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I recently learned that ELTs are required to run a maximum of 60 minutes before the power supply battery has to be recharged or replaced. This really doesn't seem like a great amount of time for this device to operate in order to facilitate locating by search and rescue crews. I would have said a more reasonable figure would be around 24 hours. Is it really true that ELTs are only effective for such a short period of time?

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  • $\begingroup$ The 406Mhz ELT's are picked up by the Cospas-SARSat satellites and sent to a dispatch center with the location (5km accuracy without GPS, 100m with GNSS) so they don't need to run a long time. The older 121.5/243Mhz models used to be monitored by the same system but are no longer monitored. $\endgroup$ – Ron Beyer Mar 9 '17 at 19:42
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The reference you quote is correct as it defines the "useful life" of the battery. that is: if you use it for more than 1 hour (that would include the cumulative test time) it must be recharged or replaced before being returned to service. But that isn't what you are asking about. You are asking for the duration of operation.

The requirement for Duration of Operation is 24 hrs.

RTCA DO-204A, MOPS for 406 MHz ELTs, states

1.5 Operational Goals

b. Operability

(1) The technical requirements of the 406 MHz beacon radiated signal are based on the COSPAS-SARSAT system.

(a) The 24-hour operating life requirement (§2.2.1) is based upon the maximum COSPAS-SARSAT system waiting time, the number of satellite overpasses required to optimize the beacon location solution and battery capacity limitation at low temperatures.

and

2.2.1 Duration of Operation

Refer to COSPAS-SARSAT document C/S T.001.

and from C/S T.001

4.5.1 Duration of Continuous Operation

The minimum duration of continuous operation shall be at least 24 hours* at any temperature throughout the specified operating temperature range.

The minimum duration of continuous operation for an ELT(DT) to meet the ICAO GADSS requirement at any temperature throughout the specified operating temperature range shall be 20† hours.

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You're misinterpreting a section of FAR 91.207.

That regulation says, in part:

c) Batteries used in the emergency locator transmitters … must be replaced (or recharged, if the batteries are rechargeable) —

  1. When the transmitter has been in use for more than 1 cumulative hour; or

. . .

This does not say the ELT/battery only has to operate for one hour, it says that if the ELT has been transmitting for one hour you must replace the battery, even if it's still perfectly good.
The one hour period is cumulative and includes the required annual ELT tests (91.207 paragraph D) and any accidental or incidental activations from a hard landing, inadvertently bumping the panel switch, etc.

This battery replacement requirement exists to ensure that the ELT will be able to function properly in a real crash & will transmit for at least the amount of time it's supposed to.

Per COSPAS/SARSAT specifications new 406MHz ELTs are required to be able to transmit continuously for a minimum of either 24 or 48 hours (depending on the type of beacon).
I believe the older 121.5 MHz ELTs also have a 24-hour continuous transmission requirement.

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Edit: Damn, I type slow. 3 other (and better) answers were posted while I was typing.


I think the premise of your question is not quite accurate. The only thing "required" (within this context), is that the battery must be recharged or replaced IF it has been transmitting for 1 cumulative hour. That does not really imply that an ELT battery is only "required" (i.e. there is a regulation somewhere) to operate for at least one hour.

An ELT transmitter might have been in use for 1 cumulative hour for several possible reasons, the most likely of which is accidental triggering, or bench testing it multiple times. I've been involved in several ramp searches because a random ELT was going off in a parked, unoccupied, plane.

The FAR that applies is 91.207. Paraphrased as:

No person may operate a U.S.-registered civil airplane unless... there is attached to the airplane an approved automatic type emergency locator transmitter that is in operable condition. Batteries used in the emergency locator transmitters must be replaced (or recharged, if the batteries are rechargeable) when the transmitter has been in use for more than 1 cumulative hour; or When 50 percent of their useful life (or, for rechargeable batteries, 50 percent of their useful life of charge) has expired, as established by the transmitter manufacturer.

So, if the ELT has been (or did) "go off" for at least one hour, the plane may not be operated until the ELT's battery is replaced or recharged.

Other documentation:

  • TSO-C126 describes requirements for the newer 406 Mhz ELTs.
  • TSO-C91 describes requirements for the older, 121.5 Mhz ELTs.
  • RTCA Document No. DO-188 describes the technical aspects of the batteries that must be used... type (lithium, etc.), ventilation requirements, mounting requirements, etc....
  • RTCA DO-204A (no public link that I could find) specifies a 24-hour continuous operation capability
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I presume you are referring to this FAR.

§91.207 Emergency locator transmitters.

(c) Batteries used in the emergency locator transmitters required by paragraphs (a) and (b) of this section must be replaced (or recharged, if the batteries are rechargeable)—

(1) When the transmitter has been in use for more than 1 cumulative hour; or

(2) When 50 percent of their useful life (or, for rechargeable batteries, 50 percent of their useful life of charge) has expired, as established by the transmitter manufacturer under its approval.

If you read it carefully you will notice that it doesn’t say that they have a maximum life of 1 hour. Just that they must be replaced if they have been in use for more than 1 hour. This only applies to TSO C91 (121.5MHz) ELTs. The 406Mhz batteries have different lifetimes.

Getting information on how long they must transmit is surprisingly hard. Many of the technical documents cost money to access, e.g. DO-204A, Minimum Operational Performance Standards (MOPS) for 406 MHz Emergency Locator Transmitters (ELTs) so I can’t read them.

I found one reputable source for the lifetime once activated.

"Depends on a lot of factors. The design criteria is that they transmit for at least 48 hours at 0-degrees, some will transmit much longer if the weather is warm, the battery fresh, etc. Or they will transmit for less if it's colder, if the battery hasn't been replaced when it was supposed to, etc." --Richard A. De Castro -N6RCX NREMT SAR Tech

http://pansar.voices-inc.com/ELT.htm

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