Why do airplanes fly at the height they fly at?

I think it is 30 000 feet. I think that is either 10 miles or 10 kilometers.

Is it because it's just above the cloud tops?

Is it because it's just above the cloud tops?

That's half the reason, they fly above most storms.

The other reason is the atmosphere is thinner and allows the engines to run at peak efficiency and use less fuel.

When looking at anything large companies do, go right for the money.  The answer is almost always "because it's cheaper."

@faded mike

As jja said, there are 2 main reasons.

1.  To avoid weather - ideally.  Much of the volatile weather takes place below the clouds.  You may notice from take off to the cloud layer (and especially going through it) can be a bit "bumpy".

2.  To take advantage of the jetstream / thinner air to spend less on fuel and avoid other traffic.

Is it even true - that airplanes actually fly at 30 000 feet? or theirabouts?

How do we know this is true - i have seen things that would lead to questioning the ability of a barometer to determine height above ground.

Yes Mikey it's true, when you are one, things below are very very tiny, and when you look up at one sometimes the only thing you see is the contrails (not chem) or a flash as the sunlight catches them.

FYI, I have flown over the Alps several times going to Italy, the alps are quite high and they looked impressive but a long way down.

Sigh.

@faded mike

Good questions - well worth demanding answers to.

I am not completely sure how the altitude is measured, but if it is purely by barometric pressure - it is not very precise.  I almost expect everything to be gps handled these days.

In any case, there are many methods available to us to try and calculate, measure, or otherwise ascertain the actual height of the airplane while "cruising".

Perhaps the most straightforward/economical is by taking avantage of the law of perspective.  Things that are twice as far away, appear half as large.  First you take a photo of a plane of known size, from a known distance away, and then you compare that to a photo you take of that plane in flight.  Alternatively, if you are going on the plane - you can take photos of known landmarks/geographical features and then again from altitude.

That process would, like the barometer, give you a crude estimate for the rough distance of the plane from the observer (ideally the plane would be right above your head when you took the second picture, so that distance would be purely the altitude)

I think whether it is 20k or 40k, the pilot just wants smooth sailing above the cloud level.  A perfectly precise altimeter is not needed to do that.

@faded mike

Good questions - well worth demanding answers to.

I am not completely sure how the altitude is measured, but if it is purely by barometric pressure - it is not very precise.  I almost expect everything to be gps handled these days.

In any case, there are many methods available to us to try and calculate, measure, or otherwise ascertain the actual height of the airplane while "cruising".

Perhaps the most straightforward/economical is by taking avantage of the law of perspective.  Things that are twice as far away, appear half as large.  First you take a photo of a plane of known size, from a known distance away, and then you compare that to a photo you take of that plane in flight.  Alternatively, if you are going on the plane - you can take photos of known landmarks/geographical features and then again from altitude.

That process would, like the barometer, give you a crude estimate for the rough distance of the plane from the observer (ideally the plane would be right above your head when you took the second picture, so that distance would be purely the altitude)

I think whether it is 20k or 40k, the pilot just wants smooth sailing above the cloud level.  A perfectly precise altimeter is not needed to do that.

Aside from GPS which is already mentioned, the most accurate method used in airliners is radar which gives results down to a few feet.  No guesswork needed for most planes which use a combination of methods to ensure no single failure will cause a crash.

Altitude via a picture does work well. Here is a photo of a plane coming in for a landing that I took.  It's altitude at this point is about 600m using calculations based on my cameras settings and the length of that model of Airbus A321.

Easy enough to check a plane in flight if you have a camera and good weather.

Hi folks,

Air traffic controller here.

Aircraft (well, their operators mainly) want to fly at the most efficient altitude. For modern high-bypass turbofan engined-airliners, that’s usually 30k to 40k feet. It will depend on weight, so a fully loaded 777 at max take off weight flying for 15hrs might only be able to initially climb to, say, 32k ft but a few hours later can then climb again to 34 or 36k ft.

We all still use the humble altimeter to determine what we call level (‘level’ in altimetery being a generic term, ‘altitude’, ‘flight level’ and ‘height’ all have specific meaning). So the altimeter is really just a barometer with the needle reading off feet, rather than hectopascals or inches of Mercury. The flight crew will set the altimeter with a pressure datum against which the altimeter will compare outside air pressure.

If the altimeter is set to what’s called QFE, the altimeter will read 0ft if the aircraft is on the ground, and if in the air will give the HEIGHT above ground level where the QFE reading was taken.

