5:51:514:514

5.1.4 Ice

Ice formation and ice ingestion can change (i.e. damage) the operating behavior of an engine and/or its components in many different ways. Certain aircraft/engine combinations may be more susceptible to this than others. The deciding factor could be, for example, the location of the engines (on the fuselage in the rear, etc.) or the engine type (turboprop, etc.).

Damaging influences are:

  • Ice entering the compressor and causing blade failures, altering the blading (e.g. deformation, altering stator blade position), causing a compressor stall, or causing instable combustion including flame-out.
  • Changes in the air intake through ice build-up (for example, icing around the inlet as shown in Fig. "Ice buildup"), ice formation in the core engine, blocked protective grills and filters in helicopters. These occurrences reduce the amount of air entering the engine (Example "Sensitivity for quenching at low power"), decrease engine performance, and/or cause compressor surges.
  • Ice buildup around the engine, especially on actuator and indicator cables in older engine types, and also on guide vane adjusters.
  • Ice formation in fuel with overly high water content, which can result in ice buildup in filters and vents, reducing engine performance. In extreme cases, this can lead to engine failure.
  • Ice buildup on sensors (such as temperature or pressure sensors, see Chapter 4.2, Example "Available engine thrust") causes false indications that can lead to improper action by the flight crew and/or cause control system malfunctions.

There are various causes for ice formation, and it must not necessarily be connected with overcooled rain (Fig. "Conditions"). Other causes include ice forming on leaky toilet outlets (Example "Blue ice damage") or ice formation due to poor air intake conditions (Fig. "Air speed influence"). This so-called “blue ice” can form large chunks and damage the wings and fuselage, as well as cause serious damage to the engine (Example "Special sensitivity for quenching"). With some aircraft/engine combinations the flow irregularities caused by ice on the wings can induce a compressor stall in the engines (Ref. 5.1.4-1).
Experience has shown that dangerous ice accumulation can also occur on shut-down aircraft and their engine intakes. This ice must be removed before start-up, because otherwise it will enter and cause damage to the engine after start-up or, even worse, during takeoff or flight. Even though there have only been reported cases of single engines failing due to this (Ref. 5.1.4-2), it is entirely possible that multiple engines would fail simultaneously.
The subjective impression gained from a mere visual assessment of whether or not snowfall has caused a serious ice situation is evidently not sufficiently reliable (Fig. "Visibility", Ref. 5.1.4-14).
Verification of acceptable ice-strike behavior is also not entirely without difficulties (Ref. 5.1.4-3). For example, an engine type flamed out several times, and ice buildup was observed at the connection between the intake duct and the engine intake housing. Further investigation showed that the behavior of the engine during ice ingestion normal for operation was actually worse than the certification tests showed! It was determined that the certification tests did not lend enough weight to certain realistically occurring conditions such as extremely damp snow. The impact tests to verify sufficient FOD behavior of the compressor do not provide enough information regarding flame out. Engines with leading edges, such as for a particle separator (Fig. "Areas of high risk") can be especially susceptible to ice formation under damp snow conditions.
Engine certification usually requires verification of operability under all expected weather conditions, including extreme cold. However, experience has shown that special situations can result in problems (Fig. "Internal ice formation" and Fig. "Ice accretions in fuel systems").
Various measures are taken in order to prevent ice buildup in particularly susceptible areas. In the years when anti-icing systems and de-icing technology was not yet very advanced, ice caused engine damage on a relatively frequent basis. Between 1959 an 1963 about 40% of the ice-related damages were caused by ice buildup on the engine itself. This shows the importance anti-icing systems for engines.
However, de-icing can sometimes even increase the danger of an ice strike. If, for example, de-icing systems were shut down in order to ensure maximum engine performance during takeoff, or if the pilot did not activate them in time (contrary to regulations), then late activation of the de-icing system during flight can cause large ice chunks to come loose (Example "Flame-out by melting ice", Ref. 5.1.4-19). This risk seems to be especially high (at least in military aircraft) during training flights at low altitude under high fog layers in winter.

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.

Example "Ice formation on low-pressure-stator" (Ref. 5.1.4.1-18):

Excerpt: “…The US FAA has issued an emergency airworthiness-directive (AD) following incidents in which ice broke loose from low-pressure-compressor stators at full take-off power and caused damage to high-pressure-compressor blades… Following five incidents of ice-caused compressor damage during severe US winter of 1993…The AD reminds operators that the engine anti-icing system only protects the inlet cowl and cannot prevent or remove ice accumulation on the stator stage just behind the fan.The directive requires that engine run-ups be based on air temperature and visible moisture rather than indications of icing on the airframe.”

