# AD 47-21-09: Hydraulic System Modifications

- Type: Airworthiness directive
- Status: Current
- Products: Sierra Hotel Aero Navion (Army L-17A)
- Superseded by: Nothing supersedes this AD
- Product type: Aircraft, Small airplane
- Registered aircraft: 373 may be affected, by airframe or engine model (OpenAirDocs)
- FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/30236AD133D55BF186256A56006E200E.0001
- DRS record last changed: May 30, 2024
- Page: https://openairdocs.org/ad/47-21-09

## The airworthiness directive

47-21-09 NAVION: (Was Mandatory Note 8 of AD-782-3.) Applies to All Serials Up to and Including Number NAV-4-947.

Compliance required prior to September 1, 1947.

Due to malfunctioning of the hydraulic system which has resulted in the failure of components of that system, including actuating cylinders, the following modifications must be accomplished:

  (a) If an engine-driven hydraulic pump is installed in the airplane, rework the hydraulic power system to install the manually controlled relief valve, North American P/N 145- 58027, and replace the existing placard "Hyd. System Pull Off" with new placard "Hyd. Power On".

(North American Field Service Bulletin No. 26 covers this same subject.)

  (b) Install the swivel head type nose gear actuating cylinder, North American P/N 145-58014.

(North American Field Service Bulletin No. 19 covers this same subject.)

  (c) Visually inspect all flap and landing gear actuating cylinders for evidence of cracks or other damage. Any cylinders found to be damaged should be replaced.

## Affected data sheets (1) (OpenAirDocs)

The type certificate data sheets whose models the AD names.

- [TCDS A-782](https://openairdocs.org/tcds/A-782): Sierra Hotel Aero Inc.; names 1 of 8 models: Navion (Army L-17A)

## 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: 373 (373 by airframe model, 0 by engine model)
- Makers: North American 203, Ryan 170

Models with the most aircraft:

- [North American NAVION](https://openairdocs.org/model/north-american-navion): 203 aircraft (airframe model)
- [Ryan NAVION](https://openairdocs.org/model/ryan-navion): 170 aircraft (airframe model)

All 3 models: https://openairdocs.org/ad/47-21-09/models.md
Every aircraft: https://openairdocs.org/ad/47-21-09/aircraft.md

Its text cites no advisory circulars or STCs.

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