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Airline engines are designed to work at very high temperatures. Yet, when a plane crashes they're burnt (see below). Is it something in their design?

enter image description here
(bostonherald.com)

Regmi
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    The engine in the photograph is not melted. – Michael Hall Mar 12 '19 at 23:10
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    @MichaelHall: That's on me, I've fixed it. OP originally wrote burnt. Although judging by the LP section, it did melt. –  Mar 12 '19 at 23:36
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    I have to challenge the premise. From the picture, it looks like the external housing of the engine burned and/or melted. The internal parts look like they've just suffered impact damage. The external housing does not experience high temperatures. Same reason the combustion temperature of your car's engine may reach 2000 C or so, yet the plastic components sitting nearby don't melt. – jamesqf Mar 13 '19 at 04:25
  • related: https://aviation.stackexchange.com/a/16842/1467 – Federico Mar 13 '19 at 09:11
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    I think that the only thing we can say about the external housing is that it is missing from the picture. Besides melting or burning away it could have broken up on impact and pieces shed away as the core slid along the ground. Tough to surmise what happened from a single still photo. – Michael Hall Mar 13 '19 at 16:54

3 Answers3

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How The Jet Engine Works:

Inside the typical commercial jet engine, the fuel burns in the combustion chamber at up to 2000 degrees Celsius. The temperature at which metals in this part of the engine start to melt is 1300 degrees Celsius, so advanced cooling techniques must be used.

You can read more about some of those cooling mechanisms in How are temperature differences handled in a jet engine?

See also, How do you stop a jet engine melting?:

Neil - The normal melting point of the nickel blade alloys that we use in the turbine is typically about 12-1400 degrees. But what you do, and this is the clever bit, is you actually cool these blades. You have internal cooling passages, which effectively has air that flows through and it's about 7-800 degrees. And this cooling air then exits from small little minute holes that have been drilled on the surface of the blade and this air then forms a kind of a film on the surface of the blade, and this technology is typically called a 'film cooling.'

What you also do - you coat these blades and typically use something called a thermal barrier coating. The thermal barrier coating, effectively, is ceramic, typically about quarter of a millimeter in thickness, but they have got very, very low thermal conductivity. So, effectively, even though the gas stream is at a much higher air temperature, the effective metal that exists beneath the thermal barrier coating is much colder, and you get thermal grade of the order of about 100 degrees C between the hot and the cold surface. So all of this put together - this whole cooling technology effectively helps to keep the blade below its melting temperature.

The engine is designed to manage the intense heat in a controlled way, by restricting it to certain components, injecting cool air around the hot parts, and choosing different materials for different parts of the engine. If the engine is severely damaged, doused in jet fuel, and set on fire, none of those mechanisms function; the entire engine (or whatever is left of it), as opposed to just the portions intended to manage heat, will be hot, and none of the cooling mechanisms will be working.

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Zach Lipton
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  • Those blades are little marvels of engineering. Something you can hold in one hand but costs some ~5000 dollars (and there's a whole lot of them in one turbine) – mbrig Mar 13 '19 at 02:37
  • Just out of interest, @mbrig , is it actually ok to hold in your hand (and then safely use)? I know its robustness would point one way but the way it's been micro-optimised for a different environment might indicate the opposite. I know, for exmaple, quartz envelope bulbs rather dislike skin oil: I don't mean this in particular, but this kind of thing. – Dannie Mar 13 '19 at 17:57
  • @Dannie I wouldn't know unfortunately, the one I got to hold was from an areo-derivative generator that had been rebuilt, and wouldn't be going back into use either way. – mbrig Mar 13 '19 at 19:05
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    @Dannie Yes, turbine blades can be handled with your bare hands and used. The metals are nickle based super alloys and they sometime have a ceramic thermal barrier coating they are very environmentally stable. When assembling an engine, they actually use beeswax to hold seals in place and it just burns off once the engine is ran. – OSUZorba Mar 14 '19 at 04:13
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    I'd like to add, that although the superalloys can make it to ~1400C before they melt, they will oxidize ("burn") at lower temperatures. The picture to me looks mostly like impact damage to me with some oxidization. Part of the reason it looks like impact damage to me is that materials with low melting/oxidation points are still there, but then materials with very high melting points are not. – OSUZorba Mar 14 '19 at 04:30
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    So you're telling me that a jet engine actually operates above the melting point of the turbine blades? And that they are kept solid by some sophisticated aerodynamics? And if that fails, the entire engine will presumably turn into a blob of lava? I wish I hadn't read that now... – Oscar Bravo Mar 14 '19 at 07:35
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    @OSUZorba it's even cleverer than that. The surface isn't the same as the bulk, but a thin layer of nickel aluminide (some blades anyway) with a high thermal conductivity and high melting point (~1600-1900C). This reacts in service to form Al2O3 (sapphire), MP ~2000C, but if blades are overhauled the sapphire must be removed before recoating. Paper from some former colleagues of mine. I've handled the blades in question, which I believe were miltary. – Chris H Mar 14 '19 at 09:23
  • @OscarBravo Yes, the gas path air can be nearly 2000F above the capability of the materials it is interacting with. The cooling isn't really aerodynamics, but a system of using compressor bled air to actively cool the materials. While the designing the cooling is very complex, the operation is very simple. There are just a series of orifices and metering holes that allow a specific amount of air through at each point. There are no valves or other moving parts to fail. Of course, there have been failures but generally the airfoils oxidize away before they melt. – OSUZorba Mar 14 '19 at 23:57
  • @ChrisH Yes, that is correct. Every HPT airfoil I ever worked on had a surface coating, even if it didn't have TBC, although not the specific one you mentioned. It was used for environmental protection, but it was too thin to really help with cooling the base material. – OSUZorba Mar 15 '19 at 00:05
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Peter's answer to another question has a nice chart that shows internal jet engine temperatures:

