Transponders and runway status lights

3 min read

ASDE-X noteAt first glance you’re probably wondering what Runway Status Lights (RWSL) have to with aircraft transponders. The short answer is that they both help make the airport surface environment safer by helping to reduce runway incursions. Both provide additional information to pilots and air traffic controllers to allow for better decision-making and an in increase situational awareness.

I recently flew to Orlando International Airport (KMCO), and while en route I began my normal review the airport diagram to become familiar with the layout and to see if there were any changes since the last time I was there. After looking over the path from the expected runway to the FBO, I noticed two notes in the top left regarding ASDE-X & transponder operation and Runway Status Lights. I had heard about the FAA’s plan to install this light system at the larger airports, and was now looking forward to seeing them in person at Orlando.


Let’s begin with the first note about ASDE-X. ASDE-X stands for Airport Surface Detection Equipment, Model X. This uses a surface movement radar to accurately determine the position of aircraft and vehicles on the taxiways and runways. The ground controller is presented with a color screen to show the position of these targets with pinpoint accuracy. This system interrogates your transponder, which explains why you’re required to operate your Mode C transponder on the movement areas. This system is a great supplemental tool for the ground controller to monitor the position of all aircraft, especially when the visibility is low and it’s tough to see the runways from the control tower.

Runway Status Lights

The second note simply states that Runway Status Lights are in operation. These provide in-ground warning lights on the runways and taxiways, illuminating red lights when it’s unsafe to enter, cross or take off on a runway. This makes use of the airport’s ASDE-X system to detect aircraft movement on taxiways, runways and the final approach path.

RWSL Here’s how it works: when you’re holding short of a runway on a taxiway and there’s an aircraft on final approach, you’ll see a string of red lights (called Runway Entrance Lights) on the taxiway centerline leading from the hold short line onto the runway. Once the aircraft on final flies past your position, the lights will extinguish. Another set of lights are built into the threshold of the runway (viewable to aircraft waiting on the runway or beginning a takeoff roll), and illuminate red any time another aircraft is on that runway’s surface area. The practical thing to remember is that if you ever see these red lights, don’t taxi or takeoff, even if you have a clearance from ATC. You should hold your position and question the controller.

Runway Status Lights are operational at about 23 of the larger U.S. airports. For more information, and to see a great video of how this system works, visit the FAA’s page on the program.

Transponder Ops Change in A.I.M.

While we’re on the topic of transponder operation, it’s a good time to point out a recent change to the A.I.M. The publication now states that:

“Civil and military transponders should be turned to the “on” or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems.”

Even if you’re operating out of a pilot-controlled airport, make it a habit to turn the transponder on as you begin your taxi so that it becomes part of your flow. This will ensure you’ll always have it on at airports that have surface detection equipment regardless of the instructions posted on the airport diagram and A/FD.