static port and carb heat in airplanes

Factual Questions
1

Hi -

I’ve recently started taking flying lessons, and there are a couple things I’m confused about right off the bat.

An airplane uses the combination of a pitot tube and a static air port to determine things like air speed, elevation, etc. I can understand the basic principle - that the pitot tube has a pressure reading that varies with forward air speed (since air is being shoved into the tube while in flight), and that the static air port is used as a base reference. But where I get confused is that this seems to contradict with the ‘hi speed low pressure’ rule of physics, in a way.

It seems to me that the static air port would not record the barometric pressure correctly - or at least while the plane is in motion. It is essentially an open cavity, that is exposed to a surface where air is rushing by the opening, which would therefore lower the air pressure inside the cavity (similar to driving 80 mph on the highway, and then lowering the window - air pressure in the car decreases as it is ‘sucked’ out due to the high speed of air passing over by the now open window). Is there some sort of compensation for this built into the calculations used to determine airspeed and elevation to take this into account?

The other question is that concerning carb heat. The procedure manual states that you should turn on carb heat in a descent, and then once at the new cruise altitude, you shut it off. It would seem to me that carb heat is to prevent or reduce icing, and therefore the decision to turn it on or off would be based on the temperature, rather than the particular manuever being performed.

Any insight by other pilots would be greatly appreciated!

2

I am not a pilot, but given the number of reports on aircraft accidents I have read where failure to turn on carb heat was a factor, I wonder what the reason is for ever turning it off?

3

Carburetor icing doesn’t have much to do with the ambient temperature. Indeed, it’s often at its worst on hot humid summer days. On really cold days, there is often too little moisture for icing to form.

It’s mostly caused by expansion of the air/fuel mixture as it pass the restriction of the throttle plate in the throat of the carburetor (when you expand a gas, its temperature decreases). Because of this, carburetor icing is more of a problem at low power settings (typical of a descent) - when you close the throttle, the restriction is greater and so the expansion/cooling effect is greater.

In other words, your manual is giving sound advice.

4

When you heat the air on its way to the intake manifold you reduce the total amount that gets there (the number of molecules per second), which reduces the power the engine can produce - sort of the opposite of turbocharging.

5

The static port is designed to not be affected by the relative wind. It’s typically a small pinhole perpendicular to the relative wind, and at those sizes the viscosity of air is such that it won’t flow into the port. Static ports are also often duplicated on both sides of the aircraft, so that ram air forced into the port by a sideslip does not effect readings.

As for carb heat, there are several reasons why you don’t use it continually. One is that hotter air can cause detonation. Another is that you lose power when you apply carb heat, because you’re feeding hotter, and therefore less dense air into the engine. A third is because carb heat typically pulls unfiltered air, which is to be avoided as much as possible. And finally, leaving carb heat on all the time defeats its purpose. You may enter a condition in which the carb heat moves the venturi temp into the icing region, and ice will form. Then you’ll have no way to get rid of it.

Carb heat is not a guarantee against ice formation. It’s a way to change the temperature of the air going into the carb so that you can change the status quo. If ice is forming, adding heat may get rid of it. If ice isn’t forming, it may be because it’s too cold, and adding heat will put you into the ice formation region.

Incidentally, your instructor has probably told you this already, but if you apply carb heat at cruise because the engine starts running rough and you suspect icing, do NOT just pull it on and then switch it off right away. Intermittent application of carb heat may just melt ice and cause it to reform once you shut it off. Once you turn on carb heat, leave it on until the engine is running smoothy, and then wait a little longer. This is also why you pull carb heat on in the pattern - not because you’re more likely to get ice at low power, but because IF you do, the application of carb heat itself can cause the engine to quit completely if it dislodges a chunk of ice into the carb. So once you put on carb heat, you leave it on while you’re near the ground. Just remember to put it back in again on the go-around, or you won’t develop full power.

I imagine you’ve been told this already, but carb ice forms because of the temperature drop in the venturi of the carb - a drop that can be as high as 30 degrees C. So you can get carb ice at almost any ambient temperature.

