Here's a question that has me wondering if this is true: do winglets on 737NGs help at all in improving performance and saving fuel in hot and high conditions?
There's a reason for this question - for example, Qantas, Virgin Blue, Kenya Airways, Aeromexico, Southwest Airlines, SAA, and Turkish Airlines have ordered 737NGs with winglets or are planning to have them fitted with winglets. What these airlines have in common is that they are based out of countries that have brutally hot summer weather and/or high-altitude regions. Of course, hot and high conditons are a hindrance to engine performance, particularly during takeoff. Air tends to get less dense during hot weather and in high altitude regions, and that means engines have to consume more fuel for takeoff.
Winglets are normally supposed to improve a 737NG's range and even lower noise levels a bit. I have the impression that they seem to reduce the amount of power that engines require upon takeoff and in flight, which would also definitely save on fuel and probably mechanical stress, too.