When you’re settling into your seat at the airport, you’ll notice some interesting mechanical sounds coming from outside the plane. If you’re afraid of flying, this mystery can cause a great deal of anxiety. It’s a good thing there’s nothing to worry about pre-flight checks. In fact, you want everything to go smoothly during this time frame. One of the methods airports use to keep planes ready to go is by supplying portable power supplies that literally jump start the plane on the runway.

According to Start Pac, that’s the mechanical sound you hear in many cases.

Standard airplane battery packs function like a car, so the plane needs to be moving in order for the pack to charge. Unlike a car, a plane’s battery must power a great deal of electronics so passengers can sit comfortably prior to takeoff. These systems power everything from air conditioning to in-flight movies.

In order for the plane to function properly, it needs a jump start from a portable battery pack. This will also give the turbine engines a boost, which enables the plane engines to turn over and the plan to function properly.

Airports service many planes in a single day, so portability is key. There are power packs embedded into the concrete, but many commercial airports will taxi a portable unit to the runway and charge the plane through that method. Portable power can also recharge quickly, so multiple units can be used for a boost while others are charging for the next shift.