Air Safety

We all remember the forces acting on an aircraft- lift, weight, drag and thrust- and the balance point, known as the centre of gravity. But there is more to balanced flight – there is the factor called ‘pitching moment’ – the tendency for the nose to move upwards or downwards due to the relationship between the resultant forces of lift and weight or thrust and drag. The plan area of the surfaces behind the centre of gravity all tend to resist any deviation from the streamlined flight – that is they are like the flights on a dart and continuously try to keep the nose pointing into the relative wind.

Similarly, the surface area ahead of the centre of gravity is destabilising and tries to cause the aircraft to deviate further from straight flight. The designer of course, makes sure that the stabilising effect is dominant- but not too dominant as the elevator has to overcome this stability to change the angle of attack. This is why the position of the centre of gravity is so critical – but I’ll discuss that some other time.

So where are we?

The aircraft in steady, balanced flight has the forces in equilibrium and the resultant pitching moment is zero. Any final tendency for the nose to deviate further or to return to a streamlined position has been finally balanced by the elevator trim. The aircraft’s natural stability will now try to maintain that angle of attack.

If there is a disturbance- say a gust from below – the aircraft will pitch nose-down to maintain the same angle of attack relative to the new airflow direction. Or, if the pilot adjusts the power setting (and as you know an increase in power causes a nose-up pitching moment), the aircraft will still try to maintain the same angle of attack and hence will change its flight path. Try it yourself. Trim the aircraft carefully in level flight at constant airspeed and then slightly increase the power setting. The airspeed (which represents the angle of attack remember) stays constant but the nose rises slightly and the aircraft will climb.

Why is this significant? Because it illustrates the harmonious relationship between stability and control and it illustrates the significance of trim and CG position.

Now to the approach. If we want to take advantage of the stability of the aircraft to help us to fly accurately, then the way to do that is to accurately trim the aircraft for the angle of attack that we want (we don’t have alpha displayed so we use attitude/IA$) and use small power adjustments to achieve the desired descent angle. This is why the argument for control of airspeed with elevators developed. But there’s more to it. The airspeed indicator, like the altimeter and VSI is a performance instrument. It displays second-hand information. It tells the pilot the result of what he has done – not the expected result of what he is about to do.

As you learn to fly smoothly and accurately on instruments you will find that your visual flight is more accurate and more steady also. Why? Because you fly by setting the attitude and the power and by letting the aircraft stabilise. ‘Power plus attitude equals performance’, is the way many teach it.

If you take a little time to accurately set an attitude and the power you will find that the aircraft will settle down to a performance figure – airspeed or rate of climb – close to the one you want. A small adjustment is made to the attitude if required and the aircraft is trimmed accurately. It will then fly as if on rails. Its stability is helping you to achieve your desired flight path.

CHANGE, CHECK, HOLD, ADJUST, TRIM – is the routine that is used.

Visual flight is no different – if the pilot uses the same technique and doesn’t ‘chase’ the performance instruments. Set an attitude and the power and hold it steady until the aircraft has time to settle down – and then trim it accurately. Note also that a constant attitude gives you constant speed – hence my preference for this way of approaching the threshold.

The other ‘target’ for the approach was the aim point. You will remember that power determined the rate of climb or descent (hence the school for controlling rate-of-descent with power). The aircraft’s stability will maintain the angle of attack. Now let’s consider the effect in relation to a fixed point on the ground. Up till now we have been examining the aircraft’s flight path in relation to the air or the horizon – we have been talking about vector or path through the air. For the approach we have to consider the path in relation to the ground so that wind has to be taken into account- we are considering a ground-referenced vector.

In nil wind conditions, the two paths are the same. If we set the aircraft in a trimmed descent we will be able to pick a point on the ground where the aircraft would hit. If we increase power slightly the point on the ground will be slightly further away. If we reduce power the point on the ground will be slightly closer – the flight path will be slightly steeper.

As you will recall from your BAK studies, the wind has exactly the same effect. A slight headwind will steepen the flight path relative to the ground and a tail wind will make the approach shallower.

It’s just like watering the garden.

Imagine you are standing on your back lawn and playing the hose on a particular spot. There are two ways to adjust the fall the impact point of the stream. If there is an excess of water pressure (velocity), you can tilt the nozzle up to reach a further impact point or down for a closer point. If you are playing the water as far as it will reach, then raising the nozzle will cause the impact point to be closer. An aircraft is exactly the same. If you have an excess of speed you can directly adjust your flight path and aim point by raising or lowering the nose. If you are at the minimum speed to reach your aim point, then raising the nose will cause your flight path to fall short of the aim point (like trying to stretch the glide).

The other method of control is to set the angle of the nozzle close to where you want it and adjust the impact point by varying the pressure of the water. Again the aircraft is the same. You can set the attitude and vary the aim point by small power adjustments.

Also the adjustment for wind is more direct as it acts in the same way – a slight increase in headwind is compensated for by a slight increase in power and a slight reduction in headwind is balanced by a small reduction in power.

This constant attitude approach is similar to the Navy’s constant angle of attack approach and gives accurate glide path control while maintaining the optimum approach speed – and those pilots know how to arrive on speed and hit the aim point – accurately and consistently. Constants and Trim – the smooth but ‘tighter’ approach.