A320 Sidestick Forward

[Another St Ivian]

Tourist:

I will caveat this reply by saying that I am not privy to any inside knowledge on the A320, however I am approaching this from a large-aircraft performance flight testing background.

In short, application of nose-down elevator input will permit an increase in directional nosewheel effectiveness, and a reduction in Vmcg, which in turn brings about beneficial runway performance figures.

Considering Vmcg in isolation, FAR 25.149 defines it as;

VMCG, the minimum control speed on the ground, is the calibrated airspeed during the take-off run, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with the use of the primary aerodynamic controls alone (without the use of nose-wheel steering) to enable the take off to be safely continued using normal piloting skill.

Now, as you can see, you’re not allowed to use the nosewheel steering, however positive nosewheel contact provides additional stability (the straight nosewheel will resist yaw related sideloads). The amount of sideways force the nosewheel can react against is related to the friction it can generate, which simplistically will be the perpendicular force the tyre contacts the runway with, multiplied by the coefficienct of friction (F = mu*R). The more force pressing the tyre against the tarmac, the more sideload it can achieve before skidding, the more it can help you keep going straight down the runway.

Ideally for performance reasons you want your Vmcg figure to be as low as possible to minimize required runway length (assuming you’re not otherwise limited in some other regard). Considering the aircraft at high-speed on the runway with a take-off stab. trim setting, typically the nosewheel would ordinarily be light on the ground at speeds near Vr. This leads to a good natural take-off rotation, however you lose the benefit of the nosewheel’s ability to counteract any yaw force, and so your Vmcg speed would have to be declared at a much higher speed where a mix of rudder and residual nosewheel force can balance thrust asymmetry. It should also be noted that for most large aircraft, Vmcg is a real cliff-edge speed. One or two knots can make the difference between spearing off the runway edge to maintaining control of the aircraft.

But hang on…why not just drive the nosehweel into the ground to make up for it?

Well this is exactly what Airbus advocate in this instance. The next step is to then approach the certification authorities with this procedure to ask permission. In considering an operational procedure to supplement the aircraft’s performance, one overriding aspect in assessing the permissibility of such a proposal is the crew workload.

If you came up with a procedure where the crew start the take-off roll with neutral pitch input, and then at >60 KIAS applied nose down, and then washed it off again by rotate – I can imagine there would be concerns with workload. For a light aircraft using no flex or derates, that would make for a lot of handwaving in an otherwise busy phase of flight. So the answer is to simplify the technique – have our man at the controls hold pitch down from brakes release, and slowly wash it off by the time you get to rotate. It has absolutely no benefit at lower speeds, however in the crucial cliff-edge region where Vmcg occurs, it absolutely makes a difference.

So on the balance of it – the ‘cost’ of holding a nose-down input where it makes no difference, versus the simplification in workload and crucial enhancement of runway directional control – it’s an easy call.