Planes on PHL Approach transmitting on fire freqs?

[bettlewood]

I've been back into scanning for a few months now, and I noticed this oddity today when I turned Close Call on (BC246T).

I live along the PHL flight path in the Camden area and I'm getting close call hits on 166.6875, which looks to be a decommissioned National Incident Radio Support Cache/fire frequency per

National Incident Radio Support Cache - The RadioReference Wiki

Not every plane is transmitting this out, I'd say I'm getting it about 1 out of every 6 planes. All I hear is a repeated "buzz-buzz-buzz-buzz" noise, and it stops when the planes go out of range of my scanner.

Any ideas?

 

[nd5y]

If all you hear is buzzing how do you know it is coming from an airplane?

 

[bettlewood]

I live on the flight path, buzzing starts, plane passes over seconds later, buzzing stops after engines are out of earshot. Pretty sure it's the plane!

 

[nd5y]

It is probably an image, intermod or receiver overload.

 

[N2SCV]

Probably an image. Look up the IF frequencies in the manual, deduct the IF (or 2x or 3x the IF) from 166.6875, it will give you the air frequency. If your IF is 21.6 then 166.687-21.6=145.087-21.6=123.487.
So the air freq might be 123.490 or near there.