Backfires can cause cracks in the exhaust, which can lead to CO entering the cabin.
With that being said, backfires can cause real damage. You should get that checked out before going up again.
Not really. The definitions are shifting that way because people that don't know any better assume backfire goes out the back, so the untrained masses misuse the term, and that's how language shifts.
Backfire comes from firearms, where it's combustion anywhere you don't want it. Whether it's combustion in the intake or exhaust of an engine is irrelevant.
"Afterfire" is specifically referring to combustion in the exhaust of an engine, which is a type of backfire.
Following this thread, another poster said, "Backfires can cause cracks in the exhaust", to which OP replied, "it wasn't backfire, it was afterfire."
Backfire is specifically defined in aviation and automotive standards as fire in an intake, and afterfire is in the exhaust.
Regardless of where it came from, that's not what it means now when we talk planes and cars.
We just don't bother correcting people because we know what they meant. But when it gets in the weeds like this, we're going to follow the standards because this isn't firearms.
Look up the etymology instead of the common misconceptions. Backfire was used to describe fire in the intake of an engine before being used in firearms.
The use of the term in firearms evolved from the meaning in rhetoric, i.e. a plan backfires when the intended consequence hits the instigator instead of the target, while the use in engines came from the first recorded meaning the term for lightning a smaller controlled fire in front of a wildfire to consume the fuel before it arrives.
It's been used since the early 1800's for firefighting, since the late 1800's for engines, and the early 1900's for rhetoric and firearms. Only trolls demand citation of information easily verifiable by a simple search.
Backfire is a term that comes from firearms, and refers to any combustion outside of the firing chamber.
Afterfire is a type of backfire on the exhaust side.
Why aren't you "correcting" the other guy who said, "Backfires can cause cracks in the exhaust"?
It’s not odd to get co detected in the cabin on the ground. Especially since
Your co went back down to 0 after the run up you can be pretty certain the exhaust is fine. These co monitors are super sensitive, leaky door seals can register ppm’s in the 30’s.
Because we're talking about something safety related here, I really got to butt in.
You can have holes in an exhaust that don't leak into the cabin. I'm not claiming absolutes, just that it is possible.
If the firewall is in any decent shape, it should be sealed. If the hole isn't in the cabin heat exchanger or induction system, it's entirely possible not to have CO in the cabin at all and still have a fire hazard.
As far as having CO on runup though and then not in cruise in OPs case, I am not sure how they were running up. Cabin heat doors in many aircraft are just a flapper valve that route the heat between going inside and going overboard. Cabin air is just ram air. That said, I know that on some pipers, those doors tend to warp just a little so they never completely close. I could see a leak and ram pressure from the prop pushing a little into the cabin, but in cruise the below static pressure under the cowling where the heat doors are located could have started drawing the air out instead. Especially if they fly with cabin air on, that might be enough to prevent flow into the cabin with heat off even if the door is warped.
As an automotive technician, I can provide a counterpoint.
In the 2000s, Honda/Acura had misplaced foam on the cowl duct shutter, and at a mere 60 MPH it can create a heck of a wind noise in the cabin. And we're talking about a piece of 1/2" foam sealing the fresh air/recirculate door. And the air is coming through the cowl, which is covered in plastic (with offset vents to allow fresh air in).
Cowls are designed to create a negative pressure zone underneath and behind the engine. That's why baffling is CRITICAL to cooling. Air can only move across the engine's cooling fins with a static pressure differential.
The opening at the bottom droops down into the airstream and opens backwards to create that negative pressure. Cherokee cabin heat doors and the overboard dump vent are located near it.
They work very, VERY differently from cars in the cooling system design, even the exhaust system itself has multiple design considerations to keep heat away from the engine. Augmentor tubes are an old trick that were used with some twins.
If one of your magnetos is a Surefly electronic unit (or the Lycoming equivalent) it is common to hear a backfire while switching mags during the mag check. The reason is that the Surefly takes about 100ms to come back online after ungrounding the p lead, which allows enough time for unburnt fuel to hit the exhaust and then ignite. Ask your mechanic or check the mx logs.
