We have compiled some relevant information below. There are links to full articles and GAPs here too, and you will find more information on the CAA website.
Plan Continuation Bias
Determination to reach your destination despite changing circumstances is something most pilots have experienced or witnessed. It is commonly referred to as ‘get-there-itis’. The technical term for this is plan continuation bias – continuing with a failing plan despite evidence that it’s not working.
Plan continuation bias is most often reported in the approach-to-landing phase of flight, when a pilot’s goal is to land the aircraft, and their focus is on progress toward that goal. It is a powerful but unconscious cognitive bias to continue the original plan, and it can prevent pilots from noticing subtle cues that the original conditions have changed.
Cognitive bias is a general term used to describe many distortions in the human mind that are difficult to eliminate, and that lead to perceptual distortion, inaccurate judgment, or illogical interpretation. Research has shown that plan continuation bias (or 'get-there-itis') can combine with other cognitive biases.
Confirmation bias is a tendency for people to favour information that confirms their preconceptions, regardless of whether the information is true. As a result, people gather evidence and recall information from memory selectively, and interpret it in a biased way. Essentially, you see what you want to see. Confirmation biases can therefore maintain or strengthen beliefs in the face of contrary evidence, leading to potentially disastrous decisions.
This is the tendency to revert to high-frequency actions, beliefs, and interpretations. Frequency bias can lead you to see a routinely observed object as it normally appears, even when this differs from its actual current appearance. Similarly, when making decisions, frequency bias manifests as a tendency to do what you most frequently do in that situation, even when you have previously decided to do otherwise. In simple terms, it's your brain thinking, “it’s always worked before”.
Full Vector article, Get-there-itis here.
In general, every successful landing has an element of pre-planning, followed by actually getting the aircraft to a point from where a landing can be made.
The key to any good landing is a stabilised approach – this means simply that you are at the right height, the right speed and on the right approach path (ideally on the runway extended centreline) by a predetermined point.
Know Your Aircraft
It is useful to work out the takeoff and landing distances required for your aircraft in ISA conditions at sea level, with nil slope, and nil wind on a sealed runway. This can serve as a basis for determining whether it is necessary to carry out performance calculations in any given situation.
Know how your aircraft performs when flying slowly and at different density altitudes.
There are too many performance factors to list, but here are some relevant to this accident.
The gross weight of the aircraft directly affects stall speed, a 10 percent increase in weight increasing the stall speed by 5 percent.
Liftoff speed is generally about 15 percent above the stalling speed, so an increase in weight will mean a higher liftoff speed.
In addition to the higher speed required, acceleration of the heavier aeroplane is slower. Hence, a longer takeoff distance will be required. As a general rule of thumb, a 10 percent increase in takeoff weight has the effect of increasing the takeoff run by about 20 percent.
Heavier landing weights require higher approach speeds, which means that the aircraft will have greater momentum and require more runway in which to land and stop. A 10 percent increase in landing weight has the effect of increasing the landing distance by about 10 percent.
As air density decreases, both engine and aerodynamic performance decrease.
As altitude increases pressure reduces. Air is free to expand and therefore becomes less dense.
Temperature generally also decreases with altitude. This causes the air to contract and become denser. However, the drop in pressure as altitude is increased has the dominating effect on density when compared with the effect of temperature.
Water vapour is lighter than air; consequently moist air is lighter than dry air. It is lightest (or least dense) when, in a given set of conditions, it contains the maximum amount of water vapour.
Density altitude represents the combined effect of pressure altitude and temperature. As air density decreases, aircraft performance decreases.
The effect of a high-density altitude on the power developed from the un-supercharged engine is adverse, and less power will be available for takeoff
High-density altitudes are found most commonly at high-elevation aerodromes when the temperature is high. Low atmospheric pressures will accentuate the effect.
A crosswind situation will affect takeoff and landing performance, mainly because of the reduced headwind component.
A gusting wind situation will require that you keep the aeroplane on the ground for a slightly longer period of time to provide a better margin above the stall, thereby increasing your overall takeoff roll. Gusty conditions also necessitate a higher approach speed, which results in a longer landing roll.
The possibility of turbulence and windshear should be considered when working out takeoff or landing distances.
The presence of windshear can cause sudden fluctuations in airspeed after takeoff or during an approach.
