5.1.1 Rain

Rain can affect the operation of all of an airplanes in several different ways (Fig. "Effects of heavy rain").
Extreme rain can cause a decrease or even complete loss of power. The decrease in power can be seen in the lower rotor RPM and is due to the severe cooling of the gas flow caused by the rain (or hail). In this case, this cooling effect is greater than the heat energy created by the fuel. If the heat loss becomes greater, combustion may become instable and the combustion chamber may extinguish, causing a total loss of power. For this reason, every effort is made to increase the stability limit of combustion chambers.
Another observable effect is the rapid cooling of the compressor housings. This is accompanied by corresponding shrinkage and/or distortion of the housings/casings, bridging the clearance gap at the blade tips and causing heavy rubbing of the blades against the housing. This can cause heavy wear of the protective coatings in the housing and also damage the blade tips (crack initiation in extreme cases) and/or result in a dynamic fatigue fracture of the blades. Even if operating conditions return to normal after this event, if there is a large amount of wear product present, it can worsen the operating performance of the compressor (efficiency, surge limit).
A further problem is the penetration of water into electric and electronic devices (also see Figs. "Environmental risks for electronics" and "Causes for electronic damage") such as regulator boosters. A typical case was moisture entering into the booster of the thrust nozzle regulator of a fighter aircraft as soon as it flew through rain.
There is a potential risk of damage due to droplet impact (Fig. "Influences on rain erosion I") when erosion-sensitive materials (e.g. fiber-reinforced synthetics, aluminum alloys) are used in the front compressor area.
The AIA (Aerospace Industries Association) has developed a definition for “especially rough weather conditions” (Fig. "Impact on engine").
Rain is not the only way for large amounts of water to enter the engine. If there is water on the runway, it can be thrown up by the wheels and into the engine, depending on the engine and landing gear configurations (Fig. "Concentrated rain").

Figure "Effects of heavy rain" This diagram matches several rain-related factors that affect engine behavior with the engine components that they affect. These effects have are taken from refs. 5.1.1-1 and 5.1.1-6.

The amount of rain which enters an engine depends on several factors:

  • Configuration of the air intake (nacelle engine, Fig. "Geometric parameters") With an integrated engine intakes are on the hull, and air reaches the engine through an intake duct
  • Geometry of the engine intake (e.g. spinner, blades)
  • Flight attitude (e.g. altitude, speed, climbing or descending flight, engine output)
  • Local atmospheric conditions In general, water will be unevenly distributed in the intake air flow. Long, cambered intake ducts will cause the water in the air flow to be considerably more unevenly distributed. These distributional patterns are covered in acceptance test procedures

It can be assumed that virtually all engine components will be affected by heavy water intake into the engine. The fan is the first affected component, but the following compressor area also clearly reacts to the water.
Water intake affects the stage-characteristics and efficiency of the blades (ref. 5.1.1-3). It reduces power output of each stage, and the decrease in air flow in the compressor is especially pronounced at high RPM. This effect combines with the change in the ram temperature and causes severe alterations in the surge behavior and also the throttling characteristics of the compressor. This must be seen in relation with the changes in water distribution due to centrifugal force.
In the combustion chamber, both the efficiency and fuel flow can be affected. Flame instabilities (flickering) can occur and in extreme cases, the flame may be extinguished.
Heavy water intake in the compressor can indirectly affect the behavior of the regulators by changing the aerodynamic and thermodynamic parameters as well as the parameters of combustion. It is also possible that the rotor inertia may change, affecting the response times after RPM changes.

Example "Leaky electronics casing" (Fig. "Leaky electronics casing", Ref. 5.1.1-8):

Excerpt: „Modifications have been developed to correct power-fluctuation problems…(in a small turboshaft engine)…A specialised team, consisting of …(engineers of the operator, the OEM and the control system provider) have identified the recurring problem as one of moisture entering the engine limiting systems (ELS). Since…(the enginetype) was introduced into service in 1989, numerous uncommanded engine power fluctuations, or rollbacks, have occurred at critical flight stages, but these could not be reproduced in ground runs. Investigations… confirm that moisture ingress into ELS harness was a major contributing factor. Further Tests showed that a combination of loose connectors, especially that for the torque transducer, and moisture ingression into the connectors via various paths, caused most of the rollback incidents. Trial installation and flight testing of…modified ELS components is now complete and…are being incorporated into the…fleet.“

Comment: About controller problems also from other aeroengine types is always again reported (Fig. "Environmental risks for electronics"). Behind the term “rollback” may hide a drop of the aeroengine power respectively the gas producer rotation speed (see chapter 5.1.4). There is the question how the proof of the serviceability of those systems took place. A markedly influence at such problems have atmospheric pressure and changes of the environment temperature when the altitude alters.

