# AD 55-09-01: Magnetic Circuit Breaker

- Type: Airworthiness directive
- Status: Current
- Products: Aero Spacelines 377, 377MG and 377SGT
- Published: December 4, 1956
- Issued: December 4, 1956
- Superseded by: Nothing supersedes this AD
- Product type: Aircraft, Large airplane
- Registered aircraft: 1 may be affected, by airframe or engine model (OpenAirDocs)
- FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/F3EA383F51D1EDCE8625682A00652F03.0001
- DRS record last changed: November 21, 2025
- Page: https://openairdocs.org/ad/55-09-01

## The airworthiness directive

55-09-01 BOEING: Applies to all Model 377 aircraft with Hamilton Standard Propellers. To be accomplished as soon as practicable but not later than June 30, 1955. Loss of manual r.p.m. control can occur as a result of tripping of the fast-acting magnetic circuit breaker before the slow-acting fuse is blown on a faulted branch. Improved circuit protective coordination and resultant reliability in the manual control function should be provided by replacing the "Manual" 10-ampere magnetic circuit breaker with a 5-ampere thermal type breaker, and replacing the four 5-ampere fuses in the synchronizer unit with 2 ampere Slo-Blo fuses. (Hamilton Standard Service Bulletin No. 283 dated December 21, 1953, covers this same subject.)

## Affected data sheets (1) (OpenAirDocs)

The type certificate data sheets whose models the AD names.

- [TCDS A-812](https://openairdocs.org/tcds/A-812): Aero Spacelines Incorporated; names 3 of 3 models: 377, 377MG, 377SGT

## Registered aircraft it may apply to (OpenAirDocs)

Matched by airframe and engine model; the AD's applicability may narrow it by serial number, part number or equipment.

- Aircraft: 1 (1 by airframe model, 0 by engine model)
- Makers: Boeing 1

Models with the most aircraft:

- [Boeing 377MG](https://openairdocs.org/model/boeing-377mg): 1 aircraft (airframe model)

All 1 models: https://openairdocs.org/ad/55-09-01/models.md
Every aircraft: https://openairdocs.org/ad/55-09-01/aircraft.md

Its text cites no advisory circulars or STCs.

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