Hangars, buildings and areas of trees all influence the flow of the wind near them. The mechanical turbulence resulting from this disturbed airflow may become very marked in the lee of the obstruction. In winds below 15 knots, turbulence occurs in the lee of the obstruction and may extend vertically to about one-third higher again than the obstruction. In winds above 20 knots, eddies can occur on the leeward side to a distance of about 10 to 15 times the obstruction height and up to twice the obstruction height above the ground.
Getting to the stabilised-approach point is best achieved by flying an accurate circuit, and completing all pre-landing checks early so you can focus on accuracy and finesse.
The standard height for the downwind leg is 1000 feet agl, but some aerodrome operators specify different heights for terrain or mixed-traffic reasons. If you are flying to an unfamiliar aerodrome, check the data on the relevant AIP aerodrome chart beforehand.
The turn on to base leg is usually begun when the landing threshold is 45 degrees back over your left shoulder for a left-hand circuit, and right shoulder for a right-hand circuit.
In nil wind conditions, roll out of the turn when the aircraft is at right angles to the runway extended centreline. If there is wind present, make an allowance for drift.
Adjust the power on base leg to complete the turn on to final by about 500 feet agl.
Using the three-degree glidepath as an example; at 1.6 NM from touchdown you should be 500 feet agl; at about 2.4 NM you should be 800 feet.
Once the aircraft is established on final, select landing flap as required, adjust approach speed if necessary, and trim. Check your aim point to verify that you are on the desired glidepath.
The aim point is where you want the wheels to gently kiss Mother Earth on arrival, so the task from here on is to make that happen.
If the aircraft is maintaining the correct glidepath, the aim point should remain static in your field of view. If it is changing, adjust power as soon as you spot the trend, but be prepared to readjust as soon as you’ve achieved the correct glidepath.
Importantly, rate of descent is controlled with power, and airspeed is controlled by adjusting the nose attitude. If you are descending too quickly and your aim point is moving up the windscreen, then adjust this by adding power. If you are flying slower than your approach speed, then lower the nose to increase the speed. Remembering that these two do not operate in isolation; an adjustment to one may well mean an adjustment to the other.
Depending on how you were taught, you will either make the approach at a constant speed, or progressively reduce to your pre-calculated target threshold speed on short final. The latter method is necessary when your performance calculations show that there’s just enough runway length for you to land comfortably.
If you habitually approach at an airspeed higher than the target threshold speed VTT (1.3 times the stalling speed in the landing configuration), your calculated landing performance figures won’t be achievable.
In a gusting wind situation, it’s normal to add half the gust spread to your approach speed – say the wind is 20 knots gusting 32, take the difference of 12 knots, halve it (6) and add the result to your approach speed.
You are not doing yourself any favours by adding unnecessary speed to the approach for ‘mum and the kids’. Extra speed means extra floating and extra landing length. Make your slow flight accurate and your landings will improve.
You should always nominate a decision point where you will abandon the takeoff or discontinue the approach if things are not going as expected.
For takeoff, this is the point at which there is sufficient distance available to safely stop the aircraft should it accelerate slower than expected or suffer a power loss.
For landing, the decision point should be a height where there is sufficient room to effect a safe go-around if you are not happy with the approach. An important factor here, and one that is often overlooked by pilots, is the assessment of groundspeed while on approach. A check of the windsock and an estimate of whether your groundspeed is about what you would expect for your airspeed when on short final is good practice. Any tailwind component very significantly increases the landing distance, so a go-around is usually the best course of action should this be the case.
On very short final, you cross the fence and the runway threshold, and now it’s time to slow down the rate of descent to a point where it’s close to zero as the wheels touch the runway.
Ideally, you should ‘hold off’ until just before the wing reaches the stalling angle, then allow the aircraft to settle gently on to the main wheels. (We’re assuming a tricycle-gear aeroplane here.) Lower the nose wheel gently to the ground while you still have elevator effectiveness, and concentrate on keeping straight.
Brake as required, raise flap when it is safe to do so, and taxi clear of the runway.
If at any stage on final, it appears the approach is diverging from optimum – for example, 150 feet too high or 12 knots too fast and this will result in a long landing, go around. There’s no shame in doing this, and some day this action may save your life.
