Ted
The pilot formerly known as Twin Engine Ted
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- Oct 9, 2007
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iFlyNothing
Well, it's the moment you've all been waiting for. Below is my list of recommendations for how to make your engine last while it's running. Yes, we will be addressing LOP and ROP. I am hoping this will be worthy of a sticky to match the "ground" thread, and provide a good reference for pilots of piston aircraft. This will focus on things that you can do in your operation of your engine to make it last longer. This will not cover things that fail that aren't your fault, and assumes that you were smart enough to put oil in the engine at a rate faster than it leaks/burns. Also, to go into full details of engine operation is beyond the scope of this writing, so there will be a few specifics, but mostly general rules. First, some background.
Fundamentally, there are two primary things that kill engines (assuming you put oil in them): temperature and pressure. Temperature kills engines by weakening parts, making them more susceptible to damage (and damaging them directly). In some cases, temperatures. Pressure is force, which when exerted on parts can break them.
Temperature and pressure are also two facts of engine operation. Internal combustion engines take a fuel and burn it, creating heat. Gasses heat up, increasing pressure in the cylinder. That high pressure pushes down on the piston, through the connecting rod, to the crankshaft, causing the crankshaft (and thus the propeller) to spin. This is the simplified version, obviously. The point is that you can't have an engine without temperature and pressure.
Given the above background, the primary question for engine longevity comes down to how to best manage temperature and pressure. We'll go through the phases of flight.
1) Start-up/Warm-up:
When starting your plane, an idle of around 1,000 RPMs is good. If you're routinely hitting 2,000 on your starts, stop it. Figure out how to hit lower. Your car may hit 2,000 RPM on start, but your car probably also uses significantly thinner oil than your plane does and has an engine that is designed in such a way that it is more tolerant. It takes a bit of time for the oil to get pumped through these large engines. Also, at 2,000 RPM, your engine is putting a lot more heat out than your car engine is, since your plane engine is having to turn a prop and not simply spin at idle.
You do want to let your oil temperature get into the green prior to takeoff. This has to do with making sure the engine is warmed up with clearances being what they're designed to be, as well as making sure that you don't accidentally run into an overpressure situation. Pre-heating minimizes the time required for this, so that's another benefit. Turbo engines (especially GTSIO-520s) care more about warm-up time, 172 engines care less. Some factory oil temperature gauges aren't very useful in this regard, so one must guess. My Aztec was this way.
2) Takeoff/climb/cruise
For takeoff, one should apply power smoothly and steadily. I see a lot of people just slam the throttle forward. On certain trainers this is less of an issue because they don't make much power, have no counterweights on the crankshaft, and have a fixed-pitch prop. In spite of this, a slow, steady throttle advance is a good habit to get into. Certain larger engines have dynamic counterweights, which are essentially pieces of metal attached to the crankshaft that will swing around to absorb vibrations. Because these aren't fit rigidly and are designed to move, too rapid of an acceleration can cause damage to them. On higher power engines especially, there is some concern about "shock heating", that is going from idle power to takeoff power very rapidly with the associated increase in heat from such a change. Some would argue this point just as they argue shock cooling. I know that it's more professional to make gradual power changes and the passengers like it better, so it's just good practice whether or not you think it matters. In my opinion, it certainly doesn't do anything good for the engine to make rapid power adjustments. I generally do my accelerate to takeoff power in 1-2 seconds. This doesn't have to a rolling throttle up, so you can hold the brakes if you've got a short runway to work with.
Now we get into the most controversial part: the LOP/ROP discussion and power settings.
What you want to use for climb and cruise settings will vary somewhat depending on your personal preference for speed vs. economy and reliability, as well as the specifics of the engine you're flying. For example, a 172 with a fixed pitch prop is fine to leave at full throttle from takeoff until you need to slow down. For most naturally aspirated engines, there's not much (if any) harm in leaving them at full throttle, provided you are keeping temperatures reasonable. As you climb in altitude, power will drop off naturally. In the Aztec, for instance, I went full throttle for takeoff, sync'd up the props at 2500 RPM, and just did my climb that way. Pull the props back to 2300 RPM for cruise. Throttles didn't come back until it was time for descent.
