Flight Lieutenant R.M. Aldwinckle and his crew were joining the escort for convoy ON-207 when they sighted U-420. The first attack was marred by depth charges that did not explode, possibly because of condensation freezing the hydrostatic fuses. In a second attack, Aldwinckle released a homing torpedo, described is his report as a 600 lb depth charge. U-420 sank after a third attack. The following excerpts are taken from the secret Command Provisional U-Boat Assessment Form.
Department of National Defence, Directorate of History and Heritage, 181.003 (D1354).

Aircraft crew:

Narrative:

While flying at 8000 feet over 6/10 S.C., with a base of 2500 feet, to avoid icing and on a course of 356 deg.T., the Captain sighted at 1101Z a U-Boat fully surfaced, 60 deg. to port at 20 miles distance. U-Boat’s course 110 deg.T. at 10 knots. The aircraft manoeuvred so as to keep the sun behind it and dived on the U-Boat, taking advantage of cloud cover. The submarine appeared to sight the aircraft at approximately five miles distance and then trimmed to hull down position. The attack was carried out from three o’clock at 1105 GMT in position 5049N – 4101W. Six 250 lb DC’s [depth charges] set for 25 feet and spaced at 60 feet for 215 mph. were dropped from 75 feet at an I.A.S. [indicated air speed] of 220 mph. aimed to straddle the C/T [conning tower]. The tail gunner states that he saw at least three DC’s enter the water at three o’clock, of which one was seen to explode forty yards short of C/T. At least two splashes of entry were seen between the explosion and the U-Boat. One of the splashes was close to the hull. None were seen on the port side of U-Boat. Two or more men were seen on the C/T and guns were seen to swing towards A/C [aircraft] as it went in for the attack, but no fire came from the U-Boat during run-in. The aircraft did not fire during approach to attack because the upper turret was fogged from icing (picked up in cloud) which obscured the gunner’s vision. The rear gunner fired a short burst as the aircraft crossed the U-Boat. The aircraft then made a sharp turn to port and climbed to 1000 feet. Aircraft “A”/10BR was not fitted with a nose gun.

The U-Boat began to fire flak, putting up a heavy curtain of fire with at times as many as fifty bursts in the air at one time. The aircraft returned fire intermittently for one hour and nine minutes, approximately at 1211/Z the U-Boat was observed to start crash dive and aircraft turned in for attack. At 1212/Z the U-Boat submerged at 1_ miles from aircraft and the aircraft began attack with the 600 lb DC. On the first run over the target, the conditions of height and speed made it impractical to execute an attack. On the second run, the 600 lb DC was dropped under very good conditions of height and airspeed, namely height 250-300 feet. and I.A.S. of 158 mpg. It was released two mins. after U-Boat submerged, The aircraft was heading directly into wind and was in level flight. The path of the 600 DC in the air was not noted, nor was the point of entry observed, however, the navigator states that the unit was aimed to strike 150-200 yard ahead of the C/T swirl which was still visible. The approach was made from eight o’clock to the U-Boat’s track which was estimated at 205 deg.T. The sea was rough and there was no evidence as to the functioning of the unit nor were there any signs of damage to the U-Boat. The aircraft turned to port in order to observe the result of the attack.

Approximately four minutes after the 600 lb DC was dropped, a periscope was definitely identified by three members of the crew. It was on a course of 100 deg.T. The aircraft made its approach from two o’clock to the periscope’s track and dropped two 250 lb DC’s at 1218/Z from 50 feet. The I.A.S. was 100 mpg. The point of entry of the DC’s with relation to the periscope was not observed but photographic evidence indicates that No. 2 DC fell directly over periscope track and slightly to the rear of periscope itself. Both DC’s were seen to explode with a third explosion or black eruption observed slightly of the rear of the mid-point between the DC plumes. The tail-gunner states that this third explosion shot up like an oil gusher as under great pressure and rose to a height of fifty to sixty feet. It rose like a water spout. These explosions took place four minutes after 600 lb DC was dropped. This description is borne out by photographic evidence. The Captain states that on circling the spot, the waster was stained black with a reddish tinge and this was distinct from DC scum, over an area approximately one and one half times that DC residue.

Following the attack, a two hour smoke float was dropped. The aircraft remained in the vicinity for one half hour and then S/C for the C/V [convoy]. Enroute to the C/V it altered course to investigate smoke seen on the horizon, and discovered a smoke float and three naval vessels in the vicinity, thirty miles at 310 deg.T from C/V ON-207. Information concerning attack passed to vessels by V/S [visual signal]. The aircraft then proceeded to C/V and also passed the information to it using R/T [radio transmitter] on instruction from S.O. [Signals Officer] Patrol carried as ordered by S.O. At 1548/Z the navigator sighted a suspicious object in position 5051N – 4056W which proved to be a fully surface U-Boat on Co.140 deg.T/8 knots. Engaged U-Boat with M/G fire at about one mile distance. The U-Boat set course for C/V at 1616/Z This information was passed to S.O. while aircraft was enroute to C/V.

It is worthy of note that in both instances of U-Boat sightings, the A.S.G. failed to give any indication of the U-Boat’s presence, despite the fact that the set was serviceable and functioning properly on both occasions.

Remarks on U-Boat Tactics

At 1105/Z the U-Boat employed the following tactics viz: turned in tight circles so as to keep the stern – heavily armed – directed towards the aircraft. The A/A fire was from rear bandstand. AT 1548/Z, in contrast to the above, the U-Boat adopted entirely different tactics viz: pivoting practically within its own length, thereby keeping the aircraft on the U-Boat’s beam and presenting heavy fire from guns mounted on bandstands fore and aft on C/T. The crew consider that the U-Boat’s fire came from two quadruple mounts located, one forward and the other aft of C/T. There were no deck guns. This second U-Boat was bluish white in color while the first was covered with rust.