ELT transmitting question

[Irish_Armada]

So on my solo XC last Saturday to Modesto, NorCal Approach was asking a plane to check a frequency to see if they could confirm an ELT was transmitting. The guy responded that he thought it was, and it was a strong signal. I also thought I heard him say "in the vicinity of..." as if he knew where it was transmitting from. This all happened while a lot of stuff was going on for me so I don't remember the exact phrasing of the conversation. Is this something that ATC can ask any of us? Also, how would you actually do this, would you flip over to 121.5 on Comm 2 and listen there? And how do you get locational information from this? I remember on one of my first lessons my instructor did this with Travis Approach but I wasn't really aware at the time what he was doing -- hell, I probably thought the right rudder was the throttle then... But on both occasions, it sounded like an ELT was transmitting in error, rather than there actually being an incident.

 

[markb5900]

Yes you go to 121.5 and listen.
The closer you are the louder it will be for you.

Yes, ATC may ask you to do it, if you can't, just say unable. They will understand, but if at all possible, IMHO you should. It COULD be a down plane they are trying to find a general area for.

If you only have one radio, then tell them you will be off the frequency for a few minutes to listen and report when you are back on their frequency.

 

[Palmpilot]

I also thought I heard him say "in the vicinity of..." as if he knew where it was transmitting from.

Unless the pilot has direction finding (DF) capability, the best he/she can do as far as locating it is to report his/her own location and the perceived signal strength.

As for finding the signal without DF equipment, Civil Air Patrol teaches its pilots ways of doing this, such as wing shadowing, build-fade, etc., but they are not normally relied upon, because CAP airplanes have DF equipment installed. I did have to use those techniques to find an ELT in Sacramento once when the airplane's DF equipment was malfunctioning.

 

[Threefingeredjack]

Once the 406 becomes ubiquitous the process will be much more accurate, and wing shadowing to create LOPs will become a lost art. It is possible for one aircraft to get a fairly accurate location by using the wing shadow technique coupled with a plotted arc and working inwards.

 

[Irish_Armada]

Thanks for the responses, question answered!

 

[denverpilot]

You can also get a feel for signal strength by tuning slightly off of 121.5. If you're able to hear the sweeps clearly from a 121.5/243.0 ELT on modern radios more than one notch off the center frequency, you're right on top of the bloody thing.

Let 'em know it's "very strong" at your location and give 'em really good coordinates for where you are. (Radial and DME distance off a VOR or better, GPS lat/long.)

406 MHz beacons put out a tiny signal on 121.5 and it's interrupted every few seconds. You'll never hear one very strong, but if you hear regular timed breaks in the transmission, you can let the controller know its likely a new beacon with 406 capability so they can send an appropriate note on to SAR crews looking for it. Maybe it's had the antennas ripped off in the crash and satellite isn't picking up the 406, but a close in SAR team (air or ground) will know to listen for 406 data bursts and if they have the correct gear, they can get the ELTs coded information which may include last good GPS coordinates or at least identify the aircraft through 406 beacon registration information.

And folks with 406 beacons, please register and keep your beacon information updated.

 

[flyingron]

I've had ATC ask me to listen on 121.5 for ELTs a few times. In the busy mid-atlantic it's probably a reasonable place to narrow the search...

 

[flyingmoose]

Like they said, without DF equipment it's tough to locate direction. Depending on where the antennas on your airplane are located then you can wing shadow, but that is not normal stuff for someone just listening for a signal. You can also "detune" your radio to a few steps and see if you still hear it. 121.55, 121.6 and you play with the squelch to see how string the signal is. Long story short, you can do it with just a regular radio, but it takes practice and it's not something ATC would ask or expect you to do.


Sent from my iPad using Tapatalk HD

 

[flyingron]

I'm quite familiar with the techinques. Did some ham radio fox hunting it the day...also did some ad hoc df'ing to find a stuck mike once. One handy way if you're reasonably close is to remove the antenna from your handheld and then you can use your body as a shield.

 

[Capt. Geoffrey Thorpe]

Like they said, without DF equipment it's tough to locate direction. Depending on where the antennas on your airplane are located then you can wing shadow,

That sounds interesting...

I doubt that the fabric wings on my ride would block much (leading edge is aluminum), but the antenna is sitting in the middle of aluminum sheet about 3 - 4 foot square on top of the fuselage - would that block a signal that is right "underneath" the ground plane?

 

[flyingmoose]

That sounds interesting...

I doubt that the fabric wings on my ride would block much (leading edge is aluminum), but the antenna is sitting in the middle of aluminum sheet about 3 - 4 foot square on top of the fuselage - would that block a signal that is right "underneath" the ground plane?

