Instrument Written Question Thread

[jesse]

So, my written is a couple days away, and it is basically crunch time right now for me studying.

[Rant] A lot of this test is stupid.[/End Rant]

It is likely that I'll be posting multiple questions in this thread over the weekend when I just don't quite understand the answer. So please quote the question you are replying to.

Question #1

Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

a) Atmospheric pressure lower than standard.
b) Air temperature warmer than standard
c) Air temperature lower than standard.

FAA Answer is: B

Sigh. Okay. This one just doesn't make sense to me.

True Altitude = MSL (unless the FAA is stupid and considers true altitude to be indicated altitude which would make no sense)

Lets say your airplane is sitting on the beach at sea level. The pressure is standard (29.92). Your altimeter indicates 0 feet. Now the hurricane comes over you, the pressure continues to fall, and your altimeter is now indicating dramatically lower then you actually are..

IMHO--at that point--your altimeter is indicating a lower altitude then flown (or parked).

I clearly am not understanding something.. Someone smart please correct me.

 

[SkyHog]

So, my written is a couple days away, and it is basically crunch time right now for me studying.

[Rant] A lot of this test is stupid.[/End Rant]

It is likely that I'll be posting multiple questions in this thread over the weekend when I just don't quite understand the answer. So please quote the question you are replying to.

Question #1
Sigh. Okay. This one just doesn't make sense to me.

True Altitude = MSL (unless the FAA is stupid and considers true altitude to be indicated altitude which would make no sense)

Lets say your airplane is sitting on the beach at sea level. The pressure is standard (29.92). Your altimeter indicates 0 feet. Now the hurricane comes over you, you're in the eye and the pressure decreases to 28.92. You twist your altimeter to 28.92 and *shock* you're indicated altitude is -1000 feet.

IMHO--at that point--your altimeter is indicating a lower altitude then flown (or parked).

I clearly am not understanding something.. Someone smart please correct me.

To get true altitude, Jesse, you need to convert indicated altitude somehow. True altitude = actual altitude MSL, yes, but if all you have is the altimeter in front of you, you get True altitude by converting.

Remember, warm air is less dense than cool air.

 

[jmaynard]

Question 1:

Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

a) Atmospheric pressure lower than standard.
b) Air temperature warmer than standard
c) Air temperature lower than standard.

FAA Answer is: B

The Pilot's Handbook of Aeronautical Knowledge explains it this way: On a hotter than standard day, the atmosphere expands to a larger volume than it would occupy on a standard day. Therefore, the level at which the pressure is such that an altimeter would indicate 5000 feet on a standard day is actually above 5000 feet (the air is pushed upward); this would cause an altimeter to indicate 5000 feet when the aircraft is above that altitude.

 

[jesse]

To get true altitude, Jesse, you need to convert indicated altitude somehow. True altitude = actual altitude MSL, yes, but if all you have is the altimeter in front of you, you get True altitude by converting.

Remember, warm air is less dense than cool air.

Nick--I think you're thinking density altitude. I'm not sure that I've ever seen formulas for correcting pressure altimeters for temperature errors in at least my level of aviation.

Basically you correct for non-standard pressure and you're kind of hosed for non-standard temperature at least on the altimeter itself.

 

[dmccormack]

This is a typical "too much data and not all is relevant" question.

(I'm not sure why they do this, as it causes analytical types to waste time and actually think through, such as you are doing).

I feel your pain -- I'm doing this test over again for the CFII (it's the exact same test -- don't ask me why).

I feel like I forgot half the test stuff since I don't fly with an RMI and last time I used an ADF was in March.

 

[jesse]

What I don't understand yet--is why A isn't right too.

 

[dmccormack]

Nick--I think you're thinking density altitude. I'm not sure that I've ever seen formulas for correcting pressure altimeters for temperature errors in at least my level of aviation.

Basically you correct for non-standard pressure and you're kind of hosed for non-standard temperature at least on the altimeter itself.

No.. there is a temperature component to DA...

think....

