Wind Gradients & the Aiming Point

[Guest TOSGcentral]

OVERVIEW.

 

 

The Aiming Point (to refresh the memories of those who are unsure of the term) is a visual control method to terminate the approach for landing at where you wish to make the round-out into the pre-landing float.

 

 

The Aiming Point needs to be something that is visually obvious to the pilot from some distance away. Often this will be the runway ‘piano keys’, perhaps a mentally envisaged line across the runway between two edge markers, or a discoloured patch of grass on less well endowed runways.

 

 

The basic premise is that if the aircraft is taken from some point in the sky (typically the Final Turn) and it descends on a constant slope to the Aiming Point (AP) then the AP will remain stationary in the pilot’s field of vision and just become larger the closer that you get to it.

 

 

As an example, which most of us should have seen by now, is the nose camera view from an air launched ground attack missile. Let us say the target is the door to a bunker. This will be central in the screen and remain there, just becoming larger until the missile strikes. The target neither moves up nor down in the field of vision.

 

 

The central point of AP usage is a constant air speed (Approach Speed) from an adequate altitutude, at a suitable power setting that provides a ‘cone of control’ ie if the aircraft is overshooting then you have the facility to reduce power and steepen the approach, or if undershooting then you have adequate surplus power to increase aircraft energy and make the approach angle more shallow.

 

 

Gliders use exactly the same approach control technique despite not having an engine. Their cone of control is dictated by the steepest descent angle with full deployment of spoiler/airbrakes, or their shallowest approach angle when fully cleaned up. Typically you will position the final turn such that fully clean the glider will reach the upwind end of the runway and with full air brakes it will go through the downwind boundary fence!

 

 

In visual terms what you see in the case of an undershoot is the AP steadily moving up the windscreen with no attitude change of the aircraft. In an overshoot the AP will be moving down the windscreen and eventually vanish below the nose as you commence to overfly it.

 

 

A large point that I wish to emphasise here is that all of this should be happening at a constant aerodynamic energy level (ie airspeed) with the facility available to increase or decrease total aircraft energy via the engine energy and thus the aircraft’s flight path.

 

 

A second important point to have in mind is that movement of the AP up or down in the windscreen may be arrested via fore and aft movements of the stick (raise or lower the nose). HOWEVER this will change the aerodynamic energy by raising or lowering the airspeed. There is not much point in arriving at the AP going at the speed of sound because you may then need all of the available runway before you are down to landing speed. Equally, it is even less desirable to stall before you get there!

 

 

Bear that little lot in mind while we take a look at approaches in Wind Gradients and what this does to the AP.

 

 

WIND GRADIENT SCENARIO.

 

 

(nb – You can get information on wind gradients and what they are on the “Wind Gradients and All That†thread. I have started a new thread for this because doubtless there will be more to and fro while we knock off some of the rough edges via debate!)

 

 

Let us first put things in perspective. The wind gradient is usually there, to some extent, but is really only of major consequence with surface winds of 15 knots or upwards vis a vis normal approach energy levels that are a part of basic training (but may go unremarked as to why). But if you do not know what wind gradients are, nor have been trained to deal with them, then they are a dragon waiting to take a big bite out of you and you cannot see it coming!

 

 

Typically you could have left on a local jaunt and the wind is light etc etc. A few hours later the wind is up and higher than forecast – it is doing 20 knots on the surface and you can expect an established wind gradient in those conditions. But you have to get down through it so best that you know something about the situation?

 

 

The main attribute of a Wind Gradient is that as you descend through the thing the air energy begins to fall off and this has an immediate (for all practical purposes) impact on the aircraft’s total flight energy. So your airspeed begins falling. If you lower the nose to maintain airspeed then the AP starts going up the windscreen and you are moving into an undershoot situation. In addition your sink rate increases and you penetrate further into the wind gradient at a greater vertical rate – so the damn thing starts taking more energy from you more quickly!

 

 

If you entered this scenario at too low an initial aerodynamic energy level then you may well have already cooked your goose because the situation is not going to get better – it is going to become worse quite rapidly!

