Re: A brief commentary

From:         Tony Maddern <tmaddern@cse.unsw.edu.au>
Organization: University of New South Wales
Date:         18 Jul 96 01:59:30 
References:   1 2 3 4 5 6 7
Followups:    1 2
Next article
View raw article
  or MIME structure

With regard to the Chicago DC10 accident Glenn Carroll wrote:
>
>V2 is the speed one is supposed to fly with one engine out.  In
>this case, with the outboard slats inappropriately retracted, V2 was
>not sufficient to prevent a stall.  The aircraft could have been
>flown and landed at a higher speed, so:  why is V2 so slow?  Put
>another way:  what are the factors that go into choosing V2 for a
>particular aircraft?

To my mind there appears to be confusion between V2 and Vmca.

V2 is a speed that is chosen that gives the greatest ability to
achieve the legally required climb gradient and to clear obstacles in
the departure flight path in the event of the critical engine failing.
Rules vary but it is generally about 1.2 times the stall speed as a
minimum. A speed higher than V2 will give an improved gradient but
more distance will be used accelerating to the higher speed and the
aircraft will be closer to the obstacle. At some airports with long
runways it is permissible to use a higher V2 to allow higher take-off
weights and still clear the obstacle.

Vmca is the minimum control airspeed in the air with the critical
engine failed.Vmcg applies on the ground. By definition, if the
airspeed falls below Vmca the pilot will not have enough control
authority (aileron and rudder) to control the aircraft. Vmca is
determined in flight test with the critical engine shut down and 5
degrees of bank applied towards the operating engine.

IMHO the Chicago DC10 did not stall but when the speed was reduced to
V2 it came below Vmca for the aircraft with one engine failed and
asymmetric leading edge flaps. This is an abnormal configuration and
is not considered in the calculation of Vmca. The effects of the
engine failure and the loss of leading edge devices on the same side
were additive and the end result was the pilot did not have enough
aileron and rudder capability to control the aircraft which then
rolled into the subsequent dive and crashed.

If the pilot had lowered the nose and maintained the airspeed above
the Vmca for the existing configuration he had could have maintained
control of the aircraft. Of course he had no idea what that speed was.

Rgds, Tony Maddern