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Actually, in earlier flight manuals for the 737 there was a description for the so called roller-coster-maneuver for exactly the case that the trim was so far o

by _ph_ 7y ago

Actually, in earlier flight manuals for the 737 there was a description for the so called roller-coster-maneuver for exactly the case that the trim was so far off that it couldn't be operated manually any more. The maneuver consisted of pushing the yoke forward to take the pressure off the stabilizer and quickly trim back and then put the nose up again. So it was documented, that in certain conditions the forces could be too high for the pilots.

lmilcin 7y ago
Think of it for a sec. Your plane tries to kill you by pointing the nose down. With all your force you are barely able to make it only loose the altitude slowly. Now, would you point your nose down knowing that you may not be able to point it back up again?
- _ph_ 7y ago
  Well, the roller-coster maneuver assumes you are high enough up that you can trade off some of your height for correcting the trim. Here is a link to the description: http://www.b737.org.uk/runawaystab.htm#rc http://www.b737.org.uk/runawaystab.htm#rc This obviously wouldn't work in cases like the Ethiopian, where the plane had very little height over ground due to the high mountains around the airport. Edit: the point isn't that the maneuver had prevented the accident, but that its existance in the flight manuals shows that it was known that the trim forces could be higher than what the pilots could apply to the trim wheels.
  - lmilcin 7y ago
    I'm not a pilot but I do sail some and the same manouver is used. You know, sails also generate huge forces which might not be possible to physically counter. What you do is, if you want to trim and are courteous to your crew, you point your yacht upwind for a moment to let them trim and then get back on your course.
    - _ph_ 7y ago
      I also did a bit of sailing and remember it was also standard to steer a bit more upwind if the boat is listing too much. That is, if there isn't anything in the way, if you steer more upwind :p
      - lmilcin 7y ago
        This will actually depend on your course with regards to wind. There will be a course that will generate maximum heeling moment.
      - odonnellryan 7y ago
        Generally you'll heel over the closer you are into wind while your sails aren't luffing. Taking off the wind will make the boat heel less.
  - cjbprime 7y ago
    Yeah, and I think MCAS ended up fully deflecting the stabilizer on Ethiopian. Even with a somewhat more reasonable altitude, I wonder how long it takes to physically manually retrim from one end of the jackscrew to level flight. I think even the electric motor takes at least six seconds to do that.
    - _ph_ 7y ago
      From what I read the situation was made worse by the fact that they where flying a bit on the fast side even when they were not going down yet, increasing the load on the stabilizer.
      - secabeen 7y ago
        A bit of the fast side is an understatement. They were at full takeoff power the entire time they were fighting the trim. They never reduced engine power, which would have reduced the forces on the stabilizer. An easy thing to miss certainly, and it was one of many factors that contributed to the crash: https://nymag.com/intelligencer/2019/04/what-passengers-experienced-on-the-ethiopian-airlines-flight.html https://nymag.com/intelligencer/2019/04/what-passengers-expe...
        cjbprime 7y ago
        A possible ameliorating factor: they had an IAS Disagree warning (unreliable airspeed) since takeoff, and it may be policy to keep airspeed up in that case -- not knowing your airspeed means that reducing it may cause a stall. (Still, they could have asked ATC for an speed reading or used other measurements to convince themselves that it was safe to slow down.)
- cjbprime 7y ago
  A normal person wouldn't, but you're probably overstating the reluctance an actual pilot would have. Being reluctant to push the nose down when in a bad situation would cause pilots of any aircraft to fail their first hour of stall training. It's more worrying that, since the procedure was removed from manuals and training, the pilots might simply not have realized that it would help.

by _ph_ 7y ago

lmilcin 7y ago

Think of it for a sec. Your plane tries to kill you by pointing the nose down. With all your force you are barely able to make it only loose the altitude slowly. Now, would you point your nose down knowing that you may not be able to point it back up again?

_ph_ 7y ago
Well, the roller-coster maneuver assumes you are high enough up that you can trade off some of your height for correcting the trim. Here is a link to the description: http://www.b737.org.uk/runawaystab.htm#rc http://www.b737.org.uk/runawaystab.htm#rc This obviously wouldn't work in cases like the Ethiopian, where the plane had very little height over ground due to the high mountains around the airport. Edit: the point isn't that the maneuver had prevented the accident, but that its existance in the flight manuals shows that it was known that the trim forces could be higher than what the pilots could apply to the trim wheels.
- lmilcin 7y ago
  I'm not a pilot but I do sail some and the same manouver is used. You know, sails also generate huge forces which might not be possible to physically counter. What you do is, if you want to trim and are courteous to your crew, you point your yacht upwind for a moment to let them trim and then get back on your course.
  - _ph_ 7y ago
    I also did a bit of sailing and remember it was also standard to steer a bit more upwind if the boat is listing too much. That is, if there isn't anything in the way, if you steer more upwind :p
    - lmilcin 7y ago
      This will actually depend on your course with regards to wind. There will be a course that will generate maximum heeling moment.
    - odonnellryan 7y ago
      Generally you'll heel over the closer you are into wind while your sails aren't luffing. Taking off the wind will make the boat heel less.
- cjbprime 7y ago
  Yeah, and I think MCAS ended up fully deflecting the stabilizer on Ethiopian. Even with a somewhat more reasonable altitude, I wonder how long it takes to physically manually retrim from one end of the jackscrew to level flight. I think even the electric motor takes at least six seconds to do that.
  - _ph_ 7y ago
    From what I read the situation was made worse by the fact that they where flying a bit on the fast side even when they were not going down yet, increasing the load on the stabilizer.
    - secabeen 7y ago
      A bit of the fast side is an understatement. They were at full takeoff power the entire time they were fighting the trim. They never reduced engine power, which would have reduced the forces on the stabilizer. An easy thing to miss certainly, and it was one of many factors that contributed to the crash: https://nymag.com/intelligencer/2019/04/what-passengers-experienced-on-the-ethiopian-airlines-flight.html https://nymag.com/intelligencer/2019/04/what-passengers-expe...
      - cjbprime 7y ago
        A possible ameliorating factor: they had an IAS Disagree warning (unreliable airspeed) since takeoff, and it may be policy to keep airspeed up in that case -- not knowing your airspeed means that reducing it may cause a stall. (Still, they could have asked ATC for an speed reading or used other measurements to convince themselves that it was safe to slow down.)
cjbprime 7y ago
A normal person wouldn't, but you're probably overstating the reluctance an actual pilot would have. Being reluctant to push the nose down when in a bad situation would cause pilots of any aircraft to fail their first hour of stall training. It's more worrying that, since the procedure was removed from manuals and training, the pilots might simply not have realized that it would help.