 

 

Getting into trouble could be expressed as simply as responding via the AP to normal approach control methods for normal conditions and that is not going to work! The AP is going up and you are undershooting. On a high drag, low inertia aircraft with a small engine then you may rapidly run up to maximum engine power and this will especially happen if you have responded to the stimulus of the rising AP and started to raise the nose, thus lowering your airspeed. Now you are moving right onto the low speed end of the Total Drag Curve, drag is increasing rapidly and you need yet more energy to combat that. If you are low on aerodynamic energy and have used most of your engine energy then you are energy bankrupt and there is nothing that you can now do about it except possibly arrange where the undershoot arrival will be so that you still have some measure of control left! That is in ideal and clinical mental state conditions sitting in our chairs reading about it. The reality is that you are highly likely to get into a low speed loss of control situation and totally lose it!

 

 

So let us talk briefly about flight management.

 

 

MANAGING THE WIND GRADIENT.

 

 

Your normal AP approach control technique will work in a wind gradient as well as it does in benign conditions. But the situation has to be anticipated and the key factor in very light aircraft becomes knowledge of the sky that you choose to enter and then wish to exit.

 

 

Now, there is nothing really complicated or difficult about this but the key is anticipation and putting yourself at an energy level that will deal with the situation more or less normally BEFORE you get into the situation.

 

 

This is as simple as applying the standard approach speed energy calculation of 1.5 times stalling speed plus one third of the winds speed that you may observe from the wind sock. This must be the MINIMUM air speed that you enter the top of the wind gradient.

 

 

From a positioning point of view you put yourself closer in and maybe higher than normal for your usual approach. It does not matter if you are too high or too close and have overestimated the wind gradient effects because you may then open up and go around and be a bit wiser about actual conditions. It will matter if you are engine out, forced landing and have made no provision for the gradient!

 

 

The basic approach speed calculation will keep you safe but it is not going to be the normal ride. You have to be involved in active piloting and maintain your total energy level to ensure that the situation stays safe and that you may control the machine normally.

 

 

We can now apply normal AP procedures. As the wind gradient begins to steal your aerodynamic energy then you steadily lower the nose to maintain airspeed. This pushes the AP up your windscreen and you use increasing engine energy to enable you to raise the nose a bit to keep the AP central but the airspeed constant. This will probably be a continuing process therefore it is critical that you started with sufficient aerodynamic energy such that you have sufficient engine energy left in reserve to preserve the approach slope to the AP.

 

 

AFTERWORD.

 

 

The wind gradient is not some form of monster – just another part of flight and piloting. I have attempted to keep the above clear and easily understood. But if some of my terminology is unfamiliar to you then let me say that it is basic stuff that you MUST know about to be a pilot – so go get some more education.

 

 

Finally – my apologies to Blueline for once more being so long – I do try to keep them short but I tackle some big subjects mate!i_dunno

 

 

Aye

 

 

Tony

 

 

[poteroo]

Tony,

 

I happen to use 1.33 x the stall speed of the aircraft in landing configuration and at gross weight,(which gives some margin as you are going to be under gross), plus 1/2 of the gusting difference in the forecast or AWIS.

 

So, in the case of the Jabiru 160, using 45 KIAS as the stall, and with a wind of 18,gusting 32....I'd add 7 kts to 45 x 1.33 = 67 KIAS.

 

Your calculation in this case will read 45 x 1.5 + 1/3 of 18 = 74KIAS

 

My reading of these results is that it could be 67 for the more experienced pilot and towards 74 for the student??

 

The anemometer is going to record ever larger gust differences as the wind speed increases. The directional fluctuation also increases in amplitude with speed and surface type. So, I always intend to carry a little more power in situations where the windsock, or other cues, indicate a strong, gusting and directionally fluctuating wind - allowing me better directional control,as well as vertical, in the flare.

 

 

[Guest TOSGcentral]

Hi Poteroo,

 

 

Thanks for chiming in with a damn good post. You go into areas that I did not want to go with the initial post – rather I wanted to establish some basics. But your input is very relevant to a ‘broad brush’ user base and what works for one will not work for another. So let us go a little further.

 

 

Firstly the basic figures that I used are valid for high drag/low inertia ultralights and gliders. Let us look at that a bit more. This category of ultralight has limited aerodynamic potential, but does have an engine and the power to weight ratio wise may be quite efficient when applied from a suitably high aerodynamically energy level.

