Human Factors Incident Reporting — Page 322, Lesson 529

I want to walk you through a case study in Human Factors Incident Reporting. This excerpt takes us into the real-world chain of events leading up to a specific accident, and it's a classic example of how design, procedures, and human behaviour can combine into a trap. Let's start with the aircraft system in question. We're talking about the Hawker Siddeley Trident, and specifically its high-lift devices. The Trident had both flaps and something called droops. Droops are leading-edge devices that extend forward and down from the wing's leading edge to increase lift at low speeds. They are not the same as flaps, which are on the trailing edge. Now, here's the critical speed envelope for these droops. The excerpt gives us two key speeds. There is a droop retraction speed at 225 knots. Below that speed, the droops should be extended. Then there is a limit: above 250 knots, the droops could not be extended at all. Why? Because the air pressure on the wing at that speed would be so great that it would literally rip the droops off the wing structure. So the safe operating band for having the droops extended is between 225 and 250 knots indicated airspeed. To help the crew manage this, the Trident had an amber 'droops out of position' warning light in the cockpit. Now, listen carefully to what this light actually told you. It did not tell you whether the droops were retracted when they should be extended, or extended when they should be retracted. It simply told you that the droops were not in the correct position for the indicated airspeed. That is a significant design limitation — it's an ambiguous warning. It just says "something is wrong," not what is wrong. To make matters worse, this amber light was located right next to the warning light that advised the crew of a lack of pneumatic pressure in the stick push system. The stick pusher is a system that physically pushes the control column forward to prevent the aircraft from stalling. So you have two critical warnings — one about your high-lift configuration, one about your stall-protection system — sitting right next to each other. That's a classic human factors issue: poor warning-light grouping and potential for confusion. Now let's move to the run-up to the flight, the procedural and cultural background. In the months before the accident, BEA — British European Airways — had issued flight safety bulletins telling pilots that it was totally unacceptable to perform flapless take-offs in the Trident using only the droops. So the official procedure was to use flaps for take-off. But here's the human factors problem: several pilots were known to do this flapless take-off anyway. Why? Because although it increased the rotation speed at take-off — meaning you need a higher speed to lift the nose wheel off the runway — it gave a far better climb performance. So pilots were trading a procedural violation for a performance benefit. Now, here's where the design of the cockpit and ingrained habits created a deadly trap. On several occasions after performing a flapless take-off, as the aircraft approached the normal flap retraction speed, the Captain would still call for "flaps" out of habit. The First Officer, hearing that command, would immediately retract the droops. But remember — if you retract the droops at that speed, you lose lift. On all those previous occasions, the Flight Engineer had noticed what was happening and reversed the droop travel before the aircraft fell out of the sky. So the system was relying on the third crew member to catch a mistake caused by a design-induced error. Finally, we have the maintenance factor. Several days before the fateful flight, the aircraft underwent routine maintenance, and during that visit, the stick pusher system was overhauled. Unfortunately, on reassembly, the mechanic made an error. The excerpt cuts off there, but you can already see the layers: a confusing cockpit warning, a known procedural violation, a habit-driven error chain, and a maintenance mistake. That is the anatomy of an accident waiting to happen.