ADVENTURES IN THE AIR
Frankly, this is one of my favorite sections of the DMG. I was absolutely floored at just how solid the aerial combat rules in first edition are, given that nobody since then can seem to come up with a really great set (SWSE being a possible exception, and hopefully my upcoming rules in the WWII book for AFMBE--cheap plug).
The section begins by making the (very true) point that aerial combat takes place in a far different environment than normal combat, which is why it needs its own set of rules. It goes on then to detail various Flying Mounts and the benefits and drawbacks of caring for and training each. Griffons, for example, are carnivorous and require "enormous amounts of food, especially after prolonged aviation." They cannot be stabled with normal horses, as they will eat the horses. Hippogriffs, in contrast, are easier to train and handle, but aren't as reliable or devoted as mounts.
Following this information, we get the ratio of flying time to rest required, and the amount of in-game training time needed to master battle on a flying mount.
The next section dives full bore into the Aerial Combat rules. It outlines the function of speed, maneuverability, and attack modes in the context of the system.
For speed, the DM is directed to each creature's entry in the Monster Manual, and instructed to convert aerial speed per turn into speed per round. It then gives guidelines for how fast a creature can climb and dive, based on its speed. Diving creatures double damage from attacks due to momentum. The rules don't take terminal velocity or the force of gravity into account specifically, save to say that creatures can dive three times as fast as they can climb (roughly) but that's likely because if they did the system would become overly complicated.
Maneuverability is rated from Class A: creature can turn 180 degrees per round, requires one segment to reach max speed, can hover in place, stop and turn on a dime...to Class E: creature can turn 30 degrees per round and requires 4 full rounds to reach maximum speed). Interestingly, the most feared creatures in the game--dragons--are rated at Class E, due presumably to their bulkiness and lack of aerodynamics.
Attack Modes: This section breaks down every flying critter in the Monster Manual, and gives its maneuverability, strategy and M.O. for aerial combat in a neat, brief stat block. Quite useful, really. Part of me thinks this information should've been in the Monster Manual, but truthfully, it probably fits better here.
Following monsters, it breaks down the means by which "Men" (read: humans, demihumans, and humanoids) can fly, including spells and magic items.
Following this breakdown of information, we get into Conducting Combat, the actual system. It recommends the use of miniatures (one area that I will readily concede miniature figures would be of enormous use, given the complexities of the system), but states that the combat can be conducted "on paper," with nothing more than a pencil and a hex grid, which is also quite true.
There are two methods given. The simple method has combatants only able to turn at the beginning and end of their move, i.e. they can only move in a straight line during their turn.
For those desiring more realism, detailed rules are provided outlining how turns within each maneuverability class can be handled during the round. A character with Maneuverability A, for example, can do a full U-turn, or can make up to six, 30-degree turns in a round, or three, 60-degree turns, etc, provided that the flier does not exceed his or her speed in the round.
Aerial Missile Fire is next. Basically, the rules here are that all range increments are increased by one in the air (so short becomes medium).
Finally, we are given rules for remaining airborne when the mount or flier has sustained Damage. Greater than 50% of hit points taken forces the mount to land. Greater than 75% results in an uncontrolled plummet. Feathered wings, being more difficult to damage, gain extra hit points for this purpose, but only to the percentage total. For example, a Griffon with 50 hit points still has only 50 hit points, but instead of being forced to land after taking 25, because its wings are feathered it can stay airborne until it has taken 37 points of damage, and will plummet only if killed. The basic formula is to add half-again the total hit points to the base hit points (in this case, 50 + 25=75) and determine "forced to land" and "plummet" totals based on that modified number.
Finally, we get a note that made me chuckle. It's just a reminder that if players don't remember to have their characters take a minute to strap in when they leap in the saddle, they'll fall out within the first round and take 1d6 damage from the impact per 10 feet of the fall, while their mount goes off into the sunset alone.
Overall, this is a very fast, workable, intuitive and reads as though it would be a lot of fun, aerial combat system. Some aspects, I recognize, were kept into third edition, but honestly I never engaged in aerial combat in 3e so I don't know how similar the systems are. In any case, a lot of modern designers (of any game, not just D&D) could do worse than to look back at the roots of the hobby to find inspiration and techniques in some areas.