Bjorn’s Corner: hot summer, hot engines

By Bjorn Fehrm

By Bjorn Fehrm

17 July 2015, ©. Leeham Co: It is summer in south of Europe and we have had over 30°C/86°F for weeks. It makes one realize the conditions where the engines have to work over their flat rating point in the Middle East.

Aircraft engines are a bit fidgety. They don’t like temperature although they are made to sustain that their hottest parts, the nozzle and first turbine after the combustor, gets scalded to 1700°C/3,092°F or more.

Go down to the very back end of the engine and we come to where the key engine parameter, EGT (Exhaust Gas Temperature), is measured. It determines a lot of things, among them the time the engine stays on wing. Things are typically 700°C/1,832°F cooler here and this is where a reliable temperature measurement probe can be placed. Based on its values, the total health of the engine’s core is determined. It is also a key input whether the engine shall be throttled back in a hot take-off like in the Middle East.

Whereas directly after the combustor we would talk degrees and tens of degrees in terms of engine parameters, at the end of the turbines we talk about fraction of degrees. These fractions determine whether the engine will continue to be used or it has to be taken off for service.

We talk about the EGT deterioration rate for the engine.

EGT

Imagine that at EGT redline, let’s say 1,000°C (engine EGT is given as Celsius world-wide), it is finished. The engine can no longer be started and asked to perform yet another take-off. It will be reported as used up and ready for service, often a hot part performance restoration overhaul.  It will bring back most of the EGT margin that we had on the engine when new. Typically a new engine has a margin to the point where it can’t stay on wing anymore of around 50°C for a long haul engine and 100°C for a short haul engine.

The engine’s wear each time it is cycled through a take-off (which is the real stress of an engine) is determined to a fraction of a degree. Turbofans typically wear fast the first 1,000-2,000 take-offs after service or new, chewing up 20 of the degrees they had as EGT margin, then they settle down, chipping along at 0.5°C per 100 take-offs. This means we have typically 6,000 cycles before a long haul engine is due, and 16,000 cycles or more before our short haul engine has reached red-line EGT.

Those that work with airline engines knows things are a bit more complicated than this but “grosso modo” this describes the life of an engine, small or large, short haul or long haul. I thought it might be fun to know that it is all down to fraction of degrees in this thing which is running at least twice the melting point of most of its internals.

Corner point

The EGT also determines if the engine will deliver full power at a hot take-off. Normally engines keep their performance up until a certain external temperature, then the engine computer start to throttle back the fuel it injects in the combustor and we are past the flat rating point or the corner point. This is typically the 30°C/86° F we experience now, meaning most airlines don’t get full take-off thrust from Nice Airport these days.

Engines can be given “thrust bumps.” This means that for a very limited amount of time, typically minutes to allow a take-off to be performed, the engine computer can allow a higher corner point than normal. Let’s say it starts to throttle back the engine at 40°C instead.

This does not come for free. It increases the EGT wear rate, so instead of the normal 0.5 degree per 100 cycles we might see closer to one degree. That would half the time on wing of the engine and this is why Tim Clark, president and CEO of Emirates Airline, says “we don’t do thrust bumps;” he wants his engines on the wing for the full 5,000-7,000 hours or whatever is standard from the engine OEM.

The engine EGT margin is also what gets squeezed when one specifies a stronger CFM56 or IAE V2500 for one’s 737 or A320, a so called “throttle-push” engine variant. For a “throttle-push” variant the OEM simply modifies the parameters in the engine computer to inject more fuel in the combustor, thereby spinning the engine faster. As it is a rotating air pump it will push more air out the back at higher speed, voilà more thrust!

But once again nothing is for free in this world. Example: a CFM56-7B27 for Boeing’s 737-800 or -900 with 27,000 lb thrust can have an EGT margin when new of around 50°C. Specify the 26,000 lb variant instead and you get 75°C or the 24,000 lb variant (more for the -800) and you have a nice 100°C to chip away at. The deterioration as described is still the same, “grosso modo,” 20 degrees initial wear for the first 2,000 cycles and then five degrees per thousand. So after the first 2,000 cycles, you have another 6,000 on the stronger engine or 16,000 on the smaller. In reality it might a little bit different but it makes one understand why higher thrust is not always better; rather specify what we need but not more.

