Timing for flying into soybeans: Dean has found that flying on the cover crop when leaves are turning yellow, but have not yet dropped is best for beans. If the cover crop is flown into the beans after leaf drop, the cover crop seed might germinate, but the cover crop roots might not penetrate the soybean leaves, preventing establishment.
Timing around forecasts: Since aerially-applied seed sits on the soil surface as opposed to getting drilled into moist soil, it is important that rain is forecasted within the next week or so after flying on cover crops.
However, Dean recommends not to get caught up in the details of picking the perfect time to plant cover crops. He recommends just getting the cover crop planted, because it is difficult to find perfect conditions.