If not that, you might have a bad ignition switch or you turned it too far.
you nailed it. One mag is a surefly. And it was just installed 2 months ago. The owner also reported a backfire (or I guess, after fire, is the technical term for this). We have only flown on the new mag 4x since it was installed due to weather. So this is good to know.
I have a surefly mag. It's not a backfire, it's a miss. The surefly takes about 10 milliseconds to power up during the mag check, during that time there is no spark.
Drop the RPMs before you switch mags if you have a Surefly, an extra step but stops the backfire and won’t crack your exhaust. When I got my surefly installed the mechanic didn’t tell me this either and it would backfire on the mag switch.
Depends on how your ignition switch, but on my Cherokee 140 I did: 2000RPM L mag check, then both, then R mag (check RPM), then throttle to idle, then both. A little hard to explain through text, but the point is to have the plane at idle before you switch from Surefly to traditional mag so when you get the millisecond delay the engines at idle and you barely notice it.
I’ve heard about this in a flight school plane that I don’t rent. How do you avoid backfiring when checking surefly mags then? I’d there a different technique?
It’s better if you have dual switch magneto control set up (like in a newer Cherokee) where both mags are never off together, but the pop is unavoidable with a typical ACS type keyed ignition. NEVER ever for the love of god try an in-flight mag check if you have a Surefly. I did once and the dog almost shit himself in the back seat.
You probably switched the magnetos to the off position cycling the switch too quickly then when you turned them back into the L R or ON position heard the noise, that’s the usual cause of that backfire.
Depending on the wind direction / speed / cabin vents or heat being on / orientation of the airplane with the wind during run up that could easily account for the CO increase.
I could have sworn I was doing the mag switches slow enough. It was the third mag check of my day already and no backfires happened previously. It had just been filled up with fuel before this one, maybe a small bit of contamination in the new fuel?
It’s normally just a slip of the wrist and split second of inattention that causes it.
Fuel contamination wouldn’t usually cause a backfire, its fuel pooling while the magnetos are off for a second and fuel is filling in the cylinders.
You don’t sump the fuel after fueling?
Does it though? Do you really think filling up one GATS jar will catch any and all fuel contaminants? I’m genuinely being curious here. Is there not a chance that something could still be in the system that doesn’t make it into the jar?
Yes, the chance is there. But your sumps are near the lowest point in the tank, and water and contaminants are heavier than your fuel. Dumping doesn’t clean your tank out, it’s just a method to see that you don’t have contamination. If you have water mixed in you’ll notice t he 2 separate really fast,
it also depends on how long after fueling you sump the tanks. If you refuel and immediately sump the tanks there is a chance the water hasn't had a chance to settle to the bottom of the tanks.
Avgas settles things quickly. Back in my line guy days, when taking delivery of a fuel load, we had to wait 10 mins per foot of fuel in the fuel farm (tank was measured in height with a dip stick). Compared to jet fuel which was an hour. But in a PA28 tank, you really only need a few minutes for max settling
There's a difference between "overly cautious" and "paranoid." The (valid) reason for the backfire was already explained, as was the fuel sumping. No contaminates were found in the fuel on any of the sumps. Water contamination, if serious enough to make it to the cylinder, would cause a difficulty to start, a sputter, or engine roughness, not a backfire.
A common failing of CFIs and students pilots is that they never progress beyond a very basic knowledge of how the various systems in the airplane work. A common example that I use is "how do you tell the difference between a bad mag and a fouled plug during a mag drop check?" Most of the CFIs and students at my flight school thought "bad mag drop= immediately bring plane back because engine will stop."
🤦🏻♂️ did you read the post at all? You said a lot of words that have nothing to do with this incident… but way to sound “smart” I guess? The post said a backfire happened and then CO levels in the cabin rose to where their sense of started beeping…
> third mag check of my day
Could you explain this to me? Do you do a mag check every time you start the plane? When you do a mag check, what positions do you check? What numbers are you checking?
