From the B-727 NWA 6231 NTSB Report
The sound of the stall warning stick shaker was recorded intermittently less than 10 seconds after the onset of the overspeed warning. Five seconds later, vertical acceleration reduced to 0.8g, and the altitude leveled at 24,800 feet.The recorded airspeed was 420 kn.
The stall warning began again and continued while the first officer commented, "There's that Mach buffet, 71 guess we'll have to pull it up." followed by the captain's command, "Pull it up," and the sound of the landing gear warning horn. The FDR readout shows the following:
Two seconds later (about 13 seconds after the aircraft arrived at 24,800 feet), the vertical acceleration trace again declined to 0.8g and the altitude trace began to descend at a rate of 15,000 feet per minute.The airspeed trace decreased simultaneously at a rate of 4 kn per second and the magnetic heading trace changed from 290° to 080° within 10 seconds, which indicated that the aircraft was turning rapidly to the right. As the aircraft continued to descend, the vertical acceleration trace increased to 1.5g. The aircraft's magnetic heading trace fluctuated, but moved basically to the right, the vertical acceleration trace increased immediately,with peaks to +3g. Ten seconds later, the peak values for vertical acceleration increased to +5g.The aircraft had descended from 24,800 feet in 83 seconds.
Pitot Information (In this case, pitot heaters were not on)
A pitot head of the same type that provided pitot pressure to the first officer's airspeed/Mach indicator was exposed to icing conditions in a wind tunnel. With the pitot heater inoperative, 1 to 2 inches of ice formed over the pressure inlet port. During the exposure, a thin film of water flowed into the pressure port, some-of which flowed out of the drain hole.
Blockage of the drain hole by ice seemed to depend on the length of time required for ice to form and block the total pressure inlet port. The longer it took for ice to form and block the total pressure port, the more likely it became that the drain hole would be blocked by ice. Also, the greater the angle between the longitudinal axis of the pitot head and the relative wind, the greater the likelihood that the drain hole would become blocked with ice.
Constant altitude pressure measurements showed that when the total pressure inlet port was blocked by ice and the drain hole remained open, pressure changes occurred that would cause a reduction of indicated air- speed. However, when both the total pressure port and drain hole were blocked, the total pressure remained constant, which would cause indicated airspeed to remain fixed. Also, abrupt and small pressure fluctuations occurred shortly before either the pressure port or drain hole became blocked by ice.
In an effort to reproduce the apparent inconsistencies between the airspeed and altitude values on the FDR traces, tests were conducted with an airspeed indicator and an altimeter connected to vacuum and pressure sources. By altering the vacuum to the altimeter and to the airspeed indicator, the altitude trace could be reproduced. However, following ascent above 16,000 feet, the FDR airspeed and altitude values could be simultaneously duplicated only when the total pressure to the airspeed indicator was fixed at its FDR value for an altimeter reading of about 15,675 feet and an indicated airspeed of about 302 kn.
Following the accident, the Safety Board requested that the aircraft mailufacturer analyze the data from the CVR and FDR to determine (1) The consistency of these data, particularly the airspeed and altitude values, with the theoretical performance of the aircraft; (2) the significance and possible reason for a simultaneous activation of the overspeed and stall warning systems; and (3) the body attitude of the aircraft during its final ascent and descent.
The indicated airspeed of the aircraft when the stick shaker was first activated was calculated to be 165 kn as compared to the 412 kn recordedbytheFDR. The decrease in airspeed from 305kn to 165kn as the aircraft climbed from 16,000 feet to 24,000 feet (within 116 seconds) is within the aircraft's theoretical climb power performance. The aircraft's pitch attitude would have been about 30° noseup as stick shaker speed was approached. The stall warning stick shaker is activated by angle of attack instrumentation which is completely independent of, and therefore not affected by errors in, the aircraft's airspeed measuring systems. Boeing personnel interpreted the sound of the landing gear warning horn on the CVR to indicate that the thrust levers had been retarded to-idle. The second reduction in vertical acceleration to 0.8g which was coincident with atheoretical relationships of angle of attack, velocity, and drag were compared to the recorded rate of descent and load factor to determine the attitude of the aircraft after the stall. The comparison showed that the aircraft attained an angle of attack of 22°, or greater, during the descent. Transient nosedown attitudes of more than 60° would have been required to achieve the measured descent rate with an angle-of attack of 22°. The variations in load factors, which averaged about +1.5g, were attributed to variations in the aircraft's angle of bank.
The aircraft was probably exceeding 230 kn, with a nosedown attitude of about 50° as it descended below 11,000 feet.
The aircraft hit the ground in a slightly nose down right wing down attitude. Except for both elevator tips, the left horizontal stabilizer, and three pieces of light structure from the left stabilizer, the entire aircraft was located within an area 180 feet long and 100 feet wide.
The aircraft and altitudes are different, but there may be some similarities between this event and AF447 relative to pitot icing and the descent, time and distance.