Figure "Geometric parameters" The behavior of an engine in heavy rain is especially dependent on the engine's geometric values, such as the size, arrangement, and shape of its components. In addition to the above parts, the arrangement of air vents and bleed valves inside the compressor can have a pronounced effect on the behavior of engines with a large amount of water in them. The total amount of water taken into the engine depends on the scoop factor (calculated by the diameters of the components in the flow duct of the intake area) and the flight speed (Fig. "State of operation"). The distribution of water between the bypass and the engine core is affected by the flow feed at the intake, the spinner, the fan, and the geometry of the flow channel between the fan exit and the front of the splitter (“6”).

Figure "Concentrated rain" Large amounts of water can enter the intake flow in other ways than simply as rain. This illustration depicts a landing in conditions that comply with regulations. The runway is covered with water. The amount of water that enters the engines is probably greater than would be possible purely through rain. Takeoff under these conditions would be a particular cause for concern. In this case, however, the relatively high engine output and short duration of water intake would have less dramatic results.
An important factor in this type of water intake is the tendency of the tires to throw water up. It has become possible to use calculations to estimate (Fig. "Drop size distribution") the expected height of the “water cloud” for specific tire patterns, flight speeds, engine intake geometries (Fig. "Geometric parameters"), and water puddle geometries. It is also possible to determine the size and distribution of water droplets.
A further way water can enter into an engine is as condensation of water vapor in the air. The condensation of water vapor in the air can occur due to the temperature drop in the accelerated flow of the intake area. The higher the temperature of the humid surrounding air, the greater this effect is. The condensation zones are determined by the geometries of the engine/nacelle intake (especially the intake rim), spinner, and fan hub. There is no concrete evidence in the literature that describes the degree to which this type of water intake affects engine behavior.
If a puddle forms in the intake duct due to rain or condensation (for example, in fighter aircraft), it can be sucked into the engine during startup and the impact force can plastically deform the blading.

Figure "Impact on engine" According to Ref. 5.1.1-2 there were 48 incidents of power loss in civilian cargo aircraft between 1980 and 1989. In 14 of these cases multiple engines were affected. The greatest risk is during descent, since the low rpm decreases the blow-off effect at the intake, increasing the amount of water in the intake flow. Example "Leaky electronics casing" and Example "Combination of hail and rain" demonstrate this serious process.
Because of this, the rating and testing process requires verification of proper operating behavior with high water content in the intake air.
These tests showed that this problem can be brought under control through appropriate influencing of the rain drops and hail corns.
It was recognized that only about 10% of the rain water at the intake of a modern fan engine
reaches the engine core. The raindrops are sprayed around by the fan on entry and reach the speed of the air flow. If these particles pass through the fan, they are centrifuged into the bypass duct. The diagram depicts a recommendation for the “water and ice tolerance” of engines. This recommendation is based on an extensive study from 1994 about engine power loss due to rain and hail. These recommendations (Ref. 5.1.1-3) exceed the requirements of the regulations of that time, FAR 33.77c. Another source (Ref. 5.1.1-1) prescribes the maximum water content in the air at 15% by weight. The air flow-through rate of a large fan engine transports about 1-2 liters of rain per second (0.5-1 Kg. of ice/s) into the core engine and about 5-10 liters/second of rain (2.5-5 kg. ice/second) through the entire engine, including the fan.

Figure "State of operation" The flow contraction is greatest during standing operation, i.e. relative to the engine inlet, a larger cross-section becomes effective during rain tests. Therefore, tests in standing engines are „on the safe side.“

Example "Leaky electronics casing" (Ref. 5.1.1-2):
”…(the Aircraft) experienced a dual engine flameout while in descent from 35000 feet…The crew reported severe rain and hail and lightning…The crew reported light icing until about 30 seconds before power loss. The crew reported a sudden encounter with heavy hail, heavy rain (and) with moderate turbulence…
The crew was unable to restore power on either engine. The aircraft lands safely on a grass levee …
There were hail impact dents in the leading edge of the horizontal stabilizer, and the paint had been removed from the radome. The radome was dimpled from hail stones impact, the largest being about 1.5 inches in diameter. There was no hail damage to the wing leading edges and no hail damage to the engines.“

Comments: A similar incident occurred with the same aircraft/engine combination during descent from 8900 feet through heavy hail and rain. Both engines shut down under idle descent power, but were restarted with no problem. This shows that engines can be sensitive to these extreme weather conditions (also see example 5.1.2-1.2), even though they successfully passed the required rating tests. On the other hand, specific constructive measures can avoid this type of weakness.