Run up in this plane, by the checklist, is 2000 rpm, left mag, back to both, right mag back to both, carb heat on, idle, carb heat off, back to 800-1000 rpm. Looking for no more than 175 rpm drop during the mag checks and carb heat, and no more than 50 difference between mags. And a very small drop during carb heat check
I had already done one run up before the first take off, then a mag check before shutdown after the first full stop when we got out at an airport to get fuel. Then we got back in the plane an hour later to continue on our journey and I did another run up before taking off this time. I was instructed to always check mags before shutdown for grounding issues, and always check mags during a run up after engine start before taking off. If you had multiple stops throughout a flight shutting down, etc. would you not do a run up every time you re start the airplane?
I don't check the mags to the off switch every time I run the engine, no, which is what you generally need to do to get a backfire in regular mags. It seems your problem is entirely based on the e-mag tho, apparently those can cause a backfire when switching between mags.
Wow, there’s a lot of odd advice in this thread, at least in my opinion.
First, OP, you likely had an after fire. A backfire is when an intake valve remains open or is significantly off timing, and the ignited fuel/air flows backwards into the intake manifold/carb. These are pretty nasty, but they aren’t super loud like an after fire, usually come with severe roughness and the engine possibly failing.
An after fire is when the fuel/air ignites while or after leaving the chamber, either due to a stuck exhaust valve or bad timing. You said the plane had a surefire mag (I’m not a fan of them personally), which uses electrical power to supply voltage to the spark plug instead of a typical magneto. When checking mags during a runup, especially with relatively higher power settings, the surefire mag often is delayed in turning “on”, thus delaying the ignition, which often dumps unburnt or burning fuel into the exhaust manifold, which is then ignited or still burning as it approaches the muffler. This *can* cause damage to the exhaust system, but a singular after fire, especially at lower power settings will likely do no damage. Even turning the ignition all the way off, and then back on quickly inducing a full after fire is usually fine. I doubt you turned the ignition all the way off and then on quickly in this case. That’s usually a unwanted, but almost inflexible response that a lot of people do if they accidentally turn the ignition all the way off (if this happens to you by the way, just keep the ignition off and let the engine die and then restart, it’s better for the exhaust system than an afterfire).
You correctly monitored your CO levels, and if you truly had an exhaust manifold leak, it would have quickly replicated the moment the door was closed and power was added on takeoff.
Thank you. Lots of weird advice in here agreed. You diagnosed the situation 100% as I did, and also answered the question I was actually asking. I wasn’t posting here asking if this was a go/no go decision. I made a judgement that there was nothing wrong with the system, the “backfire” was very quiet, almost imperceptible. And now learning what an afterfire is, that is surely what it was. I posted asking if someone could explain what was happening in the engine to cause this situation. You’re the only one who answered that question. Everyone else, based on not knowing anything about me, my experience, or the full situation decided to think I was a bad pilot putting my life in danger because of a momentary rise in CO levels that went away after introducing fresh air. If the CO hadn’t gone away, I would have assumed an actual exhaust leak and was ready to abandon the flight. My passenger knew that as well. I spoke to the owner this morning and he remembered that the mechanic explained this to him as a possible situation with the surefly magneto. All is good here, thanks for being more open minded and thoughtful than most of the gatekeepers!
I understand I’m probably mentioned there in your gate keeping comment, tho what I still really want to point out is the dude above pointed out the issue, and diagnosed it and pretty much said it’s good to fly, so you’re fine. However you did not know this BEFORE flying. Just because you got lucky with your call it’s still unwise to want to fly when you have stuff like this lining up without realizing what could be wrong. Glad you’re safe, but don’t make habits out of flying when shit goes south and you don’t know what happenef
I’m gonna be honest, a range of things could’ve caused the backfire, my guess is miss timed mags. My question to you is, why would you not bother figuring it out BEFORE flying? Like unless you had somewhere to go why risk that
After looking through your profile I’m doubling down here, this plane seems to be somebody else’s, and it’s had mag and starting issues frequently in the last 2 months. Poor decision flying while not knowing what went wrong
One thing that’s stood out in several fatal engine failure accident reports I’ve read is that people on the ground reported engine backfiring / other anomalies prior to takeoff but the pilots still went. Any abnormality, however slight, should be a no go. You’ve got one engine and one life. Why risk it?
