In the term "911 plane," the word "plane" is a noun, serving as the core subject. The numeral "911" functions as an attributive noun (also known as a noun adjunct), which acts like an adjective to modify "plane." Therefore, the entire phrase operates as a compound noun or noun phrase, with "plane" being the main point, specifically identified and contextualized by "911."
An attributive noun is a noun that modifies another noun that follows it, specifying its type, origin, or context. In this case, "911" is not a quantifier or a simple descriptor; it is a proper noun representing the specific historical events of September 11, 2001. Its function is to distinguish these specific aircraft from all others. The grammatical structure pinpoints the subject not as the event itself (9/11), but as the physical objects (the planes) directly involved in that event.
For the purpose of an article, this grammatical analysis establishes that the main point is the aircraft. The focus should be on the planes themselves: their specific flight numbers (e.g., American Airlines Flight 11), their routes, their passengers and crew, and their role as the instruments of the attacks. The modifier "911" sets the definitive scope, ensuring the article's subject is narrowly defined as the four aircraft hijacked and crashed on that specific day, rather than aviation or the 9/11 attacks in general.
