5 ms·

Interesting analysis in this post on the PPrune forum: http://www.pprune.org/rumours-news/447730-af447-wreckage-found-30.html#post6477527 http://www.pprune.org/

by wazoox 15y ago

Interesting analysis in this post on the PPrune forum: http://www.pprune.org/rumours-news/447730-af447-wreckage-found-30.html#post6477527 http://www.pprune.org/rumours-news/447730-af447-wreckage-fou...

ColinWright 15y ago
For a non-pilot that's a difficult read, but worthwhile to understand the issues involved and implications. Thank you.
- BrianLy 15y ago
  Wikipedia has some coverage of the flight control modes mentioned at http://en.wikipedia.org/wiki/Flight_control_modes_(electronic) http://en.wikipedia.org/wiki/Flight_control_modes_(electroni...
johno215 15y ago
From what I understand, the instruments may have shown that airspeed was increasing when it was not. So the pilots tried to pitch up to reduce excessive airspeed but this in reality just caused them to go deeper into the stall. There was a stall sensors but they chose to ignore it in favor of the faulty airspeed sensor. A case of lack of situational awareness or rather faulty situational awareness brought on by a sensor failure.
- epochwolf 15y ago
  A little background on the warnings. There are no specific sensors for them. They are issued by the computer when it detects a dangerous situation, which it uses a variety of inputs to determine. With unreliable airspeed and altitude (pitots tubes not working) the overspeed warning should have been ignored completely and the stall would have to be verified with other data if possible.
lotharbot 15y ago
My simplified summary/explanation [0]: Several flight instruments rely on air pressure. If one of your pressure gauges gets blocked, you can get very misleading data. This guy's theory as to what happened in this case is that the forward pressure tube (pitot tube) had an ice blockage, which led to a low airspeed reading. The autopilot tried to correct by accelerating the plane. Since loaded jetliners normally fly slightly nose-up to maintain altitude [1], the autopilot kept that orientation as it accelerated, which led to a climb and ultimately a stall. When the plane stalled, it started dropping altitude rapidly -- but with a blocked pressure line, this could misleadingly report a high airspeed due to the increase in external pressure. As the pilots took over, they were (according to this guy's theory) trying to reduce airspeed by cutting the throttle and nosing up. Rather than fixing an overspeed condition, they were actually making the stall worse. They never would have realized what was happening because their instruments kept reporting overspeed. [0] I'm not a pilot, but I worked in an aerospace museum's education department, and learned a lot from the pilots and engineers who volunteered with us. [1] According to one of the volunteers, who was also an engineer for one of the big jetliners, loaded jetliners get about half of their lift by keeping their wings 3-6 degrees above level. The exact angle is selected based on load and airspeed; get it wrong, and you'll accidentally climb or descend.
- rorrr 15y ago
  Why don't they use GPS to determine the altitude and the speed? Even a $100 GPS in my car is more precise than the speedometer.
  - jwr 15y ago
    GPS will give you groundspeed, not airspeed. And since you don't get lift from groundspeed, it isn't as relevant to keeping the airplane flying.
    - rationalbeats 15y ago
      Example of a Cessna flying backwards lift generated by headwind. http://www.youtube.com/watch?v=u2tiaWPNamE http://www.youtube.com/watch?v=u2tiaWPNamE

by wazoox 15y ago

ColinWright 15y ago

For a non-pilot that's a difficult read, but worthwhile to understand the issues involved and implications. Thank you.

BrianLy 15y ago
Wikipedia has some coverage of the flight control modes mentioned at http://en.wikipedia.org/wiki/Flight_control_modes_(electronic) http://en.wikipedia.org/wiki/Flight_control_modes_(electroni...

johno215 15y ago

From what I understand, the instruments may have shown that airspeed was increasing when it was not. So the pilots tried to pitch up to reduce excessive airspeed but this in reality just caused them to go deeper into the stall. There was a stall sensors but they chose to ignore it in favor of the faulty airspeed sensor. A case of lack of situational awareness or rather faulty situational awareness brought on by a sensor failure.

epochwolf 15y ago
A little background on the warnings. There are no specific sensors for them. They are issued by the computer when it detects a dangerous situation, which it uses a variety of inputs to determine. With unreliable airspeed and altitude (pitots tubes not working) the overspeed warning should have been ignored completely and the stall would have to be verified with other data if possible.

lotharbot 15y ago

My simplified summary/explanation [0]: Several flight instruments rely on air pressure. If one of your pressure gauges gets blocked, you can get very misleading data. This guy's theory as to what happened in this case is that the forward pressure tube (pitot tube) had an ice blockage, which led to a low airspeed reading. The autopilot tried to correct by accelerating the plane. Since loaded jetliners normally fly slightly nose-up to maintain altitude [1], the autopilot kept that orientation as it accelerated, which led to a climb and ultimately a stall. When the plane stalled, it started dropping altitude rapidly -- but with a blocked pressure line, this could misleadingly report a high airspeed due to the increase in external pressure. As the pilots took over, they were (according to this guy's theory) trying to reduce airspeed by cutting the throttle and nosing up. Rather than fixing an overspeed condition, they were actually making the stall worse. They never would have realized what was happening because their instruments kept reporting overspeed. [0] I'm not a pilot, but I worked in an aerospace museum's education department, and learned a lot from the pilots and engineers who volunteered with us. [1] According to one of the volunteers, who was also an engineer for one of the big jetliners, loaded jetliners get about half of their lift by keeping their wings 3-6 degrees above level. The exact angle is selected based on load and airspeed; get it wrong, and you'll accidentally climb or descend.

rorrr 15y ago
Why don't they use GPS to determine the altitude and the speed? Even a $100 GPS in my car is more precise than the speedometer.
- jwr 15y ago
  GPS will give you groundspeed, not airspeed. And since you don't get lift from groundspeed, it isn't as relevant to keeping the airplane flying.
  - rationalbeats 15y ago
    Example of a Cessna flying backwards lift generated by headwind. http://www.youtube.com/watch?v=u2tiaWPNamE http://www.youtube.com/watch?v=u2tiaWPNamE