We recently posted a story about U.S. Air Force F-16Cs flying a long-range counter-air patrol mission into the South China Sea while fully armed with air-to-air missiles. The unusual weapons configuration, with five AIM-120 AMRAAMs on wingtip rails and underwing stores pylons and one AIM-9 Sidewinder underwing, as well, spurred a flurry of very familiar questions that I have gotten repeatedly for a decade: Why do F-16s carry AIM-120 AMRAAMs on their wingtip rails almost exclusively, instead of AIM-9 Sidewinders? I thought it was time to settle this once and for all with an answer straight from the F-16’s manufacturer, Lockheed Martin.
The AIM-9 was carried on the Viper’s wingtip rails for well over the first decade and a half of F-16 operations. After the AIM-120, introduced in the early 1990s, proliferated throughout the USAF’s F-16 fleet, the Sidewinders largely moved onto the outer underwing stations. Even today, with the advanced AIM-9X and its high-off-boresight targeting ability, the Sidewinder is still largely mounted under the Viper’s wings. Some have even mentioned this may impact the missile’s ability to lock up targets far off the jet’s centerline, but that doesn’t seem to be a major concern for the USAF.
There have been various explanations given in forums and on social media for the near-omnipresence of the AIM-120s on the Viper’s wingtip rails, even during training where air-to-air combat isn’t the focus. These have included issues with the missiles themselves, as well as the idea that the AIM-120s act similar to winglets, offering the F-16 better efficiency when they are mounted. Another common explanation is that this configuration cuts down on wing flutter.
Wing flutter is defined as “a dynamic instability of a flight vehicle associated with the interaction of aerodynamic, elastic, and inertial forces (aeroelastic phenomena).” Flutter has airframe fatigue and even performance impacts in particular areas of the flight envelope and can be minor to catastrophic in nature. If you have ever seen an armed F-16 taxi by you in person, the AIM-120s on the wingtips certainly bounce around, almost to a comical degree.
With all this in mind, I asked Lockheed Martin directly about this for a definitive answer. Here is their response:
The F-16 is able to carry different missiles on multiple stations at the same time, such as the AIM-120 and AIM-9. Because of that capability, the AIM-120 is positioned on the wingtip to reduce wing flutter, while still ensuring mission performance.
So, the wing flutter explanation is indeed the right one. This would explain why most F-16s fly with AIM-120 captive training rounds, which weigh 335 pounds, just like their live counterparts, on their wingtips regardless of the training situation.
Some air arms that fly the F-16 are not cleared to purchase the active radar homing AIM-120, such as Iraq and Egypt. So, these aircraft are often seen flying with AIM-9s, which weigh about half as much as an AIM-120, on their wingtip rails, as the semi-active radar homing AIM-7 Sparrow that they are allowed to buy is not capable of being mounted there.
With over 4,600 F-16s delivered to air forces around the globe, roughly half of which are still flying, the F-16 has been studied incredibly closely over the decades and evolved accordingly. This includes insights into wing flutter, including with different stores configurations, and their impact on performance and airframe life. In fact, these studies continue to this very day:
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