TAT and Anti-Ice Operation

[richjb]

Most aircraft AFM's provide temperature limitations below which you need to activate the anti-ice systems, for example TAT +10 or TAT + 5 and below. My question is low must the temperature drop before you can de-activate the anti-ice system when referencing TAT.

Most of the recent aircraft I've flown are using SAT and the guidance we have on those aircraft is SAT - 40 C. It's been a long time since I've flown a TAT referenced aircraft but I seem to remember TAT - 20 C as being a reasonable number.

Any thoughts or suggestions? The DA2000 Falcon AFM does not provide guidance in this regard. She's a dog in the climb through the 20's with A/I on and it would be nice to be able to turn it off.

Thanks in advance,

Rich Boll
Wichita Kansas

[jtr]

Why not use a TAT/SAT chart to determine when you reach -40 SAT (I assume you have no SAT in the a/c?)

There are a lot of formulas around for ram rise

Mach # * 3 + 5 is usually close for the FL's and Mach #'s you are going to need it.

[Old Smokey]

Richjb,

Does this help?

TAT for SAT -40°C and Mach Number –

Mach: 0.60: 0.62: 0.64: 0.66: 0.68: 0.70: 0.72: 0.74: 0.76: 0.78: 0.80: 0.82: 0.84: 0.86: 0.88: 0.90
TAT :-23.2:-22.1:-20.9:-19.7:-18.4:-17.2:-15.8:-14.5:-13.1:-11.6:-10.2: -8.6: -7.1: -5.5: -3.9: -2.2

[Temperatures stated to one decimal point of accuracy to allow for more accurate interpolation].

[Based upon TAT(°C) = (SAT + 273.15) X (1 + .2 X M^2) - 273.15]

Regards,

Old Smokey

[richjb]

Thanks folks and Old Smokey for the conversion chart. Is that chart a generic conversion for TAT to SAT conversions?

Has any seen in an aircraft that they fly a recommendation for when to turn A/I off using TAT as reference? The Lear's I've been flying use SAT and all say SAT - 40 C, which seems reasonable.

Dassault proivdes both TAT and SAT in the Falcon but references only TAT in their A/I guidance and then they provide only the temperature at which it should be turned on, not off.

Thanks,

Rich

[Old Smokey]

richjb,

Yes, it is a generic TAT for SAT and Mach Number conversion, applicable to all aircraft with TAT indication. Be aware that it is not valid for aircraft with RAT indication, i.e. where the Temperature Recovery Factor (Ct or K) is less than 0.99. Most modern aircraft (including the Learjet 45) have TAT indication, not RAT.

The upper temperature threshold (+10°C in your case) is representative of the aircraft's skin temperature, and therefore indicative of it's ability to form and carry ice if water droplets are present. The lower limit is almost universally -40°C (but do check for the aircraft type), and this is indicative of the atmosphere's ability to sustain super-cooled water droplets, and is therefore a meteorological, not an aircraft phenomenom.

Regards,

Old Smokey

[richjb]

Thanks Smokey,

Rich

[Dog Driver]

Smokey,

You seem to be a wealth of information so I will ask you ...

What would be a simple way to explain the difference between TAT and RAT ??

Is it just a correction applied in the CADC to correct for a probe error ??

Seems any book I look in only addresses one or the other but never both.

[777300ER]

If your AFM does not provide you with a temperature at which anti-ice is no longer required, then I suggest you cover your a$$ and leave it on.

[lomapaseo]

I believe the recommendation regarding engine operation, was the use of ant-ice below XX deg in visible moisture which of course take into account ice crystals even at high altitude.

I'm not sure that this relates to your subject though.

[barit1]

That sounds like a good, reasonable question for Dassault product support. I'm sure you're not the only one with that question.

[Old Smokey]

Dog Driver,

The difference between TAT and RAT

The Total Air Temperature (TAT) is the sum of the Static Air Temperature (SAT) increased by compressibility arising from Mach Number or TAS. These are given by either of the following formulae -

TAT = SAT X (1 + .2 X M^2), or

TAT = SAT + (TAS/87.1)^2,

where SAT and TAT are in degrees Absolute (add 273.15 for °C), M = Mach Number, and TAS is in Knots.

For example, if you are at 300 KIAS at 20000 feet, and a SAT of -20°C, your Mach Number = 0.6512 and your TAS is 403.8 Knots. Applying the formula -

TAT = (-20+273.15) X (1 + .2 X .6512^2) = 274.62°A - 273.15 = +1.47°C, OR

TAT = (-20+273.15) + (403.8/87.1)^2 = 274.62°A - 273.15 = +1.47°C

The rise, therefore, is from -20°C to +1.47°C, a rise of 21.47°C

If the efficiency of your Temperature probe (or Temperature probe and ADC in combination) is such that it indicates +1.47°C, then it is 100% efficient, and is measuring TAT. Such a Temperature probe has a recovery factor of 1.0, many probes / systems however, do not recover all of the temperature rise. For example if it was 80% efficient, only 80% of the 21.47° rise would be recovered, i.e. 17.18°, giving an indication of SAT + Rise, -20° + 17.18° = -2.82°C. This is the RAM temperature. (TAT is also the RAM temperature, but labeled as TAT to differentiate from a system without full recovery).

The Recovery factor is usually known as K or Ct. If you know the Ct for your aircraft, the formulae then become -

RAT = SAT X (1 + (.2 X M^2) X Ct ), or

RAT = SAT + (TAS/87.1)^2 X Ct

Many light aircraft with TAS up to about 150 Knots have RAT indicators and a Ct in the region of 0.8. Faster aircraft with RAT indicators, e.g. the DC9 fitted with the Rosemount probe have a Ct of 0.958. As a general rule, a Temperature indicator is considered as a TAT indicator if the Ct is 0.99 or better.

Regards,

Old Smokey

[Dog Driver]

Smokey,

Thanks, about what I figured, but I have been known to figure wrong from time to time.

You hit it on the head with the dc-9. Went from the lear (RAT) to DC-9 (RAT) and then on to md-80's (both.. in some). Never could get a straight answer from anyone about which to use or why they offer both.

Just wondering what your source material is ?? Would like to read up on a few things but wish more detail than the books I am finding now.

dog

[Old Smokey]

Dog Driver,

My sources are many and varied, having learned 'my trade' from Boeing, McDonald Douglas, and BAe, but the best reference book that I go gack to is US Navy publication HO219 (Air Navigation) which has excellent information in the area of instruments, instrumentation, and a mountain of good algorithyms for performance measuring instruments. It's an old publication, but some things never change. Frankly, I doubt that it would still be on the market.

Regards,

Old Smokey