Douglas DC-9-31 accident at St. Thomas, Virgin Islands, 12 Aug 1969

Accident narrative

On August 12, 1969, at 1409 Atlantic standard time, Caribbean Atlantic Airlines (Caribair) Flight 340, a Douglas DC-9-31, overran Runway 9 during landing at Harry S Truman Airport in Charlotte Amalie, St. Thomas, Virgin Islands. The aircraft continued 323 feet beyond the far end of the runway, striking several vehicles before coming to rest in an automobile repair shop. Of the 113 passengers and five crew members aboard, one passenger sustained minor injuries. Three occupants of the ground vehicles were seriously injured, including one who required a leg amputation, and one was slightly injured. The aircraft was substantially damaged.

### The flight

Flight 340 was a regularly scheduled passenger service originating in San Juan, Puerto Rico, at 1245. The captain had accumulated 9,529 total flying hours, with 417 hours in the DC-9. The first officer, who was flying the aircraft, had 3,826 hours, including 831 hours in the DC-9.

### Sequence of events

St. Thomas Approach Control cleared the flight for an approach and advised that there was a light rain shower and the runway was wet. At 1401, the crew stated they would hold southeast of the field at 1,300 feet to wait for the rain showers to clear the approach path. The tower noted the flight was turning final one mile out and reported the wind as 120 degrees at 5 knots.

The crew reported that the landing checklist was completed, the antiskid system was on, flaps were fully extended to 50 degrees, and spoilers were armed. The approach was flown at the reference speed of 124 knots indicated airspeed plus 10 knots. Following touchdown, the crew felt the wing spoilers deploy. The first officer applied full reverse thrust, stated by the captain as 2.0 engine pressure ratio, and applied pressure to the brake pedals. The captain assisted with both the reverse thrust and the brakes, but the aircraft did not slow as expected. The crew continued applying maximum available reverse thrust and brake pressure until the aircraft came to rest.

Witnesses and the crew provided varying estimates of the touchdown point, ranging from 800 to 1,800 feet beyond the runway threshold. Several passengers and ground witnesses stated the aircraft bounced after the initial touchdown. The airport fire chief observed that the aircraft was churning up considerable water spray, fishtailing near the far end of the runway, and producing loud sounds of heavy engine reversing.

### What the investigation found

The aircraft sustained substantial damage to its nose section, wings, and fuselage, and the nose landing gear collapsed aft. Examination of the engines, structures, and systems, including the brakes, antiskid, and spoilers, revealed no pre-accident malfunctions.

The weather in the area had featured intermittent rain showers since early morning, with 2.74 inches of rain recorded at the airport over a 24-hour period. Runway 9 was 5,150 feet long and ungrooved, with a 1-percent transverse slope but no center crown for drainage. The airport was situated in a low, flat area surrounded by higher ground, and the area north of the runway had historically been used as a fresh water catchment. The fire chief estimated water on the runway was one-half inch deep and draining to the north side.

Flight recorder data indicated the aircraft touched down at 135 knots indicated airspeed, which was 11 knots faster than the specified reference speed. The recorder showed the aircraft hit a raised concrete sidewalk 5,282 feet from the landing end of the runway at 57 knots. The Board arrived at this speed by adjusting the recorded data to account for positive overpressures from the reverse thrust on the flight recorder pitot tube.

Investigators found white tire streaks typical of hydroplaning in the last 1,400 feet of the runway. Skid patches were found on the left main landing gear tires, oriented 10 to 15 degrees nose-left, but no rubber reversion was present.

The aircraft's calculated landing weight was 91,920 pounds, which required a wet runway length of 5,050 feet. The Board believed the aircraft touched down approximately 1,000 feet from the threshold. The manufacturer calculated that with a touchdown speed of 135 knots and a complete loss of effective braking, the aircraft would require 4,403 feet to stop from the touchdown point using only spoilers and maximum reverse thrust. NASA tests with a diagonally-braked vehicle at the airport indicated that while the aircraft could have stopped on a standard wet runway, a flooded condition would require a stopping distance 2.21 times the dry distance, exceeding the available runway length.

The Board believes that dynamic hydroplaning commenced almost immediately upon touchdown. The aircraft was decelerated only by reverse thrust until it reached a point 1,400 feet from the overrun end, where the white tire streaks indicated a change to viscous hydroplaning as the tires broke through the deep water film. The Board noted that the 11-knot excess in touchdown speed had the equivalent kinetic energy effect of adding 16,000 pounds to the aircraft's weight.

### Probable cause

"The Board determines that the probable cause of this accident was the loss of effective braking action caused by dynamic hydroplaning of the landing gear wheels on a wet/flooded runway. Contributing factors were a higher-than-normal touchdown speed and the location of the airport and its topography which permitted excess levels of water to accumulate on the runway."