GDL 82 for my Sky Arrow?

[FastEddieB]

I was convinced in the other thread that the Garmin GDL82 is probably a better choice than the UAvionix skyBeacon. Installation will likely be easier, or at least no more difficult. Plus, I’ve always been impressed with Garmin’s products and support. My transponder is a Garmin GTX327, so its compatible.

I will need the unit with GPS built in, since my Sky Arrow has no GPS source - my navigation is via a Garmin 496. Aircraft Spruce has it for $1,795 without custom harness, or $1,849 with.

The physical installation should be pretty straightforward, and I can probably knock off a lot of the basic install.

1) There should be space for the unit itself under the pilot’s seat:



2) There’s no extra breaker or slot in the panel, but a convenient hole right below my 12v outlet:



3) I’m pretty sure I have room on top of my glareshield for the GPS antenna. My 496’s GPS and Sirius antenna live there already:



I should be able to fabricate the needed “ground plane” with metalized tape, and it has a great view of the sky, albeit through the canopy

Where I will need guidance and help is with wiring it all up and doing the required programming and testing. I’m planning to get started in the fall if all goes well, and would welcome some help then, preferably with someone with some experience with a similar install.

Open to any ideas and suggestions now for planning purposes, and thanks in advance!

[MrMorden]

You won't need a ground plane on top of the glareshield. Id try to find space behind the panel, to minimize coax runs to the transponder. Get the Spruce harness, it will save a few hours of wire work. With the harness I got mine installed and working in less than 3 hours.

[3Dreaming]

You can piggy back the power off of the transponder circuit breaker.

[FastEddieB]

You can piggy back the power off of the transponder circuit breaker.

Good to know, and confirmed by the installation manual.

Thanks!

[FastEddieB]

You won't need a ground plane on top of the glareshield. Id try to find space behind the panel, to minimize coax runs to the transponder. Get the Spruce harness, it will save a few hours of wire work. With the harness I got mine installed and working in less than 3 hours.

Thanks. I’ll try to give you a call tomorrow.

[ShawnM]

Great that you are reading the install manual. Remember to keep the GPS antenna cable no shorter than 6.5 feet for proper attenuation. Use RG-400 cable for all connections per the manual. The manual does call for a ground plane for “optimal” signal. I installed the anonymous switch and the fault indicator in a DIY push to test led button. Slight overkill but it works. You’ll love this unit.

[MrMorden]

You won't need a ground plane on top of the glareshield. Id try to find space behind the panel, to minimize coax runs to the transponder. Get the Spruce harness, it will save a few hours of wire work. With the harness I got mine installed and working in less than 3 hours.

Thanks. I’ll try to give you a call tomorrow.

You might want to call in the evening. I'm driving to ATL for work and it's a crap shoot if I'll be able to answer the phone during the day.

[FastEddieB]

You can piggy back the power off of the transponder circuit breaker.

My transponder circuit breaker is 5A. The GTX327 Installation manual only calls for a 3A. And the max draw on the GDL 82 is 1.5A, so that 5A breaker should be able to handle both, right?

[drseti]

I usually spec breakers at twice the expected current draw. That will protect against catastrophic failures (like a shorted wire), while preventing need for resets under normal load fluctuations.

[MrMorden]

Circuit breakers and fuses are in place to protect the wire from a current that could cause it to overheat and melt. Using a circuit breaker spec'ed far beyond the expected load means that while the breaker is less likely the trip, the wire is MORE likely to melt. It's a lot like taking a fuse slot in your car that says 5A and putting a 10A fuse in it. Bad practice, unless you look closely at the type and thickness of wire to the device and calculate the point at which it will become damaged. You want a breaker to trip when it's 50% or so BELOW that current draw.

Eddie, you should be perfect at a 4.5A load on a 5A breaker, since the numbers you're looking at should be peak numbers that you'll likely never hit in practice. If you find it tripping a lot I'd move the GDL to its own 2-3A breaker.

[3Dreaming]

You can piggy back the power off of the transponder circuit breaker.

My transponder circuit breaker is 5A. The GTX327 Installation manual only calls for a 3A. And the max draw on the GDL 82 is 1.5A, so that 5A breaker should be able to handle both, right?

The scenario was approved for installation in another SLSA, and has been working without issue.

[drseti]

Circuit breakers and fuses are in place to protect the wire from a current that could cause it to overheat and melt. Using a circuit breaker spec'ed far beyond the expected load means that while the breaker is less likely the trip, the wire is MORE likely to melt.

This is all true. But, it is standard practice to recognize that surges are normal, and not hazardous. For that reason, if you check the breakers installed by a manufacturer in a plane, their trip rating will always be roughly twice the normal expected load. That's why Eddie's transponder, with its normal peak load of 3 amps, was equipped with a 5 amp breaker, and not a 3 amp breaker.

[ShawnM]

I have to agree that doubling the amperage of a breaker is bad practice and can be dangerous in an aircraft. I personally would NEVER do this. If you read AC-43-13 it states:

Section 11-48 - Circuit protection devices must be sized to supply open circuit capability. A circuit breaker must be rated so that it will open before the current rating of the wire attached to it is exceeded, or before the cumulative rating of all loads connected to it are exceeded, whichever is lowest. A circuit breaker must always open before any component downstream can overheat and generate smoke or fire. Wires must be sized to carry continuous current in excess of the circuit protective device rating, including its time-current characteristics, and to avoid excessive voltage drop. Refer to section 5 for wire rating methods.

Manufacturers typically spec current loads, wire gauges and circuit breakers for their specific equipment for a reason, follow the manufacturers recommendation, they made the equipment. If unsure or you need to deviate from this then I recommend you follow AC-43-13.

That being said, the GDL-82 manual does say you can piggyback the transponder CB but the combined current load cannot exceed 80% of the rated current of the CB. 80% of the 5 amp breaker is a 4A. When combined you exceed that 4A load. It's best to install a 2A (manually resettable) CB for the GDL unit along with the proper gauge wire determined by the length of the run. This also makes troubleshooting of avionics much easier as it allows you to isolate each specific component if needed.

If this were your car I'd say do whatever you want..........but it's not.

[drseti]

Further to the above: the auxiliary alternator on my Rotax 912ULS is rated at 15 amps. Its breaker is rated for 30 amps, not 15. The manufacturers take peak surges into account, for breakers, wiring, and connectors.

[ShawnM]

The GDL manual states the typical current draw is 0.45A and the MAX current draw is 0.5A at 14 volts.