I've seen many images of aircraft cockpit displays and instrument or control labels that all seem to use uppercase lettering, even though there seems to be a lot of evidence indicating that mixed case lettering has superior readability. Why is that?

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    $\begingroup$ Welcome to aviation.SE! This might be an interesting question for ux.SE if you don't get a good answer here $\endgroup$
    – Pondlife
    Dec 12 '19 at 1:23
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    $\begingroup$ The fonts may have only uppercase, but you can't tell by the display. They only use uppercase, but the font itself may well contain the lowercase letters - as well as other characters. $\endgroup$
    – JRE
    Dec 12 '19 at 10:22
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    $\begingroup$ At the very least, if you see "I" you know that's an i and not an L $\endgroup$
    – BruceWayne
    Dec 12 '19 at 19:13
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    $\begingroup$ Not all avionics systems use uppercase exclusively. You can find lowercase in some of Garmin's recent offerings, for instance. See for instance the lowercase labels for the buttons in this image of a G1000. But for much of what is displayed, lowercase doesn't make a lot of sense. $\endgroup$ Dec 13 '19 at 6:06

The specific reason all-uppercase was adopted is just historical. Traditional steam gauge faces use capital lettering exclusively. The reason for that is that capital letters are easier to produce with limited tooling. When they were installed in aircraft, this cost-efficient convention was followed.

This pattern is not exclusive to gauges: historically, in Indo-European languages, uppercase letters came first. Bicameral script is a later invention, present only in a few (albeit very popular) language scripts.

Early typewriters were all-uppercase. Upper case persisted in teletype communications and early computers. Mixed case always added complexity, so its adoption in tech took time throughout the 20th century. This made all-uppercase text very familiar to the technically-minded people involved in aviation, right at the time when its conventions were being established.

The reason uppercase hasn't been replaced, even in glass cockpits, is a simple case of getting the best size:readability ratio. Capitals are fixed height and allow for twice larger letters (thus more readable) within the same amount of space.

For the same reason, exit signs in buildings spell "EXIT", not "Exit", most book covers feature all capital letters, and "NO STEP" won't be replaced with "Please avoid stepping here" anytime soon.

Mixed case has superior readability for natural English, but the language used in aviation is a specific lingo with heavy use of abbreviations. The word-recognition advantage you use reading this post would not initially work in reading "ALT" or "HDG". These are new symbols to learn. Once learned, they become more familiar than the original non-abbreviated terms they stand for.

Aviation documentation is written in mixed case, but the use of uppercase in cockpits continues to be beneficial, as uppercase helps automatically emphasize the exact terms as seen in the cockpit and elsewhere.

Will aviation displays ever switch to mixed-case? The process is ongoing, but rather than changing the old function, it's just the use of mixed-case for some new functions. These involve incorporating paperwork into the glass, which uses more natural language, for which mixed case is suitable (and, as Gerry correctly adds, officially recommended).

Aviation-specific, abbreviation-rich jargon is an artificial language, not a natural one, so there's no case for it to switch.

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    $\begingroup$ The majority of text on the PFD and ND are labels. The primary information is the numbers and the symbols. As you point out, labels from the steam gauge days were in all caps and that carried over as pilots don't like change. Current FAA guidance in AC 25-11B is that all caps for labels is acceptable and text in sentence form should be in mixed case. $\endgroup$
    – Gerry
    Dec 13 '19 at 12:44
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    $\begingroup$ Even in "glass cockpits", all upper-case may have an advantage. Text rendered on a raster display can be rendered legibly on a much lower resolution display when all upper-case. When mixed-case is used, higher display resolution is required to avoid artifacts that would impair legibility. $\endgroup$
    – Anthony X
    Dec 14 '19 at 18:37
  • $\begingroup$ @AnthonyX True. I'm looking years ahead, when we'll have as much display resolution as desired. In that case, for natural language (I imagine actual paper-paperwork will eventually be gone completely), mixed case wins. It's not life-critical, it's done on the ground or in cruise, so it doesn't have to shout. $\endgroup$
    – Therac
    Dec 14 '19 at 18:50
  • $\begingroup$ @Therac I should add that in the glass cockpit scenario, software complexity is a safety issue; more software or more complex software increases risk of bugs; lower-case lettering requires complex rules of typography as well as sophisticated "sub-pixel" rendering algorithms for optimal readability. All upper-case can use simple fixed-pitch or minimal kerning and no sub-pixeling and still be easily readable. That simplifies the software, increasing software reliability and provable correctness. $\endgroup$
    – Anthony X
    Dec 14 '19 at 20:28
  • $\begingroup$ I wish I could pick multiple answers! What a cool community. $\endgroup$
    – Yobert
    Dec 16 '19 at 20:05

In low light or with vision partially obscured (like, say, smoke), all caps can be seen from farther away and/or with greater clarity. This is more important than the easier to follow at a glance word-shapes you have with upper and lower case text, which is only a significant benefit when words are strung into sentences that you scan.

