# AD 49-46-01: Vacuum System Fusible Plug

- Type: Airworthiness directive
- Status: Current
- Products: Boeing DC-4, DC-6, DC-6A and DC-6B
- Superseded by: Nothing supersedes this AD
- Product type: Aircraft, Large airplane
- Registered aircraft: 17 may be affected, by airframe or engine model (OpenAirDocs)
- FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/390774C75F7DD65C862569BC005C1D90.0001
- DRS record last changed: May 30, 2024
- Page: https://openairdocs.org/ad/49-46-01

## The airworthiness directive

49-46-01 DOUGLAS: Applies to All Models DC-4 and DC-6 Aircraft Equipped With Vacuum systems, Incorporating Oil Separators Other Than the Type Mentioned in Item 2.

To be accomplished not later than April 1, 1950.

To guard against the possibility of excessive air temperatures in the vacuum system discharge line, one of the following modifications must be accomplished to this system:

1. Install a fusible plug in the side of the vacuum pump discharger port at the right angles to the axis of the discharge port boss. Some pumps incorporate a plugged hole in the discharge port which may be enlarged to a 3/8-inch pipe tapped hole to accommodate the fusible plug. This plug should employ an AN 840-8D fitting with a binary eutectic mixture of 67.8 percent tim and 32.2 percent cadmium, which has a melting point of 351 degrees F. Figure 2 describes the design of such a plug. The 3/8-inch fusible plug fitting is intended for pumps such as the Model 3P-211 and 3P-485. For smallerpumps such as the 3P-207, and AN 840-6D fitting, incorporating the same modification as shown below, should be used. Incorporation of an overboard drain line clamped to the fusible plug is recommended but is not mandatory.

Brass fittings of the same design as the above dural fittings are acceptable. On installations which do not use an overboard discharge line the possibility exists that the plug may damage other nacelle components if it can hit them upon being out of the adapter as high velocity. Therefore, if no overboard discharge line is provided, the installation must be made in such a manner that the plug will not be directed toward any vulnerable components when it issues from the adapter.

2. Replace the present oil separator with a new oil separator, Genisco No. 40081 or equivalent. The new separator incorporates a pressure relief valve and can be disassembled for cleaning.

(Douglas Service Letter A-129-T-1271/WB-11-Q-4, dated April 1, 1949, covers this same modification.)

## Affected data sheets (4) (OpenAirDocs)

The type certificate data sheets whose models the AD names.

- [TCDS 6A3](https://openairdocs.org/tcds/6A3): The Boeing Company; names 1 of 3 models: DC-6A
- [TCDS 6A4](https://openairdocs.org/tcds/6A4): The Boeing Company; names 1 of 2 models: DC-6B
- [TCDS A-762](https://openairdocs.org/tcds/A-762): The Boeing Company; names 1 of 7 models: DC-4
- [TCDS A-781](https://openairdocs.org/tcds/A-781): The Boeing Company; names 1 of 1 models: DC-6

## 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: 17 (17 by airframe model, 0 by engine model)
- Makers: Douglas 17

Models with the most aircraft:

- [Douglas DC-6B](https://openairdocs.org/model/douglas-dc-6b): 9 aircraft (airframe model)
- [Douglas DC-6A](https://openairdocs.org/model/douglas-dc-6a): 7 aircraft (airframe model)
- [Douglas DC-6](https://openairdocs.org/model/douglas-dc-6): 1 aircraft (airframe model)

All 3 models: https://openairdocs.org/ad/49-46-01/models.md
Every aircraft: https://openairdocs.org/ad/49-46-01/aircraft.md

Its text cites no advisory circulars or STCs.

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