Figure "Drop size distribution" This diagram (Ref. 5.1.1-5) shows the distribution of rain drop size in connection with the water content of the air and the number of drops that fall on the 10 cm x 10 cm surface in one minute. The thick cross lines show the dependency on the rain intensity. The area marked with the grey ellipse shows a typical value (4% by weight of the air flow rate) for rating tests according to the table in section 5.1.1. One can see that the test is in the sufficiently safe area, at least for the conditions shown in the diagram.
From about 100 mm/hour rain intensity (marked by the black circle) onwards, individual drops will be roughly 6 mm in diameter. This is important for erosion tests, for example. Measures against problems due to water intake

  • During the development stage, sufficient trials on ground and in the air should be sufficiently close to actual conditions and conducted as early as possible.
  • Certification and rating tests for new engine types should be conducted with the newest available data, if this data seems to indicate stricter conditions than those required by the current official regulations. An example of increased strictness would be simultaneous water and hail intake.
  • Optimal design of the intake ducts in the hull of military engines and of the intake area of nacelle engines Fig. "Geometric parameters"). With large fan engines, this includes the design of the spinner, and the geometry of the blading and air ducts. Changes for optimisations should be carried out in small steps.
  • Optimization of the position, type and the shielding of sondes/probes.
  • Optimization of the control system. Especially at multi shaft aeroengines changes at the fuel admeasurement and the ratios of the main shaft speeds under consideration of the power demand and a “performance deterioration” offer itself.
  • Suitable operation specifications for heavy rain.
  • Water insensitive, respectively tight cables, connectors and casings/boxes for electrical and electronic systems. If necessary sealing measures (example 5.1.1-1). The tightness determines the safety of these systems against atmospheric influences. This has to be proved with operation relevant tests. Thereby for example it must be considered, that during change of the flight hight the atmospheric pressure and the environment temperatures markedly change. This is also true for the differential pressure to the interior of the casing/box, cables or connectors. Such an air exchange is supported by the intrusion of humidity. Even if only the intrusion of relatively dry environment air is concerned, during cooling condensate can form. Because the air is frequently salt laden, additionally with corrosion and fault currents must be anticipated.


5.1.1-1 T. Tsuchiya, S.N.B. Murthy, “Water Ingestion Into Jet Engine Axial Compressors“, AIAA-82- 0196, proceedings of the “AIAA 20th Aerospace Sciences Meeting“, January 11-14, Orlando, Florida, page 1-6.

5.1.1-2 T.L. Alge, J.T. Moehring, “Modern Transport Engine Experience With Environmental Ingestion Effects“. AGARD-DC-558, proceedings of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines“, Rotterdam, The Netherlands, 25-28 April 1994, Chapter 9.

5.1.1-3 K. Hünecke, “Die Technik des modernen Verkehrsflugzeuges“, Motorbuch Publishing, page 123

5.1.1-4 “Airlines worldwide stand by CFM56“, periodical “Flight International“, 21 January 1989, page 11.

5.1.1-5 M. Diem, “Die Struktur der Wolken und der Regen von der antarktischen bis zur tropischen Zone“, proceedings of the “2. Forschungskonferenz in Meersburg“, Germany, 1967, page 57-84.

5.1.1-6 S.N.B. Murthy, “Effect of Heavy Rain on Aviation engines“, AIAA-89-0799, paper of the “27th Aerospace Sciences Meeting“, January 9-12, 1989/Reno, Nevada, page 4.

5.1.1-7 S.N.B. Murthy, A. Mullican, “Transient Performance of Fan Engine With Water Ingestion”, NASA Contractor Report 190778, April 1993.

5.1.1-8 P.P. Cairns, “RAAF and P&W develop cure for PC-9 powerplant”, Zeitschrift „Flight International“, 9-15 November 1994, page14.

5.1.1-9 “Blow-dry your Jet”, Zeitschrift “Flight International”, 18 May 1985, page 49.

© 2020 ITTM & Axel Rossmann
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