Written test question about fuel mixture

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injb

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I'm using the Cessna Pilot Center ground material. One of the questions is:

"When you adjust the fuel/air ratio with the mixture control for a change in altitude, the ratio you are changing is the

  1. volume of fuel to the volume of air
  2. weight of fuel to the weight of air
  3. weight of fuel to the volume of air
"
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I picked 3, and they say I'm wrong. They say it's 2. Their explanation is:

"Answers 1 and 3 are incorrect because the number of air molecules in a fixed volume of air changes significantly with changes in altitude and consequently the only valid ratio is by weight."
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Who is right? Obviously the goal of adjusting the mixture for altitude is to maintain the correct ratio of fuel mass to air mass ('mass' being the usual term for 'weight'). Logically you can't keep something the same by adjusting it so that rules out answer 2.

My understanding is that the carburettor mixes a fixed mass of fuel with a given volume of air, and it's for that reason that you have to adjust the ratio at all. Therefore answer 3 is correct. Or am I missing something?
 
Welcome to the FAA. The volume of air is related to the mass or weight of air by pV=zNRT where N=Mass/MolecularWeight. Answers 2 and 3 are correct but only answer 2 is more correcter(TM) because fuel is burned on a mass basis rather than a direct mass to volume basis. Another way to look at it is the volume of the air is fixed but the mass decreases with altitude (density decrease) so the mass of fuel must be decreased.

Of course none of the FAA test question writers really understands the gas law so the answers provided appear to be stupid to anyone who understands gas behavior.
 
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injb said:
I'm using the Cessna Pilot Center ground material. One of the questions is:



I picked 3, and they say I'm wrong. They say it's 2. Their explanation is:



Who is right? Obviously the goal of adjusting the mixture for altitude is to maintain the correct ratio of fuel mass to air mass ('mass' being the usual term for 'weight'). Logically you can't keep something the same by adjusting it so that rules out answer 2.

My understanding is that the carburettor mixes a fixed mass of fuel with a given volume of air, and it's for that reason that you have to adjust the ratio at all. Therefore answer 3 is correct. Or am I missing something?
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The volume of air stays the same but as you climb the weight of a given volume decreases because its density decreases. The carburetor meters fuel according to the velocity of the air passing thru it, not the density or mass. It's not an ideal fuel metering device at all, and neither is aircraft fuel injection, though it's a lot better than carburetion. Only electronic fuel injection, with its mass flow sensors and temp sensors and EGO sensors comes anywhere close to ideal.

So the correct answer is 2. Fuel's density varies some with temperature changes, too, so weight is the number that makes sense.

Combustible mixtures of air and fuel vary between 8:1 (air:fuel by weight of each) and 18:1. 8:1 is really rich, 18:1 is really lean. Best power will be found around 12:1 and stoichiometric at 15:1. Now, if we went by volumes instead of weights, those numbers would be all over the place depending on temperatures and pressures.
 
Clark1961 said:
Welcome to the FAA. The volume of air is related to the mass or weight of air by pV=zNRT where N=Mass/MolecularWeight. Answers 2 and 3 are correct but only answer 2 is correct because fuel is burned on a mass basis rather than a direct mass to volume basis. Another way to look at it is the volume of the air is fixed but the mass decreases with altitude (density decrease) so the mass of fuel must be decreased.

Of course none of the FAA test question writers really understands the gas law so the answers provided appear to be stupid to anyone who understands gas behavior.
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I don't quite get that bit!


Dan Thomas said:
The volume of air stays the same but as you climb the weight of a given volume decreases because its density decreases. The carburetor meters fuel according to the velocity of the air passing thru it, not the density or mass. It's not an ideal fuel metering device at all, and neither is aircraft fuel injection, though it's a lot better than carburetion. Only electronic fuel injection, with its mass flow sensors and temp sensors and EGO sensors comes anywhere close to ideal.

So the correct answer is 2. Fuel's density varies some with temperature changes, too, so weight is the number that makes sense.

Combustible mixtures of air and fuel vary between 8:1 (air:fuel by weight of each) and 18:1. 8:1 is really rich, 18:1 is really lean. Best power will be found around 12:1 and stoichiometric at 15:1. Now, if we went by volumes instead of weights, those numbers would be all over the place depending on temperatures and pressures.
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In hindsight I see why I was wrong to rule out #2 - I can see a way of interpreting the wording where that's true. If the air mass/fuel mass ratio is wrong, and you adjust the control, well then you're adjusting the air mass/fuel mass ratio. Fair enough. That interpretation starts with the assumption that you've waited for the air mass/fuel mass ratio to be wrong before you do anything. I suppose that's realistic.