I have never tried it with a fabric wing but I would bet that you would get some effect. Even with the aluminum wing, it will not block the signal totally but you can def hear a difference. Basically the concept is to fly steep turns and listen to the signal strength while knowing what side of the airplane the antenna is located and on what radio. Once you have a general direction then you fly that way while listening and then do it again. Slowly narrowing it down to an area. Blah Blah So yea if your antenna is right on top of the aircraft then anything right below would have a partial blocked signal.
Cheers

 

[gprellwitz]

The question has been answerd by others already, but I'll note that you should be monitoring 121.5 on COM2 when able in any case. IIRC, there's a NOTAM out to that effect. When I hear an ELT while monitoring guard, I let ATC know. They'll ask the strength, and for me to tell them if it's getting weaker or stronger on my route of flight.

 

[Palmpilot]

One handy way if you're reasonably close is to remove the antenna from your handheld and then you can use your body as a shield.

I used that method to find an ELT at Monterey Airport one day, although I didn't have to remove the antenna. Tower asked me to check if my ELT was on, as it apparently started about the same time I landed. The funny thing is that I had been taught that method only a week before in a CAP ground school course.

For lurkers, you hold the handheld radio right in front of you, and rotate your body around your vertical axis until you get a null (where the signal sounds weakest), and then you know that the signal is coming from behind you. If the signal is too strong for you to hear a null, then you can make it weaker by tuning to an adjacent frequency, or by removing the antenna as FlyingRon suggested.

 

[Palmpilot]

That sounds interesting...

I doubt that the fabric wings on my ride would block much (leading edge is aluminum), but the antenna is sitting in the middle of aluminum sheet about 3 - 4 foot square on top of the fuselage - would that block a signal that is right "underneath" the ground plane?

One way to find out is to tune to an ATIS while airborne, do some 360 degree turns, and see if you hear any nulls.

It's very approximate, but it's better than nothing.

 

[Capt. Geoffrey Thorpe]

One way to find out is to tune to an ATIS while airborne, do some 360 degree turns, and see if you hear any nulls.

It's very approximate, but it's better than nothing.

I'll try that.

 

[PuertoRicoFlyer]

If ATC asks you to check 121.5 for them, you might not be the only aircraft they are asking. With multiple aircraft in different locations reporting "loudness" they can tell if the signal is valid and get a rough idea of where the signal is comming from. This starts "narrowing" the location for those who will do the actual search.

You are one part of a much bigger operation. B)

 

[Micha]

Basically the concept is to fly steep turns and listen to the signal strength while knowing what side of the airplane the antenna is located and on what radio. Once you have a general direction then you fly that way while listening and then do it again.
Cheers

You have to be careful though. This will probably work well when the transmitting antenna is near vertical, an accidental activation with the airplane sitting on the ramp or ATIS. However if it is a crash the ELT antenna may be at an angle to vertical. If your receiving antenna becomes perpendicular to the ELT antenna during the turn the relative polarity of the transmit vs receive antennas will case a drop in signal that is a result of the ELT antenna angle not the relative bearing between your aircraft and the ELT.

 

[denverpilot]

You have to be careful though. This will probably work well when the transmitting antenna is near vertical, an accidental activation with the airplane sitting on the ramp or ATIS. However if it is a crash the ELT antenna may be at an angle to vertical. If your receiving antenna becomes perpendicular to the ELT antenna during the turn the relative polarity of the transmit vs receive antennas will case a drop in signal that is a result of the ELT antenna angle not the relative bearing between your aircraft and the ELT.

Antenna polarization issues rarely happen in the real world of DFing ELT signals. You won't fly a ground track and exact bank angle enough times in a row to even notice them. And more often than not, if you're flying near any significant terrain (or even power lines) you'll be experiencing some multipath on receive.

(And frankly, most of the time the crash was survivable, the aircraft is vertical or nearly so. When it's not, the ELT antenna is a couple hundred yards behind the aircraft cabin, in the debris field, or smashed under the tail that's sitting on top of it.)

In 22 years of DFing ELTs, I've never seen anyone fooled by an antenna polarization null.

Overall, signal strength trumps nulls every time with ELTs. Blocking the signal with something like the wing of the aircraft or your own body, if hunting on foot, works best. You're never going to match up antenna angles and what-not from a low-level banking aircraft more than once around the circle. And you continually refine bearing relative signal strength, and signal quality when DFing anything, even non-beacons.

Here's some fun trivia... Which polarization, vertical or horizontal, is a radio signal from a vertical antenna that's leaning over exactly 45 degrees?

Or another... Why doesn't antenna polarization matter much on sky wave signals?

(hint: Vertical and Horizontal polarization are a model. They do not match all real-world conditions.)

For a more practical application, what polarization are most FM Broadcast band transmitters, and why? What are the benefits and disadvantages?

I've seen scenarios far more interesting, though... The aforementioned power lines can carry an ELT signal many miles from a crash site, as one example.

A favorite hiding place for one team member of a DF group that's now defunct, during practice searches, was in a metal culvert under a bridge that had high-tension lines running overhead.

 

[Micha]

Antenna polarization issues rarely happen in the real world of DFing ELT signals. You won't fly a ground track and exact bank angle enough times in a row to even notice them.

...

You're never going to match up antenna angles and what-not from a low-level banking aircraft more than once around the circle.

Actually if the sum of the angle of the ELT antenna from vertical, and the bank angle of the search aircraft is greater than 90 degrees then the transmit and receive antennas will be perpendicular twice during each 360 turn.