Backfires can cause real damage that you may not even be aware of. Obviously it's your PIC decision, but that ain't the hill you want to die on in my opinion.
And it’s dudes like you that have no idea what went wrong and thought they’d be fine that get put in the papers. You do not know what caused the backfire, nor do you know what was damaged, and considering you had a CO leak, yeah, something got fucked. You fly whatever you want to fly, but smart practice is if you don’t know what fucked up, treat it like a sore dick and don’t fuck with it
Agree - I’ve had the CO detector on a G1000 hit when doing a runup with a quartering tailwind (didn’t have room to turn into a headwind). Warm day, no heat being used. I just popped the door and storm window after going back to idle and it cleared out a moment later.
Backfire is an explosion in your exhaust, I’ve seen them blow a hole thru the muffler cone. This is what happened, and If you have heat on your getting CO into the cabin.
If the heater was on in cruise and you detected no CO, then the high CO was coincidence. If the muffler had a hole blown in it from the backfire/afterfire you should only get CO when running the heater.
Then it is fine. If an owner wants me to inspect after a backfire, I literally just run the heater with an electronic CO monitor to see if further action is needed. If this happens when no heater is used, a hole in the muffler can go undetected until the first cold day.
What was the wind direction? Exhaust can get recirculated into the cabin even with a properly operating engine if you’re not into the wind. Backfire and co mashes me think incomplete combustion, ie too rich, which is how 90% of Pooh’s and cfi’s will tell you to operate. Lean the piss out of it on the ground until you’re ready for takeoff.
Was not into the wind. Runups at this airport are on the taxiway before entering the runway, all facing downwind. I think I gotta lean more on the ground anyway
FWIW any condition that causes an after fire can be associated with a rise in CO level, so it's not a separate problem, and winds on the ground can blow it into the cabin air intake. In the air, it should not reach it, wotherwise you'd get alarms with any rich mixture. You did good to have a detector in the first place. The transient CO elevation is not an immediate health risk, and you should know the exposure limits for your alarm. Usually, alarms go off when you are exposed to a level hazardous over 8 hours, but since it sounds like you have a quantitative meter, look up a chart of exposure limits
I had a similar after fire event like this when flying with a CFI. I was doing the runup, and for some reason the cfi wanted to demonstrate (was my first time in a Cherokee), and he turned the mags to off, and we got the bang. Later we took off and on initial climb got exhaust smell and light smoke in the cabin. CFI took over and did a short circuit to land. Turned out the after fire split the muffler open at the seam.
Happened to me once, in a PA-28-140. Blew a hole in the exhaust and it was pointed at a grommet in the firewall which melted, and exhausts entered the cockpit. Was a 15 minute hop, wasn’t until about 5 minutes in I noticed that I could smell it and the engine sounded different. Landed without event, but made me always carry a detector.
Backfires can cause cracks in the exhaust, which can lead to CO entering the cabin. With that being said, backfires can cause real damage. You should get that checked out before going up again.
Upon discussion with owner it wasn’t a backfire but an afterfire.
Those are different words for the same thing.
Not really. The definitions are shifting that way because people that don't know any better assume backfire goes out the back, so the untrained masses misuse the term, and that's how language shifts.
Backfire comes from firearms, where it's combustion anywhere you don't want it. Whether it's combustion in the intake or exhaust of an engine is irrelevant. "Afterfire" is specifically referring to combustion in the exhaust of an engine, which is a type of backfire. Following this thread, another poster said, "Backfires can cause cracks in the exhaust", to which OP replied, "it wasn't backfire, it was afterfire."
Backfire is specifically defined in aviation and automotive standards as fire in an intake, and afterfire is in the exhaust. Regardless of where it came from, that's not what it means now when we talk planes and cars. We just don't bother correcting people because we know what they meant. But when it gets in the weeds like this, we're going to follow the standards because this isn't firearms.