Comments: This directive is understandable when considering that the temperature is lowered in the accelerated airflow at the inlet in aircraft that are standing, taxiing, and taking off. This causes icing to occur here even though the fuselage and wings do not exhibit any signs of icing due to atmospheric temperatures above freezing (see Fig. "Air speed influence").

Example "Flame-out by melting ice" (Ref. 5.1.4-19):

Excerpt: “Early reports indicate that a …double engine failure caused the crash of a …aircraft on 27 February. If confirmed, it would be the second time that such a failure has resulted in a fatal accident to the same type since January 2000. Take-off was in the darkness, but there was good visibility, 2 °C temperature and dew point of -3 °C.The January 2000 accident (was) near Marsa el Brega, Libya…The accident report indicates that both engines failed on the approach because the pilots failed to operate the engine intake icing system at altitude, and when intake ice began to melt in the decent, water ingestion caused engine flame-out.”

Comments: This accident is an impressive demonstration of the way in which even experienced flight personnel can make errors in judging weather conditions with regard to icing danger, especially at night (see Fig. "Visibility").
The other aircraft accident illustrates how heavy water intake combined with ice formation is especially dangerous.

Example "Flame-out by ice ingestion" (Refs. 5.1.4-4 and Ref. 5.1.4-2):

Excerpt: “…called a Christmas miracle, all 129 passengers and crew…survived when the aircraft crashed into a field…the…pilot reported he had lost power in both engines shortly after take-off…Flying in an altitude of only 2000 feet, he glided the aircraft past a village, cutting through the tops of a forest of tall trees at 155 mph to slow the plane down.”

Comments: The power loss was determined to have been caused by sheets of ice ingested into the engine shortly after takeoff. The ice came from the aircraft surface in front of the inlets and caused the engines to flame out and fail. Therefore the certification processes also demands today the proof of harmlessness for an entrance of sheet shaped ice (chart section 5.1.1).

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.

Example "Sensitivity for quenching at low power" (Ref. 5.1.4-2):

Excerpt: “Extended aircraft operations in icing conditions during a holding pattern or long descent, when the ambient temperatures and engine speeds are low, also have the potential for building of ice in the core engine flowpath causing local blockage, airflow distortion, and resultant thrust loss….experience includes a…four-engine flameout during low idle power descent after which the engines were subsequently restarted. Operation with higher descent idle rpm and prescribed operating procedures were implemented to solve this problem.”

Comment: Just at low power like it can be expected during descent, the combustion chambers are especially prone for a quenching by water (molten ice). This must be considered during development/certification.

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.

Example "Blue ice damage" (Refs. 5.1.4-2, 5.1.4-6, 5.1.4-7):

Excerpt: “Ground personnel serviced forward lavatory just before flight. Flight attendant stated fluid was visible in upper toilet area of forward lavatory after service indicating it had been overfilled. Enroute number 3 engine failed and was shut down. Flight diverted to Miami. On arrival a blue streak was found on the right side of the aircraft beginning at the forward lavatory service door and ending at the inlet to number 3 engine. The forward lavatory service door was found closed and the fill/rinse line cap was missing when the door was opened. The fill/rinse line check valve was found stuck open. When a cap was installed on the line no leakage occurred. the lavatory drain line did not leak. Number 3 engine had sustained fan damage due to ingestion of soft matter.”

Comments: The fluid from the toiled line leaked out during flight and froze, forming enough ice (blue ice-due to the color of the toilet water deodorant) to damage the fan and cause the engine to be shut down.
Understandably, the only engine affected by this type of “blue ice damage” is the one in the direction the leaked fluid flowed, i.e. the direction in which the ice traveled after breaking free. This makes this process considerably different from weather-related icing, which can damage all engines simultaneously.
The formation of dangerous blue ice in this aircraft type is not an isolated case and can lead to dangerous engine damage, as shown by further incidents (Example "Special sensitivity for quenching") in which a seal (O-ring) was damaged or missing after maintenance, ultimately resulting in damage to the engine in the same location.

Example "Special sensitivity for quenching" (Ref. 5.1.4-3):

Excerpt: “ …During the winter 1988 with more than 30 aircraft (with turboprop engines) in service and about 50,000 hours of flight accumulated, several occurrences were reported of a sudden temporary loss of torque. In general, the so-called flame-out incidents occurred under the following conditions:

  • flight phase: descent, power 10%-35% torque
  • IOAT -2 °C-+3 °C
  • altitude 8 000 - 14 000 ft
  • IAS 160 - 180 kts

Analyzing a total of 6 confirmed occurrences the following common factors were recognized:

  • incidents occurred in relatively low power-settings
  • together with the altitude effect, this meant that the engine massflow was low
  • powerplant ice protection system had been activated prior to incident, together with continuous ignition
  • prior to each incident the aircraft had encountered light icing conditions, while in most cases rain was reported by the flight crew and in some cases snow
  • however, no or little ice was seen on the airframe
  • in each case ambient temperatures were near freezing level

Comment: Concerned is a relatively old aeroengine type which is used in many commuter aircrafts. Therefore the question about a special sensitivity for quenching of this use and if so for the cause respectively feature arises.

