AoF: Art of Crashing

Greetings, this is dslyecxi, and welcome to another episode of Art of Flight. Today we're looking at everything relating to successfully crashing your aircraft. This includes engine loss, the usage and secrets of autorotation, how to deal with tailrotor loss when autorotating, picking landing sites, and how to deal with obstacles and other hazards that you'll encounter on the way down.

I ran into two situations recently that were polar opposites of each other in terms of autorotation and made me want to investigate more into how exactly the edge cases work in Arma, and that investigation led to me discovering some significant aspects of Arma's autorotation that I don't believe were known previously.

So forget what you knew about Arma's autorotation, and that includes my prior guide on it. We're about to talk about what's really going on and how that knowledge will greatly increase your chances for success.

Also, to preface this, everything I say in this guide is specific to Arma. This is not how real or simulation flying works. If you want to understand that, check out my DCS Huey video which you can find in the description.

So let's get on to crashing!

Basic Autorotation

Now that you've forgotten what you knew about Arma's autorotation I've realized that it poses a problem for my lesson plan here, so let's quickly go over what you used to know about autorotation.

Autorotation is the act of getting a helicopter to the ground when engine power is lost. This is preserving the energy stored in your spinning main rotor and using it to arrest your descent once you're close to the ground.

Basic autorotation is simple. In Arma's world, autorotation is safest with low forward airspeed. Your goal is to keep your rotors spinning until you're almost to the ground, then use that stored energy to provide some lift to cushion your touchdown. This is all pretty abstract in Arma, compared to a sim like DCS.

Because of these abstractions, the safest and most reliable autorotation technique in Arma works as follows.

The instant you lose engine power, press and hold down collective, aka your 'go down' key. This is key to keeping your rotors spinning and maintaining the energy in them. Simultaneously pitch your nose back to bleed off forward speed, reducing it to 50kph or lower as quickly as possible. Look for a landing spot, head towards it, reduce speed to below 30 kph, and when about 30 or so meters off the ground, hold up collective to arrest your descent.

That's it. That's basic autorotation, and that technique will work reliably. It's not the full story, though. The full story is a bit more complex.

Failed States

So with basic autorotation being pretty straightforward, what else can we talk about? The aspect I was looking into recently is how exactly failure states work during it. There are situations where it seems like you're doing things reasonably, then you fall out of the sky and can't recover. I had one recent incident in Prairie Fire where I assumed I was about to fail an autorotation, yet it mysteriously worked. I wanted to understand why that was, and now I can safely say that I do. And soon you will, too.

First, let's explain what the failed state is and how to recognize it. A failed autorotative state in Arma terms is when you no longer have the means of arresting your descent before impact. This is essentially because your rotor blades no longer have energy stored in them, which is to say that your rotor RPM is too low. The spinny thing isn't spinning fast enough.

Recognizing Failure

There are a few different ways to know when you've lost rotor energy and entered this failed state.

Visually, the rotor system will go from a motion-blurred appearance to an unblurred one once the rotors have slowed enough.

This is the blurred state, this is the non-blurred failure state. Bear in mind that seeing your rotors blurred DOES NOT mean that you're safe, since you can still be falling at fatal speeds with blurred rotors.

Audibly you may hear the rotors winding down as well, or an alarm sound. This alarm sound is probably meant to be a low-rotor-RPM sound, but the behavior can be different from helo to helo and it's not terribly useful as a result. You'll only ever hear this with forward airspeed, if at all, as any rearward velocity will cause it to cut out. Thus, if you're descending in a hover and hear this go away, it doesn't necessarily mean anything - you might be drifting slightly back but still falling with low RPM.

If you find this sound annoying, simply find the master caution button and... oh, wait. Nevermind, just deal with it.

If your aircraft has a rotor RPM gauge, and it works, the failed state occurs when your rotor RPM is at or below roughly 60%. This is the point where you'll start dropping like a rock. Different aircraft have different RPM gauges - some of them are steam gauges like this, some of them are digital like this. From what I've seen, as long as these animate, they're reliable.

