"Power lines" warning

[jrdioko]

During a flight lesson today, on frequency we heard a brief warning siren, then a recorded voice saying "Power lines" several times. What was that?

[StuSEL]

What were you flying?

[jrdioko]

A Cessna, the recording was on whatever frequency we were on (tower, I believe). I looked back at the recordings and couldn't find it.

[MikeNYC]

Jrdioko,

Where are you located? There are automated systems in certain parts of the world to alert aircraft to wind turbine farms, aerial tramway cables, and power lines. You can find more information here:

https://www.pprune.org/archive/index.php/t-341173.html
http://ocas-as.no/solutions_2/power_lines/

[jrdioko]

Interesting. Hayward, California.

[NoMad]

The transmission tower on the approach path (presumably) has a receiver that picks up the aircraft's transponder reply so it knows there is a nearby aircraft.  It is a ~ 1/2 watt transmitter on the appropriate comm channel that makes that audible warning.  So it only transmits when there is an A/C nearby and at very low power so only the nearby A/C hear it.