When you're flying a plane with a big turbocharged engine, you usually don't want to have more than an 85% climb power setting. In many cases, full power is allowable continuously without any restrictions, so theoretically you could maintain full power in the climb. Remember back to the background statement of what kills engines: temperature and pressure. To make rated power, you need the highest pressures in the engine, leading to the most wear and tear on components. In the Navajo, I would pull back from takeoff power to climb power at 500 ft. Then, climb at an airspeed faster than Vy. More airflow over the engines will make them cooler and happier. You can usually have a fairly significant airspeed reduction for not a big sacrifice in climb rate. Your engines are happier and you're going faster. What's not to like?
Does it make sense to climb at a lower power setting? That depends on your goals. Typically, I want to get up to altitude quickly, doubly so if there's a good tailwind up there. I haven't found low power/LOP climbs to be particularly useful most of the time because, although you do save some small amount of fuel, by that point you usually are cutting significantly into your climb performance, increasing your time to climb. Total block fuel burn probably balances out and block times probably go up a bit. Now, this is not to be confused with pulling back to a “standard” climb power. For example, in the short Navajo we used 35” and 2400 RPM, at 25 GPH per side for a climb power. Rated was around 42”, 2575 RPM, and 50 GPH per side. You don't want to climb at full power!
For cruise power, typically 75% power is the maximum setting that you want to run for a naturally aspirated engine, and 65% for a turbocharged engine. Again, there is a good amount of variability here. The rule of thumb that at 65% power you pretty much can't hurt an engine has a decent bit of merit. Yes, there are engines for which this seems a bit odd. For example, a parallel-valve IO-540 can make 260 HP, 250 HP, or 235 HP, and the only difference is the rated RPM. On the whole, it is a good rule of thumb, though, and worthy of note.
When you get into turbocharged engines, typically different power ratings have other changes to the engine in the form of different turbochargers, intercoolers, etc., all of which impact that temperature and pressure relationship that produces wear and tear. Stick to a percent power for that rating, and don't assume that just because an engine you think is identical makes more or less power that it actually is identical. In the Navajo I flew on 135, we'd fly 75% power and burn a ridiculous amount of fuel to do it. Why? The boss was paying the bills, and he wanted to. When I was flying the Navajo for Cloud Nine trips, we flew it at 65% power, and burned about 15 GPH less to go 10 kts slower. This was still ROP!
In the 310, we fly at what comes out to around a 75% power LOP. The primary reason for this power setting is that it gets us about 10 kts more cruise speed than we'd see from a 65% power. For the kinds of trips we do, the extra speed for a marginal increase in fuel burn is worthwhile, even though it does decrease our fuel efficiency.
At no point do I want to see CHTs exceed 380F. Continental recommends 380F as their maximum. Lycoming recommends 400F in cruise and 425F in climb as their maximum CHT. The limits are much higher, but these are goals. Remember, the goal here is more longevity. I have applied this rule to multiple airplanes with many different engine types, and they all have thanked me. For TIT/EGT I aim to keep below 1550F at all times, and for some aircraft and engine combinations, even lower. We flew the Navajo at 1450F TIT. As a rule, cooler is better.
Now, the big question: Does it matter if I fly ROP or LOP? Will either one kill the engine?
If done properly, ROP or LOP will work just fine and not kill your engine. In fact, you will have good longevity with either one (again, done properly). Here's why:
Remember the two main things that cause wear on engines: temperature and pressure. The most important thing for your engine, regardless of whether you run it ROP or LOP, is to keep the temperatures and pressures reasonable. You know what temperatures are in the form of CHT, EGT, and TIT. Pressures will correlate with temperatures for all other conditions equal. Keep your temperatures happy, and as a rule, your engine will be happy, provided you're also not using excessive power settings.
