There's been a lot of confusion online as to when the museum would be re-opened so I can confirm we visited January 4th 2020 and all but one hanger was open to the public. Great museum if you like planes especially Migs as there are a lot of them! Free entry to outdoor sections and 20rmb to go inside the mountain. All the signage outside has english translation but not as much inside. There are a lot of interesting looking planes outside at the back you are not allowed near. It looks like they are expanding that back section to include these planes but they aren't ready yet. We booked a car to take us out and back which I would recommend if visiting as the museum is down a pretty long bit of road so bit of a hike from any bus stops. If you get a taxi arrange to have them come back for you as there isn't likely to be any around when you come out. We were there for a good few hours and the only non-Chinese visitors the entire time and they all drove there so there isn't tour buses or taxis hanging around. Food options are very limited so bring drinks/snacks with you.

As noted by another traveler, the Changping museum is closed until November, 2019. However, Beihang, the Beijing Aviation University, has a small museum on its campus that has a selection of about 30 aircraft on display. The campus is conveniently located just inside North 4th Ring Road. Unfortunately, to visit you need to book tickets on the museum website, a process that can be done only in Chinese, and even then the museum is open only Tuesday and Saturday mornings from 9-12. Still, if you jump through all the hoops you are rewarded with a chance to see and assortment of Chinese aircraft and a reasonable history of Chinese aviation.