If the altimeter is set to QNH, the altimeter will read 0ft at mean sea level, and if in the air will give ALTITUDE above sea level.

So if a aircraft took off from, say, Miami airport at sea level, the QNH and QFE are the same, so would both read 0ft on an altimeter. Arriving at Denver airport, let’s say 5000ft above sea level, an aircraft has a HEIGHT of 0ft, but an ALTITUDE of 5000ft.

Now, as mentioned above in the thread, air pressure varies as one flies through the air, and over time even if one stays in the same location.....each nation will promulgate what’s called a ‘Transition Altitude’, below which aircraft fly on QNH or QFE. Above this Transition Altitude, they all set their altimeters to an arbitrary setting of 1013.2hPa (29.92in). This ensures that all aircraft are working off the same datum. This pressure setting gives us ‘flight level’, which you might have heard referenced by flight crew over the PA on a flight.

Flight Level 320 is nominally 32k ft, if the pressure at sea level is 1013.2hPa. It could actually be 30k ft, or even 33k ft depending on actual air pressure. So even though the in flight map displayed to the passenger keeps saying a constant 32k ft, an aircraft could go from 31k to 33k ft above seal level over the course of a few hours as the air pressure changes.

GNSS-derived altitude is not used at higher levels, but is becoming more common in encoding flight paths nearer airports.




Radar altimeters only work for measuring heights of 2500ft or less.

Now, as mentioned above in the thread, air pressure varies as one flies through the air, and over time even if one stays in the same location.....each nation will promulgate what’s called a ‘Transition Altitude’, below which aircraft fly on QNH or QFE. Above this Transition Altitude, they all set their altimeters to an arbitrary setting of 1013.2hPa (29.92in). This ensures that all aircraft are working off the same datum. This pressure setting gives us ‘flight level’, which you might have heard referenced by flight crew over the PA on a flight.

Oh that's fascinating, I did not know this.

So the height that the airlines fly at varies with air pressure changes but at that altitude the only thing that matters is one planes relation to another, so they all drift up and down together.

Very cool.

Quote from: Gonzo230 on January 19, 2021, 10:59:11 AM

Now, as mentioned above in the thread, air pressure varies as one flies through the air, and over time even if one stays in the same location.....each nation will promulgate what’s called a ‘Transition Altitude’, below which aircraft fly on QNH or QFE. Above this Transition Altitude, they all set their altimeters to an arbitrary setting of 1013.2hPa (29.92in). This ensures that all aircraft are working off the same datum. This pressure setting gives us ‘flight level’, which you might have heard referenced by flight crew over the PA on a flight.

Oh that's fascinating, I did not know this.

So the height that the airlines fly at varies with air pressure changes but at that altitude the only thing that matters is one planes relation to another, so they all drift up and down together.

Very cool.

That’s exactly right. Above terrain the only important thing is that aircraft avoid each other.

Should have added that the Transition Altitude is usually around the same altitude as the highest terrain in each country. So in the USA it’s 18k ft, in the UK it’s 3k ft.

Hi folks,

Air traffic controller here.

Aircraft (well, their operators mainly) want to fly at the most efficient altitude. For modern high-bypass turbofan engined-airliners, that’s usually 30k to 40k feet. It will depend on weight, so a fully loaded 777 at max take off weight flying for 15hrs might only be able to initially climb to, say, 32k ft but a few hours later can then climb again to 34 or 36k ft.

We all still use the humble altimeter to determine what we call level (‘level’ in altimetery being a generic term, ‘altitude’, ‘flight level’ and ‘height’ all have specific meaning). So the altimeter is really just a barometer with the needle reading off feet, rather than hectopascals or inches of Mercury. The flight crew will set the altimeter with a pressure datum against which the altimeter will compare outside air pressure.

If the altimeter is set to what’s called QFE, the altimeter will read 0ft if the aircraft is on the ground, and if in the air will give the HEIGHT above ground level where the QFE reading was taken.

If the altimeter is set to QNH, the altimeter will read 0ft at mean sea level, and if in the air will give ALTITUDE above sea level.

So if a aircraft took off from, say, Miami airport at sea level, the QNH and QFE are the same, so would both read 0ft on an altimeter. Arriving at Denver airport, let’s say 5000ft above sea level, an aircraft has a HEIGHT of 0ft, but an ALTITUDE of 5000ft.

