NTSB DC Midair Hearing Panel 3 Covers Frequency Issues

During the third panel session of the NTSB’s investigative hearing on the Jan. 29, 2025 midair collision between an airliner and a U.S. Army helicopter, the issue of frequency blocking was raised. Sixty-seven people were killed in the collision of an Army UH-60L Black Hawk helicopter and a PSA Airlines CRJ700 near Ronald Reagan Washington National Airport (KDCA).

Helicopter traffic in the KDCA Class B airspace operates on a separate frequency from fixed-wing traffic, so while the air traffic controller can hear all traffic, helicopter pilots can’t hear the transmissions from fixed-wing aircraft, and fixed-wing pilots can’t hear calls from helicopter pilots, although both can hear the tower controller.

An issue raised during the hearing was that transmissions can be blocked or “stepped on,” and there isn’t a way for the controller to know if a transmission was stepped on.

Clark Allen, who at the time of the accident was operations manager at the KDCA tower, was asked to confirm the above conclusion. “There’s nothing,” he said.

“Is there any technology available…that could be used to alert a controller that a transmission has been stepped on?” he was asked.

“There were voice technology systems…from many years ago, 20 years ago or so,” he said, “that were under development that would address, to an extent, stepped on [transmission]. Basically, the controller…their transmissions would essentially make the channel busy, and other transmissions would be blocked. That is not the system that we currently have, but that technology does exist in the world, but not in the [National Airspace System].”

AIN has been researching the issue of technology that would mitigate stepped-on transmissions. So far, the sole system available for this purpose appears to be software-defined radios from Rohde & Schwarz. According to the company, “The most recent innovation is detection of simultaneous transmissions. Rohde & Schwarz offers the world’s first ATC radios for detecting simultaneous transmissions.”

The Rohde & Schwarz Series4200 ATC radios can automatically detect simultaneous transmissions. “As soon as a simultaneous transmission is detected, the air traffic controller receives an acoustic or visual warning at his/her working position. Existing radios can easily be updated with this functionality via a software update.”

There was no mention at the NTSB hearing about the availability of this technology and whether the FAA is planning to implement it.