If you don't have that gauge, but have a vertical velocity gauge, you can tell that you're in the failed state if your descent rate is more than roughly 2000 feet per minute, or roughly 10 meters per second. If you're not familiar with where the vertical speed indication can be found in the cockpit, here are a few examples of the placement. Now that you've seen that, you're welcome to forget it, because those vertical speed indicators aren't terribly consistent from mod to mod, so either you need to do a visual calibration yourself or use other methods. If you want to manually calibrate and test them, simply put yourself in a hover, look at the gauge, and ascend until you see what it tops out at. Descend and see what it bottoms out at. At that point you know what the normal extents are and can judge based on that. Russian gauges are meters per second, NATO ones are feet per minute.

The fastest you can ascend in an Arma 3 helicopter in level flight is 10 meters per second. The fastest you can descend while level is also 10 meters per second. Ten meters per second is something you can get a good rough idea of just by looking at your altitude in the upper-left. Bearing in mind that this is radar altitude and thus can rapidly change due to terrain under you, you can generally look at it on level ground and mentally tick off a second and see if you went about ten meters. The fact that this is also your fastest ascent and level descent rate means that you can get a good sense of whether you're at that speed or not based on experience. If you see your altitude spinning down like this, you're probably going to have a bad day.

The Secret of Autorotation

Ok, so that's what it looks like. How does it happen? This is the part that I was working to figure out while making this guide, and this is ultimately the secret to how Arma's autorotation works.

I always thought Arma's autorotation required low speed, and that high speeds would cause it to fail, since doing a purely sim-style autorotation in Arma tended to result in an explosion. It turns out this isn't accurate, but the things you do to transition from high speed to low speed correspond to things that make Arma autorotations successful, so thinking of it that way - like the basic autorotation I described earlier - works most of the time. So the basic procedure noted earlier is basically correct, but doesn't cover all situations.

So if it's not speed, what is it?

As it turns out, it's all about attitude. The secret to successful autorotations isn't your speed, it's the angle of your nose relative to the horizon. In Arma's normal flight model, dipping your nose while your engine is dead will bleed off your rotor RPM - you can see that here, with a slow decay at 7 degrees nose down, and a much faster one at 20 degrees nose down. This is opposite of reality and unintuitive if you're familiar with flying simultaion helicopters, but it's how Arma handles it. The ideas of diving to gain rotor RPM isn't valid here, and flaring only slightly restores RPM. As your rotor speed decreases and aircraft speed increases, your maneuverability decreases, meaning that at some point you're barely able to raise the nose and at some point after that you can no longer raise it at all. Since being level is required to regain rotor speed, this can quickly result in an unrecoverable condition where you can never raise your nose back up. Until you hit the ground, then who knows where your nose is gonna go.

It's important to point out that when flying a helicopter in Arma, most of the time you will be somewhat nose low simply due to how helicopters travel forward. If you maintain this downward pitch upon entering autorotation, it may start out seeming ok, but you'll be losing RPM steadily until you eventually run out of it entirely.

For helos that have an RPM gauge that works - which is to say, certainly not every helicopter in Arma and mods - you can use this to stay aware of your autorotative state. At roughly 60% RPM you'll lose lift entirely, and the amount of RPM you have that's above that level essentially dictates how well you can cushion your fall or stretch a hover.

The critical pitch angle appears to be about seven degrees nose down. Once you reach that, you'll begin to slowly shed rotor RPM, and more downward pitch will increase the rate of loss. Once you get to around 60% RPM, you'll have lost so much lift that at a lower altitude you're almost certainly going to crash before you're able to recover. Recovery is very slow.

Note that this includes ANY pitch of the aircraft. Roll angle is less sensitive than pitch, but adding roll to bad pitch will makes the situation much worse. 40 degrees of roll is the critical angle at which rotor RPM degrades most rapidly, and even a small nose-down pitch combined with this will bleed your rotor RPM to dangerous levels in moments.

The combination of pitch and roll means that a seven degree nose-down pitch plus some roll will cause RPM loss more quickly than just pitch or roll alone. Since your nose tends to droop as you roll, the two can combine to dangerous levels very quickly.

Also note that seven degrees nose down is a very small angle compared to 40 degrees of roll. If you want to play it safe, come to some kind of nose-high attitude. It doesn't have to be much, you just want to ensure that you're well above that seven-degree-down limit.

In the event that you need to dip your nose slightly to briefly maneuver, monitor your rotor RPM constantly and use as shallow of a pitch angle as possible.

New Technique

So armed with this knowledge, what's the new technique involve? It's still fairly straightforward, it's just that the priorities shift a little and we use some adjusted methods.

