Flight recorder underwater beacons emit audio signals at 37.5 kHz. So, why use audio rather than radio for this important task?
One of my favorite televisions shows is Mayday, also called Air Crash Investigation. I enjoy this detective procedural which focuses on air crash investigation around the world. A recurring theme in these stories is the importance of flight data and cockpit voice recorders in helping figure out what happened.
Occasionally, aircraft are lost at sea. You then hear about the intense underwater search for these data recorders before the 30 day battery power expires. I thought it would be interesting to learn more about these flight recorder underwater beacons, which operate at 37.5 kHz using sound.
You may recall that neither sound nor light (electromagnetic waves) travel well in water. Sound or pressure waves work much better and can typically travel several kilometers. Interestingly, water medium provides many similar effects as the ionosphere, including attenuation, refraction, fading, multi-path and multi-hop propagation.
Similar to radio, our acoustic signals suffer absorption (path loss) which increases with frequency. There is also a lot of noise generated by waves and human activities (shipping) which tends to decrease with frequency. So, researchers found a sweet spot between 35-40 kHz for these narrowband beacons, which activate within four hours of water immersion and work down to 20,000 feet.
Acoustic transducers (shown at the left above) are kind of like “underwater speakers” that emit sound in all directions. Searchers locate the crash site using signal strength and TOA time of arrival techniques, just like with radio direction finding.
Flight Recorder Underwater Beacons in Action
As you can see above, a flight recorder contains a beacon transmitter (left), a solid state memory recorder for voice or data (center) and a power supply and aircraft interface unit (right). Beacons are powered by a 3 volt battery designed to last 30 days, moving to 90 days now. Typical transmit power is tens of watts, emitting 10 millisecond pulses every one to four seconds.
Flight recorders are solidly built to withstand impact shock, static crush and water pressure, as well as high temperature fire. Flight recorder underwater beacons must be strong enough to emit sufficient signal even when trapped within a fuselage or buried by mud and silt.
Our modern recorders capture more than a thousand channels of data, as well as four audio channels from the cockpit.