To go around apply full power, select carb heat off, set the climb attitude and airspeed, and raise the flap in stages. From there, perform another circuit and once back on final, apply the lessons learned on the first attempt.
A decision point you should identify is the end of the touchdown zone, that is, the point at which you will go around if the wheels aren’t on the ground. This is particularly critical where there is limited runway length.
The limiting factor for crosswind landings is your ability to maintain directional control about the normal axis. Although it may be easy enough to keep the aeroplane aligned with the runway during the round-out and landing, as airspeed decreases, rudder effectiveness will reduce, and it may be difficult to prevent weathercocking. Therefore, as the crosswind component increases, the amount of flap used for the landing is normally reduced. This reduces the surface area on which the crosswind can act after landing, improving your directional control.
Any landing with reduced flap will increase your landing roll and, if the crosswind is not steady, you may need to increase your approach speed to compensate for windshear and gusts. So before committing to a landing, it is important to consider the runway’s overall suitability in relation to crosswind component, approach/threshold speed, and available length.
The recommended crosswind landing technique is a combination of two methods, the 'kick straight' method and the 'wing down' method.
The Kick-Straight Method
On the approach, the aeroplane is crabbed into wind, and just before touchdown, brisk or positive rudder is used to yaw the aeroplane’s nose into line with the runway. Into-wind aileron is used to keep the wings level.
The Wing-Down Method
On short final, the aeroplane’s nose is aligned with the runway by applying rudder and sufficient into-wind aileron to prevent drift, while controlling speed until the flare with elevator. This results in the aeroplane sideslipping into wind at a rate that negates the drift.
The round-out and hold-off are flown in this wing-down attitude and the landing made on the windward wheel first.
Many modern light aeroplanes have a restriction on sideslipping, especially with flap extended, and therefore the recommended procedure is a combination of these two methods.
The Combination Method
Throughout the approach, the aeroplane is crabbed into wind, in balanced flight, preventing drift.
During the round-out, the wing down method is applied. The aeroplane’s nose is aligned with the runway through smooth rudder application and sufficient aileron into wind used to prevent drifting off the centreline.
The landing is made on the windward wheel, which will create a couple that lowers the other main wheel. The rudder is centralised and the nosewheel lowered rather than held off and some weight maintained on the nosewheel for directional control. At the same time, aileron into wind is increased as the speed reduces.
Keep straight on the runway centreline by reference to a point at the far end of the runway and apply differential brakes as required.
Calculating Crosswind Component
Here are a couple of quick ways to calculate the crosswind component you will experience on landing. First, you need to know the wind speed and direction (in magnetic), then you calculate the angular difference between runway heading and the wind direction.
Add 20 to the angular difference. This tells you what percentage of the wind speed is crosswind. For example, if the wind speed is 20 knots and the angular difference 40 degrees, 40 plus 20 is 60 percent. 60 percent of 20 knots is a crosswind of 12 knots.
Imagine that the minutes on the face of your watch are equivalent to the angular difference between the runway and the wind direction. If the difference is 30 degrees, then thirty minutes is half way around your watch face, therefore the crosswind is half the wind speed. If the angular difference is 45 degrees, then that is three quarters the way round your watch face, so the crosswind three quarters of the wind strength.
If the angular difference is 60 degrees or more then consider the crosswind component to be the full strength of the wind.
You can also see the Crosswind Circuit briefing from the Flight Instructor Guide here.
Before operating at any airstrip, pilots should have specialised flight instruction, or refresher, training, in short field operations at actual airstrips. In doing so, pilots will develop the skills, experience and confidence they need to operate safely and to avoid becoming another statistic. Remember though, that training on one day will not necessarily prepare pilots for the potential conditions of another day, or a different season.
Before using an airstrip you need the permission of the owner, and must comply with any conditions or limitations of use. Talk to the owner and discuss any safety issues or operating restrictions before you go.
Animals on Airstrips
Current farming practices require diverse land use, and this often means a wide variety of stock and animals on the farm. Sheep and cattle (with cows and deer making up the majority) but also horses, dogs, goats and birds may affect your operations.
They all act quite differently when you arrive over their farm in your aircraft.
