Because of the limited character set supported by the AFTN network, NOTAM messages are constrained to be transmitted in full uppercase. This section discusses issues related to this limitation, in particular from a human factors point of view. It also explains how Digital NOTAM can provide more user friendly text outputs by applying the appropriate mix of uppercase and lowercase text.
Problems with uppercase text in NOTAM
Reading a long document written fully in capital letters is inconvenient to most people. Formal human factors studies  have demonstrated that text written with a conventional mix of uppercase and lowercase characters is about 13% quicker to read than text that is all uppercase. Several reasons account for this, including the semantic associations with uppercase (beginning a sentence, proper noun, etc.), recognition of the shape of words and a greater geometric differentiation of lowercase letters (e.g. “FE” vs. “fe”, “BR” vs. “br”).
However, information to operational personnel in preparation of a flight is provided throughout the Pre-flight Information Bulletin (PIB) in full upper case format, because this is how NOTAM are received through AFTN. A PIB can be a document of 10 pages or more, written in full upper case!
The American Institute for Research (AIR) performed on behalf of the Federal Aviation Administration (FAA) of the USA studies during 2007-2009    addressing challenges associated with the NOTAM system. The objective was to identify and eliminate human performance barriers and hence to improve safety. The studies were performed by gathering information from commercial airline pilots, commercial airline dispatchers and general aviation pilots on their user experience with NOTAM messages.
The studies identified that the formatting of the NOTAM makes them difficult to read and understand by pilots. It highlighted issues concerning uppercase text and translation of abbreviations and acronyms. These factors were identified as causing difficulties to read NOTAM and process it, increasing the workload and the possibility of error. Among the items on the pilots’ wish-list for improvement in processes and functionality when using NOTAM data the following were identified:
- to have digitized NOTAM data would enable sorting and filtering of messages and translation into multiple formats, e.g. ICAO NOTAM format, plain language and graphical representation.
- to improve the NOTAM format by e.g. reducing/eliminating the use of contractions (abbreviations and acronyms) and the use of capital letters in the free text.
How digital NOTAM can solve the PIB UPPERCASE problem
The use of uppercase in the traditional ICAO NOTAM is due to the character set limitations of the Aeronautical Fixed Telecommunication Network (AFTN). Digital NOTAM are not constrained by this AFTN limitation and a conventional mix of uppercase and lowercase can be used when generating the Event NOTAM text. The distribution of Digital NOTAM can be made through other networks than AFTN and can be used directly for enhanced PIB (ePIB). For legacy users, the Event Digital NOTAM text can be automatically converted into the ICAO format (all uppercase), thus satisfying the constraints AFTN transmission and for the traditional ICAO Pre-flight Bulletin (PIB) briefing.
In order to achieve an optimum mix of uppercase and lowercase text in the digital NOTAM output, styling rules for the digital NOTAM decoding templates need to be specified. While sentence case formatting will be used in general, for abbreviations and acronyms the use of upper case can improve the readability of the information.
Existing guidelines and good practices
To help specifying styling rules on usage of uppercase and lowercase in the NOTAM text, guidelines on information presentation in equipment and systems Human Machine Interface (HMI) exist as well as other experienced practices, presented in the following paragraphs.
The Human Factors Integration Technical Guide 3.2. Equipment: Information presentation  provides guidelines for information presentation, including usage of uppercase and lowercase. It states that information shall be presented such that it can be quickly located and understood by the intended users under operational conditions, and where size, spacing and styling of text shall be considered. Information shall be unambiguous and meaningful to the targeted user.
Specifying rules for character highlighting such as bright or bold characters, underlining characters, or slanting the text (italics) should be avoided, since the effect varies depending on the characters to which it is applied. This might also require the use of mark-up languages, which would unnecessarily complicate the Digital NOTAM Specification.
Where coding techniques are used, the meaning of codes shall as far as possible be based on associations which the user can be expected to be familiar. Words, names and abbreviations shall be based on language and terminology used by the targeted user.
Format, style and location of the information shall be applied consistently throughout the user’s overall task context, including other systems with which users can be expected to interact. This aspect is of interest when defining upper- or lowercase rules in the digital NOTAM for abbreviations and acronyms commonly used in ATM systems, such as RWY, TWY, THR, ILS and DME (compared to rwy, twy, thr, ils, dme).
The Single European Sky ATM Research and Development programme (SESAR) delivery “Generic SESAR Information Presentation Guide”
 from 2013 provides guidance on how to achieve an effective information presentation for the main actors involved in the ATM field. It provides guidance on presenting information:
- In a clear and unambiguous form.
