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If any further proof was needed that the weather patterns in Zagreb have a mind of their own – something the author has been shy to mention in the past 😀 – one needs to look no further than Lučko’s annual Zagreb Kup precision landing championship, held this year on 20 October. One of a number of seasonal sporting events at the field (and the only one that manages to get us all together in the same place), the championship is nearly always blessed with sunny skies, moderate temperatures and generally all-round fine flying weather – often a curiosity among the month’s frequent rains, low clouds and general dreariness.

The 2013 event was – unsurprisingly – more of the same, with beautifully clear skies, uncharacteristically high temps and weather more suited to that of September. The odd man out though was a strong, gusting, 15 knot southwestern wind that had been blowing for a better part of the week – and which, coming in nearly perpendicular to Lučko’s runways 28L and R, had promised quite the spectacle 🙂 .

Tall tales of short approaches

However, while said spectacle was a delight for the photographer, for the pilots taking part it had represented a whole different kettle of fish. The more obvious – and more immediate – issue facing them was the heading correction necessary to compensate for the sideways motion of the air. While this is a perfectly straightforward procedure in level flight (especially when using a few neat mental formulas to calculate the exact correction needed), on a “competition” approach it does present a few interesting challenges 🙂 . Chief among these is the need to increase the heading correction as speed is decreased in order to provide the same level of compensation and stop the aircraft drifting away. By default, competition approaches imply low speeds* (to give the pilots time to judge their rate and angle of descent), which in turn translate into relatively large corrections necessary. Indeed, at 60 knots – a good approach speed for a Skyhawk – every knot of crosswind component has to be parried by a heading correction of one degree. At 15 knots then – conditions similar to those prevailing on 20 October – you would be looking at a 15 degree crab angle.

* however, it is not uncommon for contestants to dive into the ground effect and progressively bleed of excess speed there. Floating just a few inches off the ground, this allows them to set the aircraft down suddenly, quickly and precisely – but often requires impeccable timing and a very good feel for the specific aircraft being flown

Angles of this magnitude produce a distinctly visible sideways drift across the ground (especially in the last few hundred feet of the approach), which can play havoc with one’s perception of the aircraft’s true direction, speed and motion. An additional factor – albeit a smaller one – is the reduced forward visibility from the cockpit. With the nose swung over to the side, the pilot no longer has the luxury of staring at the landing spot straight down the nose; depending on the direction of the wind, he/she now has to either look out of the curved windshield side panels (in the case of a wind from the right), or worse, across the diagonal of the cowl with a wind from the left. Both of these effects combine to measurably influence the pilot’s depth perception – consequently affecting his/her ability to correctly judge the aircraft’s height and proper angle of descent.

A more subtle issue is the slight – but still noticeable – increase in the aircraft’s angle of descent. It is important to note here the difference between the angle of descent and the rate of descent. The rate of descent – grouped under the term “vertical speed” in everyday parlance 🙂 – is a measure of how much altitude the aircraft has lost in a unit of time (feet and minutes respectively being used in the West, and meters and seconds in the lands of the former USSR). The angle of descent on the other hand effectively shows the distance the aircraft has covered per unit of height lost.

To complicate matters further, these two measures are never on the same page when wind is concerned 🙂 . Consider an aircraft flying in still air at a constant 60 knots ground speed and descending at 500 feet per minute. Starting out at an altitude of 10,000 ft, it would need 20 minutes to reach the ground, during which time it would cover 20 nautical miles of horizontal distance. Fine. But now let’s set same aircraft flying into a 20 knot headwind, reducing its ground speed to 40 knots. Since it is still descending at 500 ft per minute, it’ll still need 20 minutes to get down; but the horizontal distance it will cover in that time will now be only 6.67 NM. So, while its rate was constant, its angle had increased dramatically 🙂 (to complicate things EVEN further, the “angle” does not refer to the aircraft’s pitch – which is the same in both cases – but rather to its flight path)*.

* when viewed “in reverse”, this also explains why aircraft have two distinct airspeeds for the climb: the best rate of climb speed (Vy) and the best angle of climb speed (Vx). Vy gives you the quickest time to altitude – that is, the highest vertical speed. Vx on the other hand gives you the highest increase in altitude per unit of horizontal distance covered – that is, the greatest flight path angle – and is used immediately after take-off to clear close-in obstacles 🙂 .

The same mechanism is at work during competition approaches. While I may say that the contestants faced “a crosswind” out of pure brevity, very few crosswinds are exactly 90 degrees on – most of the time there’s a headwind component (however small) reducing the aircraft’s ground speed by a few knots. While this hardly sounds like the end of the world (especially knowing you have an engine to compensate), the effect is still there and can be particularly troublesome on the competition’s second and third approaches which prohibit use of the throttle – and consequently decrease room for error in terms of flight path perception and aircraft control. Come in too low or too far out – like you would in still air – and you might not even make the landing zone, let alone the zero mark… 🙂

All of these effects – despite being almost negligible and easily compensated individually – had come together in force on 20 October, culminating in visually the most interesting (but also most exciting and challenging) competition we’d had in years… 🙂