The animals can be unpredictable when startled by the noise and movement of an aircraft. Especially when the aircraft is low to the ground on an approach to land, or during a takeoff.
Sheep will tend to run as a flock but, when stragglers find themselves isolated, they suddenly need to join their flock. This is usually by the shortest path and often across flat areas such as an airstrip. Cattle may simply stand on the strip and watch the aircraft approach, then suddenly run off in random directions.
Horses and deer can move very quickly when frightened, and if not confined, will cover large distances.
This is a clear danger for you, your aircraft, and your passengers.
Stock will need to be removed from the strip and adjacent unfenced areas on the airstrip prior to your arrival.
Even after all this, animals may still be hiding on-site. Never assume they have all been shifted off prior to your arrival – keep a good lookout.
On arrival at the airstrip, carry out an overhead survey flight to check the strip and surrounding areas for wind strength and direction, landing distance (and takeoff distance), wires and visible obstacles such as terrain, fences, tracks, and objects on the strip surface. Get your passengers to also look out and advise you if they see any animals or obstacles.
If the stock is still being moved as you fly overhead, stop the approach and climb out to a safe height. You should consider holding at a reasonable distance from the area, to keep the noise levels down and prevent any distress to the stock or farmer.
Plan the approach and landing with an allowance for a possible missed approach and overshoot, or unexpected obstacles on the landing.
Self-brief for these possibilities and stick to the plan.
During the critical short-final stage, and on landing, keep looking for animals.
When ready to depart and prior to engine start, you and your passengers should walk down the strip and side areas looking for stock. Do it pre-start because when you are in the aircraft, you may have a restricted view of the area. Nearby terrain, trees, and scrub provide excellent cover for stock to hide in.
It’s best to accompany this inspection with noise. Talking, some shouting, whistling or an occasional handclap will alert stock that may be settled or asleep. When disturbed, make sure you allow them to move away together. You will need to have them professionally removed.
The walking inspection is also a good time for you to check the operating environment. You can see the surface conditions, assess obstacles, and plan the departure. You need to think about a decision point to abort the takeoff and stop in the remaining distance available.
When lined up, check again for intruders during the takeoff roll and watch for them possibly coming from the side of the strip. Be prepared to stop the takeoff sooner, rather than later.
And look out for high-speed stragglers trying to join their mates on the other side of their airstrip!
Windshear is a change in wind speed, wind direction, or both. In New Zealand, on average, there are nine reported incidents involving windshear per year. Most of these incidents occur in or around the takeoff or landing phases of flight and involve all classes of aircraft from wide-body jets to microlights.
There are two types of windshear, horizontal and vertical. Horizontal windshear has a change in direction or speed at the same height, while vertical windshear has a change in direction or speed between two heights.
Causes of Windshear
The common causes of windshear are:
- Gust fronts;
- Surface obstructions;
- Frontal activity; and
- Sea breezes.
Let’s look at just surface obstructions here.
Windshear created by the wind flow around obstacles is probably one of the more serious concerns, both in severity and in its likelihood of being encountered. By obstacles, we mean anything from large hills to isolated buildings, from mountain chains to rows of trees. The effects worsen as windspeed and the angle at which the wind strikes the obstruction, increase.
Coping with Windshear
What windshear does to an aircraft is complex. Obviously, downdraughts and updraughts will have effects, but the loss of airspeed – the loss of lift – can accentuate these effects and, in the worst case scenario, make recovery impossible.
The first defence is to develop the ability to recognise the likely presence of windshear before flying into it. Clues that may be available to the pilot include the following:
- Eeffects of wind ‘dumping’ on trees and crops, or in the ripple and spray patterns on water surfaces;
- Strong, gusty surface winds, especially where an aerodrome is located near hills, or where there are comparatively large buildings near the runway, indicate the probability of local windshear and turbulence;
- Wind socks indicating different winds are an unmistakeable sign that windshear exists;
- TAFs provide the surface and 2000 ft winds. Any variation between the two provides an indication of possible windshear.
Local knowledge for operating at a particular aerodrome can be useful in making judgement calls. If the winds are strong and the aerodrome is unfamiliar, ask for advice from other pilots or air traffic services (but remember, the decisions are still yours).