Detonation is a big concern that will kill engines. While it's outside of the scope of this post to go into detonation in detail, the rule of thumb that if you're at 65% power or below you won't have detonation issues is true for virtually all aircraft engines, certainly all the ones I've seen. If you have a big turbocharged engine, don't lean out at takeoff power.
There are benefits to each and reasons why you might want to run ROP vs LOP. Engine wise, an engine that runs LOP will suffer less plug fouling. An engine that runs ROP will often hide some ignition system issues, since a rich mixture is easier to ignite than a lean mixture. So, you will likely see more misfires with engines running LOP. Since power drops off more quickly when LOP, depending on outside air temperature and other conditions, it may be more difficult to reach the CHTs you're looking for LOP without losing a great deal of airspeed, and it can be more workload intensive. I know some pilots who choose to fly ROP for the purpose of simplicity alone. Either one, if done properly, will give you good longevity. The main key comes down to respecting temperatures and making sure you stay out of detonation. Get educated on the specifics that work for your airplane.
I'm not going to go into the specifics of how you should run ROP or LOP. Each airplane is different, and while the techniques are the same, the fine points are typically a bit different for each. Suffice it to say you usually want to be greater than 100F ROP or less than peak for most engines, respecting power setting and temperature recommendations already given. For certain engines and certain power settings, that still won't work. Know your airplane! Detonation is a real concern for certain aircraft engines, and you need to be wary of it.
3) Descent/landing
The primary consideration here is the old question of shock cooling, and whether it is best to pull the power back slowly or if it matters. As with above, dynamic counterweights don't like rapid changes in power, but certainly going from cruise power to idle in around 1-2 seconds won't hurt anything as far as counterweights go.
The old rule of thumb with shock cooling is to reduce power 1" per minute to keep temperatures reducing at a reasonable rate.
Now, there are a few things to keep in mind here. The first one is that, if you are doing what was outlined above to keep your CHTs/EGTs/TITs low and happy, that will also mean that the difference between operating temperature in cruise and at a descent/approach power is minimal. I think that the shock cooling issue probably came up more with turbo engines that were being run hot and hard, and were therefore more susceptible to damage from a rapid temperature change. Meanwhile, an O-360 seems to have no problem getting power cut to idle rapidly all day long in the pattern with students. Either way, the theory about shock cooling is that it will typically manifest itself not in cylinders falling off, but gradually with decreased longevity over time, cracks in the crankcase, etc.
My opinion is that gradual power reductions and allowing your temperatures to drop slowly to prevent what is known as "shock cooling" makes sense, regardless of whether or not it is the plague some make it out to be. Again, it's more professional and passengers like it more. Plus, it makes sense that letting the engine - the whole engine, which goes beyond the CHTs, EGTs, and TITs - cool gradually can only be beneficial for it, or at the very worst not help at all. So I would advocate decreasing power at 1"/minute roughly, and behaving as though shock cooling does exist.
I also typically will push the mixture full rich (gradually) and/or open the cowl flaps when the landing gear comes down. Gear slows the plane down, and then pushing the mixture to full rich helps to keep the engine about where it was for CHTs despite the reduced airflow, and also reduces EGTs/TITs, and lowers internal combustion pressures, cooling the pistons. An engine monitor is the biggest asset you can have in helping to determine what the true temperatures are in your aircraft. Most factory gauges aren't particularly accurate and give you limited information at best. As an example, the engine monitor in the Aztec taught me that when I put the gear down I should open the cowl flaps to full to keep the engines cool. On the 310 there are no cowl flaps, so I push the mixtures rich.
4) Parking/shutdown
Nothing really special here. For a turbo engine, you do want to let the engine cool down for 3 minutes. The purpose here is to get oil going through the turbo to cool it off from its higher temperature to a more reasonable temperature and help prevent coking. Sometimes it still happens, yes. If you're naturally aspirated, the normal taxi to your hangar and to get settled is enough time to let the pistons cool down, and then you're just fine pulling the mixture back and shutting things off.