Other things getting hot

It is not only aircraft turbofans that are fidgety. Solar Impulse 2 is grounded for nine months at Hawaii because its batteries got too hot. The amount of solar energy that gets captured by its solar cell-laden wings and fuselage varies and it has therefore a large pile of high performance batteries to store energy when sunny and use when not so sunny. It will also use energy from the batteries for power peaks like take-offs or climbs.

When taking-off and climbing from Nagoya on the way to Hawaii, these batteries were working hard and got too hot. They thereby destroyed themselves so they now have to be replaced. At the same time the Solar Impulse team realized they had not calculated right when it came to how to cool and use the batteries. They will figure out how to modify that and then repair Solar Impulse 2 before they fly again. It takes nine months but this is the way progress works. Pioneers take the leap and learn, then leap again. Kudos to those that dare the leap.

40 Comments on “Bjorn’s Corner: hot summer, hot engines

    • LOL, never seen it before, something for the environmental department :).

      • Could come in handy. Like squid ink. Confuse the enemy’s targeting with a cloud of black. 😉

      • Didn’t they do something like this with the early KC-135’s with turbojet engines? I seem to recall some pretty black smoke coming out of their engines when they were taking off from Boeing Field. And I also remember seeing pictures of B-47’s with some kind of propulsion coming out from the side of the fuselage. I believe it was called “jet-assisted takeoff” or JATO for short.

    • Water injection has been done, but Tim C does not like it, I remember his vehement rejection when it was “floated” for the 777X.

      Bjorn, does water injection add other issues than tanks (weight)? (and maint complexity but would not to seem to be serious?)

    • Wow those B-52 engines must spew at least 30% of the fuel out the back….

      • significantly less than 1% soot.
        And at the time it was deemed difficult to fix due to the low percentage. We’ve come quite a bit along the way since then.
        ( Wonder what all the “noise is so terrible, can’t sleep” nimbys would have demanded in the 50ties/60ties)

  1. RE the old B-52 and black smoke. One factor in going to higher temperatures was to eliminate the” horrible” black smoke ( carbon ). Made sense for military. And of course for commercial re efficiency. But the rarely mentioned downside is that the higher temps result in more nitrogen decomposition which when dispersed in moist air is claimed nowadays to result in ‘acid rain ‘ via as I recall a weak nitrous acid type reaction. Been too many years since Chem class. 🙂

    • Hi Don, right on the money. The whole thing about TAPS combustors etc that all the OEMs are working on (TAPS being GE’s brand for its low NOX combustor) is to lower the emissions of nitrous acid type radicals. Taken very seriously and a lot of work is being made in the corner, new engines are well below new tougher norms for NOX emissions.

  2. Another great insight, a quick question, rolls Royce(triple spool) engines were always said to retain their performance better than their contemporaries, is there an easy technical explanation for this?

    • There is. They are shorter and due to the gyro forces the fan tend to want to stay put when it is gusty etc, this causes the rest of the engine to flex and wear its many rotating seals. The Tren’s being shorter are stiffer i.e. wear the seals less.

      • So with today’s lighter and slower fans that is presumably less of a problem than it was? If so, I wonder what is more significant, the weight saving or the reduced maintenance?

        I can remember reading an article in a Cathay in flight magazine back in the 1990s. It was talking about how one of their RB211s had never been off the wing and was still running to spec despite and amazing number of hours. I can’t remember exactly how long, but it made a deep impression.

        • The fan shall be viewed together with its casing, going CFRP on the fan means you can have a CFRP fan casing. The combination saves quite some weight, probably in total on aircraft level half a tonne. Each tonne of empty weight cost you half a percent in efficiency so you gain 0.25% of fuel consumption with a CFRP fan + fan casing on these large engines. The gyro forces will reduce, how much it will help with engine shaft flexing I can’t say.

