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Thread: Air Zimbabwe 767 Engine Surge, Tailpipe Flames, Mayday... Continues to Destination

  1. #21
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    Quote Originally Posted by ATLcrew View Post
    Then there's the matter of me having spent six years flying a family of jets that had neither an EGT, nor an EPR indication...
    ...so you don't need to worry about it!
    Be alert! America needs more lerts.

    Eric Law

  2. #22
    Senior Member Evan's Avatar
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    Quote Originally Posted by ATLcrew View Post
    Interesting article, thanks. They don't really explain how they came up with those (and other) numbers, and seem to use the word "should" a lot without backing it with much, but alas...

    Then there's the matter of me having spent six years flying a family of jets that had neither an EGT, nor an EPR indication...
    Hopefully they at least had some sort of CDU exceedences maintenance page.

    Quote Originally Posted by Airbus Safety First Magazine
    To protect turbine hardware, an operational limit on EGT (called “EGT red line”) is demonstrated during endurance tests required for FAR 33/JARE/ EASA engine certification. During such tests, the engine is run for 25 stages of 6 hours each. For each stage, the engine spends up to one hour cumulative time at max takeoff regime, with average EGT at redline conditions. Moreover, FAR 33 engine certification requires the engine to run for 5 minutes with N1 and N2 at red line levels and with EGT at least 42°C above the red line. After the run, the engine is disassembled and the turbine assembly must be within serviceable limits.
    This article is addressing how to avoid EGT overtemp on take-off, but it illustrates the need to take prolonged incidents of extreme exceedances very seriously. Even without an EGT indication, shouldn't this 767 scenario at the top of this thread lead you to conclude that a prolonged, extreme EGT exceedance has occurred (at least Area A, if not B or C, on the chart below) and that landing asap is the required move?
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  3. #23
    Senior Member Gabriel's Avatar
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    There is one thing that doesn't seem to make much sense in that chart: lack of continuity. What's the difference between flying 4:59 at 949C and then reduce to 914C and flying 4:59 at 949C, then 0:02 at 451C, and then reduce to 414C? You bypass A and B and go straight to C for exceeding the EGT redline by 1 degree for 1 second?

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  4. #24
    Senior Member 3WE's Avatar
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    Quote Originally Posted by ATLcrew View Post
    Then there's the matter of me having spent six years flying a family of jets that had neither an EGT, nor an EPR indication...
    Could those jets have been designed with some thought given to not be overly complex (and yes using some computer monitoring).
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    Could those jets have been designed with some thought given to not be overly complex (and yes using some computer monitoring).
    Or he uses TIT instead. TIT is more important that EGT. EGT is a "byproduct" of TIT. Imagine the EGT in an F-14 with afterburner. But the TIT remains under control.

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  6. #26
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Or he uses TIT instead. TIT is more important that EGT. EGT is a "byproduct" of TIT. Imagine the EGT in an F-14 with afterburner. But the TIT remains under control.
    If you have a diminished flow of air to the cumbustors, thus diminished cooling and diminished 'shaping', resulting in excessive heat transfer to that section (and thus a rise in EGT), would the TIT reflect that?

  7. #27
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    If you have a diminished flow of air to the cumbustors, thus diminished cooling and diminished 'shaping', resulting in excessive heat transfer to that section (and thus a rise in EGT), would the TIT reflect that?
    Yes. TIT is the turbine INLET temperature. It is the temperature between the combustion chamber and the 1st turbine stage.
    As the gas flows through he turbine and gives part of its energy to the turbine, the pressure and hence the temperature go down and result in a lower EGT after the last turbine stage.

    The mixture of air and fuel is about stoichiometric in the beginning of the combustion chamber to make the combustion sustainable and more efficient, but then more air (that bypasses the first section) is added in the combustion chamber to get the temperature down to levels that are manageable by the turbine blades that are subject not only to temperature and aerodynamic loads but also to strong centrifugal loads due to their high rotation speed. If the flow of air is less than it should, that results in a too rich mixture in the beginning of the combustion chamber and then unburnt fuel. As more air is added to the gas (to cool it down), the unburnt fuel finds the missing O2 and burns, resulting in a higher gas temperature rather than lower). The TIT will be higher and so will be the EGT.

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  8. #28
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    Or put a little differently, the TIT is a direct measure of the temperature of the gases that are going to damage the turbine if the turbine is going to be damaged due to the temperature of said gases.
    Be alert! America needs more lerts.

