TWA 514 crashed into terrain while attempting to land at Washington Dulles International Airport. from Wikipedia:
"The flight was being vectored for a non-precision instrument approach to runway 12 at Dulles. Air traffic controllers cleared the flight down to 7,000 feet (2,130 m) before clearing them for the approach while not on a published segment.
The jetliner began a descent to 1,800 feet (550 m), shown on the first checkpoint for the published approach. The cockpit voice recorder later indicated there was some confusion in the cockpit over whether they were still under a radar-controlled approach segment which would allow them to descend safely. After reaching 1,800 feet (550 m) there were some 100-to-200-foot (30 to 60 m) altitude deviations which the flight crew discussed as encountering heavy downdrafts and reduced visibility in snow.
The plane impacted the west slope of Mount Weather at 1,670 feet (510 m) above sea level at approximately 230 knots (265 mph; 425 km/h). The wreckage was contained within an area about 900 by 200 feet (275 by 60 m). The evidence of first impact were trees sheared off about 70 feet (20 m) above the ground; the elevation at the base of the trees was 1,650 feet (505 m).
The wreckage path was oriented along a line 118 degrees magnetic. Calculations indicated that the left wing went down about six degrees as the aircraft passed through the trees and the aircraft was descending at an angle of about one degree. After about five hundred feet (150 m) of travel through the trees, it struck a rock outcropping at an elevation of about 1,675 feet (510 m). Numerous heavy components of the aircraft were thrown forward of the outcropping, and numerous intense post-impact fires broke out which were later extinguished. The mountain's summit is at 1,754 feet (535 m) above sea level."
As a result of this accident, air traffic controllers now assign an altitude to fly until intercepting a segment of a published approach.
Northwest 6231 crashed after encountering an aerodynamic stall. From Wikipedia:
"The flight was chartered to pick up the Baltimore Colts in Buffalo after the aircraft originally earmarked to transport the team was grounded by a snowstorm in Detroit.
The Boeing 727-251, registration N274US, departed New York City's John F. Kennedy International Airport at 19:14 for a ferry flight to Buffalo. As the craft climbed past 16,000 feet (4,900 m), the overspeed warning horn sounded, followed 10 seconds later by a stick shaker stall warning. The aircraft leveled at 24,800 feet (7,600 m) until it started to descend out of control in a spin, reaching a vertical acceleration of +5g. At about 3,500 feet (1,100 m), a large portion of the aircraft's horizontal stabilizer separated due to the high G-forces, making recovery impossible. Flight 6231 struck the ground in a slightly nose down and right wing-down attitude twelve minutes after take-off, at 19:26."
The accident board determined that the pitot heat had been inadvertently turned OFF prior to takeoff, and as the aircraft climbed through clouds the pitot tubes froze, causing altimeter effect on the airspeed indicator, in which an increase in altitude will cause indicated airspeed to increase.
On many aircraft today, the pitot heat will automatically be turned ON when the aircraft is airborne.