Now, as mentioned above in the thread, air pressure varies as one flies through the air, and over time even if one stays in the same location.....each nation will promulgate what’s called a ‘Transition Altitude’, below which aircraft fly on QNH or QFE. Above this Transition Altitude, they all set their altimeters to an arbitrary setting of 1013.2hPa (29.92in). This ensures that all aircraft are working off the same datum. This pressure setting gives us ‘flight level’, which you might have heard referenced by flight crew over the PA on a flight.

Flight Level 320 is nominally 32k ft, if the pressure at sea level is 1013.2hPa. It could actually be 30k ft, or even 33k ft depending on actual air pressure. So even though the in flight map displayed to the passenger keeps saying a constant 32k ft, an aircraft could go from 31k to 33k ft above seal level over the course of a few hours as the air pressure changes.

GNSS-derived altitude is not used at higher levels, but is becoming more common in encoding flight paths nearer airports.




Radar altimeters only work for measuring heights of 2500ft or less.

Good stuff Gonzo, saved me some typing. My first visit here for some time.

Where do you work? I'm a 777 "driver". Not working much these days for obvious reasons.

TA at most major UK airports is 6000 ft but there is a move to make it 18K like the US. Low TAs can add to the workload on approach as it generally coincides with a Localiser and GP capture at circa 10nm.....in China and Russia when you add in a combination of QFE and metric ops, after a long night flight, in can be a complete brainf@£k!!

Quote from: Gonzo230 on January 19, 2021, 10:59:11 AM

Hi folks,

Air traffic controller here.

Aircraft (well, their operators mainly) want to fly at the most efficient altitude. For modern high-bypass turbofan engined-airliners, that’s usually 30k to 40k feet. It will depend on weight, so a fully loaded 777 at max take off weight flying for 15hrs might only be able to initially climb to, say, 32k ft but a few hours later can then climb again to 34 or 36k ft.

We all still use the humble altimeter to determine what we call level (‘level’ in altimetery being a generic term, ‘altitude’, ‘flight level’ and ‘height’ all have specific meaning). So the altimeter is really just a barometer with the needle reading off feet, rather than hectopascals or inches of Mercury. The flight crew will set the altimeter with a pressure datum against which the altimeter will compare outside air pressure.

If the altimeter is set to what’s called QFE, the altimeter will read 0ft if the aircraft is on the ground, and if in the air will give the HEIGHT above ground level where the QFE reading was taken.

If the altimeter is set to QNH, the altimeter will read 0ft at mean sea level, and if in the air will give ALTITUDE above sea level.

So if a aircraft took off from, say, Miami airport at sea level, the QNH and QFE are the same, so would both read 0ft on an altimeter. Arriving at Denver airport, let’s say 5000ft above sea level, an aircraft has a HEIGHT of 0ft, but an ALTITUDE of 5000ft.

Now, as mentioned above in the thread, air pressure varies as one flies through the air, and over time even if one stays in the same location.....each nation will promulgate what’s called a ‘Transition Altitude’, below which aircraft fly on QNH or QFE. Above this Transition Altitude, they all set their altimeters to an arbitrary setting of 1013.2hPa (29.92in). This ensures that all aircraft are working off the same datum. This pressure setting gives us ‘flight level’, which you might have heard referenced by flight crew over the PA on a flight.

Flight Level 320 is nominally 32k ft, if the pressure at sea level is 1013.2hPa. It could actually be 30k ft, or even 33k ft depending on actual air pressure. So even though the in flight map displayed to the passenger keeps saying a constant 32k ft, an aircraft could go from 31k to 33k ft above seal level over the course of a few hours as the air pressure changes.

GNSS-derived altitude is not used at higher levels, but is becoming more common in encoding flight paths nearer airports.




Radar altimeters only work for measuring heights of 2500ft or less.

Good stuff Gonzo, saved me some typing. My first visit here for some time.

Where do you work? I'm a 777 "driver". Not working much these days for obvious reasons.

TA at most major UK airports is 6000 ft but there is a move to make it 18K like the US. Low TAs can add to the workload on approach as it generally coincides with a Localiser and GP capture at circa 10nm.....in China and Russia when you add in a combination of QFE and metric ops, after a long night flight, in can be a complete brainf@£k!!

Yes, that’s true. I was thinking outside controlled airspace as I was just talking to a friend who flies a Pitts.

6000ft in most CAS area in the UK.

I’m at Heathrow. I’ve been in ATC for 22 years, and I think we’ve been working towards a TA of 18k ft for 21 of them!!!!