    Eric Law

  9. #29
    Senior Member 3WE's Avatar
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    TIT EGT OAT CHT OT CT TT BT DT FT STP CT BT GT ST

    https://www.youtube.com/watch?v=M1WemnsB98o
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  10. #30
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Yes. TIT is the turbine INLET temperature. It is the temperature between the combustion chamber and the 1st turbine stage.
    As the gas flows through he turbine and gives part of its energy to the turbine, the pressure and hence the temperature go down and result in a lower EGT after the last turbine stage.

    The mixture of air and fuel is about stoichiometric in the beginning of the combustion chamber to make the combustion sustainable and more efficient, but then more air (that bypasses the first section) is added in the combustion chamber to get the temperature down to levels that are manageable by the turbine blades that are subject not only to temperature and aerodynamic loads but also to strong centrifugal loads due to their high rotation speed. If the flow of air is less than it should, that results in a too rich mixture in the beginning of the combustion chamber and then unburnt fuel. As more air is added to the gas (to cool it down), the unburnt fuel finds the missing O2 and burns, resulting in a higher gas temperature rather than lower). The TIT will be higher and so will be the EGT.
    Ok, but who monitors TIT? I'm talking about things the crew are alerted to or aware of. The standard indicators in most commercial jets are EPR, EGT, N1 and N2, at best. I'm pointing out that, during a continuous compressor surge event, you are going to get an EGT redline indication and what is the overtemp limit for continuing to destination vs landing asap? Or is this just not defined at all?

  11. #31
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    Quote Originally Posted by Gabriel View Post
    Or he uses TIT instead. TIT is more important that EGT. EGT is a "byproduct" of TIT. Imagine the EGT in an F-14 with afterburner. But the TIT remains under control.
    No TIT on that family of aircraft either. In fact, I'm not aware of any jets that display TIT.

  12. #32
    Senior Member 3WE's Avatar
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    Quote Originally Posted by ATLcrew View Post
    No TIT on that family of aircraft either. In fact, I'm not aware of any jets that display TIT.
    Amazing, given how we used to make such a big deal about them in the more humourous days.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  13. #33
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    Quote Originally Posted by ATLcrew View Post
    No TIT on that family of aircraft either. In fact, I'm not aware of any jets that display TIT.
    II suppose it's like medical science, where a handful of vital signs are used to check for more serious conditions throughout the body. If your four primary engine vital signs are good, there's no need for more specific ones like TIT. If there is rise in TIT, that is going to show as a (albeit lower) rise in EGT, Correct me if I'm wrong, but the two cockpit indicators of a continuous engine surge are EPR and EGT. We were taliking about reducing complexity; well that seems like a pretty simple way to recognize the situation (unless, as I believe is the case for the plane you are currently flying, ECAM does that for you).

    But again, my question is, if you get a continuous (60-90 sec) surge on climb-out with a corresponding EGT redline, and then the surge self-corrects and the indicators return to normal, do you always return or is there a threshold below which it is considered permissible to continue to your destination? I'm asking because a hot start seems to have a shut-down/teardown requirement.

  14. #34
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    Quote Originally Posted by Evan View Post
    because a hot start seems to have a shut-down/teardown requirement.
    Very interesting, but not correct! This is why you can't always depend on the internet for answers.

  15. #35
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    Quote Originally Posted by BoeingBobby View Post
    Very interesting, but not correct! This is why you can't always depend on the internet for answers.
    Isn't this the internet?

  16. #36
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    Quote Originally Posted by Evan View Post
    Isn't this the internet?
    I really should have known better!

  17. #37
    Senior Member Evan's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    I really should have known better!
    The internet really has no idea as to where the limit lies with turbofans, which is why I'm asking the question here on the expertnet, but on turboshaft engines like the PT-6, the internet tells me a hot start often results in a costly hot section inspection before the engine is considered safe to return to service. So, I'm extrapolating: if the danger of compressor damage is that significant after a hot start on a turboshaft, isn't it also present on a large turbofan?

    Now, I realize with pneumatic starters and FADEC and no prop-feather drag, hot starts on turbofans are probably extremely rare, so I'm not surprised if you've never encountered one on a jet aircraft. I also realize that large turbofans are probably designed to be more robust than your typical turboshaft. But there has to be some EGT limit where even a turbofan will require an internal inspection before returning to service. I imagine that limit might also be reached during a full minute or more of recurring engine surge.

  18. #38
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    ... the danger of compressor damage is that significant after a hot start ...
    Compressor damage? EGT is in the opposite end.

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  19. #39
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Compressor damage? EGT is in the opposite end.
    Sorry, combustor damage, turbine damage, damage.

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    Quote Originally Posted by Evan View Post
    Sorry, combustor damage, turbine damage, damage.
    I'll tell you what's damaged...

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