Now, how much does all of this matter? In the course of flying, you won't be able to do everything ideally for the life of the engine. That's fine. The thing to remember with engines is that they work mostly on cumulative wear, not instantaneous. With few exceptions, there is little you can do to kill an engine within minutes. It's possible, but rare. So, take the best care of your engine you know how, use the above as a guide to help you, and you will likely get good longevity out of your engine.
Fundamentally, there are two primary things that kill engines (assuming you put oil in them): temperature and pressure. Temperature kills engines by weakening parts, making them more susceptible to damage (and damaging them directly). In some cases, temperatures. Pressure is force, which when exerted on parts can break them.
Temperature and pressure are also two facts of engine operation. Internal combustion engines take a fuel and burn it, creating heat. Gasses heat up, increasing pressure in the cylinder. That high pressure pushes down on the piston, through the connecting rod, to the crankshaft, causing the crankshaft (and thus the propeller) to spin. This is the simplified version, obviously. The point is that you can't have an engine without temperature and pressure.
Given the above background, the primary question for engine longevity comes down to how to best manage temperature and pressure. We'll go through the phases of flight.
1) Start-up/Warm-up:
When starting your plane, an idle of around 1,000 RPMs is good. If you're routinely hitting 2,000 on your starts, stop it. Figure out how to hit lower. Your car may hit 2,000 RPM on start, but your car probably also uses significantly thinner oil than your plane does and has an engine that is designed in such a way that it is more tolerant. It takes a bit of time for the oil to get pumped through these large engines. Also, at 2,000 RPM, your engine is putting a lot more heat out than your car engine is, since your plane engine is having to turn a prop and not simply spin at idle.
You do want to let your oil temperature get into the green prior to takeoff. This has to do with making sure the engine is warmed up with clearances being what they're designed to be, as well as making sure that you don't accidentally run into an overpressure situation. Pre-heating minimizes the time required for this, so that's another benefit. Turbo engines (especially GTSIO-520s) care more about warm-up time, 172 engines care less. Some factory oil temperature gauges aren't very useful in this regard, so one must guess. My Aztec was this way.
2) Takeoff/climb/cruise
For takeoff, one should apply power smoothly and steadily. I see a lot of people just slam the throttle forward. On certain trainers this is less of an issue because they don't make much power, have no counterweights on the crankshaft, and have a fixed-pitch prop. In spite of this, a slow, steady throttle advance is a good habit to get into. Certain larger engines have dynamic counterweights, which are essentially pieces of metal attached to the crankshaft that will swing around to absorb vibrations. Because these aren't fit rigidly and are designed to move, too rapid of an acceleration can cause damage to them. On higher power engines especially, there is some concern about "shock heating", that is going from idle power to takeoff power very rapidly with the associated increase in heat from such a change. Some would argue this point just as they argue shock cooling. I know that it's more professional to make gradual power changes and the passengers like it better, so it's just good practice whether or not you think it matters. In my opinion, it certainly doesn't do anything good for the engine to make rapid power adjustments. I generally do my accelerate to takeoff power in 1-2 seconds. This doesn't have to a rolling throttle up, so you can hold the brakes if you've got a short runway to work with.
Now we get into the most controversial part: the LOP/ROP discussion and power settings.
What you want to use for climb and cruise settings will vary somewhat depending on your personal preference for speed vs. economy and reliability, as well as the specifics of the engine you're flying. For example, a 172 with a fixed pitch prop is fine to leave at full throttle from takeoff until you need to slow down. For most naturally aspirated engines, there's not much (if any) harm in leaving them at full throttle, provided you are keeping temperatures reasonable. As you climb in altitude, power will drop off naturally. In the Aztec, for instance, I went full throttle for takeoff, sync'd up the props at 2500 RPM, and just did my climb that way. Pull the props back to 2300 RPM for cruise. Throttles didn't come back until it was time for descent.
