Boeing 747-121 accident at Nantucket, Massachusetts, 4 Nov 1970 (N739PA)

Accident narrative

On November 4, 1970, Pan American World Airways Flight 114, a Boeing 747-121, encountered moderate to briefly severe turbulence at approximately 27,000 feet near Nantucket, Massachusetts. The encounter lasted just over four minutes and resulted in injuries to 21 passengers and two stewardesses. The flight, which had departed New York for Paris, aborted its trip and returned to John F. Kennedy International Airport.

### The flight

Flight 114 was a regularly scheduled passenger flight from John F. Kennedy International Airport (JFK) to Orly Airport in Paris, France. Aboard the aircraft (registration N739PA) were 148 passengers and a crew of 15. The flightcrew was highly experienced; the captain had 16,145 total flight-hours, including 511 in the Boeing 747, while the first officer and flight engineer had 5,034 and 13,945 hours, respectively.

The flight took off from JFK's Runway 4L at 2117 eastern standard time. The climbout was normal, with the crew describing the turbulence as an occasional "nibble." The number 1 airborne weather radar "spoked" badly when turned on and was deemed unusable, but the number 2 radar functioned properly. The captain reported seeing lightning to the north, and the radar showed weather cells about 40 miles to the left. The flight was cleared to 31,000 feet via Jet Route J62 to Nantucket.

### The turbulence encounter

At 2146, while climbing through approximately 27,000 feet over Nantucket, the aircraft encountered moderate to briefly severe turbulence. The captain changed the autopilot from heading mode to turbulence mode and made a power reduction. He stated that he remained close to the controls during the descent but did not interfere because the autopilot was doing an excellent job of maintaining attitude.

At 2147, the flight advised Boston Air Route Traffic Control Center that it was in turbulence and descending through 25,000 feet. During the descent, the aircraft reentered clouds, and the crew activated the nacelle anti-ice. The flight leveled out at 23,000 feet before commencing a climb back to 31,000 feet.

At 2207, the flightcrew requested and received clearance to return to New York. They subsequently requested vectors around the turbulence and preferential air traffic control handling. The aircraft landed safely at JFK at 2339.

### Cabin conditions and injuries

The seatbelt sign was illuminated when the severe turbulence struck and had remained on since takeoff. Stewardesses had personally instructed some passengers to remain seated, but others were moving about the cabin. No announcements had been made from the flight deck to explain why the sign remained on or how long it would be illuminated.

Twenty-one passengers and two stewardesses were injured, ranging from minor to serious. Six passengers and one stewardess required hospitalization. Two of the hospitalized passengers reported that they struck the ceiling with their heads during the aircraft's first excursion, fell back into their seats, and maintained their seated positions for the remainder of the turbulence.

The hospitalized stewardess had been preparing the rear galley when she felt mild turbulence. Noting the seatbelt sign was on, she started toward her seat but was caught by the initial severe turbulence. She struck her head on the ceiling, fell to the floor, and held onto seat legs to prevent being thrown about, suffering a cerebral concussion and back sprain.

Injury data indicated that four people standing in the forward portion of the cabin were uninjured, while eight known injured persons were located aft of the cabin's midline. After landing, medical personnel had difficulty removing passengers suspected of having back injuries because the aircraft aisles were too narrow for standard stretchers.

### What the investigation found

Post-flight inspection of the aircraft revealed minor cabin interior trim damage. Several overhead storage bins had dropped open, spilling contents into the cabin, and several economy section headrests had detached from their seats. The investigation could not determine if the open bins or detached headrests contributed to individual injuries.

A readout of the flight data recorder showed the aircraft was in turbulence for approximately 4 minutes and 10 seconds. The maximum intensity occurred during the first 10 seconds, subjecting the aircraft to "g" forces ranging from plus 1.65 to minus 0.83. The aircraft was climbing through 27,150 feet at 354 knots indicated airspeed when the turbulence began. It encountered a severe downdraft followed less than two seconds later by an updraft. The aircraft reached a maximum altitude of 28,270 feet before descending to 22,800 feet, at which point the turbulence ended.

The Boeing 747 flight manual recommended a severe turbulence penetration speed of 280 knots or 0.82 Mach, whichever is lower, and the use of the autopilot turbulence penetration mode. The Board believed that the loss of altitude recorded during the encounter reflected the captain's power reduction, made in an effort to slow the aircraft to the recommended 280-knot penetration speed while the autopilot was in turbulence mode. The Board noted that the light turbulence during the climbout was not indicative of the sudden jolt encountered at 27,150 feet, which could explain the pilot's use of climb airspeed rather than penetration speed. The aircraft was under control at all times.

Weather data showed a deep low-pressure system near New York City with an occluded front and convective activity. The Board considered that although the aircraft was clear of clouds during the encounter, it was near the tops of cumulus buildups where strong vertical currents existed.

An engineering review of the Boeing 747's structural response and autopilot data showed no adverse interplay during turbulence. The autopilot did not drive the fuselage or increase the load factor. However, the investigation disclosed a significant variation in load factor depending on longitudinal station. Load factors sustained at the aft pressure bulkhead could be 1.5 to 2.2 times higher than those experienced at the pilot's station, which might result in considerable variance in a person's response to the turbulence depending on their location.

The Board noted that passengers probably began moving about because of the extended period the seatbelt sign remained on during light turbulence, combined with the lack of an explanation from the flight deck.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this accident was "the entry of the aircraft into an area of moderate to briefly severe turbulence associated with convective activity while numerous occupants were unsecured by seatbelts, even though the seatbelt sign was lighted."