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In commercial installations, the pressure to keep systems running is immense. Considerable money might be lost if an expensive system is shut down. Operators are often under pressure not to do this. The result has at times been tragic. Nuclear power plants are kept running longer than is safe. Airplanes have taken off before everything was ready and before the pilots had received permission. One such incident led to the largest accident in aviation history. Although the incident happened in 1977, a long time ago, the lessons learned are still very relevant today.

In Tenerife, in the Canary Islands, a KLM Boeing 747 crashed during takeoff into a Pan American 747 that was taxiing on the same runway, killing 583 people. The KLM plane had not received clearance to take off, but the weather was starting to get bad and the crew had already been delayed for too long (even being on the Canary Islands was a diversion from the scheduled flight—bad weather had prevented their landing at their scheduled destination). And the Pan American flight should not have been on the runway, but there was considerable misunderstanding between the pilots and the air traffic controllers. Furthermore, the fog was coming in so thickly that neither plane’s crew could see the other. In the Tenerife disaster, time and economic pressures were acting together with cultural and weather conditions. The Pan American pilots questioned their orders to taxi on the runway, but they continued anyway. The first officer of the KLM flight voiced minor objections to the captain, trying to explain that they were not yet cleared for takeoff (but the first officer was very junior to the captain, who was one of KLM’s most respected pilots). All in all, a major tragedy occurred due to a complex mixture of social pressures and logical explaining away of discrepant observations.

You may have experienced similar pressure, putting off refueling or recharging your car until it was too late and you ran out, sometimes in a truly inconvenient place (this has happened to me). What are the social pressures to cheat on school examinations, or to help others cheat? Or to not report cheating by others? Never underestimate the power of social pressures on behavior, causing otherwise sensible people to do things they know are wrong and possibly dangerous.

Excerpted from ‘The Design of Everyday Things’ by Don Norman