Engine Loss

When your engine turns off, this icon disappears. This can happen both from damage and from running out of fuel. If you've taken damage sufficient to kill the engine, this ENG bar will turn red, but keep in mind that it can be red while the engine is still working. The icon is reliable, so use it as your primary indicator since the bar can be misleading.

Once you've lost engine, immediately drop collective and hold it down. You'll do this for the duration of the descent in most cases.

This next step is optional and is most beneficial at particularly low altitudes and high speeds. If you're at a low altitude and high speed, quickly do a cyclic climb to gain altitude. This allows you more time to pick a landing spot and a better vantage point to do so. Make sure you're maintaining down collective while you do tihs.

Whether you climb or not, the last thing you need to do is bring the aircraft to a level or nose-high attitude, keeping the nose from drooping down below the horizon. You can reference your artificial horizon instrument to monitor your attitude. Your speed doesn't matter here, only attitude. Being level will naturally cause you to lose speed, but it's gradual and allows you to keep moving out of what's likely a hot area at a good percentage of your prior speed.

Descending

The potential distance you can travel during the descent phase is based on your altitude as well as speed. This glide indicator uses a white orb to show where velocity is leading the aircraft, while a red arc roughly corresponds to how far it can glide at that speed.

Here we'll look at three situations starting in the same configuration at 300 meters altitude - one where we maintain as much speed as possible, one where we drop to around 60kph, and one where we bleed almost all speed and end up at 30kph.

Here's maintaining our speed. After losing engine, we have a broad range of potential landing sites. What we do here will be largely dictated by the tactical situation, and in this case, a spot far from where we lost engine, with good concealment nearby, is chosen as the landing site.

Here's dropping to 60kph. This limits our options considerably compared to maintaining speed, but we still have a large enough area to offer us choices in how we approach the landing. What we end up choosing will be dictated by the tactical situation, with the primary goal being a safe landing and the secondary one being something that facilitates any onboard troops.

Finally, here's dropping to 30kph. This takes away most of our options, putting us into a highly vertical autorotation that keeps us from making it to town. While this is a fairly safe way to approach, it isn't very flexible and doesn't permit easy reaction to surprise enemy threats.

As you can see, maintaining speed can make a big difference on your potential landing sites. Unless you need to bleed off speed to avoid running out of options, you'll want to maintain as much speed as you can, as this gives you the most opportunities to find suitable landing sites. Make sure you don't flare to kill your speed right away unless you deem it necessary.

Avoid purely vertical autorotations in terrain with obstacles whenever you can. Going straight down is more difficult and gives poorer awareness than being able to keep at least 30-60kph forward speed, and more can be helpful as well. In this case, not hitting this tree on the way down was pure blind luck. Don't rely on luck.

Don't feel the need to rush to the ground, as one way or another you're going to get there.

The main dangers of the descent phase of autorotation are based around attitude.

If you come under fire and think you can dive to gain or regain speed in order to evade, there is a very high chance that you'll lose rotor RPM and become nearly or totally uncontrollable. At low altitude this will reliably kill you. While staying level and descending may make you an easier target, potentially getting you killed in the process, diving and losing rotor RPM at mid to low altitude will absolutely kill you with no chance of recovery. There actually are some techniques that can be used here, but we'll talk about those later - for now, we're concerned with the most straightforward of autorotations, figuratively and literally.

If you allow your nose to droop over time, at some point it will pass that seven degree downward pitch threshold and begin degrading your rotor RPM, which will then put you in a dangerous state. Like I mentioned earlier, having some upward pitch beyond just being level can add additional safety to an autorotation.

If you need to turn, try to do low-bank-angle turns utilizing your pedals as much as feasible.

Finally, if you've lost your tail rotor at a lower speed and have begun to spin, you might normally be inclined to pitch down into a dive to gain some speed so that the helo weathervanes to negate the tail rotor. If you're in an engine-out autorotation, DO NOT DO THIS, you'll almost certainly lose your rotor RPM and crash. You'll instead need to rely on the lowered collective to minimize the torque spin as much as possible.

Aside from issues relating to attitude, another danger is in trying to stretch your descent too far. If you're not sure you can make a given landing site comfortably, you probably want to choose another unless you simply have no choice. Don't be tempted to keep changing the site, try to have one picked out and settled on well before it's time to start the actual landing process.

