Fri, 10 June 2011
0:00:00 Comments[2]
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- 1. The AF447 is a very interesting incident. The first instinct is to call it pilot error because they stalled the aircraft; if they were getting conflicting data then they should have just gone to the standby Attitude Indicator (AI) and maintained straight and level flight. There are, however, circumstances which the layman may not be aware of. When the air data computer started having problems, they switched to a reversion mode. This disconnected the autopilot, including the auto-throttle. Pilots familiar with that type will tell you how very difficult it is to try and fly the aircraft at that altitude, at night, with conflicted readouts and without the autopilot auto-throttle. Factor in that the joystick on that aircraft has no tactile feedback to speak of, and they had a very difficult task. When the final incident report is released, I think you'll see the words "deficiencies in the human machine interface" feature prominently.
2. The "Swiss cheese" model of accident is more properly known as the Reason Model, after James Reason. Each slice of cheese is a layer in our defences against an incident (training, procedures, design, etc) but there are 'holes' in each layer (human inattention, etc). Sometimes all the holes line up, allowing a straight path through to an incident. Also have a read up on the Boeing 'Chain of Events' model, where a chain of events lead to an incident; if any one of the 'links' in the chain were broken, the incident would not have occurred. A good example of this model is the collision of two 747s at Tenerife in 1977. - Fellow air crash investigators fans – regarding the segment that criticised the aircrew for not following the flight instruments.
If the pitot tubes ice up, (one or all of them), the effect can be catastrophic, not only are the standby instruments of air speed, and true airspeed disabled, but so are the main readouts of these functions (via an air data computer). But even more importantly those air speed parameters are critical inputs to the ‘air data computer’ which also has inputs to the, cockpit instruments (readouts) auto pilot, and stall warning, to name but a few.
It’s not so much that the pilots will be ignoring the instruments and going on gut feeling, it’s that they will be getting conflicting information in the cockpit. They may well be ‘flying on instinct’ but only because they are unable to resolve the good information from the bad.
There was a similar episode of air crash investigations where a pitot tube had not been covered the night before and a mud-wasp was had built a nest it the hole, in this case one set of instruments disagreed with another set, the resulting confusion was all to predictable.
It’s been a while since I worked on aircraft, but the actual pitot static system (the pipes and tubing that feeds the computer and instruments) is not redundant, only duplicated, and even then some of the pressure and static feed in a single point of failure.
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