Enter image description here

You can see that the temperatures are highest by a fairly large factor in the combustion chamber. This means that only the combustion chamber needs to be able to withstand those temperatures. To save weight and often use less expensive and less exotic materials, the rest of the engine may be made out of materials that don't need to withstand such high temperatures. As such, in an accident where jet fuel may be dispersed in an uncontrolled way and burn with as much oxygen as it can get, it's easy to scorch engine parts and anything else around.

It also is in part a question of time. The ability to withstand heat varies with time. In a crash of a fairly fueled aircraft that may burn uncontrolled for a long time you are likely to find scorched parts like this. Whereas a plane that runs its tanks try and crashes in a field may not see the same fire marks. However, if the plane hits the ground with enough force the heat generated from the impact can also lead to markings like this.

Peter Mortensen
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Dave
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First off, the engine is running around 2000° F (NOT 2000° C/3632° F) only in a few places within it. The Turbine Inlet Temperature (TIT) can be that high but cools rapidly when the exhaust gases are rapidly expanded through the high and low pressure turbines to exit at approx 1000° F at the jet pipe (enthalpy is converted into mechanical work here). Most sections of the engine are not designed for that high of a temperature and may well oxidize or deform in the post crash fire.

UPDATE: While I do not have definitive values for the max TIT of a CFM-LEAP engine, a good estimate would be around 1500°- 1600° C (2732° - 2912° F) as this represents about the state of the art for gas turbines outside of a few military applications. This source quotes that the Pratt & Whitney F-135 can operate at TITs of 2000° C due to its use of a proprietary ceramic coating over single crystal nickel cobalt superalloys for the hot section. It is unclear whether that is a sustained engine power setting or simply a max operating temp prior to destruction. This is NOT representative of a typical aviation gas turbine, which run much cooler. I would stand by my original figure of 1000-1200° C TIT for earlier gas turbines eg PT-6, J85, J79, etc.

Romeo_4808N
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    Note: 2000° F is 1093° C – Déjà vu Mar 13 '19 at 14:18
  • .....what is your point? – Romeo_4808N Mar 13 '19 at 17:12
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    @CarloFelicione probably to inform people like me what the celsius equivalent to 2000° F is (seeing as celsius is something you provide for the other value, and most of the world understands over fahrenheit) – CalvT Mar 13 '19 at 17:22
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    This disagrees with the temperatures quoted in other answers. – Carl Witthoft Mar 13 '19 at 18:48
  • No it does not. Tf = 9/5 * Tc + 32 – Romeo_4808N Mar 13 '19 at 18:58
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    This does disagree with the answer by Zach Lipton which quotes a source as stating "2000 Celcius" (and links the source). – Martin Bonner supports Monica Mar 13 '19 at 21:10
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    Do you have a citation for this figure? The Stanford link that Zach quoted above specifically states "up to 2000 degrees Celsius", and the chart posted by Dave appears to peak at around 1788 degrees Celsius (~3200 F). – GalacticCowboy Mar 13 '19 at 21:10
  • My source, which uses vague "up to" language, is from a course about WWII, so I wouldn't treat it as authoritative on the details of every engine today. Here's another source that says 1400°C and here's an FAA manual that says "around 4,000º F" with a cooler airmass around it. It also depends greatly where you measure, as the entire point of the cooling system is that the hottest temperatures are only found in one place. – Zach Lipton Mar 13 '19 at 21:43
  • Turbine inlet temperatures on modern engines are over 2000C, far beyond 2000F. The materials used in modern turbines could survive uncooled at 2000F, yet they all have very expensive (in design and engine efficiency) cooling. The LEAP uses extremely high combustion temperatures, in the high 3000sF. The shrouds are CMC and still have to be cooled. – OSUZorba Mar 14 '19 at 04:17
  • I also want to state the every modern jet engine at least since the late 60s has used single crystal nickel cobalt superalloys for the hot section and most use TBC coatings. The LEAP even uses CMCs that have much higher temperature ranges than the nickel superalloys. The F-135 is not the hottest engine around. – OSUZorba Mar 14 '19 at 04:22
  • Evidence to back this up? – Romeo_4808N Mar 14 '19 at 10:08
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    @CarloFelicione Considering the Turbine Exhaust Temperature redline for the Leap-X is 1900F, which is after the HPT expands and cools the gas, I think that is a pretty good indication your numbers are off. Type Cert here: http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/ae71cf81844093f98625836600766d81/$FILE/E00088EN_Rev_4.pdf I also have six patents on turbine cooling designs, but can't quickly find public data on T4 temperatures. – OSUZorba Mar 14 '19 at 21:27
  • Yeah I’ve read the type cert. That is pretty high, though if I recall the max continuous operation EGT is a chillier 1700° F or so. If they are getting those kinds of CET/TITs, that’s amazing as the current state of the art is about 1700° C. – Romeo_4808N Mar 15 '19 at 00:23