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

  1. #41
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Evan View Post
    ...PT-6...hot start...
    What is a hot start and have you ever ridden a Jetstream-31?
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  2. #42
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    Quote Originally Posted by BoeingBobby View Post
    I'll tell you what's damaged...
    This thread. It started as a simple question but I guess nobody has an answer for me.

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    Quote Originally Posted by Evan View Post
    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?
    The reason you haven't found a definitive answer on the internet is probably because there are too many variables. To wit, for us and our V2500-series engines, the red "tickmark" on the EGT display is OAT-dependent and is not automatically indicative of a hot start. It indicates only that MOC should be contacted. They have a whole set of parameters and limits that they will check against to determine whether a hot start has occurred (or even just an overtemp has occurred).

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    Quote Originally Posted by 3WE View Post
    What is a hot start and have you ever ridden a Jetstream-31?
    I parked next to one a couple times.

  5. #45
    Senior Member Evan's Avatar
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    Quote Originally Posted by ATLcrew View Post
    The reason you haven't found a definitive answer on the internet is probably because there are too many variables. To wit, for us and our V2500-series engines, the red "tickmark" on the EGT display is OAT-dependent and is not automatically indicative of a hot start. It indicates only that MOC should be contacted. They have a whole set of parameters and limits that they will check against to determine whether a hot start has occurred (or even just an overtemp has occurred).
    But you have a digital temp readout as well, so you have an absolute value, right? (BTW, I'm NOT asking about hot start, that was just the precedent I based this on. I'm asking about an in-flight continuous surge) I was expecting some max exceedance value, after which the engine should be suspected damaged, left at idle and you should land asap. But I take it from your response that this value doesn't exist in any manual and it's simply left to pilot discretion. That surprises me. I wouldn't want to be sitting in 18F, watching a tail of flame for a minute and a half, followed by a resumed climb and a hot section blade to the skull five minutes later. I also wonder if FADEC will limit engine N1 after a serious overtemp event.

  6. #46
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    Quote Originally Posted by Evan View Post
    But you have a digital temp readout as well, so you have an absolute value, right? (BTW, I'm NOT asking about hot start, that was just the precedent I based this on. I'm asking about an in-flight continuous surge) I was expecting some max exceedance value, after which the engine should be suspected damaged, left at idle and you should land asap. But I take it from your response that this value doesn't exist in any manual and it's simply left to pilot discretion. That surprises me. I wouldn't want to be sitting in 18F, watching a tail of flame for a minute and a half, followed by a resumed climb and a hot section blade to the skull five minutes later. I also wonder if FADEC will limit engine N1 after a serious overtemp event.
    There are EGT limits for normal operations, you can find those in the FCOM, but I'm not aware of any set number for an overtemp during or secondary to a surge.

    If it's "flaming out the back for a minute and a half", you'll have other abnormal indications besides EGT. Like you said, there are other "vital signs" involved.

  7. #47
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    Quote Originally Posted by ATLcrew View Post
    I parked next to [a J-31] a couple times.
    Whenever I could watch the start up, the temperature marched a good bit past a red line (20 to 30%) and then when the engine really kicked in, it rapidly dropped to maybe 80% of the red line.

    The pilots never flinched/responded did anything, so I ass-umed that getting 'hot' for a short time was a normal part of startup- and it would make sense that the red line in flight would be "something is not working right" and not_necessarily "something is melting".

    Now, to complicate things- an ag pilot friend (who should know a little bit about starting PT-6 engines), stated that his goofball pilots hot started both engines the one time he puddle jumped to Flyover...then again, the ag pilots I know are just a little bit cowboy improvisational...

    Then again-again, I should probably check the Internet for what engines are on a J-31 and an AT-802...and their type-specific starting procedure...I believe that most AT-802s are powered by second hand turboprop airliner engines, too.
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    Who gives a phugoid about fire inside of tail pipes and combustion chambers and EGTs and TIT's...they are designed to handle hot gasses.

