A comment I had on another (now deleted) question states that Air china 006 didn't spin. For me it was a spin (unwanted roll due to asymmetrical stall). The comment states it was "it was a slow roll during a very-high-speed dive".

How can I make the difference between both? What makes this occurrence of uncontrolled roll not a spin?

  • $\begingroup$ I believe the key difference is that a spin occurs due to a stalled wing and consequential instability in the longitudinal axis. Wheras “uncontrolled roll” may occur in a non-stalled condition - maybe a spiral dive well beyond Vne with no aileron command authority. $\endgroup$ – ob318 Sep 19 '20 at 14:55

In the beginning of the article on Wikipedia it is stated that the plane experienced "high G forces and high speed". Not knowing the story of the flight but with this statement we can suppose that it wasn't a spin but rather a spiral dive. But when reading the full story it is stated that the lose of engine 4 leads to a decrease of the speed, that the pilot didn't had any rudder control, applied full left yoke and yet the airplane was still rolling right. Those are clear sign of a spin and not a spiral dive!!

A spin begin in situation where asymmetric stall occurs. In this situation the airplane start spinning on the yaw axis, the external wing creates more lift wherase the inner wing is stalled and drops, thus driving the nose down. But the main characteristic in a spin is that your airspeed is very small (which doesn't mean that the sink rate is not high) but prevent extreme aerodynamic loads on the aircraft and the g loading remain somewhat close to one during the spin. In one word the airplane is falling vertically from the sky spinning on itself, wing inclination can be close to 0 up to 90° depending on the aircraft.

On the other hand a spiral dive can occurs without any stall. When starting a turn if you let the nose drop, the inclination will increase and your speed will increase. At one point pulling on the yoke will only tighten the turn and increase g forces but will not move the nose up. This situation can happen when you're focused on something on the ground and start turning to see it better. If you don't pay attention the nose will drop and it's a one way trip as you are focused on the ground and not your instruments. You reach VNE and loose control of the aircraft. In this situation the aircraft is flying a tight spiral to the ground with high wing inclination always increasing which can be seen as a slow roll.

In a spin you mostly use your rudder pedal to stop the rotation and then pull on the yoke wherase in a spiral dive you need to use the yoke to reduce inclination first and then pull. Adding some yoke to stop rotation in a spin will not help and worse may even flatten it and increase rotation speed. Using rudder pedals in a spiral dive won't have much effect, you need to bring down the inclination, level the wings and this is mostly done using the yoke while keeping your flight coordinated with rudder input if needed. Therefore it is important to differentiate both situations if you want to come out of one of them alive.

  • $\begingroup$ With high wing loading and low air density, even a spin leads to very high sink rates. Also, the high roll rate in a spin can easily overstress the wing tanks due to centrifugal loads. Don't underestimate structural loads in a spin! $\endgroup$ – Peter Kämpf Sep 19 '20 at 19:46
  • $\begingroup$ Yes correct, in spin you will have a high sink rate, same as for spiral dive in any airplane. My experience comes from aerobatic flying and centrifugal loads are not really a concern there. What is sure is that your airspeed is not increasing in a spin (outside of the recovery dive) and that's why I was stating that you might not overstress the aircraft... But for an airliner the story might be different you're correct. $\endgroup$ – MaximEck Sep 19 '20 at 20:26
  • $\begingroup$ A spiral dive produces MUCH less drag and speed buildup is massive and massively dangerous. A spin is comparatively harmless and the most likely reason that Air China 006 came down in one piece. The high initial speed can be explained by low density and maybe incipient spin particularities of the B747. I've never spun one myself, so I am guessing here. $\endgroup$ – Peter Kämpf Sep 19 '20 at 20:28
  • $\begingroup$ Yes that's pretty much what I'm saying no? $\endgroup$ – MaximEck Sep 19 '20 at 20:30
  • $\begingroup$ You say "… I suppose that it wasn't a spin but rather a spiral dive". I disagree. $\endgroup$ – Peter Kämpf Sep 19 '20 at 20:31

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