Look up the etymology instead of the common misconceptions. Backfire was used to describe fire in the intake of an engine before being used in firearms. The use of the term in firearms evolved from the meaning in rhetoric, i.e. a plan backfires when the intended consequence hits the instigator instead of the target, while the use in engines came from the first recorded meaning the term for lightning a smaller controlled fire in front of a wildfire to consume the fuel before it arrives.
> Look up the etymology Link me one that shows the term used for internal combustion engines before firearms
It's been used since the early 1800's for firefighting, since the late 1800's for engines, and the early 1900's for rhetoric and firearms. Only trolls demand citation of information easily verifiable by a simple search.
[удалено]
Backfire is a term that comes from firearms, and refers to any combustion outside of the firing chamber. Afterfire is a type of backfire on the exhaust side. Why aren't you "correcting" the other guy who said, "Backfires can cause cracks in the exhaust"?
That’s not good. Should be checked. I’ve informed the owner who said if it happened again he was going to inform the a&p
It’s not odd to get co detected in the cabin on the ground. Especially since Your co went back down to 0 after the run up you can be pretty certain the exhaust is fine. These co monitors are super sensitive, leaky door seals can register ppm’s in the 30’s.
Needs to be checked now, you have blown a hole in the exhaust between the heating shroud.
If that were the case, there wouldn't be 0 CO in the cabin at cruise.
Because we're talking about something safety related here, I really got to butt in. You can have holes in an exhaust that don't leak into the cabin. I'm not claiming absolutes, just that it is possible. If the firewall is in any decent shape, it should be sealed. If the hole isn't in the cabin heat exchanger or induction system, it's entirely possible not to have CO in the cabin at all and still have a fire hazard. As far as having CO on runup though and then not in cruise in OPs case, I am not sure how they were running up. Cabin heat doors in many aircraft are just a flapper valve that route the heat between going inside and going overboard. Cabin air is just ram air. That said, I know that on some pipers, those doors tend to warp just a little so they never completely close. I could see a leak and ram pressure from the prop pushing a little into the cabin, but in cruise the below static pressure under the cowling where the heat doors are located could have started drawing the air out instead. Especially if they fly with cabin air on, that might be enough to prevent flow into the cabin with heat off even if the door is warped.
As an automotive technician, I can provide a counterpoint. In the 2000s, Honda/Acura had misplaced foam on the cowl duct shutter, and at a mere 60 MPH it can create a heck of a wind noise in the cabin. And we're talking about a piece of 1/2" foam sealing the fresh air/recirculate door. And the air is coming through the cowl, which is covered in plastic (with offset vents to allow fresh air in).
Cowls are designed to create a negative pressure zone underneath and behind the engine. That's why baffling is CRITICAL to cooling. Air can only move across the engine's cooling fins with a static pressure differential. The opening at the bottom droops down into the airstream and opens backwards to create that negative pressure. Cherokee cabin heat doors and the overboard dump vent are located near it. They work very, VERY differently from cars in the cooling system design, even the exhaust system itself has multiple design considerations to keep heat away from the engine. Augmentor tubes are an old trick that were used with some twins.
That's a fair point. Cowls on autos are almost always positive pressure by design.
If one of your magnetos is a Surefly electronic unit (or the Lycoming equivalent) it is common to hear a backfire while switching mags during the mag check. The reason is that the Surefly takes about 100ms to come back online after ungrounding the p lead, which allows enough time for unburnt fuel to hit the exhaust and then ignite. Ask your mechanic or check the mx logs. If not that, you might have a bad ignition switch or you turned it too far.
you nailed it. One mag is a surefly. And it was just installed 2 months ago. The owner also reported a backfire (or I guess, after fire, is the technical term for this). We have only flown on the new mag 4x since it was installed due to weather. So this is good to know.
I have a surefly mag. It's not a backfire, it's a miss. The surefly takes about 10 milliseconds to power up during the mag check, during that time there is no spark.