    I'm more worried about composite upper curved surface paint bubble temperature CUCSPBT...

    https://www.youtube.com/watch?v=Dont3uTVqvA
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    Quote Originally Posted by 3WE View Post
    Who gives a phugoid about fire inside of tail pipes and combustion chambers and EGTs and TIT's...they are designed to handle hot gasses.
    Combustion chamber? Sure. It is designed to withstand the temperature of the stoichiometric mixture of air and fuel, which gives you the maximum achievable temperature in a combustion. Add more air and it will be less hot because you will use the same energy in the same amount of fuel to heat more air (that is less energy per unit of air) (actually, add too much air and you won't be able to sustain the combustion because the en temperature if the combustion did happen will be less than what's required to start the combustion in the first place). Add more fuel and there will be unburnt fuel that will however absorb heat and become hotter than what it was when it was coming to the combustion chamber, therefore leaving less energy available to heat the air. So yes, the combustion chamber can withstand pretty much any temperature you can achieve by burning air and jet fuel.

    Tailpipe? Well, they are not designed to withstand any temperature you can reach by burning air and jet fuel (unless they are designed for afterburner) because of 2 main reasons: 1, there is more air in the game than that the participates in the combustion and 2, the hot air leaving the chamber (which is not nearly as hot as the max temp inside of the chamber) looses a good amount of its energy and hence of its temperature to work in the turbines which is the used to run the compressor and the fan (if it's a turbofan). But then getting too hot is not so critical.

    TIT? That is critical. Those 1st stage turbine blades are turning at over 10,000 RPM, and heat reduces both:
    1- The Young's module (or elastic module) of the material (that is, how much it resists elongation when under stress, or its inverse, how much it will elongate for each unit of stress). To maximize the engine efficiency we want to minimize amount of air that leaks past the turbine through the gap between the tip of the blades and the casing, and if we want to keep that gap small in all ranges we don't want the blades to elongate too much at the highest RPM (highest stress) which tends to coincide with the highest temps that reduce the elasticity modulus and hence increase the elongation. The reduction of the elasticity modulus is what brought down the WTC towers when the hot steel still had enough capacity to resist the stress of the weight of the building, but not enough stability not to buckle. AND
    2- The material's resistance, that is how much stress it will withstand before being unable to deform permanently and even fail. This is much more straightforward. It is the reason why steel is heated to forge it more easily. And I don;t need to tell you that it is not nice to have red hot turbine blades that were spinning at more than 10,000 RPM suddenly become loose and fly of the tangent, especially since the tangent can go through load-bearing structure, fuel tanks, hydraulic piles, signal-carrying or power-carrying electric cables, other engines, pressurized hulls, and people.

    Stage 1 turbine blades (and I am not talking about the other ones because stage 1 ones receive the hottest gasses of all the turbines) are NOT designed to withstand any temperature that burning jet fuel in air can yield. Not even close. Because of that, air is added at different points along the combustion chamber. First a chunk of air, then the fuel in a proportion that is rich, then more air to burn the unburnt fuel (and then some) to ensure that all the fuel burns and we are getting the maximum energy from it, and then more air to dilute the hot combustion gasses and bring the temperature down to a level that the turbine can manage. The reason why not all the air is added at once is that we add so much air to lower the temperature that the resulting mixture would be so poor that combustion would just not happen.

    While the temperature in the combustion chamber can exceed 2000C, the TIT is typically below 1500C, which is about the temperature that steel melts.

    So yeah, you better watch out those TITs and the resulting EGTs. While combustion chambers, turbines and tailpipes are designed to handle hot gasses, they are not designed to handle arbitrarily hot gasses.

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    Quote Originally Posted by Gabriel View Post
    Blah blah blah, lecture that tail pipes are not combustion chambers. blah blah blah.
    Dude, can you not see nor enjoy the subtle ironing that while you and Evan call for greatly increased temperature monitoring at numerous places within the engine, with new procedures and rewrites, in contrast, here we see what is clearly a non-normal start up (with a rather cold, inefficient combustion) but we blister the paint on the wings?