 

 

The glider on the other hand has superb aerodynamic energy potential but has no engine! So the two forms of flight meet in the middle with very similar energy requirements immediately pre-wind gradient entry.

 

 

Now you introduce as an example the Jab 160 so let us take a look at that situation. The Jabs are very energy efficient machines (even though light) and have good surplus of power. In common with most of the low end of GA ,and upwards, wind gradients are not really much of an issue even if prudent approach control speed does require a bit of awareness and effort. So typically we are looking at around 1.3 VS plus a suitable bit for the wind, turbulence and gradient as an approach airspeed.

 

 

I have no problem with your figures, or comments re the skilled/unskilled at all and that gives us a bit more insight into the conditions that we choose to fly in.

 

 

In your example I would be way over turbulence penetration speed in a Thruster so we get a bit of valid insight into why there comes a time when you put them back in the hangar before you are confronted with that situation!

 

 

So perhaps other users would like to put in a few bob’s worth on energy control in a wind gradient with other aviation disciplines (spare me the SR71 please!). Hi Crezzi – what do you do with Trikes? Any gyro fanatics out there – what do you do? Some insights into powered parachutes from anyone?

 

 

It is a very big world that we live in and discussion is great – but let us keep it valid for what we fly!

 

 

Thanks again Poteroo – I did not think anyone wanted to play with me anymore !

 

 

Aye

 

 

Tony

 

 

[Guest Crezzi]

Hi Tony

 

The technique you outline works pretty well for higher performance trikes though I don't think its explictly described as such in the weightshift syllabus I'm aware of. Typically the advice is simply to increase the nominated approach speed in gusty or x-wind conditions. Eg a trike with a stall speed of 30kts might increase approach speed from the normal 50kt to 55kt. These figures come directly from a trike POH (which specifically mentions wind gradient) and fit pretty well with the formulae you quote.

 

It gets a bit more interesting in slower trikes where the normal approach speed might well be less than 10kts above the stall speed. The technique on these is to pretty much to have the bar in as far as the waistline will allow ! This provides maximum energy to protect against wind gradient or low-level turbulence and the very high drag profile makes it unlikely the flare would be excessively protracted. In extremis I would use partial power as well.

 

In my experience, trike pilots tend to pick up an appreciation of this and other low-level effects fairly well - in your example the 20kts of wind thats picked up whilst we are flying is going to have a very noticable on groundspeed round the circuit. Maybe being exposed to the elements improves awareness and, although this would presumably also apply to pilots of some "old school" ultralights, trikes have the additional factor of the direct feedback from the whole wing.

 

Cheers

 

John

 

 

[Guest Juliette Lima]

Hi Tony,

 

Thanks once again for another helpful article....your offerings on wind gradient and the aiming point have renewed my awarness of some issues on final.

 

I've had a couple of experiences on final recently (same runway)....turning from base with plenty of height for a comfortable glide approach around 52kts, engine idling, all seemed well until late final, when suddenly the aiming point rapidly raised up the screen.

 

This caught me by surprise.....the lesson learned for me being watch attitude and aiming point for immediate correction rather than being caught out quite low.

 

You mention the wing gradient effect is more pronounced with winds of 15kts and upwards....in both my 'sinking' episodes, the winds were only slight so I can only guess perhaps a 'local' sink hole or permanent ground feature must exist just before the runway concerned....another consideration for aiming point monitoring.

 

Thanks again for your time and contributions....pity your recent series of very helpful articles in Pacific Flyer hav'nt found their way into the RAA mag.....a reprint of your 'Airmanship' classic by RAA might prompt regular contributions by you in the mag.

 

Cheers

 

JL

 

 

[facthunter]

Light&variable.

 

The L&V or Var 5k, particularly in thermally conditions can really be "light & VICIOUS" ( as we sometimes termed it ) Beware the dust devil, (mini twister) with or without the dust.

 

Nev.....

 

 

[Guest TOSGcentral]

No probs JL.

 

I would suggest that you have a persistent 'micro met' situation that you encountered. We all have to learn the quirks of our home airfields and superimpose these on general considerations.

 

At Watts we tend to have a 'duty thermal' just at final turn to runway 30L. This can stuff an approach before it starts if you have got it right - or at least make the situation other than expected.

