Slow Flight & hot metal smell

[skidoo]

I Had a great day today. It was full sunshine like I haven't seen for months. Anyway, we took off, got to altitude, set medium cruise power, about 22" hg, closed cowl flaps, and then leaned it to about 1625 TIT. Peak would get it to the max 1685 limit.

So after a little cruise, my instructor has me set up for slow flight at about 65 KIAS. I pull back power and adjust the nose to maintain altitude. We are now putting along at about 65 and I ask him if he recognizes that smell. Like somethings burning. He says he doesn't smell anything. He says maybe exhaust fumes due to the lower airspeed. About a minute later, he says looks like the TIT is getting warm and starts adjusting the mixture richer. I look and it was at the very top of the green. There was no digital reading on that particular screen. The immediate concern was to get the temp a bit lower immediately. At the time, I couldn't quite describe the smell. It was a light smell, nothing distinctive. Now that I had time to think about it, I would say is was similar to the hot metal of a wood stove (but without the wood smell).

Shortly thereafter, we re-established cruise speed and re-leaned. The smell went away.

I'm not sure if the TIT max event occurred during the slow flight portion or just after that when powering up to re-establish cruise speed. I think at some point there, power was up above 23" hg, which is above the power where I first leaned it.

So, what should I learn from this today? Should the Cowl Flaps have been opened below a certain airspeed? Should I be observing and adjusting lean for the TIT with any significant airspeed change, or perhaps just for an increase in power level?

Any other words of wisdom from which I can improve? I'll also discuss it further with my CFI next time.

 

[nosehair]

So, what should I learn from this today? Should the Cowl Flaps have been opened below a certain airspeed? Should I be observing and adjusting lean for the TIT with any significant airspeed change, or perhaps just for an increase in power level?

What do the cowl flaps do? What are they for?
Engine temp control, right? Airspeed is certainly a major consideration, but so is outside temp. On a real cold day, you may not need to open the cowl flaps. Practice and observe what kinds of TIT rises you get on different OAT days, and at different altitudes, to learn how to use cowl flaps and also adjusting, or increasing mixture to help with engine cooling.

 

[dillardrg]

Skidoo,

Any time you make a power change you should relean for the new condition.
When doing instruction or maneuvers a good practice is to go full rich, there is always the possibility that you may need full power (you may inadvertantly stall) and you should be ready for that or for any power setting needed.
Fuel is cheap in relation to replacing a cylinder.

 

[bbchien]

Boy, your CFI doesn't get it....
TIT? What are you training in- a Turbo Arrow?

 

[TMetzinger]

Skidoo,

Any time you make a power change you should relean for the new condition.
When doing instruction or maneuvers a good practice is to go full rich, there is always the possibility that you may need full power (you may inadvertantly stall) and you should be ready for that or for any power setting needed.
Fuel is cheap in relation to replacing a cylinder.

+1.

Too rich a mixture fouls plugs, and wastes gas. Too lean a mixture can (in certain circumstances) break stuff, especially on turbosupercharged or turbonormalized engines.

I recommend you waste the gas. Unless the density altitude is so high that the engine doesn't run at full power (and this wouldn't apply to turbo'ed engines), I use full rich for manuevers.

But back to the original situation - TIT/EGT is used for adjusting mixture and there is a TIT limit that needs to be observed. CHT is what you look for as far as engine cooling goes though, and I don't see any mention of CHT in your original post.

It may be perfectly normal to get that smell in the slow, nose-high configuration. The flow of hot air and exhaust will change and you may get fumes in the cockpit you'd never get in normal flight regimes. If it were me, I'd uncowl the engine and look for any signs of an oil leak. Absent any leaks, or temps (CHT/TIT) exceeding limits, I'd put this into the "learn what is normal for MY airplane" category.

 

[gismo]

+1.

Too rich a mixture fouls plugs, and wastes gas. Too lean a mixture can (in certain circumstances) break stuff, especially on turbosupercharged or turbonormalized engines.

To be accurate, "too lean" is too simple of a description for this complex situation and implies something that's very untrue. The most "destructive" mixture is about 25 F on the rich side of peak EGT or TIT. Leaner than that is actually less stressful and even peak EGT is less stressful than 25 ROP. In most engines when the power is limited to 60% max rated or less
the mixture can be set to anything that makes the engine run without concern for damage.

I recommend you waste the gas. Unless the density altitude is so high that the engine doesn't run at full power (and this wouldn't apply to turbo'ed engines), I use full rich for manuevers.

