Flying with a small temp/dew pt. spread

[Skyscraper]

Kind of a beginner question here guys, be gentle

I'm planning an early morning flight for tomorrow with a temperature/dew point spread very likely to be much smaller than what I'm used to. Most of the flying I do is late morning/afternoon when the OAT is warm and humidity is pretty low.

The conditions tomorrow morning are likely to be around 30F with a dew-point of 28F (Humidity of about 96%). Skies are forecast to be clear. Cruising altitude no higher than 5,500 MSL and likely to be more like 3,500 MSL (planning final details tonight)

My basic question is, under these conditions, how much should carb ice be a concern? Is applying a little carb heat while cruising considered a bad practice? Do you guys and gals normally wait until experiencing an RPM drop before applying it? I know carb ice is possible even up to 70F with high humidity, so it's on my mind.

I'm mostly just concerned for the first couple legs as there will be some terrain to consider and the weather should be warming up and drying out quickly and should be of little to no concern upon reaching my destination.

Remember, I don't have a lot of winter flying experience, so go easy if I've missed something or just plain wrong in some of my understandings, I'm still a student here

 

[poadeleted20]

The conditions tomorrow morning are likely to be around 30F with a dew-point of 28F (Humidity of about 96%). Skies are forecast to be clear. Cruising altitude no higher than 5,500 MSL and likely to be more like 3,500 MSL (planning final details tonight)

With that set of conditions, odds are the T/DP spread will increase rapidly as the morning goes on.

My basic question is, under these conditions, how much should carb ice be a concern? Is applying a little carb heat while cruising considered a bad practice?

In subfreezing temps? Yes, applying a little carb heat before symptoms show would be a bad practice. It's very difficult to form carb ice when the OAT is less than freezing, as the moisture in the air is pretty well frozen already and will usually not freeze to the carb throat. However, application of a little carb heat may warm the induction air just enough that it will go above freezing in the air box, and then freeze to the carb throat as it passes through. Now you have self-created carb ice.

Do you guys and gals normally wait until experiencing an RPM drop before applying it?

In most planes, yes. In Continental-powered carbureted Cessnas, like the C-150, early 172's, and most 182's, which are far more susceptible to carb ice than their Lycoming-powered kin, I'll pull carb heat any time I'm below the green arc on the tach (or MP gauge on 182's -- and note that with a c/s prop you won't see an RPM drop with carb ice in cruise until it's way too late). Otherwise, I wait for symptoms.

 

[dell30rb]

Close temp/dew point is usually a sign to look out for fog.

Don't sweat the carb ice issue. Follow whatever normal carb heat procedures are listed in the POH.

Its not a bad idea to pull the carb heat occasionally in cruise and see if you have any ice build up. But if you use it, make it all or nothing.

 

[bobmrg]

Absolutely nothing wrong with pulling the carb heat every time the notion strikes you....just in case. The NTSB recommends full heat whenever you reduce power below your cruise power setting. Don't ever use partial heat unless you have instrumentation that reads out temp in the carb throat or uses optics to check for ice.

Bob Gardner

 

[poadeleted20]

Absolutely nothing wrong with pulling the carb heat every time the notion strikes you....just in case.

...unless the OAT is well below freezing, in which case you could create carb ice you cannot then melt out, and there are documented cases on point. It would also be a bad idea in dusty conditions as you'd be bypassing the air filter (unless, of course, the air filter has become blocked by the dust and using the carb heat control to bypass the filter is the only way left to get air into the engine).

 

[JasonM]

Check out this thread I posted a while back. A lot of good responses about carb ice and answers to your questions I believe.

http://www.pilotsofamerica.com/forum/showthread.php?t=64354

 

[dell30rb]

It's very difficult to form carb ice when the OAT is less than freezing, as the moisture in the air is pretty well frozen already and will usually not freeze to the carb throat.

I've heard this but I did fly an airplane that did not abide by this rule. A C150. OAT was about 20F, wide open throttle and at 11,500 feet the engine would not run more than 15 seconds without full carb heat applied. This happened over two days. First day I was above a broken layer. Second day sky was clear, visibility was excellent.

Here is a pic from the 2nd day actually. I went down to 9500 to get out of whatever funky temp/humidity combination was causing my carb ice.

 

[poadeleted20]

My guess is you were overleaned, not carb iced. Using carb heat would raise the DA inside the engine and effectively enrich the mixture. But as I said, it possible although unlikely.

As you can see, the probability of carb ice in cruise at 20F is basically zero in the low humidity conditions you describe.

 

[dell30rb]

Definitely had the same problem with the mixture rich. And in between

Though I got a decent RPM drop with it on preflight, I also suspect the carb heat may not have been fully disengaging when pushed in.

 

[RV10flyer]

Looks like your carb heat concerns are covered above.

Other things to think about for cold wx flying..no frost allowed on wings, water in fuel can freeze and not show up in fuel sample(unlikely with your warm daytime temps and 28-32F tomorrow AM), engine preheat, allow engine to warm a few minutes longer than normal before applying full power, must have a good strong battery, warm clothing for preflight and emergencies, microfiber cloth to remove fog from inside of windshield and a carbon monoxide detector. Have fun as it looks like a beautiful forecast for tomorrow.

 

[dell30rb]

Looks like your carb heat concerns are covered above.