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    $\begingroup$ Do you know of some research that backs this up? In the UK all road signs are in mixed case specifically for the readability reason, and they generally (obviously there are exceptions) aren't words strung together into sentences, but things like place names etc. $\endgroup$
    – Muzer
    Dec 12 '19 at 9:51
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    $\begingroup$ @Muzer Right - that's because the human brain can "read" text by looking only at the outline shape of the word instead of the letters - but this doesn't work if the text is all-uppercase (because then all words of the same length have roughly the same shape). I wonder if the all-uppercase trend started off as a hunch in the 1950s or so and by the time that solid research suggested mixed-case was better than all-uppercase there was too much institutional inertia to change? (I wonder if it could be because in WW2 almost all military stencil prints were all uppercase for simplicity...?) $\endgroup$
    – Dai
    Dec 12 '19 at 11:14
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    $\begingroup$ @Muzer Really? UK stop signs spell "Stop" not "STOP" ? $\endgroup$ Dec 12 '19 at 11:33
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    $\begingroup$ @Muzer yeah, STOP and SLOW as well are examples where UK signs do use all-caps. I think the point is, single short words are better readable in caps, whereas place names are often long enough for the “shape” aspect to help, like it does in sentences. $\endgroup$ Dec 12 '19 at 11:45
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    $\begingroup$ @StianYttervik I think that might be because of the Vienna Convention on Road Signage. $\endgroup$
    – Dai
    Dec 12 '19 at 11:54

For at least certification under the FAA via AC 25-11B all upper case is acceptable but not explicitly required. If you're building an instrument and you know that all upper case will be accepted during certification it's only logical to build it that way.

5.4.2 Regardless of the font type, size, color, and background, text should be readable in all foreseeable lighting and operating conditions from the flightcrew station (§ 25.1321(a)). General guidelines for text are as follows: Standard grammatical use of upper and lower case letters is recommended for lengthy documentation and lengthy messages. Using this format is also helpful when the structure of the text is in sentence form. The use of only upper case letters for text labels is acceptable.

However this NASA study finds that mixed case is actually more legible. But this seems to be applied to lengthly sections of text.

There is almost a consensus among researchers that, when other factors are controlled, lower-case characters are more legible than upper-case (Hartley, 1981; Philips, 1979; Tinker, 1963). Poulton (1967) performed an experiment to determine the difference in readers’ attention between upper and lower-case in newspaper headings. He reports that lower case headings were located faster than upper case heading. Tinker (1963) tested lowercase and upper-case fonts for legibility and pleasingness. He reported that lower case was read faster and ranked higher in pleasingness.

This FAA paper suggests that all-capital letters for single words may be easier to identify

Although continuous text is easier to read when presented in mixed case versus all upper case, single words may be recognized better when displayed in all upper case (AC 25-11A; Ahlstrom and Longo, 2003).

Which may explain why it's used for instruments. They quote AC 25-11A however the language of that AC does not suggest this. But Ahlstrom and Longo, 2003 has some great info on the topic specifically Capitalization specifically subsection 3: Use of capitals. Capitalization should only be used for: headlines, key phrases or acronyms, short items to draw the user’s attention to important text (for example, field labels or a window title), the first letter in a sentence, or a single character in each word in a title or label. [Source: National Air Traffic Services, 1999]

I would check out the whole document for more info.

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    $\begingroup$ by far the best answer here! Well reasoned with references to back it up. Cheers Dave $\endgroup$ Dec 14 '19 at 13:38

Bear in mind that YOU may be comfortable with lower-case letters, but there are other cultures in the world that have different symbols for their alphabets, and they have to learn the Latin alphabet as something new. So having only upper-case letters makes it easier for non-Latin users.

Note that this idea of all upper-case also applies to the QRH. For example, there is no such drill as "Fire Engine", it's "FIRE ENGINE" because the EICAS only shows upper-case messages, and so the pilot doesn't have to understand the message, they simply look through the QRH to pattern-match the EICAS message. If a drill/procedure in the QRH has an initial-case title, then it's for a non-EICAS procedure (usually called "Unannunciated").