Learn to recognise the likelihood of hazardous windshears and avoid them. Make an early decision to avoid an encounter by going around or by delaying the approach or takeoff until conditions improve. If the windshear is strong and is likely to persist – for example eddies from obstacles – do not takeoff. Or, if you want to land, choose an alternative aerodrome.
Be prepared for windshear and be ready to take the appropriate action the instant it is required.
When taking off, configure the aircraft for maximum performance. Use all of the runway length available. If runway length permits, delay your rotation until you have reached a higher airspeed. When required, do not reduce power too soon after takeoff. Plan the after-takeoff path to avoid having to climb above high obstacles.
On approach, use a higher than normal airspeed. As a general rule, add half the amount that the wind is gusting to your approach speed typically, no more than 20 knots, higher if runway length permits. Maintain the increase until the flare.
The stabilised approach concept has much to offer the general aviation pilot, in addition to improving the ability to cope with windshear. The technique involves, as far as practicable, establishing the aircraft on the glide slope in the landing configuration as early as possible and flying the appropriate constant airspeed, pitch attitude, and rate of descent by the smooth application of power and elevator control down to the flare.
If strong windshear conditions are evident, and you experience deviation above the normal glide slope, be cautious about reducing power. If the deviation is caused by an updraught, chances are you may soon encounter an equally strong downdraught, which should position you back on the glide slope.
If you encounter a high sink rate near the ground or a significant loss of airspeed, full power is called for without hesitation, whether after takeoff, on approach, or at any other time while flying at low level.
There is a Vector article here on windshear for pilots of light aircraft here.
If terrain prevents you flying from point A to point B in a straight line – you are mountain flying, and you need to apply appropriate techniques and principles to your flight. Every pilot in New Zealand needs mountain flying skills because you are never far from terrain, or its influence on either the weather or your flight path.
Lack of Horizon
The horizon is the line where the sky meets the sea. Without a defined horizon it is difficult to maintain a consistent aircraft nose attitude. Pilots should overlay an imaginary horizon by visualizing where the real horizon would sit if the terrain or weather around them was transparent.
The lack of a defined external horizon can create aircraft attitude and airspeed problems. When flying among the mountains, or anywhere the horizon is not visible, you must learn to imagine that horizon. The useable horizon is the line where the sky meets the sea. In the mountains, visualise where this line is as if the mountains were transparent, and superimpose it on the terrain.
The mind can be fooled in the mountains. Different lighting conditions can create definition and depth perception problems, and in winter, snow cover makes it even harder to determine if what you see is real.
When you are among large mountains, especially in clear air, it is very difficult to accurately judge scale and distance.
Mountains seem a lot closer than they actually are, simply because they are so much larger than you are. It’s hard to imagine that some of the crevasses in the major glaciers would swallow an aircraft whole, leaving no trace at all. The most effective way to confirm your distance from the terrain is by picking out features on the surface that you can accurately judge the size of, such as tussocks, trees, or bush. This will help you work out how far away you are and give you an indication of your size relative to the mountain. It is important to be able to judge your distance from the terrain – this is the only way to know if you have allowed enough room for a reversal turn.
A common perception problem, of which pilots should be constantly aware, is the tendency for snow-covered faces and ridges to merge with each other. Mountain faces in the distance, with a mix of rock and snow, can very easily mask the presence of a much nearer ridgeline of rock and snow merging against the background. This can also occur with rock, bush or tussock-covered hill faces and ridges.
Wind and Turbulence
If you imagine the airflow as water, it can help you figure out how the wind will behave at different points along your route. Think about how it will flow over the terrain, where it will accelerate through passes, divert along valley floors or be forced over a ridge. Rapids (or turbulence) will occur where flows mix together, and where deceleration occurs in the lee of obstacles. Wind flows are generally predictable below 15 knots, but are more difficult to predict above 15 knots.
The lower the cloud, the more restricted your options, and the more you will confront other traffic. Understanding what conditions are required to produce different cloud types will help you figure out what flying conditions may be like in that area. Be aware that the terrain creates its own weather – this can be very changeable and limit your escape options.
Approach and Landing
Always join overhead a strange aerodrome to check for runway layout, wind, traffic and terrain. With mountain airstrips, the constraints of the surrounding terrain will dictate how this is best done.