- In a format that calls for the attention of the users.
- In the right location.
- With the appropriate level of content and in the right format.
Regarding usage of uppercase and lowercase in information the SESAR guidance recommends the normal sentence case. The use of all uppercase should be avoided because it reduces reading speed and renders words indistinguishable.
Lowercase enhances reading efficiency because word shape is helpful in word recognition. It is therefore recommended to use lowercase, which supports faster word recognition.
Further related to choice of fonts, the guidance states that text should be presented in a combination of uppercase and lowercase letters. Continuous text is easiest to read and comprehend when it is presented in mixed case letters and should also be used for messages, menu descriptions, button descriptions, or screen identification. Capitals 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.
Similar input came from the SESAR ePIB project.
Within the SESAR P13.02.02 Digital Integrated Pre-flight Briefing project, a validation trial with pilots was conducted during 2016, investigating human performance improvements based on the graphical view of NOTAM events in an enhanced pre-flight information bulletin (ePIB), enabled by the digital NOTAM. The ePIB provided the NOTAM text in a mix of uppercase and lowercase. This was compared with the traditional PIB, provided in full upper case. Pilots’ input was collected that readability of the information was improved in the ePIB as compared to text solely in uppercase.
Pilots’ ideas on how briefing applications could support identifying abbreviations in the ePIB and display the explanation on less known abbreviations/acronyms were collected. This allowed the digital NOTAM text to be enhanced with mark-up elements, so that briefing tools/applications can identify abbreviations and offer either pop-up explanations or full expansion of less known abbreviations.
Within the AIS Stakeholders’ working arrangements of EUROCONTROL, the AI Operations Sub-group discussed during 2012 the usage of abbreviations in NOTAM. The Sub-group prepared input to ICAO proposing to indicate in ICAO Document 8400 (Abbreviations and Codes)  which abbreviations/acronyms that should be used in NOTAM and which that should be spelled out. The objective was to increase the readability and the understanding of the NOTAM text by harmonising the usage of abbreviations in the free text.
It was proposed to spell out abbreviations and acronyms which are not common and can be subject to misinterpretation. Also, with the digital NOTAM developments in mind (e.g. no AFTN constraints), words such as “closed”, “available”, “activation”, “active”, “activated” and “unserviceable” were proposed to be spelled out instead of using the abbreviations. However, many abbreviations were recognised to be kept, such as “RWY”, “TWY”, “ILS”, “VOR”, “ATS”, since their meaning is so well known and the abbreviation/acronym is perceived as a word.
The EUROCONTROL Pre-digital NOTAM Templates document  applies to a certain extent the principle of spelled out text in item E on words such as “closed”, “available”, “activation”, “active”, “activated” and “unserviceable”, and on acronyms that are not commonly known.
Krokos, K.J & White, C. E. (2008). NOTAM System Modernization: The Pilots’ Perspective. Report summarizing input from the Pilot Input to NOTAM System Modernization Working Group. (FAA Report submitted to FAA AJR under Grant #99-G-048). Washington, DC : American institutes for Research. Krokos, K.J, White, C. E & Willis T. J (2009). NOTAM System Modernization: The General Aviation Pilots’ Perspective. Report summarising input from the GA Pilot Input to NOTAM System Modernization Working group. (FAA Report submitted to FAA AJR under Grant #99-G-048). Washington, DC: American institutes for Research. SESAR DEL16.05.03D06_Generic SESAR Information Presentation Guide Ed.00.01.00 - Guidance for an effective information presentation. ICAO Document 8400 (Abbreviations and Codes). EUROCONTROL Pre-digital NOTAM Templates document (Ed. 1.3).
Krokos, K.J & White, C. E. (2008). NOTAM System Modernization: The Pilots’ Perspective. Report summarizing input from the Pilot Input to NOTAM System Modernization Working Group. (FAA Report submitted to FAA AJR under Grant #99-G-048). Washington, DC : American institutes for Research.
Krokos, K.J, White, C. E & Willis T. J (2009). NOTAM System Modernization: The General Aviation Pilots’ Perspective. Report summarising input from the GA Pilot Input to NOTAM System Modernization Working group. (FAA Report submitted to FAA AJR under Grant #99-G-048). Washington, DC: American institutes for Research.
SESAR DEL16.05.03D06_Generic SESAR Information Presentation Guide Ed.00.01.00 - Guidance for an effective information presentation.
ICAO Document 8400 (Abbreviations and Codes).
EUROCONTROL Pre-digital NOTAM Templates document (Ed. 1.3).