# Density altitude (aviation)



## j-p-c

Greetings all, I recently joined this, my first language forum, and would like your help to settle a point that has long been vexing me.

In aviation there are different ways to define altitude for different purposes. One of them is called "density altitude", and is deduced from the atmospheric density.
Obviously this density decreases with an increase in altitude, and vice-versa. So, "high density" equals "low altitude" and vice-versa, all else remaining equal.

When I come across the sentence "The aircraft was being operated at a high density altitude", is there a rule in the English language that tells me wether "high" relates to "density" or rather to "altitude"?  The two meanings are mutual opposites in this case, and equally possible.
In any other example I can think of, for instance "a heavy machinery operator", context or common sense can tip the meaning either way, and I find this weirdly vague.

Thanks for your thoughts.


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## PaulQ

j-p-c said:


> is there a rule in the English language that tells me whether "high" relates to "density" or rather to "altitude"?


No, and it isn't needed. You should see "high density" as a single semantic unit. In this, it is the same as any other single word - it is a piece of vocabulary that you need to know if you are involved in flying aircraft.

You will now ask _"Isn't this confusing?",_ and the answer is _"No, because *English depends to a huge extent on context* and, in the context, _"The aircraft was being operated at a high density altitude"_ is perfectly understandable and has no ambiguity at all."_

Another way of looking at the phrase is that "if you are not a pilot, etc., then you need not know about such things, and if you are, you should."


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## lingobingo

As far as I can see (from a bit of googling) high density altitude — meaning *density altitude that’s high* and often written as _“high” density altitude_ to make that clear — is a standard term but high-density altitude is not.


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## j-p-c

"Need not know" is strictly CIA stuff, isn't it ?  : )

I'm ok with dependance on the context as a rule if you inform me so, thank you.

Isn't dependance on context the very meaning of ambiguity ?  

If, as you say, I should see "high density" as a "single semantic unit", then by your own lights "high density altitude" means "low altitude", where high density is found, right ?
Not confusing at all !

Wether I am a licenced pilot is neither here nor there, you decide.  : )


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## j-p-c

User With No Name said:


> It seems to me that whether you parse it as "at an altitude where the density is high," or "at a density altitude that is high," the aircraft is still close to the ground. Which one is more standard would depend on specialized usage in aviation, and lingobingo suggests it is probably the latter. But either way, I think the plane winds up in the same place.
> 
> Or am I confused?



As am I, by the words not the actual situation.

High density is found at low altitudes & vice-versa.


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## User With No Name

j-p-c said:


> As am I, by the words not the actual situation.


It is confusing. But either way, you do agree that the plane is close to the ground, right?

One other thing: If the "high density," used as an adjective, is a single semantic unit as PaulQ suggests, I think a careful writer would write it "high-density altitude." In theory, that should resolve any ambiguity. Of course, in practice, that means the reader is depending on one tiny little punctuation mark and a rule that writers may or may not always follow.


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## j-p-c

User With No Name said:


> It is confusing. But either way, you do agree that the plane is close to the ground, right?
> 
> One other thing: If the "high density," used as an adjective, is a single semantic unit as PaulQ suggests, I think a careful writer would write it "high-density altitude." In theory, that should resolve any ambiguity. Of course, in practice, that means the reader is depending on one tiny little punctuation mark and a rule that writers may or may not always follow.



If "high" refers to the density, the aircraft is low.
If "high" refers to the altitude, the aircraft is obviously high.  : )


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## j-p-c

User With No Name said:


> Ah, then I misunderstood. I really don't think you can read it that way. I think you can read (maybe) as "an altitude where the density is high" or as "a density altitude that is high." But not, as far as I can see, "at a high altitude."



Sorry, read what as "an altitude where the density is high" or as "a density that is high" ?

An altitude where the density is high is a low alitude.

: )  No progress so far, and punctuation is never found in usage !


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## Andygc

j-p-c said:


> When I come across the sentence "The aircraft was being operated at a high density altitude",


Do you have an example of "coming across" it? Surely there is surrounding context which makes the intended meaning clear?


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## j-p-c

Andygc said:


> Do you have an example of "coming across" it? Surely there is surrounding context which makes the intended meaning clear?



It's a very common occurence, and I was hoping for a language rule that overrides context.


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## j-p-c

Is there a name for this type of words association ?
It's neither compound, nor portemanteau, nor ...others ?


