airbags help a person avoid burial by making the user an even larger object relative to the moving snow, which forces the person toward the surface. Avalanche airbags work on the principle of inverse segregation (granular convection). Avalanches, like mixed nuts and breakfast cereal are considered granular materials and behave fluid-like (but are not liquids) where smaller particles settle to the bottom of the flow and larger particles rise to the top. Provided the airbag is properly deployed, the chances of a complete burial are significantly reduced. With freeriding becoming more popular in recent years, avalanche airbags have become (relatively) commonly used equipment, with one German producer reporting over 20,000 sales in the 2012/13 winter season. One complication is that the small explosives or compressed air canisters that deploy the airbag might need to be emptied or expended in order to fly on a commercial airplane. Though the International Air Transport Association has approved carrying charged devices, the U.S. Transportation Security Administration requires that they be discharged. For this reason, some users who travel by air buy a new canister or refill an empty one after arrival. In 2014, one company introduced an airbag system that uses a battery-powered fan to inflate its bag in around 3.5 seconds. Avoiding the use of compressed gas is claimed to avoid problems with travel restrictions. The manufacturer claims a number of other advantages for the system, such as the ability to easily and repeatedly test it, the ability of a continuously operating fan to keep the airbag inflated even if it receives a puncture or small tear and the fact that the bag can be easily deflated and repacked, making the system available for a second deployment if needed. Other devices More backcountry adventurers are also carrying Satellite Electronic Notification Devices (SEND) to quickly alert rescuers to a problem. These devices include the SPOT Messenger, Emergency Position-Indicating Radio Beacon (EPIRB) or Personal Locating Beacons (PLBs) containing the Global Positioning System (GPS). This device can quickly notify search and rescue of an emergency and the general location (within 100 yards), but only if the person with the EPIRB has survived the avalanche and can activate the device. Survivors should also try to use a mobile phone to notify emergency personnel. Unlike the other devices mentioned above, the mobile phone (or satellite phone) provides two-way communications with rescuers. On-site rescuers (usually companions) are in the