North Korea recently lifted a flight ban on most of its older warplanes. This ban was imposed in May after a MiG-17 crashed into the sea off the west coast. There was a mechanical failure and given the age of most North Korean aircraft and shortages of spare parts and fuel for flying time over the last decade it was felt prudent to ground most of the air force until the oldest aircraft could be checked for any common problems. The MiG-17s were particularly suspect since they had been in service since the 1950s. North Korea is the only country still using MiG-17s and has a few dozen of them still flyable (at least in theory).
South Korea considers the North Korean air force more of a nuisance than a threat and plans to destroy most North Korean aircraft in the air or on the ground (or in caves where many are kept) in the early stages of any future war. North Korea seeks to avoid this by using most of these aircraft first in a surprise attack and holding back some of them back, basing these in tunnels. South Korea has plans to deal with all that but will not, for obvious reasons, discuss details.
With the exception of some MiG-29s, the North Korean air force consists of 1,300 Cold War era Russian and Chinese aircraft, about half of them combat planes. The Chinese aircraft are knockoffs of older Russian designs, and most of the North Korean fleet consists of aircraft designs that were getting old in the 1970s. Recent North Korean Air Force training exercises, like the loss of the MiG-17 in May, confirms what many South Korean and American intelligence analysts already suspected: that the North Korean Air Force can barely fly and hardly fight.
The most modern aircraft the North Koreans have are 40 MiG-29s they got in the 1980s, when they were still getting freebies from the Soviet Union. The rest of their combat aircraft are poorly maintained and infrequently used (because of fuel and spare parts shortages) antiques. There are 50 MiG-23s, an unreliable 1960s design which few other countries still use. There are about 190 MiG-21s (40 of them Chinese copies of the Russian design) and about 90 each of F-6s and F-5s (Chinese copies of the MiG-19 and MiG-17, both 1950s designs hardly anyone else uses). They have 160 bombers and ground attack aircraft, most of them elderly Russian and Chinese designs. The best of this lot are the 32 Su-25s, which are a decent contemporary of the U.S. A-10 that has proven itself in Afghanistan and the Caucasus.
The helicopter force is also elderly. The best of them are 20 Russian Mi-24 gunships and 80 American MD-500D, smuggled in from Germany in the 1980s. Perhaps the most dangerous aircraft are 300 AN-2 single engine bi-plane transports. A sturdy Russian aircraft which, although designed in the 1940s, is simple, rugged, popular, and remained in production until 2002. Able to carry ten passengers, the North Korean AN-2s have been seen practicing flying low and at night. Since each AN-2 can carry ten soldiers, it is believed they are meant to deliver commandos into South Korea early on in a war. Several thousand of these troops could cause a lot of confusion as South Korea mobilized for war. But since 2007 fuel shortages have meant few AN-2s have been flying. That means the pilots are not really skilled enough to carry off a night operation, especially flying low (to avoid radar) through the mountains separating the two Koreas. Using AN-2s now would lead to a lot of them, if not most of them, not making it. Then there are whatever surprises South Korea and the U.S. have developed to counter this daring use of AN-2s.