"Aircraft " should be perfectly acceptable, in fact preferable to "airplanes" or "aeroplanes", one of which is felt to be too lightweight for adult use on the east side of the Atlantic.
I'm not a native English (or German) speaker, but this is how I understand:
Aircraft is the general term for "flying machines". It can be anything from air balloon to helicopter or airplane. Airplane is a fixed-wing, power-driven aircraft.
aircraft - das Luftfahrzeug (die Luft - air, das Fahrzeug - vehicle, craft) airplane - das Flugzeug
That's how I'd understand it too. I suppose another course contributor has seen it differently, however, as "aircraft" is now accepted. I would still caution against learning it that way.
I am concurring with this user and others; "Airplanes" sounds childish and wrong to a UK English speaker. "Aircraft" or "Planes" should be acceptable answers.
In the U.S., I don't think it sounds wrong to say "planes" or "airplanes". "Aircraft", however sounds awkward. I makes me think of an amusement park employee saying 'Keep your hands and feet inside the aircraft'
"we have many aircraft" is also a correct english translation, however it is not seen as an acceptable alternative by the app. Can someone please look into this? (aircraft is used as plural in this case)
When a translation you think is correct is not accepted, you should use the ‘report’ button to bring it to the attention of the staff. If you have doubts, you can first ask in the comments whether the translation is indeed correct; but if you're sure of your translation, commenting isn't going to be very useful.
After 50 years as an aircraft engineer I find the word "Aircraft" perfectly acceptable. I can remember a film called, One of our aircaft is missing.
Then you'd be horrified to hear someone call a helicopter an airplane, then?
As mentioned in several other comments:
Flugzeug = "(fixed-wing) airplane/aeroplane"
Luftfahrzeug = "aircraft"
All your planes may well be aircraft, but not all aircraft are Flugzeuge.
Aircraft should be acceptable. Would that mean airplanes and helicopters? What would germans call the VTOL aircraft like the V-22 Osprey?
"Aircraft" generally are Luftfahrzeuge, a category that also includes helicopters, etc. (which are not Flugzeuge). The V-22 is a Wandelflugzeug. You can explore the German Wikipedia pages to see which terms are in use.
But in English, we use the exact same word for Luftfahrzeuge and flugzeuge - aircraft. An airplane is an aircraft, as is a helicopter or an airship or a hot air balloon or an autogyro, or even a model aircraft. It's an aircraft no matter whether it carries passengers or not. If it flies and it's human-made, it's an aircraft - and it applies whether it's singular or plural. So aircraft has to be a correct translation.
But that is not what ‘Flugzeug’ means. A hot air balloon is not a ‘Flugzeug’, neither is a zeppelin, nor (in spite of what Duden seems to be saying) is a helicopter a ‘Flugzeug’ in most German speakers' minds (in fact the German Wikipedia seems to be pretty clear that Flugzeuge are fixed-wing aircraft).
In English, there's a word we can use to describe cats, dogs, lions and monkeys, but if someone asked you to translate ‘Katze’, ‘Hund’, ‘Löwe’ or ‘Affe’, ‘animal’ would not be the best word.
‘Aircraft’ is ‘Luftfahrzeug’; ‘plane’ is ‘Flugzeug’.
"flythings" (or something like "flyequipment") would be the literal translation of the individual parts of "Flugzeuge", but obviously, that's not accepted since it's not an English word. There often isn't much point in translating each part of a compound word separately, other than to maybe deduce its meaning or to admire the logic with which German words are constructed, but either way you'll end up needing to translate the word as a whole to actually find the English equivalent.
In another question, "car" was rejected as a translation for "Fahrzeug" because it was too specific. Here "planes" and "airplanes" are accepted. Is "plane" not a bit too specific? Helicopters are never called planes. Nor are lighter-than-air craft. Are there other words for these in German? Am I being too pedantic in thinking that Duo shouldn't accept "planes"?
There is an important difference to be made: there is no specific term for plane in German other than ‘Flugzeug’, which does technically mean ‘aircraft’ (and this should definitely be an accepted translation) but is commonly used to refer to planes specifically. In fact, means of aerial transport more in general can be termed ‘Luftfahrzeuge’, which would never be understood as a plane specifically.
‘Fahrzeug’ and ‘Auto’ instead map pretty well to the English words ‘vehicle’ and ‘car’. Accepting ‘car’ as a translation for ‘Fahrzeug’ is, in my opinion, the same as accepting ‘dog’ for ‘Haustier’ (‘pet’) or ‘parakeet’ for ‘Papagei’ (‘parrot’). Yes, both are very prominent and common subsets of those classes, but why be specific when the original German sentence is clearly and purposefully general?
‘Viel’ can only be used with singular nouns, in the plural it needs to be declined like any other adjective (NOM/ACC: viele, GEN: vieler, DAT: vielen).