We put aftermarket winglets on the jet I fly. I personally wasn’t around pre-winglets, but here’s what the manufacturer’s data says, and my company has found:
The winglets have improved our runway performance. We can use a shorter runway that we could before, or we can get about 8% more weight off the same runway. That may not sound like a lot, but it adds up to an additional 7 passengers or 500 miles worth of fuel.
The winglet manufacturer’s claim of 5-8% fuel savings on our airframe seems to be accurate. One reputable operator even claims 10%, which really adds up to serious $$ at the end of the year. Heavily loaded jets can’t climb to their most efficient cruise altitudes right away, because they are too heavy. They climb to some lower cruise altitude, and step up to more efficient altitudes as the airplane becomes lighter with fuel burn. Our winglets allow us to climb to an initial cruise altitude that is 2,000-4,000 feet higher that we used to use, which accounts for a significant portion of the fuel savings.
Believe it or not, airplanes have speed limits when they are at lower altitudes, mostly due to the increased density of traffic. During a normal descent, we used to be able to retard the throttles to idle and the airplane would easily slow to meet those restrictions. With the winglets, we have to be careful about managing power and descent rate so as to get the airplane slowed down in time to keep the air traffic controllers happy.
YMMV.