An advantage peculiar to the Packard among aeronautical diesels was its light weight. The English Beardmore "Tornado III" weighed 6.9 lb/hp, and the German Junkers SL-1 (FO-4) weighed 3.1 lb/hp, while the Packard weighed but 2.3 lb/hp. In fairness to the Beardmore, it was the only one of the three engines designed for airship use, and part of its heaviness was due to the special requirements of lighter-than-air craft. A contemporary and comparable American gasoline engine, the Lycoming R-680, weighed 2.2 lb/hp. To have designed a diesel aircraft engine as light as a gasoline one was a remarkable achievement. Disadvantages There are four main reasons why the Packard diesel was not successful. First the Packard Motor Car Company put the engine into production a brief three years after it was created. The only successful airplane diesel, the German Junkers "Jumo," was in development more than three times as long (1912-1929). The following tests indicate that the Packard diesel was not ready for production, and hence was unreliable. Packard Motor Car Company 50-Hour Test (Feb. 15-18, 1930): This test was identical to the standard Army 50-hour test which was used for the granting of the Approved Type Certificate. The engine tested was numbered 100, and was the first to be made with production tools (approximately half a dozen engines had been handmade previously). It had to be stopped three times, twice due to failure of the fuel pump plunger springs and once due to the loosening of the oil connection ring. These failures were attributed to manufacturing discrepancies. In addition, 4 out of a total of 103 valve springs broke.[29] U.S. Navy 50-Hour Test (Jan. 22, 1931, to March 15, 1931): The engine used in the Navy test was numbered 120. (Apparently only 20 production engines had been built during the preceding 12 months; Dorner in a letter of March 3, 1962, states that the total number of Packard diesels produced was approximately 25.) The engine had to be stopped three times, twice due to valve-spring collar failures and once due to a valve head breaking. Because of these failures this test was not completed. The following significant quotations have been extracted from the test: "The engine is not recommended for service use.... Flight tests, until the durability of the engine is improved, be limited to a determination of the critical engine speeds, and to short hops in seaplanes.... It is believed that this size engine should be made suitable for service use before this type in a larger class is attempted." This latter statement probably refers to the 400-hp model.