Ted
The pilot formerly known as Twin Engine Ted
- Joined
- Oct 9, 2007
- Messages
- 29,889
- Display Name
Display name:
iFlyNothing
Unfortunately in the past few months I've gone from having had no burned valves in nearly 3,000 hours of flying (which amounts to over 5,000 hours of engine time in the air since most of that was in twins) to having a bunch of burned valves in a short period of time. Yesterday I did a borescope and compression check on the left engine of the 414 and found three burned valves. This follows the first burned valve that I had in November, which was discovered following my first trip to St. Croix (35 hours in 3.5 days).
First let me discuss what I observed, and what I think the root cause is (which I'm sure will cause some level of disagreement). At annual keep in mind that all of my cylinders had compressions that were better than 60/80, and the borescopes looked great - burned pizza look on the exhaust valves, perfectly symmetrical. The plane had flown approximately 50 hours since annual.
Before I went to St. Croix I had new mags and harnesses put on the 414. I was concerned with the possibility of a mag or harness issue. I've had many mags fail on me in the past, and these mags were old. The nylon gear in the Bendix mag will, without warning, shear a bunch of teeth off of it, rendering the mag useless. The last thing I wanted was to have a mag failure somewhere during the 1,000 miles between Florida and St. Croix, with diversion options all international. So, I had the shop replace the magnetos. Normally, I check timing as I've had bad luck with shops getting timing incorrect. Unfortunately virtually every shop I've come across gets timing wrong. I had loaned out my tools so I couldn't get it done before the trip. But the engines ran smoothly and the plane flew.
As we flew, the first thing I noticed was the EGTs were about 100 degrees higher than normal. Typically I see EGTs in the range of 1550F on the engine monitor. They were up at 1650F for the same power settings. The plane was also noticeably slower.
Over the course of the trip, I did observe the EGT on the #4 cylinder going a bit higher, and also wandering around. Keep in mind, I flew 35 hours in 3.5 days so I was very in tune with what the EGTs on each cylinder were doing (not much else to do during those long trips). If I flew shorter, more "normal" flights, then this probably would've been harder to detect without downloading data from the engine monitor. On the last leg coming home from St. Croix, I observed during the hot start that one cylinder didn't seem to give the starter much resistance. And upon landing back home in Kansas City, the idle was very rough on the left engine with a lower EGT on the #4 cylinder, although it smoothed out by around 1300 RPM.
I called the shop the next morning and told them "Pull the plane in, check the left engine. I'll bet you the #4 valve is burned." 0/80 compression on that cylinder. I told them to also borescope the rest of the cylinders and do compressions to make sure that I didn't need to ground the plane completely at that point. Cylinders 5 and 6 were showing noticeably lower compression than 1, 2, and 3. 5/6 were in the 55/80 range (still legal) with some leakage around the valve. 1, 2, and 3 at that point were still showing 65/80 or better with no indications of issues. They replaced the #4 cylinder in time for me to do my next trip, and I expected that 5/6 might need to get done at some point.
Now comes the more interesting part. I had my ignition timing tools back and checked the timing. Timing on the TSIO-520-NB is spec'd at 20 degrees BTDC. I found the left engine set to 15 degrees BTDC, and the right engine set to 17. I was not happy about this and reset the timing to spec. My EGTs were back in the 1550 range and my airspeeds were back to normal. With the new cylinder, things were running well, idle was back to being smooth. With a burned valve, this makes sense. The valve seats poorly and thus at low cylinder pressures (low idle) too much air bleeds out and you have a dead cylinder. Once you get the cylinder pressures up a bit more, the cylinder fires and runs. At power, it will seal well enough that you won't notice a power loss.
32 hours of flying later, I was having a rough idle on the left engine again, although I wasn't seeing signs during cranking of a cylinder that seemed dead to rights. I decided before the next transport I needed to do a borescope and compression check to make sure that things still looked good. Yesterday I did that transport ahead of my upcoming transport from Houston to SoCal this weekend. I found the following:
- Very obviously burned valves on cylinders 3, 5, and 6 with the other 3 cylinders looking fine. #4 cylinder looked the best (as you'd expect, being brand new) with a beautiful burned pizza pattern.
