Narco AT150 CG for W&B Calc?

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Eric Lehto

Pre-takeoff checklist
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Hoping somebody can help me out. Recently had my Narco AT150 transponder replaced with a Garmin GTX330. The avionics shop didn't do a W&B update, and I'm pretty sure it needs to. With the current lockdown here in CA, I figured I'd spend some downtime getting everything together.

Anyway, Looking at the specs for these units, the Narco weighs in at 2.3 lb and the Garmin comes in at 4.2 lb. The Garmin install manual shows its CG 4.9" fwd of the panel after installation. The Narco install manual doesn't show the CG position. Anyone know of an official spec for this?
 
Eric Lehto said:
I'm pretty sure it needs to.
Click to expand...
What was the total weight difference between the 150 and 330 installs, not just the radios? If the total difference is less than a pound then no empty weight/balance correction needed. Regardless, any correction to an aircraft empty w&b requires a AP signoff as it is considered a minor alteration. If you think it still does require one, call them up and they should be able to send you a signed corrected empty w&b record sheet covering the 330 install.
 
If you still have the Narco see where it balances on a horizontal stick.
 
Use the 4.9. At 1.1 pounds difference the most your be off is a couple of inch-pounds. You change more than that when you adjust your seat.
 
You should have an equipment list showing install weight and balance for all your avionics. Look at that document if you can find it. Use that data to document the removal of the AT150. As trivial as it seems, you are supposed to recalculate weight and balance if the change exceeds one pound in our weight class. Larger aircraft fall under different rules.
 
My equipment list for a 1979 Warrior II (PA-28-161 ) lists the Narco 150 transponder at:
Weight: 3 lbs
Arm: 57.3"
Moment: 172

The note indicates the weight includes the cable and antenna.
 
Yeah, I've got an equipment list with the weight and arm, but obviously it references the aircraft datum. I don't have the arm for a known reference point such as the panel, so calculating the arm of the new transponder in reference to the the aircraft datum is tough.
 
My Warrior's POH, Section 2, Limitations, states:
"The datum used is 78.4 inches ahead of the wing leading edge at
the inboard intersection of the straight and tapered section."

Also, the Warrior MX manual, Section 2-3, Handling & Servicing/STATION REFERENCE LINES, states:
In order to facilitate the location of various components of the airplane which require maintenance and servicing, a method utilizing fuselage station (Sta.), wing station or buttock line (LB), and waterline (WL) designations is frequently employed in this manual. (Refer to Figure 2-2.) Fuselage stations, buttock lines, and waterlines are reference points measured by inches in the vertical or horizontal direction from a given reference line which indicates station locations of structural members of the airplane. Station 0 of the fuselage is 44.5 inches ahead of the lower edge of the fire wall; station 0 (BL) of the wing and stabilator is the centerline of the airplane; and station 0 (WL) of the fuselage vertical stabilizer and rudder is 20.5 inches below the cabin floor as measured at the rear wing spar with the airplane level. The re ference datum line is located 78.4 inches ahead of the wing leading edge at the intersection of the straight and tapered section.
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So, for the Warrior, the station for the lower edge of the firewall is 44.5".
 
Last edited:
Eric Lehto said:
Yeah, I've got an equipment list with the weight and arm, but obviously it references the aircraft datum. I don't have the arm for a known reference point such as the panel, so calculating the arm of the new transponder in reference to the the aircraft datum is tough.
Click to expand...

Do you own a tape measure?
 
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