Re: The Scoop on the A330 Accident

From:         kbarr@nyx10.cs.du.edu (Keith Barr)
Organization: /usr/lib/news/organi[sz]ation
Date:         20 Jul 94 02:13:02 
References:   1 2
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In article <airliners.1994.1441@ohare.Chicago.COM>,  <dalden@legal.com> wrote:
>Thank you, Peter, for the translation of the A330 accident.  One point
>comes to mind:  If Vmc is the minimum speed at which directional control
>can be maintained when one engine is developing zero thrust, the other
>is developing full thrust and the CG is at its most rearward limit [the
>configuration of the A330], how was directional control maintained
>between 18-28 kts below Vmc?

Vmc is not a fixed airspeed.  The one displayed on your airspeed indicator 
by a red line (not to be confused with the red line indicating Vne) is 
generally the one that is the highest.  Vmc is changed by aircraft loading
(which changes the moment arm of the rudder, so an aft CG is the worst),
by changes in altitude, by changes in aircraft attitude, and by changes in
power output from the good engine (idle the good one, and Vmc basically
goes away).

In flight, a pilot really only has control of the last two items mentioned
above.  Changine power output is fairly obvious, so I will explain how
aircraft attitude can effect Vmc. 

In an engine out situation, (we will use right engine out) we have the 
following (simplified) situation aerodynamically:

                /\                        
        thrust |  |                       
          ^    |  |                       
          |    |  |                       
          _    |  |    _                / 
   ______|_|___|  |___|_|______        /  
  |____________|  |____________|      /   Aircraft movement
               |  |                       
               |  |                       
               |  |                       
               |  |                       
             __|  |__                     
            |___\/___| -> rudder          
                                          

Summing moments, we find that the clockwise moment created by the thrust is
balanced by the counter-clockwise moment created by the rudder.  If we sum
forces in the lateral direction, we find that the rudder has now created
an unbalanced force which causes the aircraft to move towards the dead
engine, which effectively reduces the angle of attack on the vertical
stabilizer.

By raising the dead engine, we "tilt" the lift vector of the wing to the
left, which creates a horizontal component of lift to balance the side-slip
condition created by the rudder.  If we continue to increase the bank 
angle, we then induce a slip in the other direction, and start increasing
the angle of attack seen by the vertical stabilizer, which increases its
effectiveness, thus lowering Vmc.

Normal procedure after an engine failure is to raise the dead engine about 
5 degrees, which counter-acts the side-slip condition, and lowers Vmc
somewhat.  

As a side note, when I was working on my multi-commercial certificate, I
had to demonstrate approach to and recovery from Vmc.  It is usually
pretty apparent that you are at Vmc because even though you have full 
rudder in, the nose of the aircraft starts to yaw towards the dead engine.
to fix the situation, you immediately decrease power on the good engine, 
lower the nose, and as the airspeed comes above red line, you start 
increasing power on the good engine again.  This was my least favorite 
demonstration in the multi-engine aircraft. (Emergency descents were the 
most fun--idle the engines and dive for the ground! :^)	
 _____________________________           _____
| Keith Barr                  \           \   \__      _____                  
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