Figure "Visibility" (Ref. 5.1.4-14): It is interesting to note that several aircraft accidents caused by icing (but not engine-related) occurred under relatively good visibility conditions (top left diagram)

Investigations showed that the icing danger cannot be accurately estimated on basis of visibility. Visibility depends on the type of snowflake and varies greatly at the same snowfall rates. Evidently visibility is considerably worse with dry snowflakes than wet ones at the same snow melt rate. The accidents in the marked region of the top left diagram mainly occurred during snowfall with wet snowflakes. This corresponds to the example depicted in Fig. "Icing danger in helicopter engines".
The lower diagram shows typical snowfall parameters in a certain area, but some of the insights gained from it should be universally applicable:
The heaviest snowfall is observed at temperatures around -1 °C. The “snowfall time”, i.e. the time required to fly through a snowstorm, is also longest at this temperature, meaning that these snowstorms cover the largest area. The heaviest snowfall as measured in melt water depth per hour is found between -1 °C and +3 °C.
Contrary to expectations, the top right diagram shows that visibility in snowfall is considerably better at night than during the daytime (see Example "Flame-out by melting ice"). This can be explained by the difference in light refraction between daytime and nighttime.

Figure "Ice buildup": The above illustration shows various areas of origin for ice chunks that can be dangerous to engines (Ref. 5.1.4-5, Example "Flame-out by ice ingestion"). Naturally, the location of the engines, the engine type, and other aircraft characteristics are important factors for the probability of an ice strike. The landing gear can also throw ice from the runway into the engine.

Figure "Areas of high risk": Icing in the engine inlet area can break free and cause serious damages and/or unstable operating behavior in compressors (e.g. stall) and combustion chambers (flame-out). If the icing blocks inlets or changes the blade geometry, it will decrease engine performance and could lead to a surge.

The top diagram (see Ref. 5.1.4-15) shows typical icing areas in and around the engine fan. The closer the icing is to the axis of rotation, the greater the danger that the ice chunks are ingested by the core engine rather than passing through the bypass duct.

“A”: Nose of the engine nacelle

“B”: The rotating nose cone (spinner); in engines with an intake guide stator this may be a fixed nose cone.

“C”: Fan rotor blades; centrifugal force should prevent icing in outer areas of the radius.

“D”: Sensors protruding into the intake duct. There is a risk of sensor malfunctions (see Example "Available engine thrust") in addition to damage to the engine itself.

“E”: Intake guide vanes into the core engine (booster guide vanes).

“F”: Fan outlet guide vanes. Here there is no danger of ice entering the core engine.

“G”: Casing struts at the fan outlet. As long as the sensors (pressure sensors, etc.) are not affected icing in this area is not a large threat to the engine.

“H”: Front area of the core engine (Example "Sensitivity for quenching at low power").

“I”: Inner contour of the fan flow duct.

“K”: Casing struts in the bypass duct.

“L”: Intake duct of the core engine.

“M”: Sensors in the bypass duct (see “D”)

Bottom diagram: This is a depiction of a low-performance turboprop engine with its typical icing areas (Ref. 5.1.4-4). Experience has shown this type of engine to be especially vulnerable in case of ice ingestions (Ref. 5.1.4-1).

“a”: Rotating nose cone (spinner).

“b”: The relatively low centrifugal force at the inner areas of the propeller blades makes these areas susceptible to ice buildup.

“c”: Nose for the boundary layer separation.

“d”: Blow-off duct for the particle separator.

“e”: Nose for the particle separator (Ref. 5.1.4-3). In general, all leading edges in the intake air flow are potential areas for ice buildup.

Figure "De-icing zones" (Refs. 5.1.4-9 and 5.1.4-10.): There are many different anti-icing systems. The difficulty, however, lies in timing the de-icing process so that dangerously large ice chunks have not yet formed, as their release could be dangerous (see Example "Engine separation"). Naturally, it is most desirable to avoid ice buildup by activating the de-icing system at the right time.

Figure "Icing danger in helicopter engines": Icing around the engine is especially dangerous for helicopters. The relatively low flight speeds do not create protective heating of the boundary layer (see Fig. "Air speed influence") and the relatively small engines react strongly to the ingestion of large amounts of fluid. Even remarkably small amounts of snow can cause flame-out. In this way, a relatively small 350 kW helicopter engine flamed-out after ingesting only 10g of snow. However, with an activated continuous ignition system, even much large amounts of snow did not cause permanent flame-out. The engine relit after about 0.5 seconds.
Certain conditions can lead to extensive compressor blade damage, and in extreme cases destroy the entire compressor rotor blading. If engines have protective screens (Ref. 5.1.4-16) or filter systems upstream, these can become blocked up or ice can form behind them and threaten the engine.
The diagram shows a helicopter type in which the compressor blading of one or both engines was completely destroyed after ice ingestion (see Fig. "Compressor blade damage").

