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Old 11th Dec 2021, 16:51
  #66 (permalink)  
safetypee
 
Join Date: Dec 2002
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Hidden assumptions

Assuming that all crews (any crew) will be able to fly an aircraft, in any situation, etc, does not match the reality of human behaviour; each of us understand situations differently, according to our view at the time, influenced by experiences, knowledge, training.

Concluding a reason for an event without considering the situation as the crew might have done, not appreciating the context without adequate thought, then our views are no better than a guess. Safety is not based on ‘a guess’.
Although safety is not fact, the requirements are based on judgements, including assumptions (although rarely published). A regulator might assume that the tasks (physical, mental) for flying a developed aircraft can be accommodated with a ‘same type rating’.

The 737 has changed; from a conventional swept-wing low-thrust design where controls (and trim) were harmonised with the thrust pitching moment, and a simple (by modern terms) dual channel AP and AT.

The need for Cat 3 auto-land, automatic trim up, an advanced dual FGS requires alternative procedures for GA, for AP/FD, single or dual modes; added complexity. Increased demand on awareness, knowledge and recall, different skills.
More recently increased engine thrust; the manual control system has to cope with a larger pitching moment, but unable to match previous flying qualities exactly - but ‘the crew will manage’ (differences training). And a final mitigation, an afterthought at the end of the checklist ‘If TOGA thrust is not required, then …’; more demand on awareness, decision, action; more complexity in identifying a safety alleviation.

Within this, a critical assumption that the aircraft can be flown with the same basic techniques taught ab-initio; control for pitch, trim to reduce force, a sequential action.
Uplinker identifies the 737 weakness - pitch and trim simultaneously, where any delay adds workload, getting being behind the aircraft.

In this incident, given a need to discontinue an approach: the crew assess the situation, altitude, system status, auto or FD, is the procedure ‘GS’ or ‘discontinued’, and then decide.
A button press on previous variants; now preceded by a multiple choice actions dependent on understanding the situation.
We assume that new technology aids operation (747, 76, 77, 78), but retrofit in an old aircraft changes human activity, with higher mental workload.
Without changing the aircraft or operational situations (money, money, money), all we might change is our thinking - what we assume and why; and with this understanding adjust how we learn from incidents.


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