 

[jesse]

No.. there is a temperature component to DA...

think....

I know one can figure DA based on pressure altitude and temperature.

I don't think one can figure true altitude based on indicated altitude corrected for non-standard pressure if non-standard temperature exists. Well I'm sure you could but I don't think I've ever seen anyone do it or talk about it.

 

[jesse]

<snip> this would cause an altimeter to indicate 5000 feet when the aircraft is above that altitude.

This seems to conflict with the question. The question states the aircraft will be lower then indicated.

 

[dmccormack]

I know one can figure DA based on pressure altitude and temperature.

I don't think one can figure true altitude based on indicated altitude corrected for non-standard pressure if non-standard temperature exists. Well I'm sure you could but I don't think I've ever seen anyone do it or talk about it.

They want you to know which way the error would be if you had a temperature below or above standard. (It's a "do you know how an altimeter works?" question)

 

[jesse]

They want you to know which way the error would be if you had a temperature below or above standard. (It's a "do you know how an altimeter works?" question)

I suppose so. I get why they have their answer. I don't get why the first one isn't right too.

If the pressure drops and you don't adjust the altimeter you will indicate lower then you are.

 

[dmccormack]

I suppose so. I get why they have their answer. I don't get why the first one isn't right too.

If the pressure drops and you don't adjust the altimeter you will indicate lower then you are.

Pressure drops..what happens to kollsman and which way do the needles turn?

 

[jesse]

(I'm not sure why they do this, as it causes analytical types to waste time and actually think through, such as you are doing).

I feel your pain -- I'm doing this test over again for the CFII (it's the exact same test -- don't ask me why).

I feel like I forgot half the test stuff since I don't fly with an RMI and last time I used an ADF was in March.

You're right with that. I am completely incompatible with the FAA written tests. I don't really care what the answer is--I always want to know *why*. I'm not a person that is fond of memorizing answers without understanding the reasoning behind it.

I keep having to force myself to not question the question and just go with the damned answer they want...which is completely against how I function.

 

[jesse]

Pressure drops..what happens to kollsman and which way do the needles turn?

Let me rethink that....

29.92--Pressure drops to 28.92. That will be a thousand foot change... Altimeter will indicate a thousand feet lower... So you're flying higher then it indicates.

Do I have that backwards?

 

[dmccormack]

You're right with that. I am completely incompatible with the FAA written tests. I don't really care what the answer is--I always want to know *why*. I'm not a person that is fond of memorizing answers without understanding the reasoning behind it.

I keep having to force myself to not question the question and just go with the damned answer they want...which is completely against how I function.

Well.. in most cases (85-90% depending on the question pool), you'll be able to reason through the logic.

There are always 3-5 questions on every FAA test I've taken where I thought they messed up. Upon further study I find out I was wrong on one or two, but there are always a couple...

 

[Everskyward]

I don't think one can figure true altitude based on indicated altitude corrected for non-standard pressure if non-standard temperature exists. Well I'm sure you could but I don't think I've ever seen anyone do it or talk about it.

There's a way to do it on an E6B (getting it out of the drawer where it's been for 8 years). I used to do it all the time when doing mapping because we had to be a certain altitude above the ground.

OK, here you go.

 

[jmaynard]

This seems to conflict with the question. The question states the aircraft will be lower then indicated.

Nope. The question is worded in a way that the sense is backward from that:

Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

The altimeter is what's lower, not the aircraft.

 

[flyingcheesehead]

Lets say your airplane is sitting on the beach at sea level. The pressure is standard (29.92). Your altimeter indicates 0 feet. Now the hurricane comes over you, the pressure continues to fall, and your altimeter is now indicating dramatically lower then you actually are..

IMHO--at that point--your altimeter is indicating a lower altitude then flown (or parked).

Read through this again.

Pressure goes down, indicated altitude goes which way?

 

[TMetzinger]

Indicated altitude goes up, true altitude goes down.

"From Hi to lo, look out below", which means your airplane will be LOWER than the altimeter indicates.

