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Friday, April 1, 2005

Avionics

ELT Testing: What is Required?

Most aircraft are required to carry an emergency locator transmitter, including all private and commercially operated jets since January 1, 2004. While there are still plenty of ELTs operating on 121.5/243.0 megahertz, after 2009, these will need to be replaced by the new 406-megahertz ELTs.

Like any other piece of installed and required equipment, ELTs must be maintained, but there is no consensus in the industry as to what kind of maintenance and testing ought to be done to keep an ELT healthy.

Consider an older TSOC91a 121.5/243.0 ELT. The regulations a clear that the ELT must be tested each year, according to 14 CFR 91.207(d), but the nature of the required test is ultimately up to the ELT manufacturer and the operator.

The FAA regulation states:

91.207(d) Each emergency locator transmitter required by paragraph (a) of this section must be inspected within 12 calendar months after the last inspection for--

(1) Proper installation;

(2) Battery corrosion;

(3) Operation of the controls and crash sensor; and

(4) The presence of a sufficient signal radiated from its antenna.

But what exactly constitutes "sufficient signal radiated from its antenna?"

There are two ways to test for sufficient antenna signal on a C91a ELT. One is to evaluate the signal strength using a test box like those made by QCAvionix or Whiffletree. The other method is to listen to the signal using an AM-band radio receiver.

The FAA endorses the AM radio test and has codified this check in the most recent update of Advisory Circular 43.13-1B (CHG 1). The test for sufficient signal radiated from the antenna is as follows (Par 12-22):

"Active the ELT using the ON or ELT TEST switch. A low-quality AM broadcast radio receiver should be used to determine if energy is being transmitted from the antenna. When the antenna of the AM broadcast receiver (tuning dial on any setting) is held about 6 inches from the activated ELT antenna, the ELT aural tone will be heard."

"It has to be a cheap AM radio," said Bob Glorioso, president of QCAvionix. "It can't be too well shielded. You're trying to overwhelm the AM detector."

Many mechanics, perhaps unaware of the FAA's recommendations, listen the ELT signal on the aircraft's VHF radio, set to receive on 121.5 megahertz. But this isn't a good test, because the aircraft radio's receiver is extremely sensitive. The AM radio test, says AC43.13, "is not a measured check; but it does provide confidence that the antenna is radiating with sufficient power to aid search and rescue. The signal may be weak even if it is picked up by an aircraft VHF receiver located at a considerable distance from the radiating ELT. Therefore, this check [VHF radio] does not check the integrity of the ELT system or provide the same level of confidence as does the AM radio check."

What the FAA is saying is that simply listening to the ELT signal on the aircraft's VHF radio does not meet the requirements of 91.207(d).

ELT manufacturers agree with the FAA's recommendations. The maintenance manual for the ACK Technologies E-01 ELT, for example, recommends the AM radio test, and complying with the manufacturer's manual meets 91.207(d). The manuals for Ameri-King's and ARTEX's ELTs also recommend the AM radio test, although Narco permits the use of the aircraft VHF radio for tests of its ELT910.

Canadian aircraft operators face some more stringent requirements for testing ELTs. These include testing power output, frequency, audio modulation, and the automatic actuation system (g-switch) by an approved Canadian repair facility using a spectrum analyzer and other test equipment.

When the phaseout of the C91a ELTs takes place by February 1, 2009, a whole new set of ELT testing requirements will apply. Anyone who currently operates with a 406-megahertz ELT already knows about these requirements, which rely on sophisticated, expensive equipment and more stringent testing.

The C91 ELTs aren't going to disappear instantly because they can still be useful for locating a crash. After February 1, 2009, however, the International Cospas-Sarsat Program will not process distress signals transmitted on 121.5/243 megahertz.

Once all the old C91 ELTs are weeded out, the old procedure of testing an ELT by listening to it will no longer be appropriate and formal tests will be the norm.

The new 406 ELTs, however, are so much more accurate that some operators are already switching and moving away from the C91a ELTs. Forward-looking maintenance companies will not only try to show their customers the benefits of the more accurate locating capabilities of the 406 ELTs but will also get ready for the required annual testing by buying and learning how to use the test units. -- By Matt Thurber

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