- Corresponding compressions of between 10/80 and 15/80 on the three cylinders with burned valves, all leaking out the valve.
A few things to note for everyone:
1) There are lots of ways to detect a burned valve if you listen to your engines. Listening is both physical (as I noticed from watching my engines crank over during a hot start and also the poor idle). Don't ignore these warning signs - it means there's a problem
2) You can also listen using your engine monitor. Chances are you won't be as in tune with your EGTs as I was on this trip, but if you download the data you'll be able to identify a problem valve
3) Here's the controversial point that I'm sure will cause some disagreement. A lot of people have said there is no published EGT or TIT limit for Twin Cessnas, and that engine monitors have so many variations that you can't believe the EGT/TIT values anyway. I have always disagreed with this. The metal doesn't know whether or not there is a published limit, all it knows is what it will tolerate before it will tolerate no more. The turbochargers on Twin Cessnas actually have published limits for TIT of 1650F (you have to dig deep to find the spec for the TH08A turbo itself, Cessna doesn't publish it), and Cessna published power settings with manifold pressure, RPM, and fuel flow that would ensure you never reached a point that would have a limit. So, there was no point in publishing one.
While there are inaccuracies that can occur with the EGT/TIT measurements, I saw a significant increase due to mistimed engines, followed by exhaust valves in the following hours. Now, my guess is that the damage was already starting and that those valves would have burned anyway given enough time. However, I also think that the mistimed magnetos and significantly higher EGTs I observed on my first trip to St. Croix accelerated the wear and burned the valves faster.
I've said before that most shops get ignition timing wrong. I see them incorrectly find TDC (some will look for the physical position of the piston through the spark plug hole, which is wrong) and most will use a gravity needle which will have inherent inaccuracies with it. Normally I see timing being set in the range of 2-3 degrees off. Seeing 5 degrees off is a new one for me. Still, this is the inherent issue with the tools that most shops use. When you look at plane-to-plane variation in speed, I bet this is part of it.
I've attached a few pictures and posted some pictures and videos on the fundraiser page for the new engines of the burned valves, you can see there:
https://poundwishes.com/donate/2473881/engine-overhaul
Yes, a shameless plug for the engine fundraiser for Cloud Nine - RAM is giving us an incredible sponsorship on these engines but we still have a lot of money to raise. Cloud Nine is an all-volunteer 501(c)3 non-profit that relies on donations. These engines will put us in a position to save many, many more homeless pets.
First let me discuss what I observed, and what I think the root cause is (which I'm sure will cause some level of disagreement). At annual keep in mind that all of my cylinders had compressions that were better than 60/80, and the borescopes looked great - burned pizza look on the exhaust valves, perfectly symmetrical. The plane had flown approximately 50 hours since annual.
Before I went to St. Croix I had new mags and harnesses put on the 414. I was concerned with the possibility of a mag or harness issue. I've had many mags fail on me in the past, and these mags were old. The nylon gear in the Bendix mag will, without warning, shear a bunch of teeth off of it, rendering the mag useless. The last thing I wanted was to have a mag failure somewhere during the 1,000 miles between Florida and St. Croix, with diversion options all international. So, I had the shop replace the magnetos. Normally, I check timing as I've had bad luck with shops getting timing incorrect. Unfortunately virtually every shop I've come across gets timing wrong. I had loaned out my tools so I couldn't get it done before the trip. But the engines ran smoothly and the plane flew.
As we flew, the first thing I noticed was the EGTs were about 100 degrees higher than normal. Typically I see EGTs in the range of 1550F on the engine monitor. They were up at 1650F for the same power settings. The plane was also noticeably slower.
Over the course of the trip, I did observe the EGT on the #4 cylinder going a bit higher, and also wandering around. Keep in mind, I flew 35 hours in 3.5 days so I was very in tune with what the EGTs on each cylinder were doing (not much else to do during those long trips). If I flew shorter, more "normal" flights, then this probably would've been harder to detect without downloading data from the engine monitor. On the last leg coming home from St. Croix, I observed during the hot start that one cylinder didn't seem to give the starter much resistance. And upon landing back home in Kansas City, the idle was very rough on the left engine with a lower EGT on the #4 cylinder, although it smoothed out by around 1300 RPM.