During a test flight, the depicted ice formation was observed from the cockpit and the following report made:
“The nose of the front gear cowling was covered by a 15 mm-thick milky layer of ice in a matter of seconds.”

The atmospheric conditions during the flight were:

• Heavy snowfall at a temperature of -2 °C
  
• Flight speed about 100 kt
  
• Pressure altitude about 1000 m (altitude at which the measured air pressure occurs under normal conditions)
  

Figure "Compressor blade damage": This diagram shows a 2500 kW helicopter engine (from Fig. "Icing danger in helicopter engines") after a heavy ice strike. All the rotor blades are mostly destroyed. The sudden relaxation of the combustion chamber blew the fragments forward and most of them lay in the engine inlet.
Similar damage was reproduced in a test by throwing a handmade snowball with about 10 cm diameter into the running engine.
The relatively filigreed structure of the compressor blading, the front section of which is made of titanium alloy, makes it even more vulnerable to this type of damage. Less damage would be likely occur in a radial compressor due to its more robust blading.

Figure "Internal ice formation": Dangerous ice formation can occur in the compressor itself, especially in the so called booster at the fanshaft (upper sketch). It is astonishing, that also in the high pressure compressor also dangerous symptoms of an ice formation have been observed (sketch below, Fig. "Sensor icing"). This can influence in different ways the power/thrust of the aeroengine up to the failing.

• Disturbance of the flow due to geometrical change of the blade profiles and reduction of the cross section. The result are drop of the compressor efficiency and/or the triggering of a surge and excitation of dangerous blade vibrations.

In the Example "Unexpected icing" ice accretes in the region of the exit vanes from the booster. This reduced the intake cross section of the high pressure compressor (HPC). So it came to the drop in rotation speed of the gas producer/core engine as well as in the low pressure region and the fan. Drop of the Performance respectively thrust and the increase of the turbine inlet temperature lead to the shut down during flight.
Fig. "Sensor icing" shows a case where it came to ice formation with flow disturbance in the high pressure compressor.

• Ice ingestion in the high pressure compressor causes as FOD the deformation of the blading. Especially endangered are the blades of the first stage compressor rotor behind the `swan neck'. This is the S-shaped channel from the booster to the high pressure compressor (upper sketch). The consequence can be besides surging also blade vibrations up to a fatigue fracture with catastrophic secondary failures (Example "Ice formation on low-pressure-stator").

• Drop of performance/power respectively rotor speed (rollback) up to flame out/quenching of the aeroengine caused by melting water. For this, especially prone are relatively small aeroengines like turboprops (Example "Flame-out by melting ice").

Figure "Ice accretions in fuel systems" (Ref. 5.1.4-27 and Ref. 5.1.4-28, Example "Flame-out during landing"): In two cases a dangerous thrust drop of aeroengines occurred. Once during landing approach after a long distance flight and once obviously during cruise. The low outside temperatures may have permitted the ice formation in the fuel system. In both cases the same type of aeroengine and aircraft was concerned. This argues for a design feature. Fuel system of the fuselage side and the aeroengine side are schematically displayed.

The analysis of the electronics memories allow the conclusion at a blockage of the fuel system between the low pressure system (LP-P.) at the aeroengine side and high pressure pumps (HP-P., sketch below right).
Fuel samples showed, that the fuel meets the specifications. Also the water content was rather low. This does not explain a dangerous ice accretion.
During tests under rather unrealistic severe conditions (water injection into the intake of the fuselage sided pump) ice crystals clogged partly the fuel flow at the entrance of the FOHE (sketch above right).
It was also shown, that adherent ice can form in them pipelines and the intake surfaces of them pumps. The thickness of the ice layer depends from the fuel temperature. Anyway no alarming disturbance of the flow-through occurred. It is possible, that such deposits suddenly come loose (power increase) and other regions of the system like the FOHE, get clogged.
The only failure indication which can be seen in connection with a blockage of the fuel flow in front of the high pressure pump are cavitation traces. Those could be reproduced within 60 seconds (!) with unusual low intake pressure.
The findings till now rule out a fuel outside the specifications. Additionally the test took place according to the experiences till now, under extreme unrealistic conditions. With this assured findings as a precondition for targeted remedies lack.

As a preliminary, not fully satisfactory remedies the responsible OEMs plan:

• Sporadic accelerations in nonhazardous flight conditions.
  
• Development of a suitable FOHW.

Figure "Warm environment" (Ref. 5.1.4-25): Danger of an ice strike, caused from cold fuel in the wing tanks.

