Density altitude (aviation)

j-p-c

Senior Member
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.
 
  • PaulQ

    Banned
    UK
    English - England
    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." :D
     

    lingobingo

    Senior Member
    English - England
    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.
     

    j-p-c

    Senior Member
    "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. : )
     

    j-p-c

    Senior Member
    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.
     

    User With No Name

    Senior Member
    n/a
    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.
     

    j-p-c

    Senior Member
    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. : )
     

    j-p-c

    Senior Member
    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 !
     

    Andygc

    Senior Member
    British English
    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?
     

    aceofpace

    Senior Member
    English (American)
    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.
     

    Andygc

    Senior Member
    British English
    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.
     

    manfy

    Senior Member
    German - Austria
    Yes, that ambiguity is based on a flaw in the English language! :p
    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 :cool:).

    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.
     

    j-p-c

    Senior Member
    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.
     

    j-p-c

    Senior Member
    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.
     

    PaulQ

    Banned
    UK
    English - England
    "Rules are made to be obeyed by fools and for the guidance of the wise1"
    :D Ah! that is what my signature should have been!

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

    dojibear

    Senior Member
    English - Northeast US
    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.
     

    Delvo

    Senior Member
    American English
    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?
    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.
     

    Andygc

    Senior Member
    British English
    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.
    "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".
     

    Andygc

    Senior Member
    British English
    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. :rolleyes:
     

    dojibear

    Senior Member
    English - Northeast US
    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.
     

    PaulQ

    Banned
    UK
    English - England
    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. :)
     

    Delvo

    Senior Member
    American English
    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.
     

    j-p-c

    Senior Member
    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.
     

    manfy

    Senior Member
    German - Austria
    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.

    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? :rolleyes:

    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!
     

    j-p-c

    Senior Member
    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

    Senior Member
    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

    Banned
    UK
    English - England
    It's a pity that you did not notice this earlier:
    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.
     

    j-p-c

    Senior Member
    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.
     

    manfy

    Senior Member
    German - Austria
    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.
     

    j-p-c

    Senior Member
    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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