Douglas C-54B-DC accident at Boston, Massachusetts, 10 Mar 1964

Accident narrative

On March 10, 1964, Slick Airways Flight 12, a Douglas C-54B-DC cargo aircraft, crashed in a lumberyard on Castle Island while making an Instrument Landing System (ILS) approach to Logan International Airport in Boston, Massachusetts. The aircraft struck the ground in a steep nosedown attitude about 7,000 feet from the runway threshold. The captain, first officer, and a freight handler were killed, and the aircraft was destroyed by impact and fire.

**The flight**

Flight 12 was a scheduled cargo trip from John F. Kennedy International Airport in New York to Boston, with an intermediate stop at Bradley Field in Windsor Locks, Connecticut. The crew consisted of Captain Irwin S. Zadwick, who had 6,000 flying hours including 814 in DC-4 type aircraft, and First Officer Salvatore J. Conilio, who had 5,824 hours with 4,340 in the DC-4.

Prior to departure, a presumed crew member received a weather briefing indicating expected ceilings of 400 to 600 feet, fog, and light drizzle or rain. The briefer advised that freezing or frozen precipitation would occur north of the route, though the company's clearance form did not mention this.

The flight departed New York at 0613 and arrived at Bradley Field at 0650. While on the ground, a crew member telephoned the Flight Service Station and received updated weather reports, which indicated light snow and sleet at Boston and light freezing rain at Worcester. The aircraft departed Bradley Field at 0735. Its takeoff weight of 57,048 pounds was well below the maximum allowable 73,000 pounds, and its center of gravity was within prescribed limits.

**The approach**

At 0801, Flight 12 contacted Boston Approach Control and was cleared to 3,000 feet. The controller provided the 0800 Boston weather, which included a 700-foot overcast ceiling, 1-1/2 miles visibility, light sleet, and fog. At 0803, the flight reported encountering moderate rime icing at 3,000 feet.

Shortly after 0809, the crew requested a lower altitude and was cleared to 2,000 feet, reporting "moderate to heavy" rime icing as they descended. The flight was then cleared for an ILS approach to Runway 4R. At 0815, the local tower controller advised approach control that visibility was 1-1/2 miles. This information was not given to the flight, though the Board later concluded this omission did not contribute to the accident.

After reporting the outer marker inbound at 0818, Flight 12 was cleared to land. A Precision Approach Radar (PAR) controller provided advisories, noting the aircraft was on course and on the glidepath. At 0821:09, the controller advised the flight it was passing the stacks at Castle Island. About one to one-and-a-half miles from touchdown, the aircraft's target disappeared from the radar scopes. The local controller observed a large ball of flame erupting from the ground at Castle Island.

**What the investigation found**

The aircraft crashed in a lumberyard on the extended centerline of Runway 4R. Witnesses in the yard reported the aircraft passed to the right of a light tower before suddenly pitching nosedown steeply into the ground.

The investigation determined the aircraft struck the ground in an approximately 60-degree nosedown attitude. The captain was found in the right seat and the first officer in the left seat. The crew had been on duty for 15 hours and 33 minutes, just under the 16-hour limit, and medical examiners found no evidence of incapacitation.

The landing gear was down and locked. The elevator trim was found in a 5-to-6-degree nosedown position, and the flaps were extended to about 30 degrees. Full flap extension is 45 degrees. Examination of the engines and propellers revealed no evidence of malfunction prior to the accident. Fire damage precluded determining the pre-impact condition of the de-icer system.

Radar data indicated the aircraft was at 480 feet above sea level and flying at a true airspeed of about 130 knots when the pitchover began. Other pilots flying near Boston that morning reported moderate to severe freezing precipitation and ice buildup.

The Board noted that the weather briefing obtained at Bradley Field showed dropping temperatures and changing precipitation, which should have alerted the crew that the Boston forecast was out of date and that airframe icing could be expected.

The company's Eastern Division Chief Pilot testified that in such icing conditions, he would maintain an increased airspeed of 130 knots, lower the flaps to 15 degrees at the outer marker, and wait to extend them to 30 degrees until the approach lights were in sight. He noted that ice on the wings would cause the aircraft to pitch up, requiring nosedown trim.

The Board analyzed the aircraft's motion and calculated that the 60-degree pitchover was a negative accelerated stall producing about -1.4g. The Board ruled out pilot action and structural failure of the tail. Instead, the Board concluded that the aircraft collected ice on its tail during the flight. It is believed the crew extended the flaps to 15 degrees at the outer marker and further extended them to 30 degrees when passing the Castle Island stacks. The Board concluded that the increased downwash from this final flap extension changed the stabilizer angle of attack, which, combined with the ice formation, destroyed the tail lift and disrupted the balancing tail loads. The resulting pitchover was too severe for the crew to recover at that altitude.

**Probable cause**

The Board determines the probable cause of this accident was loss of balancing forces on the horizontal surface of the aircraft's empennage, due to ice accretion, causing the aircraft to pitch nosedown at an altitude too low to effect recovery.