If it helps, here is an extract from a book wot I wrote. I would welcome any comments to put me straight (no flames, please!). Sorry about the lack of pictures, I haven't yet worked out how to get them in a message.
cheers
phil
Mountain Flying
In the mountains, general principles common to other areas will be vastly different. You must be prepared to adapt your flying techniques as the need arises, for the peculiarities of the region and the type of aircraft. In other words, have not only Plan A, but Plan B, C, etc. up your sleeve, because, very often, once you’ve looked at a site and gone round for finals, you will find a cloud has got there before you! You cannot afford to assume that a particular situation is the same as, or similar to, any other you might have encountered previously. You can also expect fog, especially in the early morning, which will often stick to the sides of valleys for quite some time.
In UK, mountainous areas include Scotland and Wales, the Lake and Peak Districts, and generally any hilly country above 1500 feet amsl, although a geologist would probably expect to see 2000. In many other parts of the world, these would be considered as just foothills. In Canada, look out for Designated Mountain Areas, which naturally include the Rockies, extending into the USA.
Performance changes drastically when both temperature and height increase—just the opposite to flying in cold weather, but you knew that anyway. As far as altitude is concerned, low-level operations (below about 5000 feet) probably won't need you to get too concerned, apart from taking notice of airspeed placards and power limitations, because some of the power lost with altitude is regained with cooler temperatures. You will find that at least 75% power is available to a fair height, but be careful.
Power available is reduced with height (and temperature), and rotors turn at the same speed, so, as you increase altitude, higher pitch and power settings will be required (in some helicopters, like the 500C, the rotor blades will stall before you reach engine limits). The dynamic pressure applied to the ASI is also reduced, so IAS will read less in relation to TAS, so, if you maintain a particular airspeed, your groundspeed will increase accordingly, and you will be going faster than you think.
Density Altitude is your real altitude resulting from the effects of height, temperature and humidity. The idea is that the more the density of the air decreases for any of those reasons, the higher your machine thinks it is, with all the problems that that entails. The effects are found at sea level, as well as in mountainous areas, when temperatures are high – for example, 90 degrees (F) at sea level is really 1900 feet as far as your aircraft is concerned. In extreme circumstances, you may have to restrict your operations to early morning or late afternoon in some areas. Here is a chart of Density Altitudes:
(can't get the chart in)
12000’ 13000 13600 14200 14750 15400 16000 16550 17150
Larger control movements will be needed, with more lag, so controls must be moved smoothly and gradually, or the effects may well cancel each other out – you may find yourself on the ground well before that large handful of collective pitch even takes effect! Rotor RPM will rise very quickly with the least excuse.
Your maximum weight for a given altitude (and vice versa), as well as cruising speed in relation to them both should be known, at least approximately, in advance. You also need to know the Hover ceiling In and Out of Ground Effect (HIGE/HOGE) for any weight, so you know you can come to a low hover properly, however briefly, and recover from an unsuitable landing site (hovering should actually be minimised, partly because you can’t rely on ground effect being present, and you have less power anyway, but also because you need to keep a little up your sleeve if the wind shifts, or you begin to lose tail rotor authority. Having said that, no-hover landings are not recommended, because of the chances of snagging the skids on something). Check the performance charts in the back of the Flight Manual, and start practising hovers about 1-2 feet off the ground, bearing in mind, of course, that the said charts were established by test pilots, in controlled situations.
If you allow for these effects as part of your flight planning, fine, but it's easy to get used to a particular place with a particular air density and a corresponding take-off run, base leg, etc., and you may get caught out one day when things change.
Mountain flying involves a bit of psychology, as it requires a good deal of self-control, because you will have to overcome a certain amount of fear and tension, which is not good when you really need to be relaxed on the controls, and some optical illusions.
Almost the first thing you will notice is the lack of a natural horizon, and maybe want to use the mountain tops or sides as a substitute. This, however, will probably cause a climb, or other exaggerated attitudes, and make it difficult to estimate the height of distant ground, either from a cockpit or on the ground itself, so you will find it best to superimpose a horizon of your own below the peaks. This is where using your instruments will help, both to keep attitude and give you a good idea of your height and speed (however, you’re not supposed to be instrument flying!).
Close to the ground, you will get an impression of increased speed, especially near to a ridge. For example, climbing along a long shallow slope is often coupled with an unconscious attempt to maintain height without increasing power so, unless you keep an eye on the ASI, you will be in danger of gradually reducing speed—if your airspeed is reducing, then either the nose has been lifted or you're in a downdraught (downdraughts will be associated with a loss of height or airspeed for the same power).
