Mentioned in http://www.airwaysim.com/forum/index.php/topic,48694.0.html
some models would be more popular if there was more access to the planes. But the brokers don't get them, because they're not deemed to be popular.
If an airline has a 5+ plane order of a model, allow them to flag that model as 'popular', so that the brokers acquire planes from empty slots. If the airline drops below 5 planes on order, then their flag is automatically turned off. As long as someone has a model flagged, unused/cancelled production slots produce a plane for the brokers.
Lets players who don't want to go the 707/DC8/f27/BAC/a320/737/etc route have a little better chance of growing their fleets. Worst case (say I'm the only one to order NAMC, and keep it flagged), it adds a couple of hundred planes that might not get leased to the global stats. But when the game starts with 2000 Curtis Commandos, 5000 DC-3s, I don't think that's much of a problem.