In SEOW, we now have the ability to specify how often aircraft breakdowns occur. In the DCS, under the Flight Modes tab, the "Aircraft Breakdown Rate" sliders for Allied and Axis flights determine what chance any given flight has of suffering equipment malfunction before or during takeoff. This is a fiendish addition to SEOW, since breakdowns are beyond the control of the commander and happen *after* the commander makes his flight orders, i.e. at DCS Build time. SEOW does not distinguish between AI flights and human flyable flights - all flights are treated the same when assessing breakdown probabilities.

Here is what happens. If the breakdown rate for the side in question is greater than 0%, then any one (or none) of the following effects can happen for any flight.
Canceled Flight
The entire flight can simply be canceled, with a probability set by the breakdown rate. If this happens, warning text will be placed in the side's pilot briefing screen in game, at the top. That's it - the flight is canceled and will have to be re-tasked next time.
Weakened Flight
The flight may actually be present in the mission, but at reduced strength. For each planned aircraft in the flight, the breakdown rate is applied. For example, a flight of 4 will likely reduce to 2 if the breakdown rate is 50%. Flight leads will just have to make do if they are a plane or two short - no room for tears.
Aborting Flight
The flight may appear and takeoff, but suddenly turn back to base and land. This is an aborting flight, e.g. engine running hot, wrong fuel loads, no radio, guns jammed, recall from base etc. Obviously this affects AI flights most of all, since human pilots can just ignore waypoints and fly on. But it will be most disconcerting if you are flying escort on a bomber flight that just turns tail for home after 30 km, or a recon flight just doesn't bother to fly to target. Aborted flights happen with a probably equal to half the breakdown rate.
So this feature is very useful, but in an annoying way. For people running large public campaigns, using sizeable aircraft breakdown rates may be contrary to running a reliable multiplayer campaign where most pilots EXPECT to get a seat every time. However, for those people interested in running historical scenarios where equipment reliability on one or both sides was low, this feature is just great. One scenario that springs to mind is the Stalingrad encirclement in late 1942. Russian machinery was much more reliable in the cold than German machinery. Now you can simulate this effect in SEOW.
If you don't want to tangle with aircraft breakdowns, set both sliders to 0%.
Cheers,
4Shades