I called the shop the next morning and told them "Pull the plane in, check the left engine. I'll bet you the #4 valve is burned." 0/80 compression on that cylinder. I told them to also borescope the rest of the cylinders and do compressions to make sure that I didn't need to ground the plane completely at that point. Cylinders 5 and 6 were showing noticeably lower compression than 1, 2, and 3. 5/6 were in the 55/80 range (still legal) with some leakage around the valve. 1, 2, and 3 at that point were still showing 65/80 or better with no indications of issues. They replaced the #4 cylinder in time for me to do my next trip, and I expected that 5/6 might need to get done at some point.
Now comes the more interesting part. I had my ignition timing tools back and checked the timing. Timing on the TSIO-520-NB is spec'd at 20 degrees BTDC. I found the left engine set to 15 degrees BTDC, and the right engine set to 17. I was not happy about this and reset the timing to spec. My EGTs were back in the 1550 range and my airspeeds were back to normal. With the new cylinder, things were running well, idle was back to being smooth. With a burned valve, this makes sense. The valve seats poorly and thus at low cylinder pressures (low idle) too much air bleeds out and you have a dead cylinder. Once you get the cylinder pressures up a bit more, the cylinder fires and runs. At power, it will seal well enough that you won't notice a power loss.
32 hours of flying later, I was having a rough idle on the left engine again, although I wasn't seeing signs during cranking of a cylinder that seemed dead to rights. I decided before the next transport I needed to do a borescope and compression check to make sure that things still looked good. Yesterday I did that transport ahead of my upcoming transport from Houston to SoCal this weekend. I found the following:
- Very obviously burned valves on cylinders 3, 5, and 6 with the other 3 cylinders looking fine. #4 cylinder looked the best (as you'd expect, being brand new) with a beautiful burned pizza pattern.
- Corresponding compressions of between 10/80 and 15/80 on the three cylinders with burned valves, all leaking out the valve.
A few things to note for everyone:
1) There are lots of ways to detect a burned valve if you listen to your engines. Listening is both physical (as I noticed from watching my engines crank over during a hot start and also the poor idle). Don't ignore these warning signs - it means there's a problem
2) You can also listen using your engine monitor. Chances are you won't be as in tune with your EGTs as I was on this trip, but if you download the data you'll be able to identify a problem valve
3) Here's the controversial point that I'm sure will cause some disagreement. A lot of people have said there is no published EGT or TIT limit for Twin Cessnas, and that engine monitors have so many variations that you can't believe the EGT/TIT values anyway. I have always disagreed with this. The metal doesn't know whether or not there is a published limit, all it knows is what it will tolerate before it will tolerate no more. The turbochargers on Twin Cessnas actually have published limits for TIT of 1650F (you have to dig deep to find the spec for the TH08A turbo itself, Cessna doesn't publish it), and Cessna published power settings with manifold pressure, RPM, and fuel flow that would ensure you never reached a point that would have a limit. So, there was no point in publishing one.
While there are inaccuracies that can occur with the EGT/TIT measurements, I saw a significant increase due to mistimed engines, followed by exhaust valves in the following hours. Now, my guess is that the damage was already starting and that those valves would have burned anyway given enough time. However, I also think that the mistimed magnetos and significantly higher EGTs I observed on my first trip to St. Croix accelerated the wear and burned the valves faster.
I've said before that most shops get ignition timing wrong. I see them incorrectly find TDC (some will look for the physical position of the piston through the spark plug hole, which is wrong) and most will use a gravity needle which will have inherent inaccuracies with it. Normally I see timing being set in the range of 2-3 degrees off. Seeing 5 degrees off is a new one for me. Still, this is the inherent issue with the tools that most shops use. When you look at plane-to-plane variation in speed, I bet this is part of it.
I've attached a few pictures and posted some pictures and videos on the fundraiser page for the new engines of the burned valves, you can see there:
https://poundwishes.com/donate/2473881/engine-overhaul
Yes, a shameless plug for the engine fundraiser for Cloud Nine - RAM is giving us an incredible sponsorship on these engines but we still have a lot of money to raise. Cloud Nine is an all-volunteer 501(c)3 non-profit that relies on donations. These engines will put us in a position to save many, many more homeless pets.
