Rule number one is to strictly follow your company's SOP. This is a legal requirement.
Generally, the rules are:
On the ground, when icing conditions may be encountered ("anticipated")or
are encountered, use the engine anti-ice.
In flight, again, the use of engine anti-ice is required whenever icing conditions are encountered or are anticipated.
Usually, most manuals include the caveat to not wait until you actually begin to accrue ice before turning on the engine anti-ice. In fact, there is usually a specific procedure for turning on the engine anti-ice when it is realized that you already have ice.
So, what criteria constitute 'icing conditions'?
Again, read your company's SOP...read your flight manuals...for this definition.
But, generally, when on the ground, if you have precipitation of any kind and the temperature on the ground is +10 or less (not "below ten", but plus ten or less), you should use engine anti-ice. Or, if in low visibility and the temperature on the ground is +10 or less. For the purpose of using engine anti-ice on the ground, low visibility, in the U.S, is defined as one statute mile or less. So, foggy conditions would be one example of this.
Keep in mind that, puddles of water, snow patches, wet or damp surfaces...all...constitute an icing hazard to engines when the temperature is +10 or less. So, it may be a pretty, sunny day, but if you're taxiing over water puddles, snow patches, wet or damp surfaces and the temperature is +10 or less, the engines may pick up ice.
In flight, we look at
both the TAT
and SAT. Again, +10 or less is the magic number. There are exceptions!!!! Let me explain....
Normally, "during climb or cruise", if in moisture, if the SAT is
colder than minus 40, we don't have to use engine anti-ice. However, during descent, we would use engine anti-ice, if in moisture, regardless of the SAT (as long as the TAT is +10 or less).
The exception to the above is, if operating in or near CBs (TRW), we would use engine anti-ice, assuming the TAT is +10 or less, regardless of phase of flight, regardless of SAT.
All of the above is delineated in both my former company's SOPs (I'm retired.) and the Airbus manuals (FCTM and FCOM). It's been many years since I've flown a Boeing, but, if my memory serves me correctly, the above general rules apply to Boeing, as well.
A common error and misconception occurs when, during cruise, pilots are hesitant to turn on the engine anti-ice. Their interpretation of the rules is that use of engine anti-ice is expressly prohibited during cruise if the SAT is below minus 40.
Not quite the case.... The problem with turning on anti-ice at cruise is that, under certain circumstances, turning on the engine anit-ice reduces the amount of thrust available....thrust that may be needed to keep the thing flying at high weights, high altitudes, etc. We generally try to fly at the 'optimum altutude' for fuel economy, and under certain circumstances, total thrust available may be an issue. Add turbulence due to flying in and around CBs and TRWs (which you shouldn't do, anyway, but we all have at one point or another in our careers)...speed decays...thrust comes up to compensate....not enough thrust available with the engine anti-ice on....well, you get the picture. But, on the other hand, the thrust becomes greatly limited when all engines flame out....
.
Many problems occur when flying in or near CBs and TRWs in tropical climates. There have been several incidents where engines have flamed out....most have involved the CF6 engine, most were during descent (thrust idle or close to idle), visible moisture, and SAT colder than minus 40. In almost all cases, the crew followed the SOP and had the engine anti-ice on. Despite this, there were still problems.
In tropical climates, where the tropopause is very high, CBs and TRWs grow very tall. In these climates, there is a lot of moisture. That is to say, the clouds are relatively wet. (as opposed to a TRW over Minnesota in March) In these cases, there is a lot of 'wet' packets of moisture at high altitudes and at low temperatures. Again, during descent, when the thrust settings are low...when flying in these conditions...be sure to have the anti-ice on. Or, as the Airbus FCTM suggests, even during cruise, in these conditions, run the engine anti-ice.
Hope this helps...hope this answers some of your questions.
Fly safe,
PantLoad