Today's post is launching a new series called: "Ask Oscar, The Pilot", where I aim to answer popular questions commonly asked to pilots starting with: Why Do Pilots Say Niner?
Question: Why do pilots say Niner instead of just saying nine?
To answer this question, we have to introduce a few concepts first:
1. Aviation English (AKA. Pilot English):
English is widely used in Aviation. In fact, ICAO states the following:
184.108.40.206.1 The air-ground radiotelephony communications shall be conducted in the language normally used by the station on the ground or in the English language. 
220.127.116.11.2 The English language shall be available, on request from any aircraft station, at all stations on the ground serving designated airports and routes used by international air services. 
2.31.2 Except when communications between air traffic control units are conducted in a mutually agreed language, the English language shall be used for such communications. 
Furthermore, pilots need to hold a sufficient level of proficiency in English. Pilots - Aviation - has a 'special' language, a secret* one that I will unveil to the world now:
*A bit of exaggeration for dramatical purposes.
"Standard Phraseology." is a concept in Radio Communications. It enables pilots to communicate using what we call for the sake of this article: "Standard Aviation Language".
Now: What is this "Standard Aviation Language"?
It's a tweaked version of English or, more precisely: "A particular way to pronounce English".
1.2 How do pilots pronounce the alphabet? ICAO Phonetic Alphabet:
The standard alphabet has a phonetic pronunciation for each letter. The sound should be the same, whatever the speaker's natural language!  Find the ICAO Phonetic Alphabet in the picture below:
1.3 How do pilots pronounce numbers? ICAO Phonetic Numbers:
Like the alphabet, numbers have a unique way of pronunciation in Aviation. Find the ICAO Phonetic Numbers in the picture below:
2.0 What's the point of pilots doing all of that?
Enough of introductory concepts, the question remains:
Why do pilots say NINER instead of Nine?
Why do pilots say TREE instead of THREE?
The answer to all of this is to avoid confusion for the following reasons:
- The cockpit is a noisy environment, and you need to speak with high volume (Shout), so the other pilot can hear and understand you, let alone speak over the radio with the controller.
- The reception can be poor sometimes over the radio, so it would be difficult to understand the tower and vice versa.
- Pilots tend to speak faster in busy airspaces, so they don't block the frequency, which may lead to skipping letters and numbers.
- There may be words that sound similar and could be confused. For example, the letter 'A' could be confused with the number '8', or the letter 'C' (see), which sounds like 'D' (dee) or 'V' (vee). 
- Pilots and controllers are from all different backgrounds and nations, they all have different accents, and some can be hard to comprehend.
- Callsign confusion. Callsigns with similar sounds on the same radio frequency. A crew might think a specific clearance is directed towards them, but it's intended for the other aircraft.
Therefore, to avoid confusion due to the reasons mentioned, pilots can be asked to repeat a clearance or read back a frequency using individual letters or numbers. Here is where the 'Special' alphabet and numbers pronunciations come to play.
*Made-up example - not an actual frequency, neither radar name nor aircraft callsign*
"Oscar 520, contact AvPosts radar 123.495"
- You mishear the controller and you read back the frequency incorrectly.
- The controller will notice the mistake, repeat the frequency and ask you for a readback.
- You read back this time using ICAO Phonetic Numbers to avoid any further confusion. In other words, your transmission will sound like this:
"Contact AvPosts radar WUN TOO TREE DAYSEEMAL FOWER NINER FIFE, OSCAR FIFE TOO ZERO"
I hope that this answers your question! If you have further questions, please send them my way. I'll be happy to answer.
   ICAO (2016). Language to be used. In Aeronautical Telecommunications. Annex 10 to the Convention on International Civil Aviation (7th ed, vol. 2, pp. 80). https://store.icao.int/en/annex-10-aeronautical-telecommunications-volume-ii-communication-procedures-including-those-with-pans-status
  CAE Oxford Aviation Academy. (2015). Transmission of Letters and Numbers. In Communications (EASA FIRST EDITION REVISED FOR NPA 29, Vol. 14, Ser. ATPL GROUND TRAINING, pp. 3–3). https://www.caeoxfordinteractive.com/index.php?route=product/product&path=82&product_id=194
ICAO (2007). GENERAL OPERATING PROCEDURES. In Manual of radiotelephony. Doc 9432-AN/925 (4th ed., pp. 19–20). https://store.icao.int/en/manual-of-radiotelephony-doc-9432