Tuesday, March 9, 2010
Safety’s Ideal World
Unfortunately, we don’t always learn from example
In 1975, the NTSB reported on the December 1, 1974, crash of a Northwest Airlines Boeing 727 near Thiells, N.Y. The Safety Board found that contrary to standard operational procedures, the flight crew hadn’t activated the pitot heat. The airplane had departed John F. Kennedy International Airport on a ferry flight to Buffalo, N.Y. The three crewmembers, who were the only people on board, were killed. The airplane was climbing to FL310; it stalled after climbing above FL240. Earlier, the airplane had been climbing at 2,500 fpm with a 305-knot airspeed, according to the flight data recorder. As the altitude increased above 16,000 feet, the airspeed and climb rate displayed on the cockpit indicators began to increase. The first officer, who was the flying pilot, remarked, “Do you realize we’re going 340 knots and I’m climbing 5,000 feet per minute?” The second officer responded, “That’s because we’re light.”
Soon, the rate of climb exceeded 6,500 fpm, and the airspeed reached 420 knots. The first officer said that he couldn’t reduce the climb rate or slow the 727. The captain told him to “just pull her back. Let her climb.” Then, the stall-warning stick-shaker activated, and the airplane started to buffet. The first officer said the buffeting was because they were close to the airplane’s critical Mach speed as shown on the airspeed indicators: “I guess we’ll have to pull it up.” A high-speed buffet is caused by the formation of a shock wave on the airfoil surfaces and turbulent separation of the flow aft of the shock wave. But there also is buffeting associated with a stall.
The captain commanded, “Pull it up.” Two seconds later, the airplane began to descend, making a rapid turn to the right and reaching a descent rate of 15,000 fpm. The crew issued a “mayday” call, and advised controllers they were descending through 12,000 feet in a stall. At about 3,500 feet MSL, part of the left horizontal stabilizer separated.
NTSB wind tunnel tests of the pitot heads found that when exposed to liquid in freezing conditions with the pitot heat inoperative, a thin film of water flowed into the pressure port and out of the drain hole. Then one to two inches of ice formed over the pitot’s pressure inlet port. Ice also blocked the drain hole.
Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other NTSB news. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.
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