    Should there not be temperature gauges there and procedures to abort the flight? (Did I really have to make that blue?)
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    Quote Originally Posted by 3WE View Post
    Dude, can you not see nor enjoy the subtle ironing
    Oh, I did. But it's not just you and me and Even. This is a public forum and there might be children out there reading these post.

    Tell me that nobody would find my previous post informative, educational or interesting, and I will delete it.

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    Quote Originally Posted by Gabriel View Post
    Oh, I did. But it's not just you and me and Even. This is a public forum and there might be children out there reading these post.

    Tell me that nobody would find my previous post informative, educational or interesting, and I will delete it.
    It's informative, educational and interesting. But I'm not calling for "greatly increased temperature monitoring at numerous places within the engine". Whether it's TIT on your P3 or ITT on your King Air or Bell 212 or TOT on your Jetranger or EGT on your turbofan aircraft, it's all just different ways to indicate the same threat. I genuinely hope that the Air Zimbabwe incident was just a rare case of bad judgment and that 99.9999% of pilots would consider the engine fried at that point until a hot section inspection deems it safe again.

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    Quote Originally Posted by Evan View Post
    But I'm not calling for "greatly increased temperature monitoring at numerous places within the engine".
    Neither I am.

    Whether it's TIT on your P3 or ITT on your King Air or Bell 212 or TOT on your Jetranger or EGT on your turbofan aircraft, it's all just different ways to indicate the same threat.
    Exactly. I never said "add a TIT indicator" or even "scrap EGT and use TIT".

    I just introduced TIT as an alternative to EGT when ATL said that they don't have an EGT indication. But then he went on to say that they don't have TIT indication either. I am guessing that they do have an indicator for some of the hot-section temperatures that you mentioned, he is just not saying which one.

    I genuinely hope that the Air Zimbabwe incident was just a rare case of bad judgment and that 99.9999% of pilots would consider the engine fried at that point until a hot section inspection deems it safe again.
    Lacking more information, for the time being I give them the benefit of the doubt*. A surge and flames coming out of the tail pipe don't necessarily mean exceeding the red line of whatever temperature of the hot section they are using.

    * At least regarding the exceedance of red lines. I would not have complained if they landed as soon as practical after a 90 seconds surge even if no red line was exceeded.

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  14. #54
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    Quote Originally Posted by Gabriel View Post
    A surge and flames coming out of the tail pipe don't necessarily mean exceeding the red line of whatever temperature of the hot section they are using.

    * At least regarding the exceedance of red lines. I would not have complained if they landed as soon as practical after a 90 seconds surge even if no red line was exceeded.
    Is that even possible? Again, a short burst of flame from a surge or two, fine, but 60-90 seconds of continuous flame (i.e lack of sufficient axial airflow, lack of cooling, lack of flame shaping)? How is that not going to run your turbine into the red, melty side if things?

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    Quote Originally Posted by Evan View Post
    Is that even possible? Again, a short burst of flame from a surge or two, fine, but 60-90 seconds of continuous flame (i.e lack of sufficient axial airflow, lack of cooling, lack of flame shaping)? How is that not going to run your turbine into the red, melty side if things?
    The correct answer is I don't know if this can happen in any specific application, but it may be possible (at least in theory).

    There are a few of factors here:

    - First, the difference between temperature, heat (energy) and heat transfer (power). What creates heat damage is neither the temperature or the heat but the heat transfer. If we live the specific heat and thermal conductivity at a side (because it is mostly constant in this case), the main factor in heat transfer is heat content which in turn is proportional to the temperature (in an absolute scale like K) but also to the mass. That's how sparks from grinding steel, that can be above 1000C, don't burn your skin. The particles of steel are super hot but have very little mass, and hence they carry very little heat. In the hot gas mixture, the vast majority of the mass comes from the air, not from the fuel. And what is severely restricted during a surge? Yes, the flow of air. So yo can have a hotter gas inside the combustion chamber but less air, and the total amount of heat can be more or less depending on how constrained the flow of air is.