Example "Engine separation" (Ref. 5.1.4-8): The same aircraft type as in Example "Sensitivity for quenching at low power" is affected again.

Excerpt: ”…. While cruising at flt level 350 in clear smooth air, a loud noise was heard, accompanied by a severe jolt on the right side of the cabin, as the number 3 end (engine?) separated from its mounts.Subsequent investigation disclosed that the forward lavatory was leaking deodorant fluid and water.
Deodorant stains existed along the right side of the fuselage & were subsequently identified on the number 3 engine nose cowl & inlet nose cone. All but 6 of the first stage fan blades were recovered at the engine impact side, & these 6 blades were located within a 180 degree segment of the fan. Leakage of the lavatory waist drain valve was the result of a damaged O-ring seal. Leakage of the dump valve was the result of disbonding of a rubber boot on its shaft.“

Comments: Evidently the icing resulting from the toilet leakage caused the fan blades to fracture and led to extreme imbalances which overloaded the engine suspension and resulted in the engine breaking off the aircraft.
The damage only occurred because another safety seal failed in addition to the failure of the O-ring seal. This is an excellent example of failure of an apparently redundant (seal) system.

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").

Example "Unexpected icing" (Figure "Internal ice formation", Ref. 5.1.4-24):

Excerpt:”…Engine anti-ice was selected on at the top of the decent…When passing between the storm cells…, visible moisture was encountered but at no times was any airframe icing present on the wings, windshield or wipers or identified by the ice detector. As the aircraft was clearing the weather…the commander became aware that the fanspeed of engine No 4 was decreasing slowly with an associated increase in Turbine Gas Temperature (TGT)… After about one minute, by the time the commander had retarded the No4 engine thrust lever, the TGT was 890 °C and rising. engine No4 was shutdown and the Rollback drill completed. The crew…diverted to Paris, Charles De Gaulle without further incident.

Rollback: The term 'Rollback' is used to describe a particular uncommanded thrust reduction…It manifests itself as a slow reduction in the high pressure spool speed associated with an increasing Turbine Gas Temperature (TGT) and a failure of the engine to respond to Pilot thrust lever…the cause of rollback was determined to be due to the build up of ice in the engine core super charger exit guide vanes in very specific meteorological conditions. The build up of ice on the guide vanes progressively `chokes' the engine core.

Comment: It is astonishing, that at this aeroengine type also behind the booster there can be still icing conditions, although there have been no icing indications at the outer surfaces of the airplane. The explanation may be found in Fig. "Air speed influence". Probably the flow was so accelerated in the icing region at the exit stator of the booster, that the temperature decrease together with the ingested mist was sufficient for icing conditions.

Example "Flame-out during start" (Ref. 5.1.4-12):

Excerpt: “The aborted takeoff that began this sequence was due to engine inlet icing. Moisture from the wet ramp was ingested into the engines as the aircraft taxied to the runway. No engine anti-ice was used because no visible moisture existed. Following the abort the pilot turned the engine anti-ice on, but unbeknown to him the system was unable to de-ice the engines. Suspecting, but not sure that ice was his original problem, he did a full-power run-up prior to his next takeoff. Since his engine indications looked good for prior to brake release. He performed the takeoff but aborted the flight shortly after liftoff due to a power loss on both engines. One-fourth inch of ice was found on the engine inlet bullets after the aircraft taxied back to the ramp.”

Comments: A so called `inlet bullet' is a non rotating nosecone. Here without de-icing (heating) the icing may occur especially intense. `Ramp' means the runway. It was obviously wet and the swirled up, respectively ingested humidity promoted the ice formation without visible humidity haze. The icing conditions were most likely created by the cooling of the accelerated ingested air (see description of Fig. "Air speed influence").

