Why do anti-ice systems on airplanes with automatic ice detectors still need to be manually selected on?

Question

As even minuscule amounts of ice accumulation on aircraft can cause tremendous loss of lift, increases in drag, decreases in engine thrust, and/or flight-control impairment, some method of detecting ice is essential for safe flight, even for aircraft not certified for flight into icing conditions. Various methods exist for doing this, ranging from examination with the Mk. I eyeball (for many light general-aviation craft, and even some larger planes) to sophisticated electromechanical systems utilising vibrating probes which change frequency when made more massive by ice accumulation and set off an alarm in the cockpit.

For aircraft not certified or equipped for flight in icing conditions, the detection of ice lets the flightcrew know that they need to get out of the ice ASAP. For aircraft which can fly in ice, it signals the need to activate the aircraft’s built-in deice/anti-ice systems; these come in three main varieties (melting the ice using hot engine bleed air, melting it with resistance heating from electrical heating elements, and mechanically breaking it off with pneumatically-operated deicing boots), but all of these still need to be manually turned on by the pilots, even for aircraft that can automatically detect icing.

Given the potentially-catastrophic consequences of allowing ice to accumulate unchecked on an airframe, and the ease with which a flightcrew (especially if fatigued or already task-saturated) could overlook or forget about an ice warning, why don’t aircraft with automatic ice detectors and built-in anti-icing link the two together, and have the ice detector automatically turn on the anti-ice system when it detects icing?

Answer 1

Because most of the time the system has to be on even when there isn't detectable ice accumulation. Because of what I like call "technical icing conditions" but which are just called "icing conditions":

ICING CONDITIONS:

1. TAT (total air temperature - accounts for temperature rise from compresibilty) less than 10C

2. Visible moisture (cloud, haze, anything that is water)

3. SAT above -40C

You also have to have WAI on for takeoffs when OAT is less than +5C with visible moisture (including visibility under 1 mile) regardless of whether there is actual ice formation (water streaming back on the wing on takeoff can freeze upon rotation due to the temperature drop from the low pressure over the wing).

When you get the wings deiced, you still have to have the WAI on at the start of the takeoff roll if departing in technical (or actual) icing. You don't want to cook the thick type IV holdover fluid, so you leave WAI on selection the very last thing as you pull out to line up for the TO roll (the CRJ200 has an evaporative WAI system which meant the leading edges get VERY hot, about 105C, which could cook the fluid and make it crusty).

When flying, the only time you can leave the WAI off when in technical icing conditions is above some speed (on the CRJs it was 230kt), unless you get an ICE caution from the ice detector.

The result is a system that automatically turns the WAI on when ice is detected would only cover about, oh, maybe 10 - 20% of the time you are legally required to have WAI on.

Cowls are similar, but have to be on even more often because there isn't the 230kt threshold for actual accumulations only. They have to be on all the time you are on the ground less than +10C with visible moisture (freezing around cowl inlets from velocity temperature drop).

Without the 230kt exclusion the wings have, you have to have the cowls on pretty much all the time you are in cloud, actual icing or not, so you are turning them on and off quite a lot if climbing/descending through layers (some pilots may just leave them on), whereas outside arrivals and departures the wings are only on when you get actual ice.

So you could automate the system like that, if you could figure out how to make the system reliably detect "visible moisture" outside somehow (probably possible, but very expensive). Otherwise, the decision to turn it on is a crew decision based on the view outside, probably 80% of the time.

Answer 2

On some aircraft, it does actually work automatically. E.g. on the Boeing 777 both engine and wing anti-ice have an automatic position:

In flight, when an ENGINE ANTI-ICE selector is in AUTO and engine icing conditions exist, the respective engine anti-icing valve is commanded open. [...]

In flight, when the WING ANTI-ICE selector is in AUTO and wing icing conditions exist, the wing anti-icing valves are commanded open.

^{(Boeing 777 FCOMv2 3.20.1 - Anti-Ice, Rain - System Description)}

The 787 and 747-8 also have the auto position. I'm not sure about the 737 MAX.

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Airbus does not seem use any automatic system to turn on anti-ice. Even in the A380 and A350, anti-ice needs to be selected manually by the crew. There are of course cautions about detected icing:

If at least one ice detector detects icing conditions, and if engine anti-ice is off: The ECAM triggers the A-ICE ICE DETECTED caution. [...]

If at least one ice detector subsequently detects severe icing conditions, and if wing anti-ice is off: The ECAM triggers the A-ICE SEVERE ICE DETECTED caution.

^{(Airbus A380 FCOM 30-10-10 - Ice and Rain Protection - System Description)}

Answer 3

In answer to the question: Many airplanes do have automatically activated anti-ice systems.***

• Anti-Ice technology evolves,
• Later production airplanes of a type that maybe in production for decades can be markedly different from early ones.
• There are also 'Operator chosen' options as automatic anti-ice operation is generally not mandated by the rules.
• Different manufacturers can have different systems too.

What I'm trying to say is that similar cockpits may have different combinations of Anti-Ice system operation.

My own airline fleet had it as follows:

The B747-400 (P&W4056)

• Wing Anti-ice (WAI) and Nacelle anti-ice (NAI) from engine bleeds.
• WAI manual only, NAI auto or manual, on ground only manual NAI.
• auto is based on "nacelle icing conditions are detected"
• WAI only available in the air in CLEAN configuration (no slats, flaps extended)

B777 (GE90)

• Wing Anti-ice (WAI) and Engine anti-ice (EAI) from engine bleeds.
• WAI and EAI auto or manual, on ground only manual EAI.
• auto is based on "engine icing conditions exist"
• WAI only available in the air in CLEAN configuration (no slats, flaps extended)

B787 (GEnx)***

• Wing Anti-ice (WAI) is Electrical and
• Engine anti-ice (EAI) from engine bleeds**.
• WAI and EAI auto or manual, on ground: speed>75kts auto WAI available. only manual EAI.
• auto is based on "When icing conditions are detected"
• WAI available in the air and on the ground with speed>75kts.

**The left and right engines have identical, independent anti-ice systems as bleeds are not used for cabin Airconditioning and Pressurisation.

*** A completely automated system is the GEnx Ice Crystal Icing 'Function'