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Does the Emergency Locator Transmitter (ELT) on smaller aircraft activate automatically in case of an accident? If so, what triggers it? Is it a certain amount of G's that triggers it or are there other parameters taken into account?

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    $\begingroup$ Striking the ELT smartly against one of the aircraft tires is a great way to set it off. That is one of the best ways to test the ELT for its annual ops check. $\endgroup$ – J Walters May 23 '17 at 14:21
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    $\begingroup$ I managed to activate an ELT the first time I landed a 210. I call it "landing with authourity." It was the first time I had landed a single engine piston plane in years. Flared a bit high... $\endgroup$ – acpilot May 23 '17 at 19:42
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There are four different types of ELTs:

  • Automatic fixed ELT (ELT(AF)). An automatically activated ELT which is permanently attached to an aircraft.

  • Automatic portable ELT (ELT(AP)). An automatically activated ELT which is rigidly attached to an aircraft but readily removable from the aircraft.

  • Automatic deployable ELT (ELT(AD)). An ELT which is rigidly attached to an aircraft and which is automatically deployed and activated by impact, and, in some cases, also by hydrostatic sensors. Manual deployment capability is also provided.
  • Survival ELT (ELT(S)). An ELT which is removable from an aircraft, stowed so as to facilitate its ready use in an emergency, and manually activated by survivors.

All the aircraft I fly in (all two of 'em) have the third type. If the aircraft is submerged or experiences a high G-force, the ELT will be triggered. There's also a button on the panel to activate it manually.

I'm not sure what G-forces are required to set off the ELT, but they must be fairly significant—I've slammed my plane down pretty hard while misjudging my landings, and never had it go off.

According to https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-44A.pdf:

To meet the "g" force requirements of TSO-C91, automatic fixed-type inertially activated ELT's (except overwater type) must activate at any inertial force, parallel to the longitudinal axis of the aircraft when installed in accordance with the manufacturer's instructions, of 5(+2, -0)g and greater for a time duration of 11 (+5, -0) milliseconds or longer.

So, there you have it. You need 5–7 Gs for 11–16 milliseconds—but only in the direction the plane is pointing. Misjudging your landing flare and stalling the plane 20 feet off the runway won't set it off, as that's not on the axis they're measuring.

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    $\begingroup$ ELTs can also be turned ON or off manually at the device. $\endgroup$ – mongo May 23 '17 at 19:55
  • $\begingroup$ I know some can; is it possible with all of them? Of the four types I listed here, two of them specifically mention that they can be triggered manually. $\endgroup$ – Aquarello May 23 '17 at 20:13
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    $\begingroup$ @Aquarello The Automatic deployable ELTs that are in general aviation aircraft have a switch for ARMED and ON. ON is used for testing them at annual and after battery replacement. Many (most?) also have an OFF position so they aren’t accidentally triggered during maintenance. Some of them have a switch on the panel that will turn them on. $\endgroup$ – JScarry May 23 '17 at 20:33
  • $\begingroup$ @jscarry Note that the annual testing requirements also include a requirement to test the crash sensors, etc. So while turning the switch to "ON" for testing is fine, it is not sufficient. $\endgroup$ – J Walters May 24 '17 at 14:58
  • $\begingroup$ @JonathanWalters Never heard of that requirement, but you are correct, it is in 91.207. If I am reading the manual correctly, the listening for the sweeps when the unit is switched to ON is all you need to do. From the ACK website, "Model E-01 Emergency Locator Transmitter The model E-01 ELT must be inspected yearly to insure continued airworthiness. The procedures as described in section 7 (Periodic Maintenance) of the installation and operation manual part number E-01-M (all revision dates) should be followed. These tests also fulfill the requirements of FAR 91.207." $\endgroup$ – JScarry May 24 '17 at 17:14
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There is another way that an aircraft ELT (or for that matter, any ELT) can be set off: By a mechanical or electronic failure of the unit. This is usually caused by corrosion and is more common on maritime ELTs than on ELTs designed for aviation use. Believe me ... it is more common than you might think. As a USCG pilot, I have many times located an active ELT that did not appear to be activated by any of the previously mentioned methods. If your ELT mysteriously activates (especially if that activation is intermittent), consider that possibility. We once flew over 800 miles to a large freighter where the ELT appeared to have been deliberately activated (i.e., manually turned on) only to have the surprised crew discover the ELT safely untouched in its bracket. They were impressed that we came that far to see if they were okay; we had left Shemya Island in the Aleutians in the middle of the night and just wanted to go back to sleep.

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I believe that it is a requirement that the ELT transmitters have an ON switch on the transmitter housing. I don't have a cite, but I might be able to dig it out. There is a similar requirement for marine emergency beacons.

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