. It does seem to generate airframe icing, but I haven't worked out if this is only in cloud. It cirtainly makes any aircraft unflyable in freezing rain very quickly! I have never flown in icing conditions myself yet, so it is all thoery to me.

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Phil I quote from my 757/767 Flying manual. Engine icing conditions can exist or be anticipated when precipitation, standing water or visible moisture(Visibility less than 1500 metres) in any form is present and the OAT or TAT is 10C or below. Engine anti-icing must be on when the above conditions exist or are anticipated during taxy and the entire flight, except that engine anti-icing is not required at climb and cruise power with the temperature below -40C TAT. In icing conditions ie. below 10C and in visible moisture engine anticing goes on immediately after start and remains on until out of icing conditions Wing anti-ice should be OFF below 500feet to meet performance criteria ie. after take-off or on apprtoach in case of a go-around.Obviously diferent types and engines vary in their requirements for the use of anticing but the actual atmospheric conditions required for ice to form remain a constantRegardsChris

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