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## aceofpace

As far as I'm concerned, there is a separate term for "density altitude" that is different from just density or altitude so "high density altitude" is just that: density altitude that is high.


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## j-p-c

aceofpace said:


> density altitude that is high.



Which of the two.....?  Density or altitude ?  : )  The alternates are opposites.


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## Andygc

You can't have a rule with a phrase which can be parsed in two valid ways - {high-density} altitude or high {density altitude}. However, given that "density altitude" has a defined meaning in aviation, reading the phrase as {high-density} altitude would seem ludicrous. If you have a high density altitude you must be well above sea level.

PS ... and have reduced thrust and reduced lift compared with what you would expect at the same pressure altitude at standard temperature.


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## j-p-c

Andygc said:


> You can't have a rule with a phrase which can be parsed in two valid ways




I can live with that.  But all three pairs have defined meanings in aviation, none are ludicrous.

P.S.: Thrust can be reduced, lift results from inputs.  : )


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## manfy

Yes, that ambiguity is based on a flaw in the English language! 
Sincere apologies to all native speakers, but the idea that a compound noun can be either truly compounded as in _toothpaste_, or hyphenated as in _dry-cleaning_, or actually separated and yet figuratively compounded as in _full moon_ doesn't really make much linguistic sense to me as a native German speaker (in German we really do compound nouns when they are called compounded, and the amount of compounded words is theoretically unlimited, as long as the final compound noun makes sense ).

And yet, I still have to agree with Paul and his post #2: it is not really a huge problem in English and it usually doesn't cause serious ambiguities.

The term "density altitude" in isolation can only be interpreted as a compound noun -- whether the meaning is known or not.
"High density altitude" will normally be interpreted as 'high-density altitude' because of the left to right reading order of western languages.
Those few that are familiar with the set compound noun 'density altitude' might stop and think about the meaning of {high density} altitude vs. high {density altitude} for a millisecond or so, but will quickly settle on the intended meaning based on context.


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## PaulQ

j-p-c said:


> I was hoping for a language rule that overrides context.


See my signature. You are putting far too much faith in "rules" and nowhere near enough faith in context.


PaulQ said:


> *English depends to a huge extent on context*


*"Rules" never override context.* <- that's not a "rule", it is guidance...


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## j-p-c

Thank you manfy for this most interesting among interesting posts.

Do I understand correctly that in German the last element of a compound noun takes precedence for its ultimate meaning as a rule, eliminating any ambiguity, no millisecond context considerations required ?   Very German-like no-nonsense !

I never said it was a huge problem in English, just a vexing isolated occurence, and so it will remain for me in my favorite language, English !

Thank you all.


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## j-p-c

PaulQ said:


> See my signature. You are putting far too much faith in "rules" and nowhere near enough faith in context.
> *"Rules" never override context.* <- that's not a "rule", it is guidance...




PaulQ, I hear you, my favorite quote, "Rules are made to be obeyed by fools and for the guidance of the wise", don't know who said it.

There's  no deep disagreement here.

Thanks for your input.


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## PaulQ

j-p-c said:


> "Rules are made to be obeyed by fools and for the guidance of the wise1"


 Ah! that is what my signature should have been!

1attributed to Harry Day, the Royal Flying Corps First World War fighter ace.


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## j-p-c

Germany, one point.  : )


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## dojibear

Grammar rules do not tell you what a "high density altitude" is. It is meaning, not grammar, that decides this.

"Altitude" is measured in feet -- feet above sea level, or feet above the local land. Quite literally, an "altitude" is a "height".

So it would make no sense to apply the adjective "high density" to "altitude". Altitudes do not have a "density". It would be as silly as saying a "bright red altitude".

But it makes perfect sense to apply "high" to "density altitude", since altitudes are heights.


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## Delvo

j-p-c said:


> When I come across the sentence "The aircraft was being operated at a high density altitude"...





Andygc said:


> Do you have an example of "coming across" it?





j-p-c said:


> It's a very common occurence


Do you mean the general structure (adjective noun noun) is common, or the specific example about altitude is? I've never seen the specific phrase you asked about, and I've read more about flight and aircraft than most people do. It looks like a bad translation from another language. Normally, descriptions of altitude ignore how it was measured and just call it "altitude", and when the data source really needs to be specified, although "density altitude" is technically not wrong, "pressure altitude" or "altimeter reading" (standard altimeters are always pressure-based unless specified otherwise) would be more natural and direct.


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## j-p-c

"Pressure altitude" begs the exact same semantic/grammatical question, answered by manfy's post as far as I'm concerned.


