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From my related question, it seems that an engine suddenly shut off would suffer from dangerous differential cooling from the airflow. This differential cooling would cause the engine to warp.

So now I'm wondering about a jet engine that suddenly enters rain. Wouldn't this initially cause severe differential cooling and thus warping, maybe even cracking? (P.S. would the same thing happen from entering a thick, dark cloud?)

How does the engine handle a huge amount of sudden rain ingestion? To me it seems like the sudden cooling on one side of the engine would cause thermal contraction, causing all sorts of problems.

EDIT: There seems to be a lot of comments about rain certification for engines and the fact that jets fly through rain all the time with no problem. I'm not questioning any of that. What I want to know is, how a jet engine handles the differential cooling caused by rain without warping or cracking?

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    $\begingroup$ Engines are tested extensively for ice and water ingestion, that video shows water being ingested at 4.5 tons per minute. $\endgroup$ – Ron Beyer Apr 29 '16 at 0:28
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    $\begingroup$ Pretty sure the tests involve getting the engine up to steady state then turning on the water/ice ingestion, otherwise it's not much of a real world scenario. $\endgroup$ – Ron Beyer Apr 29 '16 at 1:16
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    $\begingroup$ @jamesqf It should be noted though that the hail was so bad it cracked the windshield, and a majority of the engine failure can be attributed to hail. While its speculation, I'm under the belief that if that flight did not encounter hail, the engines would have continued to run. Rain was a contributing factor at that point, but not the main cause. $\endgroup$ – Ron Beyer Apr 29 '16 at 15:43
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    $\begingroup$ @jamesqf ... and engines flaming out when ingesting rain doesn't happen anymore because the industry does what it's always done - makes the testing and certification better so that something can't happen again. Modern engines are tested during certification with more water than any rain storm could introduce. Watch the videos of this linked above and others. $\endgroup$ – Simon Apr 29 '16 at 16:45
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    $\begingroup$ I edited the OP to clarify that what I'm interested in is not the rain certification, but rather the differential thermal cooling initially caused by rain. How does an engine handle that without warping/cracking? $\endgroup$ – DrZ214 Apr 30 '16 at 0:23
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In another question there is an excellent answer for how thermal expansion is handled in a jet engine. It's an established fact that jet engines can handle fast temperature swings. Have you watched how fast an ITT gauge goes up when starting an engine? My answer will focus on rain's effect on a jet engine.

The first thing to understand is that jet engines take a lot of air. That's why they can handle bizarrely heavy rainfall or even water on the runway splashing into the engine from the tires (this water jet has the fun name "rooster tails"). The Airbus 320, for example, goes through 27,000 liters of air every second during takeoff. 2 That means even during rigorous FAA certification testing the concentration of water ingested never gets higher than 20 g of water for every cubic meter of air. See FAA Part 33.78 Appendix B

In order for you to understand why the water has little cooling effect I have to describe what happens when an engine takes in water, assuming you know the basic parts of a jet engine. The air/water mixture enters the compressor, and the fan blades in this compressor are spinning rapidly enough to catch a lot of the water and like a centrifuge this water is spun towards the walls. This helps prevent water from entering the combustion path on a bypass engine, but it does not prevent water from following the gas path, especially on an engine with no bypass around the gas path.

The engine must now adjust for the extra water in the air, and engines are very good at adjusting for these kinds of changes in conditions. If there's more than one independent compressor rotor, for example, the rotor speeds will change to keep the air moving smoothly and ensure a correct pressure drop across the compressor, called "compressor rematching." The engine will also increase the fuel to air ration to compensate for the water. These two processes evaporate the extra water through thermodynamic heating, not through heat transfer. In essence, the first stages of the engine are providing the extra energy to evaporate the water, so little differential cooling occurs, even if the amount of rain is rapid.

One thing to note here is that the engine actually runs hotter than normal instead of cooler than normal during these events, due to the reasons noted above as well as the extra thrust provided from the water injection.

All the parameter changes noted above reduce your margins for engine flame out and compressor stall. In practice, an engine should flame-out or surge before water can cause a big enough thermal change to cause engine damage. If you think about it, the amount of temperature change needed to cause a problematic warping of an engine should be a lot more than the amount required to cause the engine to quit running.

So we've established that jet engines are good at expanding and contracting due to thermal changes. We've also established that relatively speaking we're actually not talking about a ton of water here, just a few grams per cubic meter of air, which would take a small percent of the engine power to evaporate (~1% by my calculations). Most of the extra water is evaporated not by cooling down engine parts, but by making the early stages of the engine work harder. Finally, the reduced margins caused by water intake mean that you'll surge or flame-out before you get to warping engine parts. These principles should apply regardless of whether the water ingestion begins in a matter of seconds or ramps up over several minutes.

In conclusion, in "Investigation of Engine Power Loss and Instability in Inclement Weather,"(pdf) the FAA noted "Engines typically are not damaged during a heavy rain encounter. Usually the most severe situation is to experience multiple engine flameout or surge with a subsequent pilot initiated engine shutdown." The report then went on to note that the two incidents where engine damage had occurred from rain were over-temperature events, not from differential cooling. So your biggest worry when flying through even storm-of-the-century rainfall should not be whether your engine will warp from rain cooling things down.

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  • $\begingroup$ Thank you, this is a detailed and well-sourced answer. If possible, though, can you add more on the sudden change from flying in dry air to flying in rain? This is where I suspect a sudden temperature change might happen. BTW I invite you to weigh in on my other question, since it apparently has to do with differential cooling as well: aviation.stackexchange.com/questions/21464/… $\endgroup$ – DrZ214 May 5 '16 at 3:16
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    $\begingroup$ Edited my answer to better cover sudden changes. My argument here is that little or no sudden cooling would occur because water does not cause significant cooling of the engine parts without causing a flameout. Is there a part of the engine you think would be exposed to a significant amount of heat differences (without causing flameout), or is this a hypothetical question? $\endgroup$ – Cody P May 5 '16 at 5:59
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A flameout is defined as a loss of engine power that is not caused by a mechanical failure. The three items needed to keep a jet engine operating are fuel, air, and a source of heat to make them burn. Loss of any one of these three can result in a flameout.

Basically if the temperature in the combustion chamber gets too low you get a flameout. But the engine's controller detects lowering temperatures (due to the rain or ice ingestion) very quickly and adds more fuel to compensate for it.

There are (old) aircraft which have used water injection to increase engine performance, usually for take-off or engine failures, where it is needed the most. The reason the thrust is increased is because more mass (the water) is accelerated, but also that the temperature of the hottest section of the engine decreases, sod you can then add even more fuel to get a higher thrust without cooking the engines. If you add water to an oil based fire, it just explodes - a similar kind of energy is in play here.

Even in a strong storm, however, the main consequence of water ingestion a jet engine experiences is a reduction in the efficiency of the combustion process. This efficiency is a function of the fuel-air ratio that is changed by the presence of water vapor. This effect is negligible under most conditions since the percentage of water present in the large volume of air entering an engine is still relatively small in most storms. The high temperature in the engine's combustion chamber quickly evaporates this level of water into steam that has little influence on the engine's power output.

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