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?
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.
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.
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.
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.