Convair 240 accident at La Guardia Field, New York, 6 Feb 1953

Accident narrative

On February 6, 1953, a Northeast Airlines Convair 240, operating as Flight 825, crashed on Runway 13 at La Guardia Field, New York. During the final approach, a violent yaw to the right developed rapidly at low altitude, and the aircraft became uncontrollable. The right main landing gear, right propeller, and right wing tip struck the runway, and the aircraft came to rest 765 feet from the point of initial impact. There was no fire, and six passengers reported minor injuries.

### The flight

Flight 825 originated at Portland, Maine, and arrived at Boston, Massachusetts, for a crew change. The new crew consisted of Captain Alva V. R. Marsh (pilot-in-command), Captain Eugene P. Rooney (first officer), and stewardess Joan Sexton. Captain Marsh had 14,702 flying hours, including 3,093 in Convair aircraft during airline operation. Captain Rooney had 8,320 total hours, with 1,186 in Convairs.

The flight departed Boston on schedule at 1838 with 11 passengers aboard. The gross takeoff weight was within permissible limits and properly distributed. The flight proceeded on an IFR flight plan at 8,000 feet MSL. After reporting over Port Chester, New York, at 1943, the crew advised La Guardia Approach Control that they were operating in VFR conditions. The IFR clearance was cancelled, and the flight proceeded VFR. The aircraft was cleared to land on Runway 13. Weather conditions at La Guardia were not a factor, with a measured 6,000-foot overcast ceiling and eight miles of visibility.

### The approach and crash

The crew completed the landing checklist before final approach. Captain Marsh, who had flown the entire trip from Boston, was making the approach. The propellers were set at an approach rpm of 2300, airspeed was 130 mph, and the rate of descent was stabilized at approximately 600 feet per minute.

At about 100 feet altitude, just short of the field boundary, the pilots heard an unusual sound on the right side of the aircraft, which they described as a muffled, rattling, buzzing, or soft cracking sound. Simultaneously, the right propeller rpm increased, and the aircraft pulled sharply to the right.

Captain Marsh immediately applied left rudder and aileron to counteract the yaw. At about 50 feet altitude, he applied power to both engines. Captain Rooney noted that the throttles were advanced the same amount, but the engine tachometers indicated 2000 rpm for the right engine and 1500 for the left. The noise decreased, but the yaw worsened, prompting Captain Marsh to close the throttles entirely. Captain Rooney grabbed the flight controls moments before impact and found that full left rudder and aileron had already been applied. The aircraft was uncontrollable and struck the runway.

The passengers evacuated the aircraft in an orderly manner. Most exited through the left rear loading door as instructed by Captain Rooney, while several used the front main entrance. All occupants were out of the aircraft in one and one-half to two minutes. Several passengers corroborated the pilots' descriptions of the sequence, with one former bomber pilot stating he heard a sound he associated with a propeller reversing in the air.

### What the investigation found

The aircraft's right wheels struck the runway 270 feet past the threshold. During the slide, the right wing sheared between the nacelle and the fuselage, and the augmentor tubes pierced the upper right side of the cabin. The left wing remained relatively undamaged. Examination of the cabin revealed the fuselage and basic seat structures were generally intact.

Examination of both engines showed they were in good operating condition and capable of developing power. The investigation focused on the right propeller.

Normally, the Convair's propellers cannot be put into reverse pitch until the aircraft is on the ground, at which point a switch on the left main landing gear energizes a solenoid to unlock the throttle reversing mechanism. Pilots can manually release this lock in flight by pulling a spring-loaded override handle on the control pedestal. Northeast Airlines' before-landing checklist required pilots to visually check that this handle was in the "in" position. Both pilots testified they were positive they had not operated the manual override during the flight and had verified its position during the checklist.

Examination of the right propeller revealed that its blades had traveled past the low pitch stop toward the reverse pitch range. Impact marks on the barrel shelf indicated a blade pitch of plus 3.1 degrees, while the power unit was found at a position representing minus 0.3 degrees. Both measurements showed the blades were in a nearly flat pitch position at impact. The left propeller was found at the full increase rpm limit, with no indication of malfunction.

Investigators conducted extensive testing of the right propeller and its control system to determine if an electrical or mechanical malfunction caused the unwanted pitch change. The power unit, synchronizer, relays, and wiring all functioned normally during tests. The high rpm limit switch, which prevents the propeller from moving into the pitch range below the high rpm limit, functioned as intended during numerous operations, though one of its contacts exhibited considerable wear.

The Board explored every known probability but found no evidence of mechanical or electrical malfunction in the right propeller or its control system that would explain the overtravel past the low pitch limit.

### Probable cause

The Board determines that the probable cause of this accident was loss of control of the aircraft during final approach due to high drag from the right propeller. This drag was induced by the right propeller blades moving beyond the high r.p.m. limit stop since the blades were found in approximately zero geometric pitch. The cause of this unwanted propeller action could not be determined.