Putting aside this obsession that some people have here of feeling the need to question (i.e. doubt) pretty much everything that you are 'told' let's just think about it in this way.  Firstly I presume you have never stood next to a Boeing 747 or Airbus A380 so you have no idea how big they actually are?

Now have you ever noticed a trend in life where objects tend to look smaller the further away you are from them?  Now apply this to a Boeing 747 or Airbus A380 passing overhead at cruising altitude.  The clue is choose the ones which are contrailing as they pass over.  Now have you noticed how small they look?  You can cover a 747 or an A380 with the tip of your little finger.  So there's a direct visual clue there about how high the aircraft is flying.

I often plane watch with a small telescope and a laptop running flightradar24.com.  That is a website which shows you real air traffic in real time.  I can see planes approaching my location from hundreds of miles away.  I can click on one and then watch the plane directly through my telescope. The information displayed for each plane includes (along with a lot more) altitude, speed, origin, destination etc.  This information is obtained directly from the planes transponders.

When I flew back from NY to Gatwick a couple of years back the screen on the seat in front of me told me we were flying at 40,000ft.  And trust me looking out of the window I could well believe that. 

Is that evidence enough for you that airliners really do fly at 30,000ft plus?

I think whether it is 20k or 40k, the pilot just wants smooth sailing above the cloud level.  A perfectly precise altimeter is not needed to do that.

Our resident aero-experts have basically covered the accuracy and necessity of knowing ones altitude. But to say a pilot just wants smooth sailing and not accuracy of their whereabouts is lunacy. That would make sense if there was only one plane on a route through the sky at one time. But that's obviously not reality. This is reality:



To think that ATC, pilots, etc don't need to have accuracy about the altitude of a plane is, like I said, lunacy.

The more speed the higher. Because the underneath air hit the plane's body upwards. The more speed, the bigger such air mass will hit the plane upwards.

This is Downwards Universal Deceleration mechanism. (Kind of UA)

There's a wonderful stack exchange article that explains this in detail, but I'll also sum it up right here.

https://aviation.stackexchange.com/questions/1609/why-do-jet-engines-get-better-fuel-efficiency-at-high-altitudes

Thrust is the difference between the entry impulse of the air entering the engine and then the exit impulse of the heated fuel and air mixture leaving the engine. Impulse is mass times velocity, and expressed with a mass flow ṁ, and thrust T is

T=ṁ⋅(v_exit−v_entry)

The exit impulse is increased by accelerating the airflow through the engine, and the acceleration is achieved by heating up the air more.

Every gram of fuel heats up a given mass of air by a certain number of centigrades. The definition of the energy content of fuels is given as the capacity to heat a pound of water by one degree Fahrenheit.

Thermal efficiency is the ratio between the mechanical work that is extracted as thrust in addition to the heat energy spent on heating the air, and it is indirectly affected by the flight altitude.

(see the Wikipedia page on the Carnot cycle https://en.wikipedia.org/wiki/Carnot_cycle)

This cycle (and also others) describe the workings of combustion engines in thermodynamic terms.

It says that the efficiency of a combustion engine cannot be greater than the temperature ratio between the temperature increase from ambient (t_amb) to the maximum temperature t_max of the process, divided by the maximum temperature. All temperatures must be expressed as a total temperature, where 0° means 0 K or -273.15°C. Engines operating in colder air makes the ratio bigger and will improve efficiency.

Here's another equation:

ηt=t_max−t_amb/tmax

This should sum it up for the most part.

Please correct me if I did anything wrong; I'm no master in aviation.

@stash

But to say a pilot just wants smooth sailing and not accuracy of their whereabouts is lunacy.

I was speaking specifically in regards to maintaining elevation precisely.  I was attempting to explain why the loosely accurate barometer was acceptable to pilots as their altitude reading - when it is, actually, not.

Wether they are 20k or 40k doesn't really matter; it's the smooth/efficient sailing they are after.  The altimeter could be wrong (and likely is) by a significant margin and it wouldn't much matter (of course air traffic control is still involved, and you must negotiate other traffic - but this is tangential to what I was saying).

The more speed the higher. Because the underneath air hit the plane's body upwards. The more speed, the bigger such air mass will hit the plane upwards.

This is Downwards Universal Deceleration mechanism. (Kind of UA)

As usual Danang, you're talking complete bollocks.

@stash

Quote

But to say a pilot just wants smooth sailing and not accuracy of their whereabouts is lunacy.

