The 121.5 MHz will be blocked. it transmits the constant whoop whoop with just 100milliWatt power output. This signal is used for homing.
The 406 MHz signal is picked up by the sattelite and transmitted to the rescue coordination centre in that area. The 406 MHz signal is transmitted with about 5 Watt, a huge difference with the 121.5 MHz, as this one is the one that is needed to get their attention.
With 121.5 MHz the sattelite could indicate the position with an accuracy of about 12 Nm. Within this 12 Nm radius, the SAR team will use to their direction finder to home in on the 121.5 MHz signal. Using the 406 MHz technololgy they position accuracry gets better due to higher frequency / smaller wave length. Position can be indicated to a radius of 3 Nm. This is the technology that is used nowadays. The SAR team has to search withing the 3 Nm using there homing equipment, or visually search that area.
Some ELTs and most PLBs contain a GPS receiver, these will also transmit their position in the 406 MHz message making the accuracy really good.
I would recommend to active ASAP. When an ELT of PLB is in the water the signal attenuation will be huge, and make the ELT/PLB active. In water try to keep the antenne up and out of the water if possible.
The best is to use both ELT and PLB, as the ELT will activate manually but is useless as it gets underwater. A PLB won’t activate automatically, but can be kept on you. When frequently flying over water a Safelink is also worthwhile. A safelink generates an man overboard AIS message which is visible on most vessels radar/AIS screen. AIS is a marine system comparable to ADS-B.
I always carry a PLB even if the aircraft has an ELT. An ELT isn’t much use if it gets destroyed or disconnected in the accident, or sinks to the bottom of the sea…
Spoke to my avionics shop this morning but it seems that the question when to manually activate the ELT had been asked for the first time.
After a bit of internal discussion they confirmed that the ELT when activated will transmit a tone on 121.5 MHz which will effectively block the frequency within the range of the ELT for other radio transmissions. The range, however, will not be all that large (whatever that may be) and diminish further as the ELT will only be activated if the danger of a forced landing is imminent, i.e. the aircraft will get closer to the ground until an eventual landing thus further reducing range.
One point of note though, the first signal on 406 MHz will only be transmitted 50s after activation of the ELT, so it does make sense to activate early to reduce the danger of no transmissions getting out if the ELT or its antenna might be destroyed during a forced landing.
I will amend my emergency checklist to include ELT – ON whenever there is a danger of a forced landing but will try to establish contact with ATC first on a frequency other than 121.5 MHz so as not to block my own transmissions. However, this will need to be handled flexibly depending on the situation. Aviate and Navigate come before communicate and with an engine out close to the ground there might be no time at all to manually activate the ELT.
Thanks for all your input.
The ELT will definitely make communication on 121.5 very difficult. I recently listened to a recording on YouTube from the rescue of a US naval aviator from the North Sea where you could distinctly hear the ELT and the stations on the scene were struggling to communicate, switching back and forth between frequencies trying to make contact with rescue helicopters coming to the scene.
It may take some time for the ELT to get a GPS position fix. I remember reading a report saying that some took hours, and some actually never got a reception, when they were activated on the water surface.
So I’d turn it on asap. Better that it’s trying to get a fix while you’re warm and try rather than starting the clock ticking when you’re wet and cold.
I am sure an ELT will block 121.50 and probably nearby frequencies too.
However I would still turn it on immediately following an engine failure, at any altitude. And set 7700 etc. The 121.5 is used only for close-in search/rescue VDF, but the 406MHz is used for satellite position fixing and that is transmitted as a brief burst once a minute only.
I will call my friendly neighbourhood avionics shop tomorrow and try to find out whether the 121.5 tone will block the frequency. As they supposedly test the ELTs regularly they should hopefully know.
It is most likely you aren’t communicating on 121.5 anyway. I would only turn my radio off to avoid a fire, not to avoid distractions. I wouldn’t turn the ELT on at 15000ft but I would never rely on the impact switch to activate it.
I’ve heard ELT’s on 121.5 before while flying. I don’t think that they would interfere too much with communications. And in any case, S&R can easily use other frequencies.
Some useful info on https://en.wikipedia.org/wiki/File:Radio_beacon_of_distress_on_121,5_MHz.ogg
Apparently the transmission on 121,5 is continuous so yes, it is a real issue.
406 MHz does bursts of 440 mS every 50 seconds – much more reasonable.