The strength of downdraughts can frequently exceed your climbing capabilities. Strong updraughts can suspend you in mid-air with zero power – if it subsides suddenly, you will be going down faster than you can apply it (sometimes, waggling the cyclic gently will spill the lift for you). A lack of cloud above, i.e. descending air, is a possible indication of a downdraught. Do not fight it, but guide the aircraft towards a lifting slope. A helicopter might get help from the ground cushion, but the effect will be less on a slope or grass. When valley flying, upslopes or slopes exposed to the sun can produce updraughts, so place yourself on a converging course to the line of the ridge and positioned to obtain a straight flight path two thirds up the slope and one across, which is generally the area of smoothest flight. However, local conditions could vary this.
(picture here)
You could climb on a lee slope (that is, the other side from where the wind is coming from), taking advantage of the updraught formed by stronger wind returning on itself (i.e. a backlash, tending to occur with abrupt surfaces), but beware of power limitations with speed reduction.
(another piccie here)
Also, there is so little room to manoeuvre if something goes wrong, or you meet someone coming the other way. If you have to do this, converging on the ridge line at 45o gives you the best chance of an escape route.
Similarly, try and avoid flight along lee slopes, but if you need to (because life's sometimes like that), smoothest flight will be obtained by flying as close as possible to the ground, say about six inches, so you’re in the boundary layer, which is a steady movement of air close to the surface, with a vertical element. This gives even less room for error, though. If the relative humidity is high, you could watch for rotor clouds, which will indicate wind currents and turbulence.
If bad visibility and rain are likely to be a problem, choose a more mountainous route even if the winds are a little stronger—Fohn effect will often provide a clear passage.
Winds are very important – they can increase your operational ceiling, payload, rate of climb, range and cruise speed, but they can also do the opposite, and be very difficult to predict, with formidable up and downdraughts associated with them. When cruising downwind, along a lee slope or not, sudden wind reversals could make you exceed VNE or even take away your airspeed completely.
There are several types of wind, which can loosely be grouped into prevailing or local, with the latter classification subdivided into other types, such as anabatic, katabatic, etc., and which are infinitely variable. The former is steady and fairly reliable, and starts to affect you from about 6000 feet upwards. Smoke from local fires may be used to detect wind direction, as can water, but this may only give half the story. For instance, it's not uncommon for the windsocks at each end of Banff airstrip in the Rockies to be 180 degrees at variance with each other! Indeed, upper winds can come in many directions at different levels, and are usually the opposite of lower winds. Where mountains are concerned, they also acquire a vertical element, which is actually where the boundary layer comes from.
As a guide to speed, whitecaps on water foam at 10 mph. Dark depressed puddles on water are called Bearpaws and are caused by downbursts. The most important thing to watch out for is the funnelling of wind as it progresses down a valley, so although the mean windspeed may be reported as 5 knots or so, you may find it as high as 30 in some places, and not necessarily coming from the expected direction.
In fact, understanding how air moves around terrain is one of the keys to good mountain flying, particularly the demarcation lines between smooth and turbulent air (in general, that moving up is smooth, and that moving down is turbulent. You can visualise the difference if you think of a waterfall, and the state of the water before and after dropping over the edge). Close to the ground, the air moves in laminar fashion, but the depth of the laminar section and the gust spread will vary considerably, depending on the nature of the surface and its heating. The laminar flow will become broken if the ground becomes rough, or there are trees, and the wind is strong. Turbulence will occur on both sides, resulting in an updraught close to the leeward side and a downdraught close to the windward side as the air is made to curl.
The movement of air over a crest line has a venturi effect, giving an increased windspeed over the summit and a corresponding reduction of pressure, which could cause your altimeter to over-read. On passing over or round an obstacle, the air may become turbulent or have formed into rolls which have a vertical or horizontal axis. Updraughts would be on the windward side and downdraughts to the leeward.
The general effect of a series of ridges is to form rolls between the crest lines, possibly causing a dangerous situation where a downdraught can exist on an upslope where an updraught would normally be expected.
(yet another piccie)
As a result, on top of steep ridges there may be an area of nil or reverse winds which is difficult to locate on the first recce. The vertical distance to which a mountain chain will influence the movement of air is about 3-5 times its height, changing with the windspeed.