When you're flying a plane with a big turbocharged engine, you usually don't want to have more than an 85% climb power setting. In many cases, full power is allowable continuously without any restrictions, so theoretically you could maintain full power in the climb. Remember back to the background statement of what kills engines: temperature and pressure. To make rated power, you need the highest pressures in the engine, leading to the most wear and tear on components. In the Navajo, I would pull back from takeoff power to climb power at 500 ft. Then, climb at an airspeed faster than Vy. More airflow over the engines will make them cooler and happier. You can usually have a fairly significant airspeed reduction for not a big sacrifice in climb rate. Your engines are happier and you're going faster. What's not to like?
Does it make sense to climb at a lower power setting? That depends on your goals. Typically, I want to get up to altitude quickly, doubly so if there's a good tailwind up there. I haven't found low power/LOP climbs to be particularly useful most of the time because, although you do save some small amount of fuel, by that point you usually are cutting significantly into your climb performance, increasing your time to climb. Total block fuel burn probably balances out and block times probably go up a bit. Now, this is not to be confused with pulling back to a “standard” climb power. For example, in the short Navajo we used 35” and 2400 RPM, at 25 GPH per side for a climb power. Rated was around 42”, 2575 RPM, and 50 GPH per side. You don't want to climb at full power!
For cruise power, typically 75% power is the maximum setting that you want to run for a naturally aspirated engine, and 65% for a turbocharged engine. Again, there is a good amount of variability here. The rule of thumb that at 65% power you pretty much can't hurt an engine has a decent bit of merit. Yes, there are engines for which this seems a bit odd. For example, a parallel-valve IO-540 can make 260 HP, 250 HP, or 235 HP, and the only difference is the rated RPM. On the whole, it is a good rule of thumb, though, and worthy of note.
When you get into turbocharged engines, typically different power ratings have other changes to the engine in the form of different turbochargers, intercoolers, etc., all of which impact that temperature and pressure relationship that produces wear and tear. Stick to a percent power for that rating, and don't assume that just because an engine you think is identical makes more or less power that it actually is identical. In the Navajo I flew on 135, we'd fly 75% power and burn a ridiculous amount of fuel to do it. Why? The boss was paying the bills, and he wanted to. When I was flying the Navajo for Cloud Nine trips, we flew it at 65% power, and burned about 15 GPH less to go 10 kts slower. This was still ROP!
In the 310, we fly at what comes out to around a 75% power LOP. The primary reason for this power setting is that it gets us about 10 kts more cruise speed than we'd see from a 65% power. For the kinds of trips we do, the extra speed for a marginal increase in fuel burn is worthwhile, even though it does decrease our fuel efficiency.
At no point do I want to see CHTs exceed 380F. Continental recommends 380F as their maximum. Lycoming recommends 400F in cruise and 425F in climb as their maximum CHT. The limits are much higher, but these are goals. Remember, the goal here is more longevity. I have applied this rule to multiple airplanes with many different engine types, and they all have thanked me. For TIT/EGT I aim to keep below 1550F at all times, and for some aircraft and engine combinations, even lower. We flew the Navajo at 1450F TIT. As a rule, cooler is better.
Now, the big question: Does it matter if I fly ROP or LOP? Will either one kill the engine?
If done properly, ROP or LOP will work just fine and not kill your engine. In fact, you will have good longevity with either one (again, done properly). Here's why:
Remember the two main things that cause wear on engines: temperature and pressure. The most important thing for your engine, regardless of whether you run it ROP or LOP, is to keep the temperatures and pressures reasonable. You know what temperatures are in the form of CHT, EGT, and TIT. Pressures will correlate with temperatures for all other conditions equal. Keep your temperatures happy, and as a rule, your engine will be happy, provided you're also not using excessive power settings.
Detonation is a big concern that will kill engines. While it's outside of the scope of this post to go into detonation in detail, the rule of thumb that if you're at 65% power or below you won't have detonation issues is true for virtually all aircraft engines, certainly all the ones I've seen. If you have a big turbocharged engine, don't lean out at takeoff power.