Example "Flame-out during landing" (Ref. 5.1.4-27): After a long distance flight the airplane was in a landing approach in GB. Shortly after a temporary acceleration in about 240 meters above ground a power drop at the right aeroengine occurred slightly over idle. Few seconds later the left aeroengine followed. Cause was a too low fuel supply. A reaction of the electronic monitoring and the power lever adjustment was without success.
The airplane hit the runway whereupon the main landing gear was torn off.
The flight took place in the middle of January and took course over the north pole. The measured outside temperature sank down to -34 °C. During landing approach the fuel temperature was -22 °C. All these temperatures are still in the design range and not unusual.
In spite of intense investigations even after 9 months it was not possible to clear satisfactory the failure causes and the sequence in detail. The only indication which pointed at a fuel shortage are signs of cavitation at the entrance of the high pressure pump. Such traces can develop in the range of minutes, correspondent with test results, during low-pressure with vapour formation (Ill. 22.2-9 and Ill. 22.2-12).
Though the fuel samples showed no alarming water content, most likely ice formation is seen in the fuelsystem in front of the high pressure pump (Fig. "Ice accretions in fuel systems"). Thinkable is ice from the fuselage pipeline system which came loose during the temporary acceleration and clogged the intake area pf the fuel oil heat exchanger (= FOHC). This had no bypass.

Comment: Because an ETOPS certificated (Fig. "ETOPS strategy") airplane is concerned, the drop-out of both aeroengines is especially alarming. This impression is aggravated by a second case about 1 year before this time in the USA, at which only one aeroengine was affected. The problems during the clearance of the incident are typical for ice formation in the fuelsystem. This is even true for electronic data recording. If not a low-pressure before the ice accretion caused the collapse of the filters or deformed the tanks (Ill. 22.2.2-7), there will be only hints like cavitation traces at the fuel pumps (Fig. "Ice accretions in fuel systems").
Also a connection with intern ice formation (Fig. "Flame out by ice formation") is apparently considered from others.

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:

  • 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.

5.1.4.1 Measures against damage from ice ingestion

  • Verification of sufficiently safe operating behavior in case of ice ingestion must be conducted under realistic conditions. If in doubt, the verification conditions must be intensified accordingly. During verification, the planned anti-icing system must also be taken into consideration along with the size and geometry of the expected ice pieces. If an engine is to be used in conditions prone to icing, then its suitability for operation under these conditions must be verified through realistic tests.For example, the strength of the blading against ice strikes must be matched with the planned anti-icing system.
  • Flights in icing conditions with helicopters without special measures against ice strikes should be avoided.
  • Mechanisms against unallowable icing in the engine inlet area are necessary. Systems that are designed to break off already-formed ice layers mechanically should be considered critically before use in sensitive engine types (especially low-power engines with filigreed compressors and/or Ti blading). This is especially true for areas that are known to be hazardous, such as the noses of helicopter engines or the leading inlet edge at the intake.
  • Suitable geometry and material selection for the rotating nose cone (Fig. "Conditions").
  • Anti-icing systems should be designed and operated so that no unallowably large ice chunks are broken free; i.e. the anti-icing system must be activated before unallowable amounts of ice are formed.
  • Engines should be mounted in a way that minimizes the danger of ice from the fuselage or wings being ingested. This is especially true for engines that are located at the rear on the fuselage.
  • The location of fill- and drain nipples for service water (toilet, etc.) should ensure that unplanned water leakage during flight does not result in ice entering the engines.
  • Sensors (pressure, temperature) in the inlet duct that may be subject to icing conditions must be properly de-iced. Possible malfunctions can occur due to icing.
  • Use of a continuous ignition device in the combustion chamber that can be activated in case of icing in order to prevent flame-out.

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.

References

5.1.4-1 NTSB Identification DCA85AA013, microfiche number 28363A, Index for Feb 1985

5.1.4-2 T.L. Alge, J.T. Moehring, “Modern Transport Engine Experience With Environmental Ingestion Effects”, paper from the “Propulsion and Energetics Panel (PEP) Symposium”, Rotterdam, The Netherlands, April 1994, Chapter 9, page 1-7.

5.1.4-3 R. Meijn, “Vulnerability of a Small Powerplant to Wet Snow Conditions”, AGARD-CP-480 proceedings of the conference “Low Temperature Environment Operations of Turboengines (Design and User's Problems)” pages 5-1 to 5-6.

5.1.4-4 L.W. Blair, R.L. Miller, D.J. Tapparo, “Ice Ingestion Eperience on a Small Turboprop Engine”“, AGARD-CP-480 proceedings of the conference “Low Temperature Environment Operations of Turboengines (Design and User's Problems)” pages 20.1 to 20-9.