Here is are some questions for you. What is the maximum attentuation provided by a wing? What is the attenuation provided by having a receive antenna polarization oriented 85 degrees from the transmit antenna?

 

[denverpilot]

... in a perfect circle over the ground with no wind and a perfect bank angle around the second circle.

Trust me, I've done it. It averages out such that even though the effect is there, you'll never notice it.

Feel free to wander out and shoot some theoretical video to try to show it happening in a real-world aircraft.

Patterns on vertical antennas mounted on aircraft aren't even close to the theoretical perfect vertical either, and most DF antennas from the common DF receiver manufacturers are bent into a "raked" look about halfway up. They're not straight.

I wager you'll never see it, even if you try. Feel free.

 

[Mike I]

The question has been answerd by others already, but I'll note that you should be monitoring 121.5 on COM2 when able in any case. IIRC, there's a NOTAM out to that effect.

Not just a NOTAM...it's also in the AIM:

AIM 5-6-2 (a) 3:

All aircraft operating in US national airspace are highly encouraged to maintain a listening watch on VHF/UHF guard frequencies (121.5 or 243.0 MHz).

 

[gprellwitz]

Not just a NOTAM...it's also in the AIM:

AIM 5-6-2 (a) 3:

Actually, I think they may have moved it so it no longer needs to be disseminated as a NOTAM.

 

[Palmpilot]

Actually, I think they may have moved it so it no longer needs to be disseminated as a NOTAM.

Copied from the FDCG section of DUAT just now:

FDC 4/4386 FDC SPECIAL NOTICE...
NATIONAL AIRSPACE SYSTEM INTERCEPT PROCEDURES. AVIATORS SHALL REVIEW THE FEDERAL AVIATION ADMINISTRATION AERONAUTICAL INFORMATION MANUAL (AIM) FOR INTERCEPTION PROCEDURES, CHAPTER 5, SECTION 6, PARAGRAPH 5-6-2. ALL AIRCRAFT OPERATING IN UNITED STATES NATIONAL AIRSPACE, IF CAPABLE, SHALL MAINTAIN A LISTENING WATCH ON VHF GUARD 121.5 OR UHF 243.0. IF AN AIRCRAFT IS INTERCEPTED BY U.S. MILITARY AIRCRAFT AND FLARES ARE DISPENSED, THE FOLLOWING PROCEDURES ARE TO BE FOLLOWED: FOLLOW THE INTERCEPT'S VISUAL SIGNALS, CONTACT AIR TRAFFIC CONTROL IMMEDIATELY ON THE LOCAL FREQUENCY OR ON VHF GUARD 121.5 OR UHF GUARD 243.0, AND COMPLY WITH THE INSTRUCTIONS GIVEN BY THE INTERCEPTING AIRCRAFT INCLUDING VISUAL SIGNALS IF UNABLE RADIO CONTACT. BE ADVISED THAT NONCOMPLIANCE MAY RESULT IN THE USE OF FORCE.

[emhasis added]

The NOTAM says "shall" (if capable), whereas the AIM only says "highly encouraged."

So if you have a radio that you're not using for another purpose and are not monitoring guard, you're violating the NOTAM.

 

[gprellwitz]

Copied from the FDCG section of DUAT just now:

[emhasis added]

The NOTAM says "shall" (if capable), whereas the AIM only says "highly encouraged."

So if you have a radio that you're not using for another purpose and are not monitoring guard, you're violating the NOTAM.

Okay, that's about what I remembered, but that FDC wasn't showing up in the ForeFlight briefings for some reason (or maybe I just missed it?)

 

[Palmpilot]

Okay, that's about what I remembered, but that FDC wasn't showing up in the ForeFlight briefings for some reason (or maybe I just missed it?)

I don't know the options on ForeFlight, but on DUAT, a standard briefing does not include FDCG NOTAMs (i.e., general FDC NOTAMs) by default. You have to select the FDCG check box, which is right below the FDC NOTAMs check box.

 

[SCCutler]

Minor irritant for me, in that my No. 2 COM is a Narco Mark 12D (notwithstanding snarky Narco comments, save your breath Kent, an outstanding COM radio), which has a design "feature" of intentionally precluding squelch on 121.5. So, unless I want to listen to frying bacon, I have to use the 430 for monitoring Guard, and comm on the Narco.

Nothing really wrong with that, except it doesn't allow me to use auto-tuning, etc.

 

[Capt. Geoffrey Thorpe]

Copied from the FDCG section of DUAT just now:

[emhasis added]

The NOTAM says "shall" (if capable), whereas the AIM only says "highly encouraged."

So if you have a radio that you're not using for another purpose and are not monitoring guard, you're violating the NOTAM.

I guess that depends on the definition of "if capable".

 

[Palmpilot]

I guess that depends on the definition of "if capable".

Well, the way I look at it, if I need to listen to the ATIS at the same time as I'm monitoring ATC, then unless I have a third radio on board, I'm not capable of monitoring guard during that time.