What is the difference between a spin and an uncontrolled roll in an high speed dive?

Question

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?

Answer 1

How can I make the difference between both?

The main difference between a spin and a spiral dive is in the angle of attack (much higher in a spin) and, consequently, much less drag in case of the spiral dive which results in higher speed.

What makes this occurrence of uncontrolled roll not a spin?

Both can be entered from an asymmetric stall. When the cg is more forward, a spiral dive becomes more likely and vice versa. Some airplanes will only enter a spin with a rear cg location and must be held in the spin with rudder in spin direction and stick back. Other airplanes will still spin with controls neutral, and the cg location has a high influence on how easy it is to end the spin.

All symptoms look like the Air China 006 upset was a spiral dive.

Answer 2

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.

Answer 3

There has been plenty written about that B747 incident. I have seen a re-creation in a simulator. For aspiring airline pilots suffice it to say that in any abnormal situation someone has to fly the aircraft. You know, the basics….

He or she has to keep a safe airspeed and if an engine is lost at high altitude that will mean starting a descent. The yaw will have to be countered by applying some rudder. Wings level, ball in the centre. It is called flying.

Then while someone is flying, the other(s) can open the book, try to restart or secure, consider terrain, alternate airports, get on the radio etc etc.

For student pilots, you will be shown what a spiral dive looks like; increasing airspeed and bank angle and high rate of descent. Reduce power, level the wings and ease out of the dive.

You will have to demonstrate the same recovery prior to getting an instrument rating without outside reference. You will also have to demonstrate remaining in control, by hand and foot following engine failures in multi engined aircraft, securing the engine by following some procedures by memory. This will have to be done on instruments without outside reference for multi engined instrument ratings.

Wings level, ball in the centre, maintain correct speed, get rid of drag (landing gear) etc dead foot, dead engine etc etc.

It is called flying.

You will be shown how to recover from a stall and how not to get into a spin.

You can also get spin training. In a spin the airspeed starts low and stays low.

Some aeroplanes can be gotten into a spin but will not stay there long - they can transition into a spiral dive, airspeed increasing etc as above with the same recovery.

I think this is what happened to that 747. The pilot flying let the airspeed decay and did not counter the asymmetry with rudder. The a/c stalled and a wing dropped. The aircraft then transitioned into a spiral dive.

In the re-creation I saw both Captain and FO thought that their respective Attitude Indicators had failed simultaneously when the wing dropped! “I lost my horizon!” “I lost my horizon too!”

Luckily the cloud base was high enough for them to recover when visual reference was gained. But high g was put on the airframe and bits and pieces came off in the process.

Remarkable aeroplane the Boeing 747!