Other things to think about for cold wx flying..no frost allowed on wings, water in fuel can freeze and not show up in fuel sample(unlikely with your warm daytime temps and 28-32F tomorrow AM), engine preheat, allow engine to warm a few minutes longer than normal before applying full power, must have a good strong battery, warm clothing for preflight and emergencies, microfiber cloth to remove fog from inside of windshield and a carbon monoxide detector. Have fun as it looks like a beautiful forecast for tomorrow.

^ Good tips

Don't forget the microfiber cloth. I remember on the day of my instrument checkride I took off at the crack of dawn on a cold morning to fly to Burlington and meet the examiner. I was on the gauges at 100 AGL as my windscreen and side windows fogged up alarmingly fast. I may be exaggerating a bit here but its a good idea to keep a cloth handy.

 

[jesse]

Absolutely nothing wrong with pulling the carb heat every time the notion strikes you....just in case

...unless the OAT is well below freezing, in which case you could create carb ice you cannot then melt out, and there are documented cases on point. It would also be a bad idea in dusty conditions as you'd be bypassing the air filter (unless, of course, the air filter has become blocked by the dust and using the carb heat control to bypass the filter is the only way left to get air into the engine).

Which I've seen happen before. I was able to keep the engine running (barely) by priming the hell out of it. Eventually the ice cleared.

Plus as Ron mentions the whole bypassing the air filter is less than ideal if you don't need to be bypassing it.

 

[Dan Thomas]

In subfreezing temps? Yes, applying a little carb heat before symptoms show would be a bad practice. It's very difficult to form carb ice when the OAT is less than freezing, as the moisture in the air is pretty well frozen already and will usually not freeze to the carb throat. However, application of a little carb heat may warm the induction air just enough that it will go above freezing in the air box, and then freeze to the carb throat as it passes through. Now you have self-created carb ice.

Water droplets can exist in liquid form down to -20°C (-4°F) and carb ice is easily possible. It would have to be cold indeed to create carb ice using carb heat: ice crystals in the air, -15 or -20°C. The OP's 28/30°F temps are dangerous enough.

Dan

 

[RV10flyer]

My guess is you were overleaned, not carb iced. Using carb heat would raise the DA inside the engine and effectively enrich the mixture. But as I said, it possible although unlikely.

As you can see, the probability of carb ice in cruise at 20F is basically zero in the low humidity conditions you describe.

Could he have had an excessively rich mixture. The addition of hot air by the carburetor heat enriches the mixture even more and causes an additional drop in rpm. If this was the case, leaning the mixture may have helped.

 

[poadeleted20]

Could he have had an excessively rich mixture.

No, for the reason you stated:

The addition of hot air by the carburetor heat enriches the mixture even more and causes an additional drop in rpm.

He said it got better, not worse, when he pulled carb heat. Since carb heat enriches, if he was running rough and smoothed out/powered up when he pulled carb heat and enriched the actual fuel/air mix, he must have been running too lean rather than too rich to start with.

 

[RV10flyer]

No, for the reason you stated:He said it got better, not worse, when he pulled carb heat. Since carb heat enriches, if he was running rough and smoothed out/powered up when he pulled carb heat and enriched the actual fuel/air mix, he must have been running too lean rather than too rich to start with.

Sorry about that, misread. Thanks.

 

[MAKG1]

Skyscraper, you're actually at substantially more risk of ice from a morning flight in the summer.

Conditions here are dry and very, very stable. You will not be seeing supercooled water anywhere.

If you do happen to see RPMs being lost without any change in settings, altitude or attitude, pull the carb heat to see if that fixes it. Sinking air while maintaining altitude will also cause loss of RPM with a fixed pitch prop, but the attitude is not constant (it pitches up).

You're much more likely to see carb ice when the air is ABOVE freezing, sometimes even far above freezing. You may have an issue with the inversion.

More importantly, tomorrow, the forecast at STS isn't showing anywhere near a problematic dew point spread. It's 18 deg C at 7AM tomorrow. It looks like a fabulous day for flying, except there is a bit of wind at a few thousand feet (be ready for turbulence, and for the wind at TPA to be very different from the wind on the surface).

http://rucsoundings.noaa.gov/gifs/r...16&hour=14&lon=-104.67&lat=39.87&airport=ksts

From flying around yesterday with a similar skew-T, the wind at 3000 was almost twice the forecast, which was the same as it is tomorrow. Conditions are stable, so any turbulence you see is driven by the terrain. Avoid flying downwind of the mountains. It's a northeast wind, so fly on the north and east sides. Yesterday, it was smooth as glass over San Francisco, but the wind speed was 45 knots at 3000. While I was over Crystal Springs, I heard a PIREP on the NorCal frequency of severe turbulence over Half Moon Bay, less than 10 miles away but on the other side of Montara Mountain. I found some moderate myself over Oakland, due to Mt. Diablo.

And FYI, despite losing my fillings over Oakland, conditions on the ground were completely benign, though with an opposite direction pattern. 050@5 (almost a direct crosswind for 13 at PAO).

 

[Immelman]

The thing to think about in a single engine airplane is radiation fog, which is typically right down to the ground if it forms and frequently shows up just *after* sunrise when the air is coolest. You'll see it form in low lying areas such as valley floows, and will wipe out many emergency landing options. Its common in the central valley (tule fog), somewhat common (but not as frequent) in central/northern CA but still happens. If you're launching 1-2 hours after sunrise it will likely be gone, if it was a factor at all. Just something to be aware of.

But overall the weather this winter, draught concerns aside, is some of the best for flying in this region year-round. Its pretty much worry free. Sure there may be some offshore wind and bumps like yesterday, but they're just bumps.... Its pushing 80 out now and the tide's falling, so I shall go surf!