AD 52-25-01

52-25-01 DOUGLAS: Applies to All Models DC3 Aircraft Equipped With Vacuum Systems.

To be accomplished not later than May 1, 1953.

To guard against the possibility of excessive air temperatures and associated fire hazards in the vacuum system discharge line, one of the following modifications must be accomplished:

1. Install a fusible plug in the side of the vacuum pump discharge port at 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 tin and 32.2 percent cadmium, which has a melting point of 351 degrees F. A drawing describing the design of such a plug is shown in Figure 2. The 3/8-inch plug fitting is intended for pumps such as the Model 3P-211 and 3P-485. For smaller pumps such as the 3P-207, an AN 840-6D fitting, incorporating the same modification as shown below, should be used. Brass fittings of the same design as the above dual fittings are acceptable. Incorporation of an overboard drain line clamped to the fusible plug is recommended but is not mandatory. 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 blown out of the adapter at 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, or

2. Employ an oil separator equivalent in principle to the Genisco No. 40081 incorporating a pressure relief valve which can be disassembled for cleaning.

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