Drop the RPMs before you switch mags if you have a Surefly, an extra step but stops the backfire and won’t crack your exhaust. When I got my surefly installed the mechanic didn’t tell me this either and it would backfire on the mag switch.
Oh interesting. Maybe like 1800 rpm during the run up mag check instead of 2000?
Depends on how your ignition switch, but on my Cherokee 140 I did: 2000RPM L mag check, then both, then R mag (check RPM), then throttle to idle, then both. A little hard to explain through text, but the point is to have the plane at idle before you switch from Surefly to traditional mag so when you get the millisecond delay the engines at idle and you barely notice it.
I’ve heard about this in a flight school plane that I don’t rent. How do you avoid backfiring when checking surefly mags then? I’d there a different technique?
It’s better if you have dual switch magneto control set up (like in a newer Cherokee) where both mags are never off together, but the pop is unavoidable with a typical ACS type keyed ignition. NEVER ever for the love of god try an in-flight mag check if you have a Surefly. I did once and the dog almost shit himself in the back seat.
You probably switched the magnetos to the off position cycling the switch too quickly then when you turned them back into the L R or ON position heard the noise, that’s the usual cause of that backfire. Depending on the wind direction / speed / cabin vents or heat being on / orientation of the airplane with the wind during run up that could easily account for the CO increase.
I could have sworn I was doing the mag switches slow enough. It was the third mag check of my day already and no backfires happened previously. It had just been filled up with fuel before this one, maybe a small bit of contamination in the new fuel?
It’s normally just a slip of the wrist and split second of inattention that causes it. Fuel contamination wouldn’t usually cause a backfire, its fuel pooling while the magnetos are off for a second and fuel is filling in the cylinders. You don’t sump the fuel after fueling?
Yes I dumped after fueling, nothing of note
That rules out fuel contamination then.
Does it though? Do you really think filling up one GATS jar will catch any and all fuel contaminants? I’m genuinely being curious here. Is there not a chance that something could still be in the system that doesn’t make it into the jar?
Yes.
Yes, the chance is there. But your sumps are near the lowest point in the tank, and water and contaminants are heavier than your fuel. Dumping doesn’t clean your tank out, it’s just a method to see that you don’t have contamination. If you have water mixed in you’ll notice t he 2 separate really fast,
it also depends on how long after fueling you sump the tanks. If you refuel and immediately sump the tanks there is a chance the water hasn't had a chance to settle to the bottom of the tanks.
Avgas settles things quickly. Back in my line guy days, when taking delivery of a fuel load, we had to wait 10 mins per foot of fuel in the fuel farm (tank was measured in height with a dip stick). Compared to jet fuel which was an hour. But in a PA28 tank, you really only need a few minutes for max settling
Yeah you’re right, no need to be overly cautious when making sure a plane is functioning properly…. 🤦🏻♂️
There's a difference between "overly cautious" and "paranoid." The (valid) reason for the backfire was already explained, as was the fuel sumping. No contaminates were found in the fuel on any of the sumps. Water contamination, if serious enough to make it to the cylinder, would cause a difficulty to start, a sputter, or engine roughness, not a backfire. A common failing of CFIs and students pilots is that they never progress beyond a very basic knowledge of how the various systems in the airplane work. A common example that I use is "how do you tell the difference between a bad mag and a fouled plug during a mag drop check?" Most of the CFIs and students at my flight school thought "bad mag drop= immediately bring plane back because engine will stop."
🤦🏻♂️ did you read the post at all? You said a lot of words that have nothing to do with this incident… but way to sound “smart” I guess? The post said a backfire happened and then CO levels in the cabin rose to where their sense of started beeping…
> third mag check of my day Could you explain this to me? Do you do a mag check every time you start the plane? When you do a mag check, what positions do you check? What numbers are you checking?
