We Aim To Please

The aircraft touched down on the mainwheels but the pilot became concerned with the nose attitude as he lowered it. He held it off for as long as possible but towards the end of the landing roll, the propeller contacted the runway. The pilot selected the mixture to ‘idle-cut-off’ and the aircraft came to rest with the nosewheel pushed against the flipper doors into an almost retracted position.

There were tyre marks on the end of the runway indicating that the aircraft had touched down just short of the lip of the runway. It was concluded that contact with this lip had caused the nose gear to collapse. Pieces of metal from the retraction mechanism were found along the runway.

The strip was bitumen, 795 metres long and was dry at the time. The strip length was adequate for the aircraft type, at the weight and in the conditions that existed. The pilot had the threshold speed under control. Apparently he didn’t need to leave himself so little tolerance to error or the vagaries of nature. So why did he aim for the ‘numbers’- in fact why ever aim for the numbers?

The point at which a 3° glideslope intersects the runway surface from a threshold height of 50 feet, is 1000 feet into the runway. This would be about a third of the way along this particular runway.

Allowing say 200 feet for the float, the touchdown would have been halfway along the runway. Perhaps this explains why many pilots don’t aim so far in. After all, the runway behind you is like altitude above you isn’t it? But then most strips have no reference marks that are clearly visible from Base and Final. So where should we aim when the runway length is not overly long?

The following letter presents a sound point of view. Reading between the lines, I would say it was written by a RMF Caribou pilot.

Dear Sir,

I have just read your article ‘How to land an aeroplane – backwards’ (p102 Sept 87 AA).

I would like to raise a point which concerns me, and that is the apparent lack of emphasis placed upon aim point selection. I feel this matter does not receive enough attention and a quick perusal of the Aviation Safety Digest’s last accident summaries revealed three accidents where in an otherwise normal situation, the aircraft touched down or struck obstacles short of the threshold. I don’t know whether these accidents were caused by poor aim point selection or poor glidepath maintenance, a combination of both or something entirely different but I did expect an article on landing technique to address aim point selection.

About the only comment you made on aim point selection was that you use the piano keys if there are any. I feel that this may not be sound advice as it places the aircraft in an undesirable situation should anything occur (such as downdrafts/wind shear) to cause the aircraft to undershoot. If quick recovery action is not taken the aircraft is set to touchdown short of the threshold. I think that more appropriate advice would be something along the lines of that used by RAAF Caribou crews, which I will outline.

The basic aim point used on runways long enough to permit a ‘normal’ (i. e. not STOL) landing, is 300 feet in from the threshold.

On all STOL landings (even on the tightest strips in PNG) the aim point used is 100 feet in from the threshold.

On occasions where wind shear has been reported and runway length is ample, many captains elect to land even longer. This technique of defining aim point was instituted after several landing incidents (notably one at Nowra where the aircraft was written off after landing short and one in Vietnam where the aircraft touched down about 100 feet short of the threshold). From personal experience, I have seen three occasions where this technique has prevented touchdowns short of the aim point from becoming touchdowns short of the runway! – and it has the added bonus of increasing obstacle clearance on finals.

Lets face it, if a runway is so short that you have to touchdown on the piano keys, you probably shouldn’t be there anyway. I don’t know how applicable this is to light aircraft flying but I hope you give the matter some consideration – as possibly good advice to help reduce the tragic number of landing accidents your publication reports each year.

Anon

When the video ‘The Gentle Touch’ was being produced there was considerable soul searching on two aspects – threshold speed and aim point. Each deserves a program in its own right.

For the sake of simplicity the video and my previous article, avoided the complex discussion on these two aspects – but obviously they are critical. Threshold speed is a vital element in a safe landing but it is not possible to specify the particular optimum speed for the variety of aircraft types, configurations, weight and surface conditions.

Aim point is almost as complex. On a bitumen runway with standard markings and adequate length, the obvious aim points are the 500 or 1000 foot markers.

On a shorter strip, itmay be wiser to aim for a point closer to the threshold than this. Unfortunately, many strips don’t have such a reference point and this is where the difficulty begins. It is often taught to use a point one-third of the way along the strip or a quarter of the way along. Unfortunately this point can become a vague reference and there is a tendency to forget about using an aim point at all.

This is why I referred to the use of the threshold or the ‘piano keys’ as an approach reference. But there is more to it. I don’t use it as an aim point as such- that is, I don’t plan to impact on the keys – otherwise there would be an inherent risk of an undershoot and premature touchdown. What I do is to regard the threshold or piano keys, as a vertical fence along the lip of the strip- and I plan to fly ‘over the fence’. In this way, I can use the threshold or keys as a consistent visual reference for every approach on any strip (in effect an offset aim point) and at the same time, minimise the risk of an undershoot. Indeed the mental image of the fence also encourages a normal rather than shallow approach path and avoids the ‘drag-it-in-on-power’ short field approach which is very susceptible to premature touchdown.

Regarding the threshold as a fence really means that my aim point for the approach is some distance in from the threshold perhaps 200 feet and my planned touchdown point is then a further 200 feet. This total distance of 400 feet or so is, I think, a reasonable compromise for shorter strips.

I am very conscious of the risk associated with using an aim point at the threshold and I would not advocate it without the special proviso that I have explained here- i. e. it’s a reference point rather than a planned impact point.

Perhaps it would be appropriate to examine standard aim point marking for all strips used by GA aeroplanes.

What do you think?