I was speaking specifically in regards to maintaining elevation precisely.  I was attempting to explain why the loosely accurate barometer was acceptable to pilots as their altitude reading - when it is, actually, not.

Wether they are 20k or 40k doesn't really matter; it's the smooth/efficient sailing they are after.  The altimeter could be wrong (and likely is) by a significant margin and it wouldn't much matter (of course air traffic control is still involved, and you must negotiate other traffic - but this is tangential to what I was saying).

Define "significant".

@stash

Quote

But to say a pilot just wants smooth sailing and not accuracy of their whereabouts is lunacy.

I was speaking specifically in regards to maintaining elevation precisely.  I was attempting to explain why the loosely accurate barometer was acceptable to pilots as their altitude reading - when it is, actually, not.

Wether they are 20k or 40k doesn't really matter; it's the smooth/efficient sailing they are after.  The altimeter could be wrong (and likely is) by a significant margin and it wouldn't much matter (of course air traffic control is still involved, and you must negotiate other traffic - but this is tangential to what I was saying).

Im not really sure what you mean by ‘the altimeter could be wrong (and likely is) by a significant margin’.

At high level all aircraft are using the same pressure setting (1013.2hPa/29.92in)

At lower levels, ATC pass updated pressure settings regularly, and when it changes, so that all aircraft in the same area are on the same setting.

Again, an altimeter is very accurate, but it only measure the difference in level from whatever datum you plug into it.

@stash

Variable, somewhere beteen 10's and 1000's of feet off (likely tending towards the latter).  This is an off the cuff estimate.

@gonzo230

At high level all aircraft are using the same pressure setting (1013.2hPa/29.92in)

Interesting! In fact, all the details you and the other person in your industry have mentioned have been - thanks!

Again, an altimeter is very accurate, but it only measure the difference in level from whatever datum you plug into it.

Intuitively, I suspect this cannot be the case - which is what I was saying.

Any altitude that is inferred from barometric pressure will be an estimate with significant margin of error.  Barometers do not measure altitude, and barometric pressure is not static nor uniform across any part of the worlds surface.

At lower levels, ATC pass updated pressure settings regularly, and when it changes, so that all aircraft in the same area are on the same setting.

Interesting! I suspected there may be updates like that, especially if conditions change rapidly.  At the same time, because the altimeter (barometer, in point of fact) will adjust with the local conditions, we can be relatively assured that all planes in the same pressure system/variance will rise and fall, relatively with another, in actual elevation (maintaining cruising pressure as measured on the barometer/altimeter) - suggesting that as long as that ceiling is sufficiently high - you probably can get away with not updating most of the time.

@stash

Variable, somewhere beteen 10's and 1000's of feet off (likely tending towards the latter).  This is an off the cuff estimate.

Like I demonstrated, thousands of planes are inflight all over the world at any given time. You're making it seem like known altitudes of each are almost haphazard. Do you really think that is the case?

@jack44556677

In aviation there are strict definitions for the terms "height", "altitude", and "pressure altitude" (referenced to "standard" 1013.2 hpa). Do a bit of googling!! A pressure change of 1 hpa (mb) equates to approx 30ft. Also google "altimeter temperature errors" and "international standard atmosphere (isa)". If the air beneath the aircraft is colder than standard, the aircraft will be lower than indicated (and vice versa). A barometric altimeter is effectively measuring the weight of air beneath the aircraft to derive a reading.

As I'm sure Gonzo will confirm, on really really cold days aircraft on approach will be vectored in at higher platform altitudes to maintain ground and obstacle clearance. Altimeter error is approx 4% for every 10 degrees above or below isa. If it's cold, the air beneath the aircraft is denser. If flying a non-precision approach (more google research for you) on cold days we make corrections to the final approach altitude profile to adjust for this error.

Barometric altimeters are very accurate, you just have to know what they are measuring!


@Gonzo

I guess we've spoken to each other many times!!

@faded mike

Good questions - well worth demanding answers to.

I am not completely sure how the altitude is measured, but if it is purely by barometric pressure - it is not very precise.  I almost expect everything to be gps handled these days.

In any case, there are many methods available to us to try and calculate, measure, or otherwise ascertain the actual height of the airplane while "cruising".

Perhaps the most straightforward/economical is by taking avantage of the law of perspective.  Things that are twice as far away, appear half as large.  First you take a photo of a plane of known size, from a known distance away, and then you compare that to a photo you take of that plane in flight.  Alternatively, if you are going on the plane - you can take photos of known landmarks/geographical features and then again from altitude.