But I still think answer #3 is still correct! Suppose we call 2 different positions of the mixture control position A and position B. When it's at A I get a certain mass of fuel for a certain volume of air. Now, you say it's velocity rather than volume, but since that's fixed by the engine speed, that means that we get a certain volume of air passing through the carb in a given space of time, right? Now if I move the mixture control to position B, I'm getting a different mass of fuel for that same volume of air. Doesn't this make answer 3 correct?
 
FAA test questions are written by the same people who write exam questions for Mechanical, Electrical, and Plumbing Contractors license exams. They know little about the topics, they are psychologists. They take the material (FAR AIM and POH's for example) and make questions from it without understanding what they are reading. I forgot the name of the testing agency but when I found out it was the same outfit that tested me for Mechanical and Electrical I thought to myself "oh no, here we go again".
 
Clark1961 said:
Why not?
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Because it sounds like a contradiction - "answers 2 and 3 are correct but only answer 2 is correct". Unless the "because" part is to say that answer 2 is the only one that's correct for that particular reason, i.e. answer 3 is correct for some other reason. But in that case I don't get why the reason matters :)
 
All other variables constant, when you change altitude does the volume of air change or does the weight of air change?
 
injb said:
I don't quite get that bit!




In hindsight I see why I was wrong to rule out #2 - I can see a way of interpreting the wording where that's true. If the air mass/fuel mass ratio is wrong, and you adjust the control, well then you're adjusting the air mass/fuel mass ratio. Fair enough. That interpretation starts with the assumption that you've waited for the air mass/fuel mass ratio to be wrong before you do anything. I suppose that's realistic.

But I still think answer #3 is still correct! Suppose we call 2 different positions of the mixture control position A and position B. When it's at A I get a certain mass of fuel for a certain volume of air. Now, you say it's velocity rather than volume, but since that's fixed by the engine speed, that means that we get a certain volume of air passing through the carb in a given space of time, right? Now if I move the mixture control to position B, I'm getting a different mass of fuel for that same volume of air. Doesn't this make answer 3 correct?
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So if we want to define the ratio for leaning at any point, and we use volume, we now have to adjust that volume for pressure and temperature for the ratio to make sense. The engine doesn't care what the temperature or pressure of that air is; it only burns the fuel based on molecules of fuel to molecules of oxygen, which is defined by weight alone.

And adjusting for pressure and temperature is already a pain for students. They have enough trouble trying to understand density altitude and true airspeed and altitude. All they need to know about air:fuel ratios is that they're by weight. If I was still teaching Aircraft Systems I sure wouldn't want to complicate things with adjustments to volumes for pressures and temperatures. The engineers who design engines and fuels don't want to do it, either. They use numbers that the flame understands.
 
injb said:
Because it sounds like a contradiction - "answers 2 and 3 are correct but only answer 2 is correct". Unless the "because" part is to say that answer 2 is the only one that's correct for that particular reason, i.e. answer 3 is correct for some other reason. But in that case I don't get why the reason matters :)
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Welcome to FAA testing. There may be reasons that several answers are correct but one answer in particular has been selected as the correct answer for reasons that only the FAA gets to chose.
 
dmspilot said:
All other variables constant, when you change altitude does the volume of air change or does the weight of air change?
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Does leaning the mixture reduce the mass of fuel relative to the volume of air regardless of altitude?
 
dmspilot said:
All other variables constant, when you change altitude does the volume of air change or does the weight of air change?
Click to expand...

The weight changes. Which is why you must find another ratio of fuel mass to air volume that results in the same fuel mass to air mass you had before, if you want to keep the correct ratio. Hence you must adjust the fuel mass to air volume ratio ;)

Clark1961 said:
Welcome to FAA testing. There may be reasons that several answers are correct but one answer in particular has been selected as the correct answer for reasons that only the FAA gets to chose.
Click to expand...

