Descriptive essays are, of course, all about description. However the intention is not to simply provide an itemized account of what is in a picture requiring description, or in this case, to provide a simple account of what is in an airport. Descriptive essays should present the reader with a visual picture based on appearance, smells, textures and so on, based on the five senses. Care should be taken to "describe" in terms of the senses and not offer an opinion as the essay then could become more discursive than descriptive.
Consider your thesis statement and introductory paragraph. Be specific in your description; for example, "An airport is more than its essential elements of size, types of aircraft, a building, a runway and people. It is a cultural experience, a hive of activity and a place where decisions are made and lives are changed forever."
Your thesis statement will help you to remain focused and to describe each element, such as dimensions, aircraft, runway, buildings, people, cultures, activities, lives and so on.
The first step is to imagine an airport. The most striking thing about an airport is its size. By definition, it needs to be large to accommodate planes, some with huge wingspans. However, size is relative and some airports cannot cater to large aircraft because of wingspan or runway length or capacity. Therefore, in getting a sense of the airport you may be describing, the size is important in guiding the reader to a true sense of scale. The reader can already imagine which aircraft may be situated on the apron or runway. Describing those aircraft may be relevant.
Airport buildings in large cities are usually quite impressive but those in smaller, outlying areas may be more about the runway than the building and so describing the building helps to enhance that visual, descriptive picture. Some airports operate all hours and are frantic and loud and light for 24 hours without respite.
Airports are very cosmopolitan; many people, many nationalities and many cultures all converge in one place to make for a fascinating culture all of its own. Should it be an airport catering only to domestic flights, there is still a diverse population. Any airport caters to people from all walks of life. There will be the businessman or woman, the tourist, the traveler visiting beloved family members across states and sometimes countries, child travelers and airport staff themselves, including those who stay on the ground and those pilots and flight attendants who work on the planes. There will also be those who fulfill administrative functions, technical and repair functions, operations, traffic control, emergency services, guest services, and then the retail outlets and restaurants often found in airports.
Airports are incredibly happy places and unbelievably sad places. They unite and they divide. They are lonely places for some and opportunities for learning, advancement and excitement for others.
There are many ways to describe an airport while maintaining a descriptive tone. When your essay is finished, ideally the reader should want to run off and experience the real thing because your description is so inspiring!
The key to describing something this large and complex is to first taxonomize the subject, dividing it into its layers and levels of operation. In your case, for example, first separate the airport’s functions by making an outline of chapters or paragraphs (depending on the size of the assignment) demonstrating the layers, as thus: Passenger areas, staff areas, maintenance/storage areas, freight/baggage areas, navigation/radar areas, runways and runway maintenance area, etc. Then take each area and split it up into its component parts. Since we as laymen are most familiar with the terminal of an airport, especially its passenger areas, you might divide that area by arrival accommodations, departure accommodations, waiting accommodations, etc. The waiting accommodations can be divided into seating areas, refreshment areas, shopping areas, etc. Do this with each subsection, going into more and more detail as you progress (food, liquor, vending machines vs. served food, etc.) Supplies/objects to the plane might include food, fuel, crew, baggage, freight (mail, UPS, etc.), and finally passengers (first class, handicapped, etc.). The maintenance and storage areas (hangars away from the terminal) might require some research. This task could end up book-length.