Trans-Cal Altitude Encoder Model SSD120-30N FAA TSO

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StandardFor Narco Transponders
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The world's smallest and lightest altitude encoder.

There are two versions of this model:
  • Standard Version SSD120-30N
  • Version SSD120-30N-MOD1 for NARCO AT-5A, 6A, 50, 50A Transponders. The Narco AT-5A, AT-6A, AT-50* or AT-50A transponder will not accept data from the SSD120-(XX)N() Altitude Digitizer. A recent modification to remove the output decoupling capacitors is required and this version includes that modification.

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  • The best warranty in the business: 42-month manufacturer direct replacement. If your Trans-Cal encoder experiences a warranty-covered failure, it'll be replaced with a brand new unit with a new 42-month warranty.
  • Average MTBF: 9.7+ years. Thats right, Trans-Cal encoders are among the most reliable on the market, commonly providing reliable service for over 10 years.
  • Pin for pin replacement for most other manufacturers' encoders.
  • Comes with Mating Connectors, Cable Clamps, and Mounting Bracket.
  • Optional: Available with two independent RS232 serial data ports selectable message protocols for most popular avionics interfaces. Programmable for 100 and 10 resolutions. See Part# 11-06983
  • Optional: Available with adapter plates that will allow the installer to mount the unit in another manufacturer's mounting tray.


  • Size: 2.5"W x 3.42"H x 1.38"L.
  • Operating temperature: -20c to +70c
  • Operating altitude: -1000' to +30,000'(Available in altitudes from 30,000 up to 100,000)
  • Accuracy: +50' from -1000' to +30,000
  • Weight: 5.9 oz.
  • Will operate on either 14 or 28 VCD at 0.270 Amps.
  • No Warm-up time: +20c to +70c -20c to +20c 30 seconds
  • *The Narco AT-50 and earlier transponder models require a modification before thay will function correctly with any altitude encoder. This modification is outlined in Narco Service Bulletin AT-50A-5.
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Bay A Verified Purchase


October 3, 2023

Product good but shipping to high as usual

William B Verified Purchase


April 26, 2023

Adhris A Verified Purchase


February 14, 2023

Replaced an AK350 that was inop. With the adapter plate it was a perfect fit and a true plug and play as the cable going to the AK350 just plugged in pin for pin with no problems.

Keith L
July 12, 2019

Drop in replacement for my AK-350s. No wiring or mounting changes required (with the additional mounting plates). After turning the avionics on there was no warm up time and the altitude grey code was spot on for pressure altitude right out of the box.

Micah B
February 17, 2019

Altitude reporting was spot on right out of the box. A direct wiring replacement from the old Ameri-King AK-350. Very pleased.

June 18, 2018

Replaced an inoperative America-King AK-350 with the Trans-Cal SSD 120-30N. It was a perfect fit with the addition of the optional adapter plate. The 15 pin plug was wired for direct replacement with the AK-350.

May 18, 2018

Excellent certified replacement for Narco AR-850. Mounts in Narco tray without modifications. Provided plastic plug for optional static port had to be replaced with a metal plug to seal properly. Calibration was very easy.

January 13, 2017


Please note,'s personnel are not certified aircraft mechanics and can only provide general support and ideas, which should not be relied upon or implemented in lieu of consulting an A&P or other qualified technician. assumes no responsibility or liability for any issue or problem which may arise from any repair, modification or other work done from this knowledge base. Any product eligibility information provided here is based on general application guides and we recommend always referring to your specific aircraft parts manual, the parts manufacturer or consulting with a qualified mechanic.

Q: Is this Trans-Cal altitude encoder SSD120-30N compatible with the Narco AT 150 transponder?

Yes, part number 11-09916 is a Narco compatible encoder.

Q: Is the SSD120-30N compatilble with the Narco AT-165 transponder?

Yes, the SSD120-30N is compatible with the Narco AT-165 transponder.

Q: Is this Tras-cal altitude encoder SSD120-30N compatible with KT76 A Transponder?

Yes, the KT-76A is listed on the installation manual.

Q: Does this unit put out serial and Gray code to the transponder?

This unit outputs gray code.

Q: Will this unit work with the Stratus ESG transponder? Also, since I will be installing this unit on the bottom of the transponder tray, could I order a twelve inch harness instead of a 48 inch harness?

Yes, you can use this with the Stratus ESG. We can do a 12 inch harness, please contact Avionics sales for price and lead time.

Q: Is the TRANS-CAL ALTITUDE ENCODER MODEL SSD120-30N Compatible with the GTX 330 Transponder?

Yes, it is.

Q: Is this Trans-cal SSD120-30N encoder compatible with GARMIN TRANSPONDER GTX-330?

Yes, it is.

Q: Is this Trans-cal Altitude SSD120-30N unit FAA approved for use in certified aircraft?

Yes, this can be used in certified aircraft.

Q: Is this Trans-cal SSD120-30N encoder compatible with GARMIN TRANSPONDER GTX-330ES?

Yes, these two units are compatible.

Q: Is the product compatible with the Adidyne IFD440 and APX340? What is the difference between this model and the less expensive 35C

No, neither of those units are listed as acceptable units to interconnect.