"This airplane is big."
Just to unpack things alluded to in some comments. In an "X is Y" type sentence, in plain (i.e. not polite) form then:
If Y is what is known as an "i-adjective" (all of which end in "i" but some adjectives ending in "i" like 綺麗 are not), then nothing is needed after the adjective. In a sense i-adjectives are a bit like verbs (they have past tenses and so on) and contain the "copula".
The "copula" is a special verb (in plain form だ in polite formです) which you need to form an "X is Y" sentence if Y is a noun.
The other kind of nouns is a "na-adjective". These do need a copula. In plain speech you would have to add だ at the end (which you would not need to do if it was an i-adjective).
What confuses everyone to start with is that ですis added to the end of "X is Y" for an i-adjective, not as the copula, but just to make it polite. Hence the confusion.
In this sentence 大きい on its own is sufficient, unless you want to be polite, in which case you would add です.
I wrote この飛行機は大きい。 which was wrong. But in the Tips and Notes is says....... Just like with positive い-adjectives, all negative い-adjectives can drop the です ending in casual speech.
One of the many similar problems Duolingo. Things like this only confuse learners. But then I'm coming to believe this is not a program to learn just to make money.
Now it's March2020, and I read my post again. I've come to accept that there are some problems with Duo especially with the tips and sentences used in the lessons. However, languages are very difficult to translate perfectly every time. Phrases often need context to translate correctly. I have become much more forgiving with mistakes and found that Duo is helping me improve my Japanese. It is the only study that I do every day. AND IT'S FREE!