Follow as close to a standard pattern as possible, although a precise circuit pattern may not be practicable. Make sure you know the elevation of the place you are going to land. Aircraft performance on higher mountain strips is markedly reduced, especially in summer. Ensure you have sufficient landing and takeoff distance available.
An approach and landing at altitude requires accurate control of airspeed and rate of descent. Use the same speeds as you would for a landing at lower altitudes.
If the conditions are gusty, add a small amount to the approach speed (a good rule-of-thumb is to add half the gust factor to 1.3 Vs). The true airspeed and groundspeed will be higher because of the density altitude. The higher groundspeed will also lead to the glideslope being flatter when holding a 500 ft/min descent rate. Use full flap and be prepared to apply full power and raise the drag flap if you need to go around. If you are approaching a one-way strip, have a go-around point for an angled escape decided well out from the strip.
It is not unusual for pilots to find themselves approaching the intended landing site with an excess of altitude. This may occur for a number of reasons:
- In confined spaces there is a natural tendency is to hold altitude to have a greater sense of space;
- The location of the landing area in relation to surrounding rising terrain;
- The desire to extend the distance seen ahead; or
- Simple failure to recognise identifiable landmarks near the landing site.
Take care not to end up ‘hot and high’ – lowering the nose and developing a high rate of descent and/or excessive speed will prevent a good accurate landing.
Always have a clearly defined decision point where you can go around if you are not happy that a safe landing is achievable. This equally applies whether you are flying an aeroplane or a helicopter.
A bad approach will rarely end in a good landing. Because power is adversely affected by altitude (acceleration is slower) when it is needed it should be applied as early as possible. Set yourself up so that the go-around decision can be made early. Avoid an approach profile that takes you below the touch-down point.
Aircraft Performance and Your Performance
At altitude, your performance and the aircraft’s performance are both degraded. Never rely on aircraft performance to get you out of trouble – only good decision-making will do that.
Density altitude calculations are particularly important for helicopter pilots.
The density altitude for a particular landing site can vary depending on the ambient conditions. Just because a successful landing was made on one particular day, doesn’t mean it will be achievable on another.
Don’t forget the effects of pressure altitude. Many aerodromes in New Zealand are high enough for this to have a considerable impact on your aircraft’s performance, particularly on hot, low pressure days.
There is a Mountain Flying DVD you can purchase from Video New Zealand, firstname.lastname@example.org for $35.78, plus $11.22 postage and packing for each order (Prices include GST.).
Helicopter High Altitude Takeoffs and Landings
It is one of life’s unfortunate facts that aircraft performance diminishes with density altitude. For helicopter pilots who work in mountainous areas, it’s just another consideration that must be added to a long list of other factors, which can include severe turbulence, unpredictable weather, unfriendly landing sites and hostile terrain. While it’s a combination of hazards that many operators learn to manage, some don’t – and occasionally the consequences can be fatal.
Basic Rules for Mountain Approaches
- Maintain constant awareness of the direction and estimated speed of the wind.
- Take into account the temperature, keeping in mind that it may increase as you approach ground level.
- Plan the approach in such a way that you retain the option of discontinuing it at your convenience – the approach should be along a slope and preferably into the wind, so as not to gain altitude.
- If the wind is light, choose a summit or an elevation as your landing site in order to be able to anticipate and counteract every possible wind activity.
- If you are not familiar with a landing site, execute a minimum of two passes over the area.
- Identify any obstacles near the landing site.
- Do not select a landing site solely on the basis of its suitability for unloading cargo.
- Carry out power checks to determine that you have the power required for the desired landing.
- Where possible, the approach to a mountainous summit should be made along the ridge – not from the perpendicular – to provide an escape route. On the final approach use a soft touch on the controls – over-controlling can lead to a loss of rotor rpm.
Power and Density Altitude
Aircraft performance depends on air density, which directly affects lift and drag, engine power, and propeller efficiency. As air density decreases, aircraft performance decreases. Density altitude, therefore, provides a basis for relating air density to ISA, so that aircraft performance can be readily determined.
The effect of a high-density altitude on the power developed from the unsupercharged engine is adverse, and less power will be available for takeoff. An increase in density altitude, therefore, has a two-fold effect on the takeoff:
- An increased takeoff speed (TAS) is required.