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    $\begingroup$ Japanese Katakana has 46 characters - and written forms of Chinese have thousands of glyphs - while it's true that Latin has the uppercase/lowercase property distinction that other writing systems don't have, I don't think it's fair to assume that non-native English speakers wouldn't be "comfortable" with mixed-case writing. As for EICAS: I understand it's mostly uppercase because the system predates modern computer typography and because it was created in the atmosphere of legacy radiocommunications systems that were also all-uppercase. $\endgroup$
    – Dai
    Dec 12 '19 at 11:18
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    $\begingroup$ Having started to learn a bit of Japanese script, I can tell you that having hiragana and katakana adds complexity. If I had to learn to read instruments in Japanese, I'm sure that having all messages in the same script (hiragana or katakana) would help. Therefore, it seems quite reasonable to expect people from other scripts to be more confortable with all caps than with mixed cases. $\endgroup$
    – Pere
    Dec 12 '19 at 16:44
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    $\begingroup$ Hiragana and katakana, if they appear together, are used to make adjacent words look different - they're not like upper / lower case, they're more like roman and italic, say. So katakana may be used for emphasis of something, or as a marker for a foreign loan word. In this case using different scripts makes it hard to read something as a "word" ... $\endgroup$ Dec 12 '19 at 23:48
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    $\begingroup$ The "easier for non latin culture" explanation may be an a posteriori easy explanation. Historically, aviation is mostly US-habits adoption. In a all-english environment, all caps might have been choosen without taking into account non-latin culture, and then adopted as a de facto standard as many other US standard. $\endgroup$
    – Manu H
    Dec 13 '19 at 9:35

I have no true evidence that it is really related, but upper case only used to be the standard before the 80's. For example, METAR et TAF messages (meteorological observation and previsions on airports) used to be exchanged in old 5 bits Baudot code that can only represent upper case alphabets - and the code for both messages still only contains upper case Latin alphabets, decimal digits and spaces.

As security is very important in aviation, people are reluctant to change something that has never caused any problem. Long story short, people using planes are used for decades to the upper case latin alphabet whatever their first language charset is, and find no strong reason to change that.

  • $\begingroup$ I don't believe Baudot code specified letter-case. It does lack a way of encoding letter-case alternatives, though, so a given machine will support only one. Confusingly, capital letters were usually called "lowercase" (because they came from the lower teletype case) and figures and punctuation symbols "uppercase". The lowercase almost invariably used capital letters though. For example, 2 might be referred to as an uppercase W. $\endgroup$
    – Dannie
    Dec 13 '19 at 14:11

From a typography perspective, all caps does not have any descenders (letters that go below the baseline such as "p", "g" and "j".) It is easier to do labels and such when you don't have to worry about accommodating space for those. In other words, you can make the text bigger for a given space: "Eject" vs "EJECT" or "Flaps" vs "FLAPS"

  • $\begingroup$ I'd been going to add that when the reader is in a hurry, the lowercase l can commonly be mistaken for the uppercase I and then both can be confused with control/command characters like | which is commonly used as a column separator. It's a particular issue with monospace and sans-serif fonts, which are typically what you get in digital displays. I think this fits better alongside your answer instead. $\endgroup$
    – user116960
    Dec 14 '19 at 15:01

I don't have any references to back this up, but to me it does make sense to have all upper-case letters for the following reasons:

1) Cockpits are usually packed with instruments which creates limitations on how large a letter can be printed. Considering even in normal circumstances the cockpit of an aeroplane can have a level of vibration, I believe having upper-case letters will improve the readability. This is because the upper-case printed letter is larger than its lower-case counterpart -- in oppose to having words written in Capital-case where most letters are written in lower-case.

In my personal experience in an environment that is subject to a noticeable vibration, the word "OFF" is more readable than "Off", so the word "NAV" is more readable than "Nav".

2) If for whatever reason the pilot's vision is reduced e.g. blurry vision due to not being able to use their glasses, an upper-case letter seems more readable, purely due its increased size but also for some letters because of their shape too. I myself can't distinguish between lower-case letters "r" and "i" from a certain distance without having my glasses on, but I have no problem recognising "R" and "I" from the same distance.

3) To me all upper-case words carry more importance over capital-case words. As an example in emergency situation e.g. if I am looking to exit a burning building, I believe my brain would notice "FIRE EXIT" much quicker than "Fire Exit", probably because upper-case words are not used in normal communication very often and visually they are distinctive to all the other text we read everyday.

In an emergency situation when priorities are different in my brain in terms of signal processing, I don't need a text saying "If you are involved in a crash, use this red hammer to break the window glass and then exit through the window." A minimal text such as "BREAK GLASS" would do way better in such a situation than a text that otherwise in normal day-to-day life situation would have been easier and better to read. This doesn't really answer this question, but I thought it is worth mentioning as I believe this may have also played a role in choosing upper-case words rather than capital words.

And again, all of these are my personal opinion and I have nothing to back them up.


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