There are benefits to each and reasons why you might want to run ROP vs LOP. Engine wise, an engine that runs LOP will suffer less plug fouling. An engine that runs ROP will often hide some ignition system issues, since a rich mixture is easier to ignite than a lean mixture. So, you will likely see more misfires with engines running LOP. Since power drops off more quickly when LOP, depending on outside air temperature and other conditions, it may be more difficult to reach the CHTs you're looking for LOP without losing a great deal of airspeed, and it can be more workload intensive. I know some pilots who choose to fly ROP for the purpose of simplicity alone. Either one, if done properly, will give you good longevity. The main key comes down to respecting temperatures and making sure you stay out of detonation. Get educated on the specifics that work for your airplane.
I'm not going to go into the specifics of how you should run ROP or LOP. Each airplane is different, and while the techniques are the same, the fine points are typically a bit different for each. Suffice it to say you usually want to be greater than 100F ROP or less than peak for most engines, respecting power setting and temperature recommendations already given. For certain engines and certain power settings, that still won't work. Know your airplane! Detonation is a real concern for certain aircraft engines, and you need to be wary of it.
3) Descent/landing
The primary consideration here is the old question of shock cooling, and whether it is best to pull the power back slowly or if it matters. As with above, dynamic counterweights don't like rapid changes in power, but certainly going from cruise power to idle in around 1-2 seconds won't hurt anything as far as counterweights go.
The old rule of thumb with shock cooling is to reduce power 1" per minute to keep temperatures reducing at a reasonable rate.
Now, there are a few things to keep in mind here. The first one is that, if you are doing what was outlined above to keep your CHTs/EGTs/TITs low and happy, that will also mean that the difference between operating temperature in cruise and at a descent/approach power is minimal. I think that the shock cooling issue probably came up more with turbo engines that were being run hot and hard, and were therefore more susceptible to damage from a rapid temperature change. Meanwhile, an O-360 seems to have no problem getting power cut to idle rapidly all day long in the pattern with students. Either way, the theory about shock cooling is that it will typically manifest itself not in cylinders falling off, but gradually with decreased longevity over time, cracks in the crankcase, etc.
My opinion is that gradual power reductions and allowing your temperatures to drop slowly to prevent what is known as "shock cooling" makes sense, regardless of whether or not it is the plague some make it out to be. Again, it's more professional and passengers like it more. Plus, it makes sense that letting the engine - the whole engine, which goes beyond the CHTs, EGTs, and TITs - cool gradually can only be beneficial for it, or at the very worst not help at all. So I would advocate decreasing power at 1"/minute roughly, and behaving as though shock cooling does exist.
I also typically will push the mixture full rich (gradually) and/or open the cowl flaps when the landing gear comes down. Gear slows the plane down, and then pushing the mixture to full rich helps to keep the engine about where it was for CHTs despite the reduced airflow, and also reduces EGTs/TITs, and lowers internal combustion pressures, cooling the pistons. An engine monitor is the biggest asset you can have in helping to determine what the true temperatures are in your aircraft. Most factory gauges aren't particularly accurate and give you limited information at best. As an example, the engine monitor in the Aztec taught me that when I put the gear down I should open the cowl flaps to full to keep the engines cool. On the 310 there are no cowl flaps, so I push the mixtures rich.
4) Parking/shutdown
Nothing really special here. For a turbo engine, you do want to let the engine cool down for 3 minutes. The purpose here is to get oil going through the turbo to cool it off from its higher temperature to a more reasonable temperature and help prevent coking. Sometimes it still happens, yes. If you're naturally aspirated, the normal taxi to your hangar and to get settled is enough time to let the pistons cool down, and then you're just fine pulling the mixture back and shutting things off.
Now, how much does all of this matter? In the course of flying, you won't be able to do everything ideally for the life of the engine. That's fine. The thing to remember with engines is that they work mostly on cumulative wear, not instantaneous. With few exceptions, there is little you can do to kill an engine within minutes. It's possible, but rare. So, take the best care of your engine you know how, use the above as a guide to help you, and you will likely get good longevity out of your engine.
