To summarize, the aircraft model with which the crew were most familiar had a pitot heat switch that was activated by an upward movement (the ergonomic standard direction), but this aircraft required a downward movement (or the reverse, I forget which it was, but they were reversed). This meant they went through the checkist and set all the controls, but this control was set wrong through no fault of the pilots (more the fault of the manufacturer). Result: no pitot heat in winter conditions.
Next, during the after dark flight, the pitot tube froze solid in icing conditions, after which the airspeed indication became an altitude indication, at a time when the aircraft was in a climb. This made the pilots think they were overspeeding the aircraft, and they responded by pulling back on the controls. The stick shaker worked as it should have (warning of a speed approaching a stall), But the pilots interpreted this as ... get this ... mach buffet. The pitot system kept delivering seemingly higher and higher airspeeds as they climbed and (in reality) approached stall speed.
The aircraft finally stalled fully, deeply, and unrecoverably. For those who don't know this, you must not ever stall an airliner, because they are perfectly balanced front to back, to save fuel, but the side effect is if the aircraft is stalled, it cannot be recovered and will flat-spin right into the ground.
Small private planes will typically nose down in a stall, often recovering right away for an inexperienced pilot, but airliners have different priorities, one of which is economical operation. An economical airplane cannot afford to have a constant air pressure on the top of the elevator control surfaces, so this is designed out. But in trade, you must not ever stall the aircraft or you will lose it.
In the final analysis, multiple factors were involved (as usual), but the fact that the pitot heater switch had an activation direction opposite the ergonomic standard, all by itself could have prevented the crash.
It's my favorite story about the value of adopting consistent ergonomic standards -- to increase a quantity or activate something, controls should move up, or to the right, or clockwise. To deactivate or decrease a quantity, the reverse. How hard is that?
For those reading this who aren't pilots: pilots aren't ghouls, we like reading accident reports because every report teaches us something that we might use to save ourselves and/or our passengers if we encounter the same conditions.
My favorite air-crash story with interesting pilot-comprehension issues was this ferry flight with only crew onboard:
http://en.wikipedia.org/wiki/Northwest_Airlines_Flight_6231
To summarize, the aircraft model with which the crew were most familiar had a pitot heat switch that was activated by an upward movement (the ergonomic standard direction), but this aircraft required a downward movement (or the reverse, I forget which it was, but they were reversed). This meant they went through the checkist and set all the controls, but this control was set wrong through no fault of the pilots (more the fault of the manufacturer). Result: no pitot heat in winter conditions.
Next, during the after dark flight, the pitot tube froze solid in icing conditions, after which the airspeed indication became an altitude indication, at a time when the aircraft was in a climb. This made the pilots think they were overspeeding the aircraft, and they responded by pulling back on the controls. The stick shaker worked as it should have (warning of a speed approaching a stall), But the pilots interpreted this as ... get this ... mach buffet. The pitot system kept delivering seemingly higher and higher airspeeds as they climbed and (in reality) approached stall speed.
The aircraft finally stalled fully, deeply, and unrecoverably. For those who don't know this, you must not ever stall an airliner, because they are perfectly balanced front to back, to save fuel, but the side effect is if the aircraft is stalled, it cannot be recovered and will flat-spin right into the ground.
Small private planes will typically nose down in a stall, often recovering right away for an inexperienced pilot, but airliners have different priorities, one of which is economical operation. An economical airplane cannot afford to have a constant air pressure on the top of the elevator control surfaces, so this is designed out. But in trade, you must not ever stall the aircraft or you will lose it.
In the final analysis, multiple factors were involved (as usual), but the fact that the pitot heater switch had an activation direction opposite the ergonomic standard, all by itself could have prevented the crash.
It's my favorite story about the value of adopting consistent ergonomic standards -- to increase a quantity or activate something, controls should move up, or to the right, or clockwise. To deactivate or decrease a quantity, the reverse. How hard is that?
For those reading this who aren't pilots: pilots aren't ghouls, we like reading accident reports because every report teaches us something that we might use to save ourselves and/or our passengers if we encounter the same conditions.