If those maneuvers are likely to involve more than brief operation at high power below about 7000 DA I'd say that near full rich is desirable, more for the convenience of being able to apply full available power without having to fool with the mixture but otherwise I'd stick with something a lot leaner than full rich. At 5000 DA you are likely dumping an extra 25% of fuel into the engine beyond what is optimal and while some might find that wasteful the real issue is the buildup of deposits in the cylinder head and on the plugs. These deposits can lead to reduced detonation margins and pre-ignition (which is far more destructive than light to mild detonation). If you have EGT a simple way to set an adequately ROP mixture is to go to WOT and set lean the mixture until the EGT is the same as you see at full power and near sea level DA (which should be in the 1250-1350 F range).

But back to the original situation - TIT/EGT is used for adjusting mixture and there is a TIT limit that needs to be observed. CHT is what you look for as far as engine cooling goes though, and I don't see any mention of CHT in your original post.

It may be perfectly normal to get that smell in the slow, nose-high configuration. The flow of hot air and exhaust will change and you may get fumes in the cockpit you'd never get in normal flight regimes. If it were me, I'd uncowl the engine and look for any signs of an oil leak. Absent any leaks, or temps (CHT/TIT) exceeding limits, I'd put this into the "learn what is normal for MY airplane" category.

There's a good chance that the smell was nothing more than dust in the heat muff getting hotter than usual but there's also some chance that the heat muff has a leak. I strongly recommend a CO detector of some sort to eliminate that possibility.

 

[CJones]

Sounds like you were cooking some cylinders. Got a CHT? What did it say?

 

[skidoo]

We were in slow flight for perhaps only 3 minutes or so. During that time, I was just concentrating on setting up airspeed, attitude, throttle. It does have a CHT gauge, but I wasn't looking at it. But, this experience emphasizes my need to fully understand the leaning / cooling process so I can minimize the overall risks.

I can't come to grips with needing to adjust lean for every power adjustment. I'm ok with wasting a little fuel to keep it simpler as long as I maintain better margins for less wear. I can see the need for adjustment for power increases, but not necessarily for power decreases. If that is needed too, someone please explain.

The POH recommends cruise at 50 ROP or 50 Rich of the max 1685F. It can easily peak above the 1685, so I just stop at 1635. But, if I do this, and the peak is really at 1650, then I am really at 15 ROP. I really hate to bring it all the way to the limit of 1685 to determine peak.

At full rich, the temps begin quite a bit lower. And there are many times where full rich are called for and the engine runs fine. So, I am wondering what if I run it at 50 ROP, or 100 ROP, or 150, or 200, or 250 ROP? What happens if I just set for a certain GPH and don't worry if it is 75 or 150 ROP?

Anyone know of a good book on the subject?

Gismo: Can you explain why 25F ROP is the worst operating point?

 

[Dan Thomas]

Gismo: Can you explain why 25F ROP is the worst operating point?

See this article, especially the charts showing where the cylinder pressures peak in relation to various mixtures:

http://www.warmkessel.com/jr/flying/td/jd/18.jsp

Basically, a 50° ROP brings the pressure peak too close to TDC and stresses the engine. Lean mixtures burn more slowly and the peak will come a little later, converting more of the energy into HP and saving stress on the engine's components.

Dan

 

[gismo]

We were in slow flight for perhaps only 3 minutes or so. During that time, I was just concentrating on setting up airspeed, attitude, throttle. It does have a CHT gauge, but I wasn't looking at it. But, this experience emphasizes my need to fully understand the leaning / cooling process so I can minimize the overall risks.

I can't come to grips with needing to adjust lean for every power adjustment. I'm ok with wasting a little fuel to keep it simpler as long as I maintain better margins for less wear. I can see the need for adjustment for power increases, but not necessarily for power decreases. If that is needed too, someone please explain.

The POH recommends cruise at 50 ROP or 50 Rich of the max 1685F. It can easily peak above the 1685, so I just stop at 1635. But, if I do this, and the peak is really at 1650, then I am really at 15 ROP. I really hate to bring it all the way to the limit of 1685 to determine peak.

At full rich, the temps begin quite a bit lower. And there are many times where full rich are called for and the engine runs fine. So, I am wondering what if I run it at 50 ROP, or 100 ROP, or 150, or 200, or 250 ROP? What happens if I just set for a certain GPH and don't worry if it is 75 or 150 ROP?

Anyone know of a good book on the subject?

Gismo: Can you explain why 25F ROP is the worst operating point?

Dan posted a short answer and a link to one of many articles by John Deakin on the subject. I have a link that goes to a list of all of them somewhere but can't find it at the moment, you could just google Deakin + mixture +pelican to get most of them. For the complete story you might consider taking the APS online course (or the in person one if you don't mind traveling to Ada OK and spending more money).

http://www.advancedpilot.com/

And FWIW, you won't hurt anything hitting or exceeding the TIT redline briefly (10-20 seconds) to establish the peak, it takes time for the turbine to get too hot. That said, you shouldn't spend any time near peak TIT/EGT at high power (i.e. above 75%), hence GAMI's "Big Mixture Pull" to get to the lean side of peak EGT at high power.

