The aircraft was on an IFR clearance and climbing through a cloud layer when it broke up in flight following an in-flight upset. The weather conditions included multiple cloud layers from 4,000 to 13,000 feet, with a freezing level around 7,000 feet msl. An AIRMET was in effect for occasional moderate rime to mixed icing-in-clouds and in-precipitation below 18,000 feet. As the airplane began to intercept a victor airway, climbing at about 2,000 feet per minute (fpm), and passing through 6,700 feet, the airplane began a series of heading and altitude changes that were not consistent with its ATC clearances.
As the aircraft was descending through 7,000 feet, the pilot advised ATC “four Juliet victor I just lost my needle give me…” No further transmissions were received from the accident airplane and the last radar return showed it descending through 3,200 feet at about 11,000 fpm.
The PIC with 5,000hrs total time, 30 hours in the last 30 days, and over 700 hours of actual instrument time became disoriented when his pitot tube iced up due to a faulty pitot heat switch. The NTSB determined that it likely would have functioned correctly on a preflight check if he conducted one, and would have not had any way of knowing that the pitot heat was only operating intermittently.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: the pilot’s loss of control and resulting exceedence of the design stress limits of the aircraft, which led to an in-flight structural failure. The pilot’s loss of control was due in part to the loss of primary airspeed reference resulting from pitot tube icing, which was caused by the internal failure of the pitot heat switch.
Blocked Pitot Tube
What happens when you block the inlet and drain hole of a Pitot tube, but the static port remains unblocked? The Airspeed begins to act as an altimeter, the higher you go the higher the airspeed reads, regardless of what speed the aircraft is actually going. See the Case Study below:
A Northwest Airlines 727 departed JFK at 19:14 for a ferry flight to Buffalo. At 19:21 the flight was cleared to climb to FL310. The aircraft began to climb 2500fpm at an airspeed of 305 knots. As the aircraft climbed through FL160, both the airspeed and the rate of climb began to increase. Reaching FL230, the airspeed had reached 405 knots (indicated) and the rate of climb had exceeded 6500fpm. The overspeed warning horn sounded a little later, followed 10 seconds later by a stickshaker stall warning. The aircraft then leveled at 24800 feet with a speed of 420 knots until it turned rapidly to the right (stall). The airplane started to descend out of control, reaching a vertical acceleration of +5g until it struck the ground in a slightly nose down and right wing-down attitude. The aircraft had descended from 24000 feet to the ground in 83 seconds.
PROBABLE CAUSE: “The loss of control of the aircraft because the flight crew failed to recognize and correct the aircraft’s high-angle-of-attack, low-speed stall and its descending spiral. The stall was precipitated by the flight crew’s improper reaction to erroneous airspeed and Mach indications which had resulted from a blockage of the pitot heads by atmospheric icing. Contrary to standard operational procedures, the flight crew had not activated the pitot head heaters.”
Bottom line: The Pitot tubes iced up, they thought they were going really fast, but they were not. They kept pulling back on the controls trying to make the airplane slow down, but stalled the airplane, and lost control, not understanding what the instruments were telling them.