The early models were handicapped by the need to drop them low and slow - 50 feet (15 m) and 110 knots - which made the torpedo planes more vulnerable to attack.  The torpedoes themselves were found to be prone to defects.  In mid-1943, an analysis of 105 torpedoes dropped at speeds in excess of 150 knots found that 36 percent ran cold (did not start), 20 percent sank, 20 percent had poor deflection performance, 18 percent gave unsatisfactory depth performance, 2 percent ran on the surface and only 31 percent gave a satisfactory run.  The total exeeds 100 percent as many torpedoes had more than one defect.

These problems were greatly reduced by the latter years of the war.  Torpedoes had fin stabilizers, nose drag rings and tail shroud rings added, all of which worked to slow the torpedo after it was dropped so that it struck the water nose-first and at an acceptable speed.  These improved the drop characteristics such that the recommended aircraft maximum launch parameters were increased to a height of 2,400 feet (730 m) and a speed of 410 knots.

The addition of the nose drag ring improved aerodynamic performance by stabilizing the torpedo in flight and reduced air speed by about 40 percent.  It also acted as a shock absorber when the torpedo struck the water.  The tail shroud ring improved the water run by reducing hooks and broaches and by eliminating much of the water roll which had characterized the earlier Mark 13s.  Hot, straight and normal runs now approached 100 percent.  To speed availability of the much improved torpedo, the Bureau of Ordnance had tail assemblies built with the shroud ring attached and then shipped these to the fleet.  By the fall of 1944 the modified torpedo was in general use by the front-line carrier units which were enthusiastic in their praise.  On one occasion in early 1945, six torpedoes were dropped from altitudes between 5,000 and 7,000 feet (1,500 to 2,100 m).  Five out of the six were observed to make their runs hot, straight and normal.  By the end of the war, the USN considered the Mark 13 to be the best aircraft torpedo produced by any nation and it remained in service until 1950.

A lanyard was attached to the tail of the torpedo.  When dropped, the lanyard tripped a starting lever, but a water trip delay valve prevented the combustion flask from lighting off until the torpedo had entered the water.  When dropped at 150 knots or more, the torpedo would enter the water at an angle of between 26 and 30 degrees.  The water needed to be at least 150 feet deep (45 m) and the torpedo assumed its preset running depth after water travel of 300 yards (275 m).  The exploder mechanism was armed after water travel of 200 yards (180 m).  Depth could be set up to 50 feet (15 m).

Success during World War II

From "US Naval Weapons" by Norman Friedman:  "A review of war experience showed a total of 1,287 attacks, of which 40 percent (514) resulted in hits, including 50 percent hits on battleships and carriers (322 attacks, including Midway), 31 percent on destroyers (179 attacks), and 41 percent (out of 445 attacks) on merchant ships."

The following is from "US Navy Torpedoes" by Frederick J. Milford:

"340 [Fido] torpedoes [were] dropped in 264 attacks of which 204 were against submarines.  In 142 attacks US aircraft sank 31 submarines and damaged 15; in 62 attacks against submarines other Allies, mainly British, sank six and damaged three.  Most of these submarine sinkings were German U-boats in the Atlantic, but five Japanese submarines were sunk by Fidos, one, I-52, in the Atlantic and four in the Pacific.  OEG Study No. 289 , 12 August 1946, is the main source for this conclusion."

Fido would have been too slow to attack the Type XXI U-boats and could be avoided by going very deep.