Horizontally, the effect is variable and most noticeable in stable conditions with more than 20 knots of wind, when standing waves will form. As you probably know, you can recognise the existence of these by lenticular clouds, but you will also see ragged cloud around the peak. These should be avoided at all costs due to the turbulence associated with them, especially at the wind speeds that lead to their creation. In addition to shockloading, momentary loss of control may occur, not to mention coffee all over the place.
A couple of thoughts for when you’re very high up; how much time it takes to get down if you have a problem, and meeting anyone else at that height on an airway who doesn’t expect you. And oxygen.
Landing Sites
Those on peaks or crests usually present you with more escape routes than any on flanks or valley bottoms so, wherever possible, landings should be made on ground higher than the immediate surroundings, so you can vary the approach direction according to the wind and have a clear overshoot path.
Use the windward sides of a slope; leeward sides should only be used in operational necessity, because wind flowing down the slope can increase its apparent angle (you need more lateral cyclic to hold the helicopter in place, and you could run out when you reduce power to lower the downwind skid). Don't forget you will not have the full effects of a ground cushion, if at all. Where conditions allow, go as far to the windward edge as possible, to avoid suddenly finding yourself in dead or reversed airflow (as if on a leeslope) and make overshooting easier. The wind coming over the peak will have increased in speed, due to Venturi effects (remember them?), so a 15 knot wind can easily become double that, aside from your altimeter misreading.
Finding the wind direction can be interesting if the site is bare and gives you no information, and it doesn’t help that mountain flying tends to take place in high pressure conditions, that is, where the winds are light and variable. We are now talking about local winds, caused by convection, for instance, or katabatic effects, combined with the prevailing wind influenced by the ground, or even a mixture of them all. Even a cloud shadow can increase the speed of a downflowing wind from a cold surface. You could judge its effects on the machine itself, flying round the site with a constant speed and power setting, or a constant altitude. Look at your power settings, whether the air is turbulent, your groundspeed varies or whether you drift. How much pedal you use to keep the thing straight is a good help – a lot of right pedal means the wind is from the left, for example, and a fair amount of vibration means it is behind you, but it may be a good idea, if you can’t have it at the front, to get the wind off to the side that requires the use of the power pedal (the left one, in a 206), just in case tail rotor authority becomes a problem. A lot of aft cyclic would indicate a tailwind as well.
So, with constant power and airspeed (say 40-50 kts), when you rise, you will be on the windward side, and vice versa. On the other hand, you would use less power on the windward side if you kept a constant height. However, use turbulence as a guide only in lighter winds – any found in updraughts will be from mechanical effects, such as trees. Smoke grenades are often used if there’s nothing else.
Aside from picking a speed which will be slow enough to detect changes and yet give enough for a margin of safety (and cope with any turbulence), when testing for wind, you should also fly about 50-100 feet below the top of the peak you want to land on, to keep yourself away from the demarcation line and reduce the chances of getting the rotors in an updraught on the leeward side. Also, keep tight in to the side, to stay inside the boundary layer.
The demarcation line is the point at which smooth air is separated from turbulent air around a peak, rather similar to that over an aerofoil.
(piccie)
It can be horizontal as well as vertical. Above or to the side of the line, air is relatively smooth and upflowing – below, it is downflowing. It steepens as wind velocity increases (and the severity of the slope), as does the area of downflow, and moves toward the top of the hill.
Having decided on wind direction, keep an eye on the altimeter as you will lose the natural horizon. A figure-of-eight type inspection gives you the best chance of getting the most information about your landing site, making all turns away from rising ground (returning towards the site) to give you a good view all the time, so you don’t lose sight of it in the trees. You could go round in a circle, but the landing point would be out of sight most of the time.
Check for Size, Shape, Surroundings, Slope, Surface and Sun (you don’t want it in your eyes). Do a couple of passes at about 30-40 kts, including an overshoot and approach to land so you can identify a reference point on the landing site and confirm the wind direction by comparing ground speeds. At the end of each pass, climb at least 50 feet, to make sure you don’t go below the site level (as you gain more experience, you will be able to cut all this down considerably. Some pilots use one or two turns in a descending spiral). Keep any trim, or you will get confused, and check groundspeed through the side windows.
(piccie)
Turn in at around 50-60 knots, and, on finals, approach at a converging angle into wind (30 degrees or so). Take particular note of escape routes, up and down draughts and turbulent areas. Maintain a constant angle, aiming directly for the point you wish to land on, as you may not be able to hover when you get there, although this may not always be possible in a confined area needing a vertical descent, or if there’s garbage on the ground which makes you pick another spot. If you do manage to hover, make it low, somewhere between 1-2 feet, and brief, one or two seconds.