- Engine power and propeller efficiency are reduced.
High-density altitudes are found most commonly at high-elevation aerodromes when the temperature is high. Low atmospheric pressures will accentuate the effect. Taking off with a heavy aircraft in these conditions is fraught with danger. Your takeoff performance sums have got to be right.
IAS Vs TAS
Another trap, which can complicate matters for pilots new to mountain operations, is the discrepancy between indicated airspeed and true airspeed at altitude.
Consider a helicopter on an approach to a pad at sea level at an indicated airspeed of 60 knots. If the calibrated airspeed is also 60 knots and the wind speed is zero, then the groundspeed will be 60 knots. Now consider the same approach but to a landing site with a density altitude of 15,000 feet. At the higher altitude, true airspeed and groundspeed increase to the extent that 60 knots indicated airspeed now equates to a groundspeed of 74 knots. If the pilot notices the higher groundspeed on the approach they may conclude that they are landing with a significant tailwind (a logical conclusion for operators at sea level). They abort the approach and set up a landing from the opposite direction.
This does not present a problem if there is in fact no wind. But what if the pilot was flying into a 5-knot headwind on the original pass – the groundspeed would still be higher than they would be used to at sea level. If they make the decision to approach from the opposite direction they are unwittingly choosing to land with 5 knots tailwind.
If the same pilot has a gross weight which only allows them to hover in ground-effect (IGE), they are going to pass below effective translational lift short of the touchdown area, over-pitch, and land heavily on whatever happens to be below – unless they have sufficient height to lower the nose and collective to pick up airspeed and rotor rpm. (This could be a couple of hundred feet depending on the rotor rpm decay and aircraft altitude.)
The type of approach flown is also critical to the safety of operations at altitude. Because the groundspeed is significantly higher at altitude, a greater deceleration is required to bring the helicopter’s speed to zero at touchdown than if the approach was flown at sea level.
At the point in the approach when maximum decelerating attitude is reached (about 30 to 50 feet above ground level), and the pilot starts easing forward cyclic and bringing in power, it is going to take more power to keep the rate of closure constant with the greater deceleration.
The pitch angle will also have to be increased to provide the lift required to support the weight of the aircraft, because the tip path plane is tilted further to the rear, and the vertical component of lift, which governs our rate of descent, is less. This additional power may not be available, and it usually results in over-pitching and one of the following:
- Arrival on the pad with the seats set lower in the aircraft and the skids level with the floor;
- A new helicopter-shaped terrain feature short of the pad; or
- An aborted landing – if the pilot is lucky and recognises the problem soon enough to allow sufficient altitude for recovery.
So, how should we fly the approach in these conditions? A long shallow approach to the forward (upwind) edge of the usable area has several advantages. It minimises the rate of descent, ensuring that valuable power is not expended while reducing vertical momentum. It also minimises the required change in fuselage attitude and the rotor disc plane, so that excessive power is not used to reduce forward momentum. However, shallow approaches can be used only in light (or zero) wind conditions. The approach profile should be steepened with wind strength to avoid turbulence.
The Trouble with Takeoffs
Takeoffs at altitude can also be problematic.
What factors should be considered when making a departure from a mountain pad? An adequate power margin at the hover is essential. As long as altitude can be traded for airspeed, takeoffs from pinnacles, ridgelines or other sites with clear areas below, offer the fewest problems.
Takeoffs from sites that have some obstacles either slightly below, level with, or higher than the takeoff point, require thorough ground reconnaissance and planning. The helicopter must be backed up into the far corner of the usable area and an abort point worked out. If effective translational lift, or an adequate climb angle, is not reached by the abort point, the pilot then knows that the takeoff can be discontinued with sufficient clear ground left for deceleration.
Takeoffs from gullies or ravines, where higher obstacles exist behind those immediately ahead of the takeoff run, also require ground reconnaissance so that turns can be anticipated.
If over-pitching or rotor rpm bleed off occurs at full power before reaching effective translational lift, or if any limits are exceeded, the takeoff should be aborted. Remember, translational lift will occur at a higher groundspeed than at sea level, and acceleration will be slower.
If you ever require full power and rpm right at the point of bleed off or over-pitching to barely clear the obstacles on climbout – you’re cutting it way too fine.
See the full Vector article here.