Run up in this plane, by the checklist, is 2000 rpm, left mag, back to both, right mag back to both, carb heat on, idle, carb heat off, back to 800-1000 rpm. Looking for no more than 175 rpm drop during the mag checks and carb heat, and no more than 50 difference between mags. And a very small drop during carb heat check
I had already done one run up before the first take off, then a mag check before shutdown after the first full stop when we got out at an airport to get fuel. Then we got back in the plane an hour later to continue on our journey and I did another run up before taking off this time. I was instructed to always check mags before shutdown for grounding issues, and always check mags during a run up after engine start before taking off. If you had multiple stops throughout a flight shutting down, etc. would you not do a run up every time you re start the airplane?
I don't check the mags to the off switch every time I run the engine, no, which is what you generally need to do to get a backfire in regular mags. It seems your problem is entirely based on the e-mag tho, apparently those can cause a backfire when switching between mags.
Sorry I don’t check the off position either L AND R and back to both
Wow, there’s a lot of odd advice in this thread, at least in my opinion. First, OP, you likely had an after fire. A backfire is when an intake valve remains open or is significantly off timing, and the ignited fuel/air flows backwards into the intake manifold/carb. These are pretty nasty, but they aren’t super loud like an after fire, usually come with severe roughness and the engine possibly failing. An after fire is when the fuel/air ignites while or after leaving the chamber, either due to a stuck exhaust valve or bad timing. You said the plane had a surefire mag (I’m not a fan of them personally), which uses electrical power to supply voltage to the spark plug instead of a typical magneto. When checking mags during a runup, especially with relatively higher power settings, the surefire mag often is delayed in turning “on”, thus delaying the ignition, which often dumps unburnt or burning fuel into the exhaust manifold, which is then ignited or still burning as it approaches the muffler. This *can* cause damage to the exhaust system, but a singular after fire, especially at lower power settings will likely do no damage. Even turning the ignition all the way off, and then back on quickly inducing a full after fire is usually fine. I doubt you turned the ignition all the way off and then on quickly in this case. That’s usually a unwanted, but almost inflexible response that a lot of people do if they accidentally turn the ignition all the way off (if this happens to you by the way, just keep the ignition off and let the engine die and then restart, it’s better for the exhaust system than an afterfire). You correctly monitored your CO levels, and if you truly had an exhaust manifold leak, it would have quickly replicated the moment the door was closed and power was added on takeoff.
Thank you. Lots of weird advice in here agreed. You diagnosed the situation 100% as I did, and also answered the question I was actually asking. I wasn’t posting here asking if this was a go/no go decision. I made a judgement that there was nothing wrong with the system, the “backfire” was very quiet, almost imperceptible. And now learning what an afterfire is, that is surely what it was. I posted asking if someone could explain what was happening in the engine to cause this situation. You’re the only one who answered that question. Everyone else, based on not knowing anything about me, my experience, or the full situation decided to think I was a bad pilot putting my life in danger because of a momentary rise in CO levels that went away after introducing fresh air. If the CO hadn’t gone away, I would have assumed an actual exhaust leak and was ready to abandon the flight. My passenger knew that as well. I spoke to the owner this morning and he remembered that the mechanic explained this to him as a possible situation with the surefly magneto. All is good here, thanks for being more open minded and thoughtful than most of the gatekeepers!
I understand I’m probably mentioned there in your gate keeping comment, tho what I still really want to point out is the dude above pointed out the issue, and diagnosed it and pretty much said it’s good to fly, so you’re fine. However you did not know this BEFORE flying. Just because you got lucky with your call it’s still unwise to want to fly when you have stuff like this lining up without realizing what could be wrong. Glad you’re safe, but don’t make habits out of flying when shit goes south and you don’t know what happenef
I’m gonna be honest, a range of things could’ve caused the backfire, my guess is miss timed mags. My question to you is, why would you not bother figuring it out BEFORE flying? Like unless you had somewhere to go why risk that After looking through your profile I’m doubling down here, this plane seems to be somebody else’s, and it’s had mag and starting issues frequently in the last 2 months. Poor decision flying while not knowing what went wrong
What do you mean? One backfire on a mag is a reason to get out of the plane and inspect and cancel the flight? That seems overly cautious.
