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Pitot-Static System Failures

The Boeing 727-200, operating as Northwest Airlines Flight 6231, departed John F. Kennedy International Airport in New York, N.Y., at about 1926 Eastern time on December 1, 1974. A ferry flight with only crew aboard, the 727s destination was Buffalo, N.Y., a great-circle distance of 261 nm. After takeoff, the aircraft climbed to 13,500 feet msl and leveled off for about 50 seconds, accelerating from 264 knots to 304 knots.

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Key Takeaways:

  • The 1974 Northwest Flight 6231 crash, caused by iced pitot tubes leading to erroneous airspeed indications and loss of control, underscores the critical importance of activating pitot heat in cold weather.
  • Pilots must understand how different pitot-static system blockages (e.g., blocked pitot tube causing ASI to act as an altimeter, or blocked static port freezing the altimeter and VSI) affect flight instruments.
  • Effective preparation for pitot-static failures includes knowing your aircraft's alternate static system, practicing "Pitch + Power = Performance" for flying without an accurate airspeed indicator, and recognizing abnormal instrument behavior.
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The Boeing 727-200, operating as Northwest Airlines Flight 6231, departed John F. Kennedy International Airport in New York, N.Y., at about 1926 Eastern time on December 1, 1974. A ferry flight with only crew aboard, the 727’s destination was Buffalo, N.Y., a great-circle distance of 261 nm. After takeoff, the aircraft climbed to 13,500 feet msl and leveled off for about 50 seconds, accelerating from 264 knots to 304 knots. It then began to climb, at 2500 fpm, still at about 305 knots. As the jet climbed out of 16,000 feet, airspeed began increasing, followed by rate of climb. According to the NTSB, at about this time, the First Officer said, “Do you realize we’re going 340 knots and I’m climbing 5000 feet a minute?”

The airplane’s climb rate soon exceeded 6500 fpm and an overspeed warning horn sounded at 23,000 feet, with airspeed at 405 knots. Shortly afterward, the stall warning stick shaker was recorded intermittently and, five seconds later, vertical acceleration reduced to 0.88 G. The airplane briefly leveled at 24,800 feet and an airspeed of 420 knots. About 13 seconds later, the airplane began descending at 15,000 fpm. The jet’s magnetic heading changed from 290 degrees to 080 degrees within 10 seconds. The 727 had stalled and was spiraling down. It descended more than 23,000 feet in 83 seconds, striking the ground about 3.2 nm west of Thiells, N.Y. Parts of the horizontal tail were located up to 4200 feet from the main wreckage. There were no survivors.

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