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## Andygc

There is no semantic problem with "pressure altitude", just as there should not be with "density altitude". Both terms are defined and the words are not separable in an aviation context. "Pressure altitude" is what you read on a conventional altimeter, "radar altitude" is what you see on a RADALT, and "density altitude" is derived from a graph.


manfy said:


> "High density altitude" will normally be interpreted as 'high-density altitude' because of the left to right reading order of western languages.


No, it won't, simply because it is a term that is extremely unlikely to be used outside an aviation environment. And more generally, that left to right order does not apply routinely to English; it's no more difficult than an "old shopping trolley", a "plastic golf tee", or a "wooden umbrella stand". We don't put "old shopping" in a trolley, play a game called "plastic golf", or use "wooden umbrellas".

And we have yet to see a sentence in which "high density altitude" means "low altitude".


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## j-p-c

Andygc said:


> "plastic golf", or use "wooden umbrellas"



Then a "heavy machinery operator" is obviously a fat guy wielding a Dremel.  : )


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## Uncle Jack

I don't think I'd call a Dremel 'machinery', but he could be driving an excavator.


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## Andygc

j-p-c said:


> Then a "heavy machinery operator" is obviously a fat guy wielding a Dremel.  : )


 No. The whole point is that in English context determines meaning. There is no left to right rule.
A heavy machinery operator is not necessarily personally heavy. A fat machine operator would be, unless he worked in a fat-processing factory.


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## j-p-c

Time to call the SPCDH, Society for the Prevention of Cruelty to Dead Horses !


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## dojibear

j-p-c said:


> Time to call the SPCDH, Society for the Prevention of Cruelty to Dead Horses !



An explanation of this joke, for non-native-speakers:

There is an old saying in English about "beating a dead horse". People beat a live horse when it is stubborn, to make it move. This is useless if the horse is dead: it will not move. 

So any time people continue talking about something, long after they should, it is called "beating a dead horse".

j-p-c is suggesting that we are no longer adding information to this thread: we are just repeating ideas, which is useless.


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## PaulQ

Delvo said:


> I've never seen the specific phrase you asked about, and I've read more about flight and aircraft than most people do. It looks like a bad translation from another language.


There are 4 pages of articles on "density altitude" when a search with Google is made: *Click.* Bearing in mind that it is not a common subject, this should tell you that subjective experience is often unreliable, and there is no bad translation.


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## Delvo

PaulQ said:


> There are 4 pages of articles on "density altitude" when a search with Google is made: *Click.* Bearing in mind that it is not a common subject, this should tell you that subjective experience is often unreliable, and there is no bad translation.


That's two of the three words in the original question, and not in the kind of sentence we were given. When people talk/write about a plane flying at a high altitude, they just talk about a plane flying at a "high altitude". The only times when different measurements of altitude need to be specifed would be in sentences comparing them with each other, not just as a way of stating that a plane was flying high.


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## j-p-c

Delvo said:


> That's two of the three words in the original question, and not in the kind of sentence we were given. When people talk/write about a plane flying at a high altitude, they just talk about a plane flying at a "high altitude". The only times when different measurements of altitude need to be specifed would be in sentences comparing them with each other, not just as a way of stating that a plane was flying high.



NTSB accident reports are full of instances of aircraft flying "at high density altitude".

N.B.: I don't wish to discuss it anymore, just trying to stop my inbox pinging.


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## manfy

Sorry for this last ping, but you don't need to feel compelled to answer. Beside answering your specific questions, the threads in this forum are also intended to help other readers with the same or a similar question, and they might find the additional posts useful.



j-p-c said:


> Do I understand correctly that in German the last element of a compound noun takes precedence for its ultimate meaning as a rule, eliminating any ambiguity, no millisecond context considerations required ?


Yes, that's right. The last noun in a compound noun is the word stem. The same is true in English, so a _car key_ is a type of key not a type of car. The fact that separate words can create a single compound noun in English is the actual problem in some cases. In your case with "high density altitude" the reader must be familiar with this aviation/metrology/engineering term density altitude or else they might misinterpret the phrase.

And as proof for this potential ambiguity and the resulting confusion, I found several published articles where even native professionals fell victim to it:
1) an article on density altitude published in a magazine for pilots:


> _Density altitude. And if you fly without paying it due attention, you may find yourself staring down the end of a runway without hope of stopping or taking off. Even if you do make it in the air, high-density altitudes can cause you to quickly meet up with terrain that has a gradient superior to your ascent._


"high-density altitudes" is clearly wrong. Only high _density altitudes_ (i.e. thin air) would cause this problem with the ascent.