There are several schools of thought about approaches, but no real standard – as with many other activities involving helicopters, there is more than one “right” answer to this one. A fairly flat, disc-loaded (shallow) one will (in theory, anyway) minimise collective for the hover, and give you the most control as you keep translational lift as long as possible, but there’s very little up your sleeve at the end, and you need to be very aware of your winds, as forward speed will mask the effects right to the last minute, although it does give you a good idea of the level of your site. This assumes you remember your training and keep going forward and down, so the cyclic is ahead of the game and operating in the cleaner air in front of the machine that helps with translation. In other words, keep the rotor disc forward, so the flow of air is from front to back, especially where snow is concerned, but you shouldn’t use the shallow approach with powdered snow, because you will lose sight of your landing point at the critical moment in the resulting white cloud.
You could, on the other hand, use a steeper angle, say, 30 degrees, increasing with the wind strength, but this requires large handfuls of power and attitude changes in the final stages if you don’t get ground effect, so you wouldn’t try this in an underpowered piston-engined machine that really shouldn’t be there in the first place (the engine may be able to cope with it, but can your tail rotor?). Anyway, since ground effect reduces your torque requirement for the hover by up to 15%, if you approach in such a way that you need no more than that amount to stop, you should find your descent stopping nicely in the right place, assuming the surface is conducive to it, and depending on whether you have high skids or not. You also have some potential energy available for an escape.
I guess you could use whatever works—I generally turn in steep around 60 kts with the disc loaded as much as possible, consistent with descending at about 250 fpm – if the blades have some tension on them, they are less likely to be overstressed. Not only that, the controls are more responsive. The power used will give you a good idea of what you need in the hover, so you have an early chance to abort if you are using too much. This works, because you are using nearly the same air as at your landing point (it’s a steep approach, remember), and 250 fpm reduces the thrust required to transition into the hover by about 15%, i.e. much the same as for ground effect. Flare the disk without moving the fuselage if you can. 250 fpm is about 20 feet every 5 seconds, if you haven't got a VSI (altimeters usually have 20-foot segments).
In a confined area, there will be a point beyond which you're committed, so don't go beyond it until you’re sure. Pick a point to aim for where you know your tail will be clear, and try to come to a hover just before crossing the edge of the trees, or whatever, on the undershoot, which both confirms the power and allows you to throw it away if need be, bearing in mind the potential windshear as you go into the hole. The size of any surrounding trees will give you a false illusion as to the size of the clearing, in that big trees will make it look smaller and vice versa. A typical clearing will have stumps and slash all over the surface – if you don’t have logs to land on (and these produce their own problems when they are slippery), take off a cleanly as possible, to avoid your skids getting caught in something (also be aware that tall trees will sway from your downwash).
Anyway, always be prepared to break off at any time, even if only seconds from success. Never commit yourself till the very last moment. Short cuts don't exist with mountains—they've been around a lot longer than you have!
Landing sites on the bottoms of valleys often have difficult access, and frequently leave no escape route once an approach has started. In this case, it's important to have safe power reserves before committing yourself. In any case, placing the aircraft downwind near to ground should be avoided, but if you have to, go low and slow when approaching downwind with a last minute turn into wind.
In snow, try landing with the sun behind you, as the aircraft shadow will give you a useful guide to the ground slope and surface and provide a focus for a sight picture approach. Some people use the landing light. For takeoff, try not to hover too much. A jump takeoff is useful if little power is available, where you get light on the skids, proceed to the edge with full RPM and tip yourself over the edge. Good fun. In a confined area, for a JetRanger, at least, you need about 15% torque in hand to do a proper vertical takeoff, so you’re probably OK if you’re hovering at about 80%.
Log Pads and Platforms
Log pads are used when slopes are steep, on rough ground. The quick and easy one is a single log across the slope for your rear skid to a solid mat of smaller ones. They can be slippery! Platforms are still made from logs, but are much more refined. The problem with them all is, you can only land one way, and there may be no room to turn once you get there, so approaching with the wind in totally the wrong direction is often the only choice. In such cases, you need much more anticipation than normal, and the willingness to throw things away much earlier. Of course, you don’t actually have to land, but it’s often worth a try.
Here is a typical log arrangement (note the larger one at the back):
(photo)