Figure "Air speed influence" (Ref. 5.1.4-11): The top diagram shows the heating-up of the boundary layer due to the air friction on the surface in the flow stream. At flight speeds near the speed of sound and above, the boundary layer heat created by air friction and the compression are great enough that icing will not occur. However, de-icing of already iced parts through high-speed flight tends to be a very slow process. Understandably, helicopters and slow-flying aircraft cannot take advantage of this effect. On the contrary, the relatively low altitudes at which these operate further increase the risk of icing (Fig. "Conditions"). Even for most commercial aircraft this effect is not sufficient to prevent icing.
Fast fighter aircraft are especially susceptible to icing when their flight speeds are relatively low, such as during takeoff and landing, which is also when the conditions for icing are most likely to be present.
Icing around engine inlets is a known problem in stationary machines used in power plants and industry. In these cases the icing occurs without direct ingestion of rain or snow, but merely from the relative moisture in the surrounding atmosphere. Similar conditions are present in engines that have a low moving speed (helicopter, etc.) or when engines are operated on the ground (e.g. during testing or taxiing, see Example "Engine separation"). In addition, water drops can be directly ingested by the engine as rain or as spray thrown up by the landing gear.

In Ref. 4.1.4-17, H.J.Wilcocks describes important conditions for icing in compressor intakes:
The air speed and humidity are especially important factors influencing the flow temperature. In the bottom diagram, TT (T-Total) is the temperature of dry air at rest (low relative humidity). TS is the temperature of the dry air flow. The flow speed in the inlet decreases the temperature of the dry air that had been at rest by several °C. This decrease (grey area) is dependent on the relative humidity. The greater the humidity, the more energy is released by the condensation of the moisture, minimizing the change in temperature. However, the temperature decrease is also related to the inertia of the condensation process. Due to the high flow speed, the dwell time in the inlet is clearly too short to for the condensation to reach equilibrium. Therefore the actual temperature increase due to condensation is correspondingly smaller. The temperature decrease in the gas flow (with speed values typical for a compressor intake) of air with a relative humidity of nearly 100% is several °C. This is enough to make temperatures around 0° C, which are typical for icing conditions, to cross the “threshold to ice formation” (Examples 5.1.4-1.1, 5.1.4-5, and 5.1.4-6).

Figure "Conditions" (Ref. 5.1.4-13): In this passage, icing is understood to be the ice buildup that occurs on an aircraft on the ground and during flight. Engine icing can be especially dangerous during takeoff and landing, when the nacelle is already iced and the necessary high engine performance can no longer be achieved.

Heavy icing can occur, when the following conditions are met:

• The temperature of the surfaces prone to icing must be below 0 °C (Fig. "Air speed influence").
  
• The freezing point of water drops in free air lies somewhere between 0 °C and -40 °C (solidification distortion). These conditions are met relatively often as shown by the top diagram in the illustration. The small the water drops, the lower the freezing point. However, below -15 °C the amount of undercooled droplets is usually low enough that there is no risk of icing (compare with bottom diagram in illustration), since dangerous icing only occurs if the undercooled fluid content is greater than 0.5 g/m³. The impact of undercooled droplets on a surface can cancel the effect which shifted the freezing point to a lower temperature, resulting in spontaneous ice formation. Therefore there is acute icing danger to every flight through undercooled clouds or fluid precipitation at temperatures below 0 °C. If the aircraft is rolling or flying at low speed, then even if the temperatures are well above freezing, icing can occur if undercooled water droplets are present and/or the temperature in the accelerated airflow decreases sufficiently. It is interesting that the freezing of small droplets (rime ice formation) depends on the aerodynamic profile of the part`s leading edge. Thin profiles catch relatively more droplets than thick profiles. For this reason, very aerodynamic coverings, sensors, and struts (Fig. "Areas of high risk") are prone to rime icing. On the other hand, thicker profiles are prone to clear ice formation. This is due to the fact that larger, clear ice-forming droplets are less deflected by the airflow. While rime ice and clear ice are two distinct types of ice, they can occur mixed together in various ratios:
  

Rime ice:
This is a rough, opaque, milky ice that is formed by the immediate freezing of small undercooled water droplets upon impact on a icing-prone surface. This rapid freezing traps air bubbles. The ice structure is often spherical and/or sharp, which makes the ice brittle and porous. This means that the damage to be expected on parts struck with rime ice (Chapter 5.1.1 and 5.1.2) is usually no more than disturbance of the combustion due to the water content and the influencing of aerodynamic forces due to the roughness of the ice (flow irregularities, friction losses, etc.).

Clear ice:
Clear ice is smooth and transparent. Clear ice is formed by relatively a relatively long freezing process of large undercooled droplets. This type of icing can also occur on aircraft parts (especially those carrying cold fuel) which were heavily cooled by high altitude flight and then flew through moist layers of air at lower altitude.

The bottom diagram shows that dangerous icing occurs most often at temperatures around -5 °C.