          • A shorter engine is going to be stiffer, but I think this makes it harder for the bearings (which are more closely spaced) to counter the gyroscopic force and keep everything pointing the right way. RR’s engines having a good service interval suggests to me that it’s not their bearings that wear out first, that the gyroscopic loading on them is not as big a deal as all that, and that it is indeed the stiffness of the non-rotating frame that matters more.

            The counter rotating IP shaft helps reduce the gyroscopic action of the engine overall.

            Physics lessons were a long time ago…

        • Mathew:

          Also a factor has to be the larger fans and while lighter, also more leverage.

          All a balancing act

          • Bearings only have to deal with gyroscopic action when the engine moves on its pylon or if the engine experiences some sort of lateral motion. On a longer engine the whole casing can flex more which in addition to the gyro forces, causes stress on the seals. I think that is not a good thing in the LEAP vs Purepower battle.

      • So what actually wears in the engine, is it the seal that wear, giving rise to oil leakage and increase EGT? How much will the FADEC roll back the throttle to keep EGT within check. Also, when the RR engine exploded on the A380, why was there not a detectable rise in EGT?

        • The seals wearing increase the parasitic losses in compressors and turbines i.e. a loss in efficiency (pressure per hp), it shall not result in oil leaks but results in increased EGT as the FADEC compensates with injecting more fuel ie running the engine harder. The FADEC will roll back to what is needed for the EGT to stay within limits, it will also signal when the engine is no loner to spec because of this roll back.

          The A380 engine had an oil pipe bust inside the central shaft part of the engine starting a fire which weakened a turbine disk which exploded. Nothing that happened in the core gas stream where the EGT is surveying what is happening.

  3. Bjorn, how do u think 280ton 84k engined A350LR will perform in Dubai summer?

    • It will have limitations but less so than 787-10. The main difference compared to 787-10 etc is the wingloading, it is a low 605 kg/m2 with the 268t version and would be at 630kg/m2 for the 280t of the -900LR. This shall be compared to the 700kg/m2 of the 787-10. So the 787-10 requires more lift-off speed and this requires the engines to get it there and brakes to stop it if one engine goes bust when at the higher speeds. All covered in the previous articles.

      • Can an engine certified on the maximum thrust it achieved on testing ?

        • No, the engines are always driven to a higher maximum thrust than what is certified. Certification is very much a trade of hot part temperatures vs. time on wing, therefore you have 5 min Take-off Ratings which is the rating which is normally quoted as engine thrust, then a lower max continuous rating (for engine out cases) and then sometimes even a third Climb rating, both lower than the TO thrust rating.

      • Is the 787 at the limit of high lift devices to get it off the ground? Can they not add more area to the flaps or a more radical flap system (well I guess the question is can they do it economically).

        • You can always modify the wing but adding more flap area or angle increases wing stresses = beefing and new certification = costs. Question is what gains for what money.

  4. I am curious at use of water injection. Boeing proposed this for 777x early in the program, but it was killed on a cost basis. Would this extend engine life? If so, why is it not used more? How much water would they carry to be effective? Since they would use up all the water on takeoff I don’t see an efficiency issue and the system should not be that heavy. Also I know the Russians use some sort of additive in the fuel for TU-160 that results in that weird yellowish exhaust. What is that?

  5. Question from ignorance…. For large commercial aircraft, is V1 achievable with lower thrust and longer runways?

    If so, would it be easier to lengthen runways than to overengineer aircraft? Certain operators have consistent temperature constraints, so they might seek to negotiate with airport owners rather than OEMs.

    • V1 ist not a constant. reduced air density increases V1.
      Other limitations like max wheelspeed block the usefullness of a longer runway.

    • No, you always have the requirement that the aircraft shall be able to stop within the runway length if one engine goes bust before rotation, therefore you also have aircraft brake limitations (tested with the famous Rejected Take-Off test) and max speed limits for the tires. So a longer runway helps until these limits come in.

    • Todays very local newspaper (“Schleiboote”/SH.Z) here has a small article on this. “new engines, new wing” ( and notes that this is a major boost to Airbus Hamburg.)

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