My Wooden Airplane
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8/15/2013 - 9/30/2013

ELT

Pretty much all passenger aircraft must have an Emergency Locator Transmitter (ELT) installed. These devices react to a strong impact force (such as a crash) and broadcast a distress signal, allowing search-and-rescue people to locate the aircraft in the event that the pilot and/or passengers are incapacitated. Older units broadcast on frequencies of 121.5 and 243.0 MHz. Newer units broadcast on these frequencies as well as 406 MHz. The significance of the 406 MHz signal is that it is digital (whereas the other two are analog). This means specific information (such as aircraft registration and contact info, as well as position information) can be broadcast along with the distress signal on 406 MHz.

For the moment anyway, 121.5 and 243.0 MHz transmitters are still legal to be installed, but their signals are no longer monitored by search-and-rescue satellites, so their effectiveness is drastically reduced. (The FCC is currently trying to make them completely illegal, which doesn't make any sense to me, but who said a government agency needs to make sense?)
So for me anyway, the obvious choice for an ELT is one that broadcasts on 406 MHz.

Unfortunately, they are quite a bit more expensive than their analog counterparts. A 121.5 MHz ELT costs a little less than $200, while some 406 MHz ELTs can cost almost $4000! This ACK model E-04 ELT costs only $600, and the only difference that I can tell between it and the more expensive counterparts is that it does not have a built-in GPS receiver. However, it does have the capability to receive and store serial GPS data from an external source, so I spent $75 on an external GPS receiver and connected the serial output to the ELT. The only down-side to this external GPS receiver is that it requires 5V instead of the 12V that is usually available, so I had to buy a $3 switching regulator from eBay.

Some people appear to be concerned about hooking an ELT directly up to aircraft's power supply, stating that the ELT should be completely independent of the aircraft electrical system. On this ELT, the transmitter IS completely independent, but the circuitry for receiving and storing the external GPS data relies on aircraft power. The reason for this is because we don't want to run down the non-rechargeable ELT batteries for this functionality. An interesting side-effect of this arrangement is that the ELT annunciator panel emits a single "beep" whenever I turn on the master switch!

As with the Transponder antenna, I was tempted to make my own ELT antenna, but being dual band (and not multiples of each other), I decided it would be easier just to use the antenna that came with the ELT. (I also later heard that the ELT is considered to be a "system", and needs to be installed exactly as instructed in order to be considered legal.) I used some leftover galvanized vent duct material from the temporary engine shroud as the ground plane. Ideally, the antenna should be mounted in the middle of the ground plane, but that's where the elevator cables are, so I had to offset it a bit. Also, the ground plane is elevated slightly so I could run the coax to the bottom of the antenna without interfering with the fabric covering.

Total Time: 10.5 hrs.