 

But (to my usual fury) it can salvage a situation for students who have cocked up the base leg management, pulled the power going into a high final turn that they would normally come out of far too low in a Thruster, and the thermal fixes it for them and they think they did the right thing!!!

 

Sorry, I do not write for the RAA mag any more - I had some 'difficulties'. In professional writing I like a good working relationship with editors. I get on extremely well with Angela and Carole at PF and that will do me.

 

Tony

 

 

[Yenn]

The main attribute of a Wind Gradient is that as you descend through the thing the air energy begins to fall off and this has an immediate (for all practical purposes) impact on the aircraft’s total flight energy. So your airspeed begins falling. If you lower the nose to maintain airspeed then the AP starts going up the windscreen and you are moving into an undershoot situation.

 

Tony as I see it when the wind speed drops off your ground speed will increase, which will lead to an overshoot situation rather than undershoot as you describe here, and the effect will be greater in a light draggy machine than in a large plane.

 

Am I missing something in your post?

 

 

 

[vk3auu]

If you are doing an engine out approach in a wind shear situation, would it not be necessary to have your initial aiming point some way up the runway, so that you actually reach the threshold without any undershoot. In that instance, you don't have the luxury of having a source of extra energy i.e. an engine to get you there. I would have thought that even with a powered approach, it would still be prudent to do a similar thing.

 

David

 

 

[eastmeg2]

In complete agreement with John Crezzi's input as usual.

 

I had to put all of this into practise during the Xmas-NY period when landing at a short strip near Kempsey.

 

Flying a high performance trike (Streak-III) in the company of lower performance trikes (Mostly Wizards) we all landed on a East-West 250m grass strip. There was a 15kt easterly at circuit height and 100ft trees at the western end of the strip.

 

As I was able to fly at 43kts (no pax) with full reflex hands off and anticipated losing 10kts of the headwind on late final I approached low over the 100ft tree tops at 45kts and commenced a moderate dive which combined with a small aplication of power the wind shear to get me down quickly and maintain airspeed(increase groundspeed) to avoid a stall.

 

Anticipating all those factors and watching all the other lower performance trikes land before me lead to a successful landing I'm proud of even after watching a video replay.:thumb_up:

 

Disclaimer: It's up to each pilot to decide whether to land at an airstrip they've not landed at before. In this instance I'd prepared by practising short field landings over imaginary obstacles at longer grass strips in the same trike in the lead up to this.

 

Cheers,

 

Glen

 

 

[facthunter]

Undershoot/overshoot.

 

Regarding Yenn's (Ians) comments, they are quite valid, as when you plan a descent, Knowledge of the actual wind enables you to be very precise as to your descent point. IF at some point during the descent, (assuming you are right on the profile, Height & distance relationship), you get a reduction in the anticipated head-wind component, you then do better than you expected and tend to get above the profile, and the effect is more a question of time ( therefore speed ) The more time you are exposed to the difference , the more the change from the planned profile. This is not really the situation where you get a change in wind gradient when on final and you drop below target airspeed because you have to ADD energy to your aircraft, to regain that airspeed. (by either lowering the nose, and thereby going below the path so you lose height (potential energy). Trade one for the other. Or a burst of power from the engine, and you stay on slope, at the desired speed.

 

The fact that from say, 300 feet down to flare height, you have a reduced head-wind for quite a short period of time doesn't amount to much in the big scheme of things. Nev..

 

 

[Yenn]

Yes Nev.

 

People still seem to forget that we fly in the air, not on the ground.

 

As for Glen's post above I would have expected to get some downdraught from the trees on the approach in 15kts of wind.

 

I think I must be in a minority as I cut the power fully when I turn base and all landings are done as training for engine out. I sometimes get caught out and need to add a bit of power, but never to the extent that I couldn't make the field.

 

 

[facthunter]

Similarity

 

I suspect that my ship has similar characteristics to the Corby. I cut the power on early base, and because I've always flown small circuits, (a habit) I usually slip a bit off to get rid of the height, but if the air is a bit" lively", I usually put a bit of power on before I come over the fence to give more precise speed control, especially if the strip is short and I am on the low side (speed-wise) for an approach. I don't recommend power-off approaches in gusty conditions. Nev..

 

 

[eastmeg2]

Yenn,

 

The trees were at the downwind end of the strip and not arranged in a manner likely to produce downdraughts (not in a line, but scattered).