# Airworthiness Directives; Sikorsky Aircraft Corporation Model S-76B and S-76C Helicopters
> **Rule** · Final rule; request for comments. · Published 2001-12-20 · Effective 2002-01-04 · 66 FR 65629
## Document
- **Document number:** 01-31039
- **Category:** airworthiness
- **Manufacturer:** Sikorsky
- **Type:** Rule
- **Action:** Final rule; request for comments.
- **Citation:** 66 FR 65629
- **Publication date:** 2001-12-20
- **Effective date:** 2002-01-04
- **FAA docket:** Docket No. 2001-SW-51-AD
## Abstract

This amendment adopts a new airworthiness directive (AD) for Sikorsky Aircraft Corporation (Sikorsky) Model S-76B and S-76C helicopters. This action requires modifying the electrical power distribution system to change the source of the 28 volts supplied to Nos. 1 and 2 attitude and heading reference (AHRS) WARN circuits. This amendment is prompted by a ground test simulating loss of the essential bus by pulling both bus tie circuit breakers and switching off both batteries. As a result of this action, both autopilots went off-line. The actions specified in this AD are intended to prevent an AHRS fail signal to both autopilots due to a failure of the essential bus, loss of autopilot functions, and subsequent loss of control of the helicopter.

## Source
- [Federal Register document](https://www.federalregister.gov/documents/2001/12/20/01-31039/airworthiness-directives-sikorsky-aircraft-corporation-model-s-76b-and-s-76c-helicopters)
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