A transponder on a Piper Cheyenne that collided with a Cessna 172 last Friday was not transmitting altitude information.
Investigators say that's despite Federal Aviation Administration
rules that require it do so.
The National Transportation Safety Board says it's not clear
whether the transponder was malfunctioning or the pilot failed to
turn it on.
Planes flying within 30 miles of Denver International Airport
must transmit altitude information to air traffic controllers.
Five people on board the planes died in the collision that
showered a northwest Denver neighborhood with debris.
The Cessna crashed into a house and severed a natural gas line.
The home exploded a few minutes later.