    - On top of that, the combustion chamber and other parts in the turbine sections (many times the turbine blades themselves) have an active liquid cooling system. For a fixed cooling system in a fixed state, the amount of heat that it can remove increases with the temperature of what is being cooled (hotter things are easier to cool than cool things) and decreases with the mass (or increases with the diminishing of the mass) of what is being cooled, because it reduces its temperature more degrees for each unit of heat that is removed. So a smaller, hotter flow of air is easier to cool than a larger, not-as-hot flow. By the time the gas reaches the tailpipe, its temperature (EGT) may have been reduced enough to be below the red line.

    - Finally, picture this scenario: You have a mass air that is stoichiometrically combined with fuel, and then reduce the amount of air but not the amount of fuel. What will happen with the temperature? It will reduce. Why? Because the stoichiometric mixture of air and fuel will burn to the same temperature, but now you have an excess fuel that will heat up (and hence absorb heat) but not burn (and hence not produce heat), so the final temperature of the gas is lower. But there is a but. As I explained before, the temperature of the soichiometric combustion is too high for the turbines, so extra air is added in the combustion chamber after the combustion to lower the temperature of the gasses before they reach the turbine. That means that there is extra air available to burn the excess fuel that was not burned where it was supposed to*. So as you star reducing the flow of air, the temperature will increase since all the fuel keeps burning but air that was before being used for cooling is now used to complete the combustion (the amount of HEAT however will be constant, since there is only so much energy that you can produce by burning a fixed flow of fuel mass). Now if you keep restricting the flow of air more and more, there will be a point where you are using all the air to burn of the fuel and any further reduction will result in unburned fuel, and hence lower temperatures AND lower heat. How much restriction is needed to reach this point and how much restriction typically happens in a surge? I don't know. If that happens, the gas in the tailpipe can be below EGT but still hot enough to burn the excess fuel as soon as the gas meets with fresh air (and hence oxygen) outside of the engine, which can produce very impressing flames of which however don;t affect the engine at all.

    - Finally, it is not completely clear to me if the 90-seconds surge was once continuous surge with a constant flame, or a sequence of bangs, like in this video. Obviously, temperature-wise, it is a very different situation. https://www.youtube.com/watch?v=UXnl0lBT6k8


    * That is how turbine engines, including turbo jet/fan/prop/shaft, accelerate. You add more fuel and, because even before increasing the RPM there is more air available to burn the extra fuel (the air used to cool down the gases before they reach the turbine), more energy is added instantly to the engine. In a turbo-jet or low-bypass turbofan the kick in trust is immediate, even before the RPMs go up, since the gas heats up more, increases its temperature, expands, and exits the engine with ore speed. In a turbo shaft, turboprop or high-bypass turbofan, where most of the power is used to spin-up the trubines and hence the fan/prop/shaft rather than to accelerate the jet exhaust flow, you need to wait for the "spool up" before having a noticeable increase in thrust. But ALL cases have 1 thing in common: While the more energetic gas passing through the turbines will increase the RPMs, the hotter (and more expanded) gas happens immediately, before the RPMs, and hence the increased air flow, have time to build up, which will make the engine transiently work with less cooling air (that is instead used to burn the extra fuel) and the hot section will be working with a gas that is hotter than normal. When the RPMs finally spin up the temperature will go down as the cooling air is used again for cooling. The risk of moving the throttle from idle to max is obvious. But also the opposite. If the throttle is moved from TOGA to idle, the fuel flow will reduce a lot instantly, while the RPM and air flow is still high, and you can have a mixture so poor that you may have a flame-out (if you don't have the ignition on). The caveat is that "throttle" hear means not the throttle levers but the actual physical throttle, i.e. the fuel metering valve. In times where the throttle was directly linked to the fuel metering valve, you had to be very cautious on how you used them. There were certain provisions in the engines to mitigate the situation (like doors that would open to dump airflow out of the engine before they got to the combustion chamber when the throttles levers were retarded quickly), but soon enough the engine manufacturers developed mechanical engine controllers, then electronic engine controllers, and then the FADEC.

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  16. #56
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    Quote Originally Posted by Gabriel View Post
    The correct answer is I don't know if this can happen in any specific application, but it may be possible (at least in theory).