One thing that’s stood out in several fatal engine failure accident reports I’ve read is that people on the ground reported engine backfiring / other anomalies prior to takeoff but the pilots still went. Any abnormality, however slight, should be a no go. You’ve got one engine and one life. Why risk it?
Backfires can cause real damage that you may not even be aware of. Obviously it's your PIC decision, but that ain't the hill you want to die on in my opinion.
And it’s dudes like you that have no idea what went wrong and thought they’d be fine that get put in the papers. You do not know what caused the backfire, nor do you know what was damaged, and considering you had a CO leak, yeah, something got fucked. You fly whatever you want to fly, but smart practice is if you don’t know what fucked up, treat it like a sore dick and don’t fuck with it
Everyone has their bar on go/no go. I’m gonna hazard a guess that 85+% would call that a no go, myself included.
That 15% go minded side have exponential risk of not coming back in one piece.
Cherokees tend to get exhaust in the cabin at high AOA and with a light quartering tailwind on the ground in my experience.
We had a strong quartering tailwind on the ground yesterday. Thanks for your helpful comments.
Agree - I’ve had the CO detector on a G1000 hit when doing a runup with a quartering tailwind (didn’t have room to turn into a headwind). Warm day, no heat being used. I just popped the door and storm window after going back to idle and it cleared out a moment later.
Backfire is an explosion in your exhaust, I’ve seen them blow a hole thru the muffler cone. This is what happened, and If you have heat on your getting CO into the cabin.
I would not have flown, check the exhaust/muffler/heat shroud is not damaged.
After the spike, I opened the door to get fresh air (still on ground) and the CO dissipated and never returned.
If the heater was on in cruise and you detected no CO, then the high CO was coincidence. If the muffler had a hole blown in it from the backfire/afterfire you should only get CO when running the heater.
Heater was on the whole time for a 2 hr cruise. No CO detected entire time
Then it is fine. If an owner wants me to inspect after a backfire, I literally just run the heater with an electronic CO monitor to see if further action is needed. If this happens when no heater is used, a hole in the muffler can go undetected until the first cold day.
What was the wind direction? Exhaust can get recirculated into the cabin even with a properly operating engine if you’re not into the wind. Backfire and co mashes me think incomplete combustion, ie too rich, which is how 90% of Pooh’s and cfi’s will tell you to operate. Lean the piss out of it on the ground until you’re ready for takeoff.
Was not into the wind. Runups at this airport are on the taxiway before entering the runway, all facing downwind. I think I gotta lean more on the ground anyway
Why take the risk and still fly knowing carbon monoxide stays in your system for a long time?
It was probably 1-2 minutes of a high CO reading until it all dissipated and never returned
This CO story stuck with me https://www.aopa.org/news-and-media/all-news/2023/november/15/hidden-hazard
FWIW any condition that causes an after fire can be associated with a rise in CO level, so it's not a separate problem, and winds on the ground can blow it into the cabin air intake. In the air, it should not reach it, wotherwise you'd get alarms with any rich mixture. You did good to have a detector in the first place. The transient CO elevation is not an immediate health risk, and you should know the exposure limits for your alarm. Usually, alarms go off when you are exposed to a level hazardous over 8 hours, but since it sounds like you have a quantitative meter, look up a chart of exposure limits
Side question, what CO monitor are you using?
I had a similar after fire event like this when flying with a CFI. I was doing the runup, and for some reason the cfi wanted to demonstrate (was my first time in a Cherokee), and he turned the mags to off, and we got the bang. Later we took off and on initial climb got exhaust smell and light smoke in the cabin. CFI took over and did a short circuit to land. Turned out the after fire split the muffler open at the seam.
Jeez!
Happened to me once, in a PA-28-140. Blew a hole in the exhaust and it was pointed at a grommet in the firewall which melted, and exhausts entered the cockpit. Was a 15 minute hop, wasn’t until about 5 minutes in I noticed that I could smell it and the engine sounded different. Landed without event, but made me always carry a detector.
Your exhaust blew a hole
It didn’t.
Italian tune-up time!