I get what you mean now...and I won't be forgetting the answer they're looking for on this one any time soon :)
 
injb said:
Because it sounds like a contradiction - "answers 2 and 3 are correct but only answer 2 is correct". Unless the "because" part is to say that answer 2 is the only one that's correct for that particular reason, i.e. answer 3 is correct for some other reason. But in that case I don't get why the reason matters :)
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If you're gonna dispute test answers you get wrong, it will take you a long time to learn to fly. Trust me. Seen it before. A student's intuition is often way out to lunch, and if he persists in using it just because he doesn't like the real answers, he only hurts himself.

Just wait until you study turbine engine theory and run up against pressure, velocity, converging and diverging ducts. Your intuition will convince you of some things that will ultimately mean that the engine cannot run.
 
Mass, mass is constant, it doesn't change with pressure or temperature, as volume does. The correct answer is mass. Weight is actually wrong too, as weight and mass can be different in some circumstances. Just like centrifugal force isn't a real force, but that's another story.
 
One is correct because you the main jet in the carburetor is a volumetric device and you are changing the volume of fuel that is supplied to compensate for the reduced density of the volume of air metered by the throttle.
Two is correct because you are adjusting the mass of the fuel to match the mass of the air flowing through the carburetor.
Three is correct because you are changing the mass flow rate of the fuel to match the reduction in the mass of the volume of air metered by the throttle.

One is wrong because what counts is the ratio of mass, not ratio of volumes.
Two is wrong because the mass ratio between air an fuel is what you want to remain constant.
Three is wrong because what counts is the ratio of masses, not the ratio of mass to volume.

In the end, memorize that the answer the FAA wants is the ratio of weights until you pass the test, then it doesn't matter.
 
injb said:
I get what you mean now...and I won't be forgetting the answer they're looking for on this one any time soon :)
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If I recall correctly I just had that question (again) on the commercial written a couple weeks ago. I just shake my head and move on. I think half a dozen old fart SEL piston pilots could overhaul the FAA question database in a couple months and make it a heck of a lot more relevant for testing a candidate's knowledge of how to fly safely. Of course I only have a couple post-graduate degrees in engineering and have taught university courses in engineering so I really have no clue about teaching relevant material and assessment of knowledge. (yes, that was sarcasm)
 
Dan Thomas said:
... The carburetor meters fuel according to the velocity of the air passing thru it, not the density or mass. It's not an ideal fuel metering device at all, and neither is aircraft fuel injection, though it's a lot better than carburetion. Only electronic fuel injection, with its mass flow sensors and temp sensors and EGO sensors comes anywhere close to ideal. ...
Click to expand...
Yes. The way I have always understood it is that carburetors are basically volumetric flow devices and modern electronic fuel injection systems are mass flow devices.

Since the carb just mixes air and fuel in a fixed ratio based on volume, when the oxygen in a given volume of air declines (as it does with altitude), the amount of fuel in the ratio must be reduced in order to keep a correct mixture. That's why in Olden Times, cars at high altitudes/thinner air like Denver were actually adjusted to lean their mixtures. Also in Olden Times, jetting of carburetors in racing cars was fine-tuned based on air temperature.

What we are shooting for, of course, is to have the right number of oxygen molecules correctly matched to a number of fuel molecules. IOW the correct mass ratio. The modern FI system does this by sensing air intake temperature and flow and by checking the exhaust to determine if the mixture is correct.

I have always wondered about our antique airplane FI systems, whether they do any better mixture control than the carburetors. Apparently it is not much better because we still have to lean.
 
Clark1961 said:
Does leaning the mixture reduce the mass of fuel relative to the volume of air regardless of altitude?
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Leaning the mixture will reduce the mass of fuel relative to everything, for example the pilot's IQ, but that's not what the question meant. For sure it's poorly (or perhaps I should say incorrectly) worded but you can tell what they were trying to get at.

Two answers are correct, but one is more correct because of the intent behind the question.
 
This was one of three questions I got wrong on my private pilot written test, it drove me crazy. It even instigated a minor argument between my instructor and me.

I think it's important here to understand that the fuel mixture is a ratio of the mass of the air, and the mass of the fuel. I got hung up on the fact that weight and mass are not the same thing... The chemical reaction that occurs requires a specific number of oxygen molecules and fuel molecules. We measure the amount of molecules that are needed by measuring the weight (or more precisely mass) of the oxygen and the fuel.