    There are a few of factors here:

    - First, the difference between temperature, heat (energy) and heat transfer (power). What creates heat damage is neither the temperature or the heat but the heat transfer. If we live the specific heat and thermal conductivity at a side (because it is mostly constant in this case), the main factor in heat transfer is heat content which in turn is proportional to the temperature (in an absolute scale like K) but also to the mass. That's how sparks from grinding steel, that can be above 1000C, don't burn your skin. The particles of steel are super hot but have very little mass, and hence they carry very little heat. In the hot gas mixture, the vast majority of the mass comes from the air, not from the fuel. And what is severely restricted during a surge? Yes, the flow of air. So yo can have a hotter gas inside the combustion chamber but less air, and the total amount of heat can be more or less depending on how constrained the flow of air is.

    - On top of that, the combustion chamber and other parts in the turbine sections (many times the turbine blades themselves) have an active liquid cooling system. For a fixed cooling system in a fixed state, the amount of heat that it can remove increases with the temperature of what is being cooled (hotter things are easier to cool than cool things) and decreases with the mass (or increases with the diminishing of the mass) of what is being cooled, because it reduces its temperature more degrees for each unit of heat that is removed. So a smaller, hotter flow of air is easier to cool than a larger, not-as-hot flow. By the time the gas reaches the tailpipe, its temperature (EGT) may have been reduced enough to be below the red line.

    - Finally, picture this scenario: You have a mass air that is stoichiometrically combined with fuel, and then reduce the amount of air but not the amount of fuel. What will happen with the temperature? It will reduce. Why? Because the stoichiometric mixture of air and fuel will burn to the same temperature, but now you have an excess fuel that will heat up (and hence absorb heat) but not burn (and hence not produce heat), so the final temperature of the gas is lower. But there is a but. As I explained before, the temperature of the soichiometric combustion is too high for the turbines, so extra air is added in the combustion chamber after the combustion to lower the temperature of the gasses before they reach the turbine. That means that there is extra air available to burn the excess fuel that was not burned where it was supposed to*. So as you star reducing the flow of air, the temperature will increase since all the fuel keeps burning but air that was before being used for cooling is now used to complete the combustion (the amount of HEAT however will be constant, since there is only so much energy that you can produce by burning a fixed flow of fuel mass). Now if you keep restricting the flow of air more and more, there will be a point where you are using all the air to burn of the fuel and any further reduction will result in unburned fuel, and hence lower temperatures AND lower heat. How much restriction is needed to reach this point and how much restriction typically happens in a surge? I don't know. If that happens, the gas in the tailpipe can be below EGT but still hot enough to burn the excess fuel as soon as the gas meets with fresh air (and hence oxygen) outside of the engine, which can produce very impressing flames of which however don;t affect the engine at all.

    - Finally, it is not completely clear to me if the 90-seconds surge was once continuous surge with a constant flame, or a sequence of bangs, like in this video. Obviously, temperature-wise, it is a very different situation. https://www.youtube.com/watch?v=UXnl0lBT6k8