That process would, like the barometer, give you a crude estimate for the rough distance of the plane from the observer (ideally the plane would be right above your head when you took the second picture, so that distance would be purely the altitude)

I think whether it is 20k or 40k, the pilot just wants smooth sailing above the cloud level.  A perfectly precise altimeter is not needed to do that.

Thank you.

Hi folks,

Air traffic controller here.

Aircraft (well, their operators mainly) want to fly at the most efficient altitude. For modern high-bypass turbofan engined-airliners, that’s usually 30k to 40k feet. It will depend on weight, so a fully loaded 777 at max take off weight flying for 15hrs might only be able to initially climb to, say, 32k ft but a few hours later can then climb again to 34 or 36k ft.

We all still use the humble altimeter to determine what we call level (‘level’ in altimetery being a generic term, ‘altitude’, ‘flight level’ and ‘height’ all have specific meaning). So the altimeter is really just a barometer with the needle reading off feet, rather than hectopascals or inches of Mercury. The flight crew will set the altimeter with a pressure datum against which the altimeter will compare outside air pressure.

If the altimeter is set to what’s called QFE, the altimeter will read 0ft if the aircraft is on the ground, and if in the air will give the HEIGHT above ground level where the QFE reading was taken.

If the altimeter is set to QNH, the altimeter will read 0ft at mean sea level, and if in the air will give ALTITUDE above sea level.

So if a aircraft took off from, say, Miami airport at sea level, the QNH and QFE are the same, so would both read 0ft on an altimeter. Arriving at Denver airport, let’s say 5000ft above sea level, an aircraft has a HEIGHT of 0ft, but an ALTITUDE of 5000ft.

Now, as mentioned above in the thread, air pressure varies as one flies through the air, and over time even if one stays in the same location.....each nation will promulgate what’s called a ‘Transition Altitude’, below which aircraft fly on QNH or QFE. Above this Transition Altitude, they all set their altimeters to an arbitrary setting of 1013.2hPa (29.92in). This ensures that all aircraft are working off the same datum. This pressure setting gives us ‘flight level’, which you might have heard referenced by flight crew over the PA on a flight.

Flight Level 320 is nominally 32k ft, if the pressure at sea level is 1013.2hPa. It could actually be 30k ft, or even 33k ft depending on actual air pressure. So even though the in flight map displayed to the passenger keeps saying a constant 32k ft, an aircraft could go from 31k to 33k ft above seal level over the course of a few hours as the air pressure changes.

GNSS-derived altitude is not used at higher levels, but is becoming more common in encoding flight paths nearer airports.




Radar altimeters only work for measuring heights of 2500ft or less.

Sorry, "Gonzo" ?....does your name mean dragon something or other?

@faded mike,

Sorry?

It’s a nickname I had at ATC college, referencing the muppet, and if you do a search on various aviation fora you’ll find me.

No idea about any dragons, sorry.

Quote

Again, an altimeter is very accurate, but it only measure the difference in level from whatever datum you plug into it.

Intuitively, I suspect this cannot be the case - which is what I was saying.

Luckily air traffic controllers, pilots and design engineers rely on science, not your intuition and suspicions.

Let us know when you do some experiments to prove your vague feelings on the subject.

@zaphod

It is true, I didn't do any googling on this at all.  I just applied my knowledge of science, in this case the dynamic variance of pressure on earth, to make an off the cuff assessment.

Even with carefully measured reference standards, calibration before takeoff, and updates in flight from weather data - a barometer cannot measure altitude.  You can infer, or calculate, from it - but you will always be incorrect to some (likely significant) degree.

I suppose it is concievable that significant pressure/temperature variance at the upper echelons is more infrequent and that that means the altimeter is usually "pretty" accurate - but this misses the whole point I was making (albeit a minor one).

The reason the altimeter is / can be a barometer in this case, is because the relative spacing of the planes and travel above the weather / in the jetstream is what matters.  A relative "sense" of your altitude is all that is required, especially when everyone else is using the same measurement devices and "pressure altitude" INSTEAD of altitude.

Barometric altimeters are very accurate, you just have to know what they are measuring!

Exactly.  They measure atmospheric pressure and calculate the estimated altitude based on reference charts/standards.

@JJA

You seemed to have missed my point.  Experiments shouldn't be required to establish that a barometer doesn't measure altitude...