5.1.4-5 “Chronical of Aviation”, 1991

5.1.4-6 NTSB Identification MIA92IA020, microfiche number 45737A, Index for Nov. 1991

5.1.4-7 NTSB Identification DEN90FA055, microfiche number 41541A, Index for Feb 1990

5.1.4-8 NTSB Identification LAX85IA206, microfiche number 28547A, Index for Apr. 1985

5.1.4-9 I.E. Treager, “Aircraft Gas Turbine Engine Technology” second Edition, Glencoe Publishing, Macmillan/McGraw-Hill, pages 418-450, 567,568.

5.1.4-10 “The Jet Engine”, Rolls-Royce, pages 135-138.

5.1.4-11 P. König, A. Rossmann, “Ratgeber für Gasturbinenbetreiber”, Vulkan Publishing, Essen, pages 7 and 141.

5.1.4-12 NTSB Identification CHI85IA101, microfiche number28026A, Index for Jan. 1985.

5.1.4-13 Amt für Wehrtechnik, “Die Vereisung an Luftfahrzeugen und ihre Vorhersage”, Merkheft 2, Porz-Wahn 1974.

5.1.4-14 R.Rasmussen, J. Cole, R.K. Moore, M. Kuperman, “Common Snowfall Conditions Associated with Aircraft Takeoff Accidents”, periodical “Journal of Aircraft”, Vol 37, No. 1 January-February 2000, pages 110-116.

5.1.4-15 S.J. Riley, Rolls-Royce plc, “Icing Research Related to Engine Icing Characteristics”, AGARD-CP-448, paper from the AGARD Conference “Engine Condition Monitoring- Technology and Experience”, pages 29.1-29.12.

5.1.4-16 Ian Parker, “Chinook's trial by ice”, periodical Flight International, 28. April 1984, pages 1160-1163.

5.1.4-17 H.J.Willcocks, P&W Aircraft, “Icing Conditions on Sea Level Gas Turbine Engine Test Standsd”, paper AIAA-82-1237 of the 18th Joint Propulsion Conference, June 21-23, 1982.

5.1.4-18FAA acts on PW2000 icing in Boeing 757!, periodical “Flight International” 18-24 January 1995, page 10.

5.1.4-19 “Loganair 360 crashes after both engines fail”, periodical “Flight International” 6-12 March 2001, page 9.

5.1.4-20 “Loganair 360 crashes after both engines fail”, periodical “Flight International” 6-12 March 2001, page 9.

5.1.4-21 “Russian Air Force, Aircraft 6 Space Review” and “Progress Releasing New AI-22 and AI-222 Turbofans”, periodical “Air Fleet”, No 23, April 2001, pages 24 and 25.

5.1.4-22 D.L. Mann, S.C. Tan, “Application of Waterdroplet Trajectory Prediction Code to the Design of Inlet Particle Separator Anti-Icing Systems” AGARD-CP-480 Conference Proceeding page16-1 bis 16-10.

5.1.4-23 G.V. Bianchini, “Development of an Anti-Icing System for the T800-LHT-800 Turboshaft Engine” AGARD-CP-480 Conference Proceeding page 18-1 bis 18-12.

5.1.4-24 Department for Transport,”BAe 146-300, G-UKAC, 3. June 2000”, AAIB Bulletin No: 4/2001 Ref: EW/C200/06/12, Incident vom 3. June 2000, page1-3.

5.1.4-25FOD News” (www.fodnews.com), 8.11.2005, “From TWA's `Plane Safe' Newsletter”, page 1.

5.1.4-26 TSB of Canada, Aviation Investigation Report A01A0030, “Multipe Engine Flame-outs”,
De Havilland DHC-8-100 C-GANS Sydney, Nova Scotia, 03 April 2001.

5.1.4-27 AAIB Intreim Report “Accident to Boeing 777-236ER, G-YMMM at London Heathrow Airport on 17 January 2008, EW/C2008/01/01, page 1-21

5.1.4-28 I.E.Traeger, “Aircraft Gas Turbine Engine Technology, Second Edition”, Glencoe Verlag, ISBN 0-07-065158-2, 1979, page 567 and 568.

5.1.4-29 FAA, “Airworthiness Directive: General Electric Company (GE) CF6-80C2B Series Turbofan engines”, FAA-2006-25738, 1979, 24.October, 2006, page 1-5.

5.1.4-30 A.Pasztor, “Airline Regulators Grapple With Engine-Shutdown Peril-Investigators Find New Icing Threat”, Zeitschtift “The Wall Streat Journal”, April 7, 2008, page A1.

5.1.4-31 Transport Canada Civil Aviation, “Mcdonnell Douglas (Huges) 369 Series Flight in Falling Snow - Optional Engine Air Inlet Deflector”, Service Difficulty Advisory AV 2000-01 vom 31 January 2000, page 1 and 2.

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