    * That is how turbine engines, including turbo jet/fan/prop/shaft, accelerate. You add more fuel and, because even before increasing the RPM there is more air available to burn the extra fuel (the air used to cool down the gases before they reach the turbine), more energy is added instantly to the engine. In a turbo-jet or low-bypass turbofan the kick in trust is immediate, even before the RPMs go up, since the gas heats up more, increases its temperature, expands, and exits the engine with ore speed. In a turbo shaft, turboprop or high-bypass turbofan, where most of the power is used to spin-up the trubines and hence the fan/prop/shaft rather than to accelerate the jet exhaust flow, you need to wait for the "spool up" before having a noticeable increase in thrust. But ALL cases have 1 thing in common: While the more energetic gas passing through the turbines will increase the RPMs, the hotter (and more expanded) gas happens immediately, before the RPMs, and hence the increased air flow, have time to build up, which will make the engine transiently work with less cooling air (that is instead used to burn the extra fuel) and the hot section will be working with a gas that is hotter than normal. When the RPMs finally spin up the temperature will go down as the cooling air is used again for cooling. The risk of moving the throttle from idle to max is obvious. But also the opposite. If the throttle is moved from TOGA to idle, the fuel flow will reduce a lot instantly, while the RPM and air flow is still high, and you can have a mixture so poor that you may have a flame-out (if you don't have the ignition on). The caveat is that "throttle" hear means not the throttle levers but the actual physical throttle, i.e. the fuel metering valve. In times where the throttle was directly linked to the fuel metering valve, you had to be very cautious on how you used them. There were certain provisions in the engines to mitigate the situation (like doors that would open to dump airflow out of the engine before they got to the combustion chamber when the throttles levers were retarded quickly), but soon enough the engine manufacturers developed mechanical engine controllers, then electronic engine controllers, and then the FADEC.
    Informative, educational and interesting, thanks. But one thing we do know, empirically, is that, when the axial flow is disrupted (by insufficient RPM as in a hot start or by aerodynamic stall and flow reversal as in a surge, the EGT rises very quickly. In the case of an isolated surge, which recovers within seconds, the amount is negligible, but in a continuous surge (which is actually a repetitive series of surges—perhaps we should call it a contiguous surge—the EGT will redline in most—if not all— cases until the fuel flow is greatly reduced or the situation self-recovers. I've read that a hot start can severely damage a turboshaft within seconds.

    As I understand it. Correct me if I'm wrong there.

    The key issue for me is that a risk has been introduced by the event, and, until the engine is stripped down and inspected, one cannot know for certain that damage hasn't occurred. When we are talking about fan blades turning at supersonic speeds and high-pressure turbines running over 10,000rpm that can hurl a chunk of metal through the engine case and then travel distances over a mile, I think it best to err on the side of caution. I mean I think it best to REQUIRE erring on the side of caution. Uncontained engine failure remains one of the few existential threats in air travel that technology can't remove, but an abundance of caution can make it extremely unlikely.

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    Evan, look up bore scope.

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    Quote Originally Posted by Evan View Post
    But one thing we do know, empirically, is that, when the axial flow is disrupted (by insufficient RPM as in a hot start or by aerodynamic stall and flow reversal as in a surge, the EGT rises very quickly. In the case of an isolated surge, which recovers within seconds, the amount is negligible, but in a continuous surge (which is actually a repetitive series of surges—perhaps we should call it a contiguous surge—the EGT will redline in most—if not all— cases until the fuel flow is greatly reduced or the situation self-recovers. I've read that a hot start can severely damage a turboshaft within seconds.

    As I understand it. Correct me if I'm wrong there.
    As I said, I don't know.

    The key issue for me is that a risk has been introduced by the event, and, until the engine is stripped down and inspected, one cannot know for certain that damage hasn't occurred. When we are talking about fan blades turning at supersonic speeds and high-pressure turbines running over 10,000rpm that can hurl a chunk of metal through the engine case and then travel distances over a mile, I think it best to err on the side of caution. I mean I think it best to REQUIRE erring on the side of caution. Uncontained engine failure remains one of the few existential threats in air travel that technology can't remove, but an abundance of caution can make it extremely unlikely.
    And to that, I agree and moreover, as I said earlier, even if no red line had been exceeded, a 90-seconds surge is a serious and very infrequent event. I tend to think that I would like to land as soon as practical not only because of the damage that could have occurred during the event, but even if God came and revealed to me that no damage occurred DURING the event, I would still be concerned about what was so wrong with the engine BEFORE the event as to cause the event in the first place.

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  19. #59
    Senior Member Evan's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    Evan, look up bore scope.
    Do they make one that you can use from the cockpit to inspect the hot section of an engine during flight before continuing to destination? I couldn't find that on Google.

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    Quote Originally Posted by Evan View Post
    Do they make one that you can use from the cockpit to inspect the hot section of an engine during flight before continuing to destination? I couldn't find that on Google.
    No but they do make one that they can shove up your rear end and see if you have a brain.

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