 

[Cap'n Jack]

Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

a) Atmospheric pressure lower than standard.
b) Air temperature warmer than standard
c) Air temperature lower than standard.

FAA Answer is: B

Let me rethink that....

29.92--Pressure drops to 28.92. That will be a thousand foot change... Altimeter will indicate a thousand feet lower... So you're flying higher then it indicates.

Do I have that backwards?

Let's talk about barometers and altimeters first
Jesse- The barometer in my house was last calibrated off the AWOS in Millville, NJ (50-60 feet above sea level). I moved to Nebraska- it reads one inch (almost exactly) lower. I am 1000' higher than in NJ. The numbers you described are exactly what I see. As the barometric pressure decreases, an altimeter will read higher, not lower. As the hurricane passes over, the pressure decreases so the altimeter will show a higher altitude. If you were flying a WC-130 into that hurricane without correcting the altimeter and trying to maintain altitude, the plane would become lower (closer to the ground)

Now lets talk about the effects of only temperature. Please bear with me as I lead you... As you heat air, it expands. Lets put this air in a glass tube that goes into space. As the pressure is not changing, the air only gets "taller"- so you have the same amount of air, but higher in the glass column. At ground level, you have the same pressure (same weight of air pressing down), but as you climb, the pressure gradient decreases at a slower rate than standard. On a cold day, you get 1/2 way up the air column at 18,000 feet*, where 1/2 the weight of air is above you. On a hot day, you reach the 1/2 point at 20,000'.

The glass column is just for understanding, but hot air does expand as I described above.


*I don't know what the 1/2 way point really is- just an example

 

[poadeleted20]

This doesn't get to be a significant issue until it gets really cold, say, Winnipeg in January. When that happens, the altimeter error due to temperature (even with the right QNH setting) can cause the ground to rise up and smite you before you reach MDA/DH on the altimeter. That's why the Canadians require their pilots to carry the cold weather altimeter correction card. For a full explanation, see http://bathursted.ccnb.nb.ca/vatcan/fir/moncton/WeeklyTopics/Archives/20040104/CurrentTopic.html

 

[AdamZ]

Jess I think they are trying to get you to realize that just because pressure is lower than standard does not mean that your altimeter has not been set corretly to the lower pressure. so there is no indication in the question that the altimeter was set at 29.92. But IIRC when the temp changes the Baro pressue does not necessiarlily change.

 

[121Dispatch]

Indicated altitude goes up, true altitude goes down.

"From Hi to lo, look out below", which means your airplane will be LOWER than the altimeter indicates.

Correct, and one of the answer choices had to do with temperature, so add "From hot to cold, look out below"

As stated by Ron, indicated altitude vs true altitude is affected by temperature as well.

 

[Areeda]

Let me rethink that....

29.92--Pressure drops to 28.92. That will be a thousand foot change... Altimeter will indicate a thousand feet lower... So you're flying higher then it indicates.

Do I have that backwards?

You've gotten a lot of good responses. I'm just rewording it hoping something clicks for you.

Yes you have it backwards.

Remember:

• These questions ask about "true" as compared to "indicated".
• If you decrease the barometer setting in the Kollsman window indicated altitude goes down.
• In your example if you have the altimeter set to 29.92 but the real setting is 28.92 then adjusting the altimeter down to the correct setting will decrease the indicated altitude. With me so far? Therefore the indicated with the correct setting is lower than with the incorrect setting. That's the way they are asking it.
• Temperature works the same way. As Ron points out in places that can be much colder than standard there are correction tables but in general we don't correct for them (especially in SoCal). If the temperature is colder than standard then just like if the pressure is lower than our setting, the altitude after correction will be lower than berfore the correction.

Joe

 

[skyflyer8]

What I don't understand yet--is why A isn't right too.

Because the question never said whether you have your altimeter set correctly (Kollsman window). If you have your altimeter set correctly, then you're good to go (barring nonstandard temperatures).