2) Course by K-State University on mountain flying:


> _Aircraft operations in mountainous areas may differ greatly from a student’s training. The experience of reduced aircraft performance caused by high-density altitudes can be a great training into operations with aircraft exhibiting marginal performance._



Again, clearly wrong. It should read "high _density altitudes_".
....one would think that they should know what they're talking about _and_ pay careful attention on how they advertise their product, don't you think? 

Anyways, I proved my point that ambiguity exists and readers should be cautious when interpreting that term -- even when that phrase comes from native aviaton professionals!


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## j-p-c

manfy, thank you for this contibution, you did the hard work I wouldn't, and are the more valuable forum member for it.

As for me, this being a language forum, I didn't want to digress and slip into an entirely different domain. I was lazy too.

Of course those familiar with aviation know that "a high density altitude" is "an alitude at which low densities are found". Not least because it's a perilous situation, can occur close to the ground, and because pilots are repeatedly warned about it.

But the language doesn't work as it should, and your exerpts demonstrate that semantic confusion creeps even into educational litterature.
As an ex flight instructor in English, but not a native speaker, it bothered me and I wanted the language aspect explained to me. Which you did.

Thanks very much indeed for your input.


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## j-p-c

Sorry to go back on my word, but I may have found a solution based on an English language concept not mentioned before.
I found it here on this forum: SS > factoids > adjective order in English, where it's called a "social concept".

If the aviation community is seen as a social group then, although "high density" and "density altitude" are both equally valid concepts, "density altitude" is much more exclusive to that group and should dominate when aviation is the subject at hand.

It removes all ambiguity and is so simple !  : )


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## PaulQ

It's a pity that you did not notice this earlier: 


PaulQ said:


> You should see "high density" as a single semantic unit. In this, it is the same as any other single word - it is a piece of vocabulary that you need to know if you are involved in flying aircraft.


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## j-p-c

PaulQ said:


> It's a pity that you did not notice this earlier:



Sorry but no, PaulQ, you have it backwards, no offence intended.

The valid "single semantic unit" in this case is "density altitude", not "high density" as you, now twice, said.

Please read my post again, "density altitude" is the concept more exclusive to aviation, and should therefore dominate.

It is important to understand the physics involved, this is not easily intuited.


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## User With No Name

I should apologize for my posts to this thread. I utterly and completely misunderstood. I deleted the posts that I was able to.


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## j-p-c

User With No Name said:


> I should apologize for my posts to this thread. I utterly and completely misunderstood. I deleted the posts that I was able to.



I for one, never took exception to any of your posts.


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## manfy

j-p-c said:


> If the aviation community is seen as a social group then, although "high density" and "density altitude" are both equally valid concepts, "density altitude" is much more exclusive to that group and should dominate when aviation is the subject at hand.


Well....I'm not entirely happy with that explanation. It may serve as an excuse for this specific phrase, but I doubt that it would hold true for the entirety of the language.
I first encountered 'density altitude' in motor sports and its related metrology; in high performance racing you usually finetune the engine based on the density altitude to maximize performance. And in that field 'high density' and 'density altitude' are equally common terms.

Ambiguities are unavoidable in any language -- in fact, often enough such ambiguities are actually desirable and intentional. They allow us to express connotations without spelling them out, i.e. we can convey meaning that is written in between the lines. 
In the end it's up to the writer to recognize the potential ambiguity of some phrasings, and he or she should adjust phrasing or context to avoid misinterpretation if so desired. And a good/careful writer should also anticipate that their work will be read outside the intended target audience, i.e. they should not assume that every reader will know every single term from a specific jargon.


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## j-p-c

manfy said:


> I first encountered 'density altitude' in motor sports and its related metrology; in high performance racing you usually finetune the engine based on the density altitude to maximize performance. And in that field 'high density' and 'density altitude' are equally common terms.



In defense of my idea (that's all it is), isn't it somewhat better than just a "specific excuse" ?

The group or person concerned with engine performance in your example would base their calculations on barometric pressure, temperature and humidity, and so would favor "density altitude" over the less exclusive -to their area of expertise- "high density".
And they would have no concern for ground proximity, thereby eliminating the ambiguity found in the aviation context.

This idea may do no more than help in some instances, and your general observations are very much to the point.


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