Figure "Flame out by ice formation" (Ref. 5.1.4-30): In high altitude during descend both aeroengines flamed out. Shortly after this the engines started again (see also Fig. "Sensor icing"). Although this danger exists since long, about this phenomenon however little was known. Till now applies, tat in such high flight altitudes the water content/humidity of the air is too low for a dangerous icing. During heavy thunderstorms like these especially occur over the pacific, tiny ice particles can form. These are neither recognisable by the pilot nor the radar.
For these incidents, like in further 14 cases at which aeroengines and airplanes of different OEMs where concerned, internal ice formation ('crystalline icing') is made responsible. This possibility of an ice formation in the high pressure compressor is in cycles of experts seriously discussed. This phenomena was obviously observed even at the exit of the high pressure compressor before the combustion chamber at an other aeroengine type. Ice may change the blade profiles and so trigger stalls (no surge, Ill. 11.2.1.1-1). So the airflow is markedly reduced. In an extreme case the combustion chamber could quench. This can also happen if the control unit disrupts the fuel supply because of a too strong rise of the gas temperature. Ice accumulation during relatively high operation temperatures is explained with a cooling of the compressor blades by the cooling effect (melting, vaporizing) of ingested ice sludge.

Figure "Sensor icing" (Ref. 5.1.4-29): Both aeroengines quenched in about 6000 meters during descend from 10000 meters. The airplane was not in the clouds although thunderstorms existed. As well the de-icing unit as also the permanent ignition have been activated. After about 1 minute the aeroengines started again automatically.
Until now 8 of such incidents have been registered. These occurred during descend above 5300 meters and extreme icing conditions. Cause have been ice accumulations which got into the aeroengine. In all cases all aeroengines started again automatically without a repeat of the incident. A similar problem in connection with rain and hail in a thunderstorm took place at a smaller aeroengine of the same OEM' (Example "Flame-out by melting ice").
But in the current case “extreme icing conditions” seem not to be existent. However it is known, that also under such seemingly harmless conditions the danger of icing exists (Fig. "Flame out by ice formation").

About the cause the OEM of the aeroengines speculates at the time of the not yet finished investigations:

• Most probable cause stands in connection with the weather (see also Fig. "Flame out by ice formation").

• There exists the potential of an ice formation respectively ice accumulation at the sensors/probes which affect the digital control unit of the aeroengine.

• As an influence/clogging of the fuel system during low ambient temperatures seems possible (Ref. 5.1.4-30). The assumption of the OEM shows, that obviously this has already occurred or was observed. Rather improbable is, that sucked ice was concerned which got into the fuel nozzles.

Aeroengine tests under influence of the weather shall clarify the problem.

Figure "Spinners": Preventing dangerous ice formation is an important requirement for rStrike-through Textotating nose cones. This can evidently be accomplished in various ways. Icing on spinners and static nose cones, especially on fighter aircraft engines, is usually prevented with hot air from inside. Metallic materials are better suited for this than fiber-reinforced synthetics due to their relatively good conductivity and high heat resistance.
The left version depicts a proven variant made from glass fiber-reinforced plastic (synthetic resin) used in civilian fan engines. The nose is made of a elastomer cone that vibrates when rotating due to the eccentric ice buildup, preventing dangerous ice formation.
The dome-like shape of the middle spinner was chosen because the ice near the axis slips off despite the low centrifugal force. The shape of the axial contour with its relatively large diameter creates centrifugal force that is high enough to prevent ice buildup by throwing off even thin layers of ice.
The diagram on the right depicts a version that is used today in Russian engines (Ref. 5.1.4-21). This is intended to show the variety of geometries that are designed to be solutions to the problem of ice buildup.
In fighter aircraft engines, the low pressure shaft usually spins rapidly enough to prevent any ice buildup from occurring, even near the tip. If smaller ice chunks are formed, they are thrown off with a high speed so that they pass through the bypass duct and cannot damage the core engine.

Figure "Helicopter flame out" (Ref. 5.1.4-31): At this helicopter type two heavy accidents by the effect of snow occurred. The snow accumulates in the intake area. If it comes loose as sludge and sucked in, quenching/flame out of the aeroengine is caused.
It is known, that the relative small aeroengine is prone for quenching during snow fall.
During an ambient temperature under + 5 °C and visible mist, the OEM prescribes to activate the de-icing and the automatic re-ignition.
In the first case the quenching/flame out of the aeroengine leads to a crash. The de-icing was shut-off, the auto re-ignition however was activated. The optional, original deflector kit in front of the intake was not mounted. In this case there have been in the sketch above indicated environment conditions.
In the second case the de-icing was activated, however the auto re-ignition was shut-off. In this case there were 22 knots wind speed at -12 °C.
From agents of the helicopter OEM during snow fall a deflector kit in the intake is urgent advised. This is also recommended by a multitude of cases where after the installation of a deflector kit, no more drop outs of the aeroengines occurred.