On 13 January 1982 a Boeing 737 sat at Washington National through an afternoon of heavy snow, de-iced at the gate and then held 49 minutes for clearance while the snow kept falling. On the after-start checklist the captain answered the anti-ice item with off, and the engine anti-ice stayed off through everything that followed: the ground wait, the crew's attempt to melt snow by sitting close behind the exhaust of the aircraft ahead, and the take-off. This section's distinction is the whole hazard: anti-ice is switched on early, on suspicion, to keep a surface from icing at all, and the surface that mattered here was the small inlet-pressure probe that feeds the EPR gauge.
With the probe blocked, the gauge lied in the dangerous direction. Tests reproduced it: a blocked probe with anti-ice off shows about 2.04 EPR, the take-off setting, while the engine actually delivers about 1.70, some 3,750 lb of thrust per engine short. The crew set 2.04 on both gauges and got there at an odd throttle position. The first officer said so four times as the roll went on (that don't seem right, does it; that's not right; I don't think that's right), and the captain's answer was that it was, and there's eighty. The 737 lifted off slowly with snow on its wings, could not climb, struck the 14th Street Bridge and fell through the ice of the Potomac. Seventy-four of the 79 aboard died, and four people in cars on the bridge; four passengers and one crew member were pulled from the river.
The probable cause named three things together, and this card is only one of them: the failure to use engine anti-ice, the decision to take off with snow and ice on the wings, and the captain's failure to reject the take-off when his attention was called to the anomalous readings, with the long delay after de-icing and the 737's pitch-up sensitivity to leading-edge contamination contributing. Fifteen days after the accident the Board's first, urgent recommendation went to every airline: teach crews to recognise inlet-probe icing, and require them to cross-check all engine instruments as take-off power is set. Anti-ice on a probe is not comfort for the engine; it is what keeps the thrust gauge telling the truth.