Human Factors Incident Reporting — Page 322, Lesson 526

I want to walk you through a case study that really drives home why human factors and incident reporting matter in aviation. We're looking at the Trident aircraft and a specific aerodynamic problem that had catastrophic consequences. Let's start with the stall itself. When an aircraft stalls, most aircraft drop nose-first after the stall. That nose-down attitude allows them to gain flying speed and essentially 'fly out' of the problem. But the Trident was different — the Trident dropped tail-down instead. That's a critical distinction because it made recovery absolutely impossible. Why? Because all the aerodynamic control surfaces were ineffective due to the lack of airspeed. You have no airflow over the control surfaces, so you have no control authority. Now, there's a distinction between a stall and a deep stall. In a regular stall, the engine intakes are within turbulent flow, and the engines flame out. That's bad enough. But in a deep stall, the engine intakes and the stabilizer are within turbulent flow. The engines flame out, and the aircraft is uncontrollable and irrecoverable. That's the key phrase — irrecoverable. The prototype Trident had already been lost in exactly these circumstances, killing all of the flight test crew on board. To try to prevent the aircraft from entering a deep stall, the Trident was equipped with a stick pusher system. This system activated when the aircraft approached the aerodynamic stall. The stick pusher was a pneumatic system — meaning it operated using air pressure — and it acted on the pilots' control columns, pushing the nose of the aircraft down if the crew failed to respond to the stall warnings. So it was an automatic override to force the nose down and prevent the stall from developing into a deep stall. But here's an important detail about the system: the pressure in the stick push system could be 'dumped' — that is, released — by pulling a lever on the Captain's side of the central pedestal. There was also a small amber light that illuminated in the case of a lack of pressure in the stick-push system. That amber light was located on the pedestal near the levers controlling the flaps and droops. So the crew had a visual indication if the system wasn't pressurised and ready to operate. Now let's talk about the Trident's wing design. The Trident had a very efficient wing optimized for high speed flight. Unfortunately, it was not so efficient at low speeds. So for take-off and landing, it was equipped with flaps on the trailing edge and droops on the leading edge. Let me define those terms: flaps are high-lift devices on the trailing edge of the wing, and droops are high-lift devices on the leading edge — droops are also known as 'slats'. Here's where the numbers become critical. Flaps were normally retracted at about 185 knots. But the droops — the leading edge slats — could not be retracted below about 225 knots. That means there was a speed range between 185 and 225 knots where the flaps could be retracted but the droops had to stay out. Below 225 knots, the Trident's 'clean' wing — that is, the wing with no high-lift devices deployed — could not produce enough lift to keep the aircraft in the air. In other words, it would enter a stall. Both the flaps and droops produced a great deal of low-speed lift, but they also produced a lot of drag. The flaps and droops were controlled by two almost identical levers located on the co-pilot's side of the pedestal. And here's a crucial interlock: if the flaps and droops were deployed as a safety measure, the droops could not be retracted unless the flaps had first been fully retracted. That's a design logic — you have to bring the flaps all the way in before you can retract the droops. But because flaps were normally retracted at about 185 knots, the droop lever was effectively unguarded between certain conditions — and that's where the human factors problem starts to emerge. Let me show you the diagrams that illustrate these two conditions. First, the stall condition where engine intakes are within turbulent flow and engines flame out. And second, the deep stall condition where the engine intakes and stabilizer are both in turbulent flow, making the aircraft uncontrollable and irrecoverable. So to summarise what we've covered: the Trident had a dangerous deep stall characteristic where it dropped tail-down, making recovery impossible. It had a stick pusher system to prevent that, but that system could be manually overridden by dumping pressure. And it had a complex flap and droop system with specific retraction speeds and an interlock that could create confusion in the cockpit. All of these factors come together in a human factors incident reporting context — understanding why these design features mattered and how they interacted with crew actions is exactly what incident reporting is meant to capture and prevent.