As you climb and descend, the atmospheric pressure changes as well. When you climb, the carb will let in the same 'volume' of air, but that volume exists at a lower pressure, and hence a lower mass. You need to compensate for that by reducing the amount (i.e. mass) of fuel you are putting in to the mixture. By reducing the amount/mass of fuel, you are keeping the ratio of fuel to air constant. This is also the reason your engine will produce less power at higher altitudes. With less air, to keep the ideal mixture ratio, you have to decrease the amount of fuel. And when you are burning less fuel, there is less chemical energy to extract from the reaction.
 
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Weight and Mass differ by the acceleration of gravity. But since gravity is the same everywhere on planet earth, weight can be used to measure mass and mass can be used to measure weight. In the English system weight is pounds and mass is slugs. In metric, weight is newtons and mass is kilograms.

Whats weird is they teach everyone that one kilogram equals 2.2 pounds. Kilograms is a unit of mass and pounds is a unit of weight. Shows how mass and weight can be interchanged. Don't really know why they do that since its actually kinda wrong (but its accepted and works). What they should teach is there are 4.45 newtons in one pound.

The equation is F = M x A
Where
F is force (unit is Newtons or pounds)
M is Mass (unit is kilograms or slugs)
A is acceleration of gravity (units are meters/second squared or feet/second squared)
 
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coloradobluesky said:
English system weight is pounds and mass is slugs.
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Except when Mass is pounds mass, and force is pounds force, or when mass is pounds and force is poundel. And, in SI the Kg is a unit of mass, except some people also use Kg force... You may not have been exposed to these other unit systems but they exist and are valid (except kg force, I refuse to accept that as valid).

Weight is a term that long predates the concept of mass and force of gravity being different things and should not be used as a random substitute for either. Which is another reason that all three answers to the original question are wrong.

 
Weight or volume, it really doesn't matter. It's a poindexter question, and (mostly) irrelevant when flying the airplane. It's important you know to adjust the mixture when it should be adjusted, but the unit of measure (volume, weight, furlongs per fortnight) isn't of practical concern. Might be nice to ponder on in a quiet moment, or read a bit about. But even if you become an expert on stoichiometric fuel-air rations, you're still going to lean (or not lean) the same way, using the red (usually) knob when the DA or the POH suggests you should. . .
 
Sundancer said:
Weight or volume, it really doesn't matter. It's a poindexter question, and (mostly) irrelevant when flying the airplane. It's important you know to adjust the mixture when it should be adjusted, but the unit of measure (volume, weight, furlongs per fortnight) isn't of practical concern. Might be nice to ponder on in a quiet moment, or read a bit about. But even if you become an expert on stoichiometric fuel-air rations, you're still going to lean (or not lean) the same way, using the red (usually) knob when the DA or the POH suggests you should. . .
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Yep. Being able to explain why the faa correct answer is most correct will not improve your ability to pilot an aircraft in any way.
 
The answer is 2 for the following reason: combustion is a chemical reaction that forms products with a specific number of atoms in specific ratios (CO2 and H2O). So the ratio of the number of atoms of carbon, oxygen, and hydrogen are important. It's convenient to measure these by weight.

But wait! Volume and weight are related by the ideal gas law PV=nRT. In other words, if we know the temperature, pressure, volume and R (a constant) we could calculate n (the number of atoms) and thus the weight (or mass). But the pressure, temperature, and volume are all changing and it would be really inconvenient to measure (or specify) these when the most important thing is really the ratio of atoms. Thus, weight is the most appropriate way to specify the fuel/air ratio.

Hope that help clarify this tricky subject for others.
 
injb said:
I'm using the Cessna Pilot Center ground material. One of the questions is:



I picked 3, and they say I'm wrong. They say it's 2. Their explanation is:



Who is right? Obviously the goal of adjusting the mixture for altitude is to maintain the correct ratio of fuel mass to air mass ('mass' being the usual term for 'weight'). Logically you can't keep something the same by adjusting it so that rules out answer 2.

My understanding is that the carburettor mixes a fixed mass of fuel with a given volume of air, and it's for that reason that you have to adjust the ratio at all. Therefore answer 3 is correct. Or am I missing something?
Click to expand...

Its an incorrectly worded question. It should have been "adjusting the mixture control knob to maintain the same ratio of weight of fuel to the weight of air".
 
Somehow I actually remember this from 3 years ago but it took me a minute to figure out why I said it was incorrectly worded. It's because changing the mixture will change all three ratios, but only one of them is one you actually care about.
 
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