 

[gprellwitz]

Can someone explain why, if he was in slow flight and thus presumably well below 65% power, he was getting TIT readings near the top of the green? And there's probably no correlation, but I always thought that at power settings below 60-65% there were no worries about the leaning because it isn't possible to get hot enough to do any damage. Comments?

 

[TMetzinger]

That didn't make sense to me either, Grant.

to Skidoo - does the airplane have an engine monitor?

 

[gismo]

Can someone explain why, if he was in slow flight and thus presumably well below 65% power, he was getting TIT readings near the top of the green? And there's probably no correlation, but I always thought that at power settings below 60-65% there were no worries about the leaning because it isn't possible to get hot enough to do any damage. Comments?

On many airplanes slow flight, especially with a "dirty" airplane (flaps and gear extended) requires a fair amount of power. Probably less than 65% but still enough that EGT and TIT can reach "normal" levels if the mixture is leaned to peak EGT. You're probably not used to seeing this because it's common to leave the mixture fairly rich during such maneuvers. IIRC, on my engines I can achieve about the same EGTs during a runnup (far less than 65% power) as I get in cruise flight at 65%.

As far as damage goes, the engine won't suffer damage at any mixture as long as CHTs are kept acceptably low (they can get pretty hot in slow flight). The turbocharger OTOH can suffer turbine damage if the TIT is allowed to remain above the redline temp for very long, regardless of the power the engine is producing. This is strictly a blade temperature issue, as the exhaust gas drives the turbine the blades grow in length from the combination of "centrifugal" force (aka inertia) and temperature generated expansion. If they get long enough they contact the turbine housing.

 

[skidoo]

That didn't make sense to me either, Grant.

to Skidoo - does the airplane have an engine monitor?

Yes, this was in a Turbo 182 with G1000. Thinking back, the hot smell was during slow flight only. I think the TIT was high just after coming out of slow flight and applying more power using throttle changes only. I think I had set up lean for 22" and 2280 rpm, prior. I do remember seeing as much as 23.5" at some point when applying power to get back to cruise. I'm thinking that may have been the reason for the high TIT. I know my process wasn't good, and I'm looking to improve.

I am wondering, if I am expecting to do maneuvers, if I should lean at a higher power level and then cut back power as desired.

 

[gismo]

Yes, this was in a Turbo 182 with G1000. Thinking back, the hot smell was during slow flight only. I think the TIT was high just after coming out of slow flight and applying more power using throttle changes only. I think I had set up lean for 22" and 2280 rpm, prior. I do remember seeing as much as 23.5" at some point when applying power to get back to cruise. I'm thinking that may have been the reason for the high TIT. I know my process wasn't good, and I'm looking to improve.

I am wondering, if I am expecting to do maneuvers, if I should lean at a higher power level and then cut back power as desired.

When I practice formation flight (which requires occasional application of high power with only about 55% power on the average) I set the mixtures to a position I know from previous experience will be sufficiently rich for the altitudes being flown. I've established that mixture position by monitoring the EGT at full throttle with the RPM set as I would for formation (2400-2500). The correct mixture yields the same EGT I see on a sea level standard day during takeoff (about 1300 F).

 

[Dan Thomas]

Can someone explain why, if he was in slow flight and thus presumably well below 65% power, he was getting TIT readings near the top of the green? And there's probably no correlation, but I always thought that at power settings below 60-65% there were no worries about the leaning because it isn't possible to get hot enough to do any damage. Comments?

The mag timing should be checked. Really late spark can cause overheating of exhaust components.

Dan

 

[Capt. Geoffrey Thorpe]

I Had a great day today. It was full sunshine like I haven't seen for months. Anyway, we took off, got to altitude, set medium cruise power, about 22" hg, closed cowl flaps,
...
So after a little cruise, my instructor has me set up for slow flight at about 65 KIAS. I pull back power and adjust the nose to maintain altitude. We are now putting along at about 65 and I ask him if he recognizes that smell. Like somethings burning.

You might want to open the cowl flaps since you don't get nearly as much cooling air flow at low speeds.

 

[Keith Lane]

Sometimes on high wing Cessnas, slow flight, and the high angles of attack really reduce the velocity of the airflow into the cowl openings, reducing the cooling effect significantly. When
I was a student, had a CFI melt the front seal on a C-150 due to prolonged slow flight. It was in summer, however. Sure was a mess to clean up once on the ground. And the A/P had the CFI clean it up. No more prolonged slow fligts for him unless he recovered occasionaly to reduce engine temps. These planes did not even have EGT indicators on board.

 

[skyflyer8]

FWIW I always smelled a teeny hint of exhaust/burning smell in the TB20 Trinidad during slow flight, which I attributed to the nose-high angle and the position of the exhaust pipe relative to the airstream. In this case there was no engine overheating or temp problems at all.