Couple of other notes about the use of carb heat: If you don’t have a CAT (carb air temp) gauge, never use partial heat. You can induce icing that way, and then you’ll have no way to fix it. Without that gauge, you’re either full-on, or off.

Also, when you do the run-up, one of your checklist items will be to pull the carb heat control and look for a drop in RPM or manifold pressure. I know a lot of pilots who just note the drop, and push the control back in. Be more careful than that. The carb heat test has two purposes - one is to verify that the control is working properly. The other is to clear any ice that may have been formed during the taxi. Just pulling heat on and then pushing it off won’t tell you if there was ice, and can make it worse. So when you apply carb heat, watch for a drop in RPM, then a rise. If you see a rise, it means there was ice. Leave the heat on until the RPM stabilizes, then wait another 10-20 seconds to make sure everything’s okay.

Also, another little safety tip to get into the habit of - when you pull the carb heat during the run-up, keep your hand on the control until everything’s good, then push the control in. If you take your hand off the control, it’s just that much easier to forget to shut off carb heat before takeoff.

Hope this helps.

6

It’s not really the absolute speed of the air that matters, but the change: a decrease in speed gives an increase in pressure, and vice versa. The static source would give errors if it were located where the speed of the airflow is different from that of the airplane. That’s why it tends to be found on some “non-tapering” part of the fuselage. Here, the air is not accelerating or decelerating and the pressure matches ambient.

7

Also, carburetor heat on small single engine aircraft is provided by air from around the exhaust pipes. If you ask your instructor first, you might think about pulling the heat on before reducing power so as to get a shot of really hot air in the carburetor. Just do the heat first and then reduce the throttle.
Was once in a situation where I need the carb heat on as I climbed through some clouds and so I also had to use the mixture to get the best possible power as a long shallow climb was not feasible. Special attention to detonation was needed but remember that if the engine in snot producing much heat, the carb heat will not be very effective.

8

As a rule of thumb, carb heat is a possibility whenever the ambient temperature is between 20 and 70 degrees F and moisture is present (or so I’ve always been told). This covers most of flying weather in North America at any given time of the year. So, even if it’s 70 degrees, if you have mist or haze or high humidity you need to worry about carb heat. On the flip side, if it’s -10 degrees you don’t use carb heat, because doing so can induce icing - and that would be really bad since you would have no way to get rid of ice in that situation. Strictly speaking, if it’s 90 you don’t need to use it either, although doing so isn’t likely to cause a problem - except you’ll have even less power available than if you didn’t have it on. At 90 degrees, you will notice a lack of performance due to the low-density hot air. The trick there is that as you increase elevation the ambient temperature will drop. So when you reach an altitude where the termperature is in the worry-about-carb-ice range you have to start applying it again. Keep an eye on the outside temperature gauge.

Your best course of action is to follow the guidance in the POH. If it says to apply carb heat while in a descent, do so. Each airplane is different.

As for the static port - my understanding is that it is placed and sized to measure the air pressure in the boundary layer - a layer of air next to the fuselage that is relatively non-moving. But I do not claim to be an expert.

Oddly enough, I recently happened across a book that explains many of the unseen airplane parts and systems that live under the cowling and behind the panel. It’s called Aircraft Systems by David A. Lombardo, a part of the Practical Flying Series published by McGraw-Hill. It’s furthering my understanding of all those various bits and pieces, perhaps you would find it enlightening as well.

9

FWIW: I always remind my students that they may apply carb heat someday and find themselves with a rough running engine. Rather than immediately shutting off the heat, wait a few seconds to see if it smooths out (it’s likely you HAD ice, and the heat is doing it’s job, but the engine is “objecting” while it ingests it.

pullin

10

You probably wouldn’t for takeoff. But you wouldn’t have to climb unusually high before the temperature dropped into the range where icing could be a problem.

11

Finish reading the paragraph you quoted, I already mentioned that, along with a caution to watch the outside temp gauge

12

Right, you noted that point.