Requiring IFR trained pilots in a VFR operation doesn’t make sense if the pilots don’t spend a significant portion of their flying utilizing instrument skills to remain IFR proficient. LA is prone to good weather most of the year, which may be one reason Company Op Specs does not require it. Mandating IFR is a hazardous recipe for the very situation that occurred. If you don’t use it, you lose it.
Why mandate IFR to fly the one day or one week out of very 4,5 or 6 months that comes along if you don’t have to? Is your meeting, kids game, or vacation more important than your pre-mature funeral? I am sure anyone of those that perished would tell ya the right answer.
Vertigo is a medical condition of the inner ear. You can experience Spatial Disorientation (SD) w/ Vertigo but they are not the same thing. Spatial Disorientation is a result of taking away one or more sensory inputs such as your vision, inner ear, or kinesthetic senses. The brain will still process the partial input and output an improper result. SD is further broken down into 11 or more sensory illusions, such as Graveyard spin, graveyard spiral, the leans, acceleration, or deceleration errors etc. What you experience depends on sensory deprivation, lighting/ darkness, aircraft, head, and body movement.
Even proficient Pilots experience SD, but proficiency, and re-current training help you to recognize and work through SD.
Non-proficient pilots reactions to SD can range from not recognizing SD and either not react or react improperly, to recognizing SD and still respond improperly. Some recognize SD and freeze up - and then don't fly the plane. SD can happen very quickly or SD can be very subtle.