Hello Hieu, and sorry for the delay in responding.
Your essay is clearly organized. However, you have some long sentences which I would shorten, and you could make some of your ideas a little simpler.
Thus affordable airfare may be an ideal choice in line with cost-effectiveness for common people to travel and enjoy advantages of air travel, such as long distance, time-saving or fast speed and so forth. (36 words)
I would shorten and simplify this to
Affordable airfares allow ordinary people to save time and travel long distances. (12 words)
Here's another long sentence:
More significantly, cheap air travel plays a praiseworthy role in wiping out the inferiority complex in all walks of life that merely upper class had been financially capable of taking a flight previously. (33 words)
I might shorten and simplify this to:
Cheap air travel can be praised for wiping out the unfair distinction between ordinary people and the upper class, the only ones able to fly previously. (26 words)
I generally recommend that your average sentence length should be about 12 words. This means a mix of short and long sentences, but you should definitely try to keep sentences shorter than 30 words. If you have 20 sentences with an average 12 words, that's about 240 words - perfect for IELTS! Writing longer sentences increases the chance of errors in agreement and syntax.
Simplify some ideas:
Your ideas are excellent, but could be a little clearer if they were expressed more simply. Some sentences are very dense and require a lot of work for the reader. In IELTS, the writer's task is to make it easy for the reader, not necessarily to compose beautiful or intricate sentences and paragraphs.
Here's one idea I might clarify or simplify a little:
The increase in demand of air cheap travel inevitably results in high frequencies of airlines that are associated with environment concerns.
Here are two suggestions
As more cheap airlines appear, there are more environmental concerns over the pollution caused by flying.
As they compete for passengers, some cheap airlines may less attention to environmental issues.
Consider using more verbs and fewer nouns in your sentences.
I look forward to seeing more writing from you!