You are reading into the question that the pilot has failed to reset their altimeter. The question doesn't say that.

A lower than standard pressure setting, alone, has no effect unless you failed to reset the altimeter.

 

[jesse]

You are reading into the question that the pilot has failed to reset their altimeter. The question doesn't say that.

It doesn't say it isn't either

Actually the clue is this:

Atmospheric pressure lower than standard.

It says lower than standard which means you'd need a pressure in the question to be able to determine if A would be right. If it said "Atomposheric pressure decreases" then IMO A would without a doubt be correct.

 

[qbynewbie]

Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

a) Atmospheric pressure lower than standard.
b) Air temperature warmer than standard
c) Air temperature lower than standard.

FAA Answer is: B

You can disregard (a) if you remember the old "high to low, look out below" saying.

If you move to an area of lower pressure from an area of higher pressure without resetting your altimeter, you need to look out below. Why? Because the altimeter will indicate that you are higher than you really are, so you are in danger of running into that skyscraper. So lowering the pressure caused the altimeter to indicate a higher altitude than actually flown -- the reverse of what is listed as answer (a).

Then all you need to remember is that low pressure does the same thing as low temperature when it comes to altimeters. Since we know that (a) is wrong because it's backwards then (c) must also be wrong, because low pressure and low temperature both affect the altimeter in the same direction.

 

[gismo]

It doesn't say it isn't either

Actually the clue is this:

It says lower than standard which means you'd need a pressure in the question to be able to determine if A would be right. If it said "Atomposheric pressure decreases" then IMO A would without a doubt be correct.

Jesse, you have to assume that the barometric setting is correct to consider any of the choices, otherwise none of them make any sense. It was clear to me that "Atmospheric pressure lower than standard" simply means that the (correct) baro setting was less than 29.92 and that has no effect on the indicated altitude if the lower setting is dialed in. This leave B as the "most correct" answer.

For me the most difficult part of the question is sorting out that they are talking about the plane being higher than the indicated altitude.

 

[flyingcheesehead]

People, people, people.

The Kollsman window is COMPLETELY IRRELEVANT.

If atmospheric pressure goes down, indicated altitude goes UP, meaning that the altimeter would NOT "indicate a lower altitude than actually flown (true altitude)" even if the Kollsman window was not adjusted.

It's a really ****ty way to ask a question, though. FAA writtens suck.

 

[jesse]

People, people, people.

The Kollsman window is COMPLETELY IRRELEVANT.

If atmospheric pressure goes down, indicated altitude goes UP, meaning that the altimeter would NOT "indicate a lower altitude than actually flown (true altitude)" even if the Kollsman window was not adjusted.

It's a really ****ty way to ask a question, though. FAA writtens suck.

Ding..Ding..Ding

Just thought about it again and hell--I'm done thinking about this question. Time to sleep.

 

[dmccormack]

People, people, people.

The Kollsman window is COMPLETELY IRRELEVANT.

If atmospheric pressure goes down, indicated altitude goes UP, meaning that the altimeter would NOT "indicate a lower altitude than actually flown (true altitude)" even if the Kollsman window was not adjusted.

It's a really ****ty way to ask a question, though. FAA writtens suck.

OH boy...

It is most certainly is relevant as a way to think "What happens to the altimeter needles when I change the setting in the kollsman?" That's something all flying pilots do aplenty, and should be able to visualize.

If you can think through that, you can arrive at the correct solution to this problem.

 

[Cap'n Jack]

Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

a) Atmospheric pressure lower than standard.
b) Air temperature warmer than standard
c) Air temperature lower than standard.

FAA Answer is: B

I think, as a group, we are mixing temperature and pressure. I know this is how it works in the real world but the question (and desired answer) is referring to a single effect- temperature only. That is why I introduced the glass cylinder in post #20.

I think the FAA is showing that even if you set the altimeter correctly, you can still see errors. It usually doesn't affect us since on warm days, the error moves us away from the ground. We all see the same error so 5000' is the same for all of us.

