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Atlas / STC / ST521SO

FAA · Supplemental Type Certificate

STC ST521SO

Increase in maximum landing weight to 135,000 pounds and maximum zero fuel weight to 128,790 pounds with pylon tanks on and 127,000 pounds with pylon tanks off.

Current Amended Aircraft Large Multi-Engine Rev 08/24/1983 View PDF on drs.faa.gov ↗

What is an STC?

A Supplemental Type Certificate authorizes a major modification or alteration to an FAA-certificated aircraft. STCs are how operators legally install new avionics, change fuel systems, add floats, swap engines, etc. — every change beyond a minor modification needs one.

Applicable aircraft (2)

Certification basis — 14 CFR

Specific sections

  • Sec. 111
  • Sec. 113
  • Sec. 115
  • Sec. 117
  • Sec. 119
  • Sec. 120

The STC PDF on drs.faa.gov is the authoritative document — it lists installation instructions, ICA requirements, and any limitations. Open PDF ↗