Really cold days- different matter.

 

[kevin47881]

And this is just the first question.

 

[gismo]

People, people, people.

The Kollsman window is COMPLETELY IRRELEVANT.

I'm not sure what you mean by that. I do believe that the FAA assumed that the altimeter in question would be adjusted to the correct baro setting. If not then the altimeter could already be indicating lower (or higher for that matter) than actual even if the temp and pressure were "standard".

If atmospheric pressure goes down, indicated altitude goes UP, meaning that the altimeter would NOT "indicate a lower altitude than actually flown (true altitude)" even if the Kollsman window was not adjusted.

Answer (A) never mentions pressure going up (or down), just that it's "lower than standard". It could have been lower than standard for the last 20 days and actually rising at the time. All that is meant by "lower than standard" is that the air pressure at that altitude is less than the standard model predicts.

It's a really ****ty way to ask a question, though. FAA writtens suck.

On that we agree completely.

 

[flyingcheesehead]

OH boy...

It is most certainly is relevant as a way to think "What happens to the altimeter needles when I change the setting in the kollsman?" That's something all flying pilots do aplenty, and should be able to visualize.

OK, fine.

The Kollsman window is completely irrelevant TO THIS QUESTION. (Hence, no altimeter settings are given.)

 

[jesse]

The worst question on the test so far in my opinion:

The gyrosopic heading indicator is inoperative. What is the primary bank instrument in unaccelerated straight-and-level flight?

a) Magnetic compass
b) Miniature aircraft of turn coordinator
c) Attitude indicator

FAA Answer: A

Thanks--but no thanks FAA.

 

[dmccormack]

The worst question on the test so far in my opinion:


Thanks--but no thanks FAA.

The whole Primary and Supporting scheme is something you memorize for the test and then FORGET as soon as you have grade in hand.

 

[Sam D]

The worst question on the test so far in my opinion:


Thanks--but no thanks FAA.

I take the whole primary/supporting discussion as a way to drill into folks what the different instruments are telling them. For instance, you could have the little plane lined up perfectly in straight and level flight but be way off the altitude you're supposed to be at. It's a matter of what's important for any phase of flight and what instrument is best to read. Maybe if the FAA said "you're supposed to be in straight and level flight and your DG is dead, what instrument should you use to make sure you're not wandering all over the place?" Now replace "wandering all over the place" with "banking". The point being that small attitude changes are not what's "primarily" important in straight and level flight.

 

[jesse]

I take the whole primary/supporting discussion as a way to drill into folks what the different instruments are telling them. For instance, you could have the little plane lined up perfectly in straight and level flight but be way off the altitude you're supposed to be at. It's a matter of what's important for any phase of flight and what instrument is best to read. Maybe if the FAA said "you're supposed to be in straight and level flight and your DG is dead, what instrument should you use to make sure you're not wandering all over the place?" Now replace "wandering all over the place" with "banking". The point being that small attitude changes are not what's "primarily" important in straight and level flight.

In the event of the failure as they describe I'm going to be trusting the turn coordinator for bank and magnetic compass as a reference to verify heading. It is way too unstable to be using for primary bank reference when there is a working turn coordinator.

I also take the ground track from the GPS into consideration.

 

[Capt. Geoffrey Thorpe]

Question #1

I clearly am not understanding something.. Someone smart please correct me.

The altimiter measures pressure but displays feet. The conversion from pressure to feet is based on assumptions about how fast pressure changes with altitude and what the pressure is at some particular altitude.

When you set the barometric pressure on the altimeter, you are correcting for the day to day changes in barometric pressure.

But, what you are not correcting for is the day to day difference in how much the air pressure changes with altitude.

On a warm day, the air is less dense, the "atmosphere" expands upward, so you have to climb higher to put a given amount (as measured by a change in the pressure) below you. So, on a hot day, the pressure change per foot of altiude is lower than on a cold day.

On a hot day, the altimeter is going to read lower than your actual altitude. Answer B.

I think.