Landing Sites

An autorotation requires you to find a place to land successfully, so the terrain you're operating in and the altitude you're flying at are significant factors to consider. The more open the terrain is, the more likely you can do a successful autorotation at any random moment. The denser the terrain, the more important it becomes to fly higher up so that you have time and distance to reach an open area on your way down.

In forested or jungle environments, tighter landing spots might only be visible from higher altitudes, since at low angles the trees will seem uniform.

When searching for a landing site, you'll generally be looking for something in the direction you can see most easily, which generally corresponds to what side of the aircraft you're piloting from. Developing a slightly out-of-trim attitude can help you acheive better visibility in the direction you're headed. If you have a copilot or gunner, they can assist with finding a landing site, either by looking out the other side of the aircraft from you or by using their sensor to look below the helo.

If in a helicopter like an Apache, Cobra, or Hind, where visibility to the front is restricted when level, your best bet is to find a landing site that you can keep visible off your front left or right, traveling towards it in a slow descending spiral.

Remember that picking a landing site is the same as picking an LZ, except you don't get the luxury of waving-off and coming around for another attempt. Look for something you can achieve easily - both the ease of landing in it as well as the ease of reaching it in your current flight state. Sometimes you don't have good sites, and we'll talk about that a bit later. For now, let's assume you've found something that will fit your helo with room to spare.

Landing

In Arma, with mouse & keyboard controls, a helicopter will maintain a hovering state if you aren't pressing either collective key. If you hold up collective, you'll raise a virtual collective all the way up, but it will take a few moments before it's at that maximum rate. When you release that key, it will take several moments for the collective to return to the hovering state.

When descending with collective down, you have the collective lever bottomed out. When you release that key, it will take a few moments to go back to hover. Because of this, the new technique is not to immediately apply collective up to arrest your fall unless you think you're descending much too quickly. Instead, in a well-controlled autorotation, you first release collective down, observe how much you're slowing down, and only then apply collective up if you need it. You can then finesse your landing by tapping raise and lower collective as-needed.

The nuance here is that doing it this way preserves rotor RPM, meaning that you have a little bit more time to finalize your landing, and it also prevents you from balooning excessively.

Here you can see how much energy is in the blades after the engine fails, and how long it takes to decay from a hover.

Landing at zero forward airspeed isn't strictly required, and generally speaking landing with some forward speed is good practice. You also want to land aligned with the terrain as best as possible, especially if landing hard, and aligned with your direction of movement if possible. You want the weight of the helo to come down on all the points of contact at roughly the same time, to spread impact and lessen the chance for rollover.

If you're landing on a slope, rolling slightly into the slope can help prevent rollover on some landings.

To help stick your landing, once you've touched down, hold down collective to use the remaining lift to push you into the ground securely. This can also help to prevent rollovers or sliding.

Whatever happens, you don't want to let the helo flip or roll over. Arma's damage model will cause a helo to instantly explode if the top of it touches the ground.

Once you're down safely and it's safe to do so, jump out and run from the helo. This is Arma, and that helo could and probably will become a bomb in a moment.

So to summarize, when you lose your engine, drop collective and hold it there, raise your nose, and either get to a level or nose-high attitude, maintaining as much forward speed as possible. Figure out where you want to land, turn towards it via level pedal turns and slight banks, avoid diving as much as feasible, find a spot that looks like it will be easily reached and is open enough to allow for comfortable landing, head for it, and when close to the ground, release your down collective key to begin to arrest your descent and use taps of up and down collective to fine-tune. Once safely landed, get out and get away from the helo.

Advanced Techniques

There's still more to be said on this new method. So far we've covered autorotations that keep you level and use pedal turns and slight movements to adjust where you're heading, avoiding pitching or rolling too much. While it's true that maintaining a negative pitch attitude will decay your RPM, there are techniques that can be used to temporarily leave the safety margins in order to accomplish some brief task. I call these excursions. Let's look at them and how they work, as well as situations where they can become relevant.

Excursions

Excursions are situations where you temporarily enter a potentially dangerous flight regime, but do so in a manner that is deliberately short in duration and takes specific precautions. Excursions are temporary moves used to change either your speed or direction in a significant way. You can use these to reverse direction, turn sharply, increase your speed, drop altitude, dodge enemy fire, and so forth.

Here are two examples.

In this first one, since we're at a higher altitude, we're able to do a split-S of sorts to completely reverse our direction while also significantly decreasing our altitude.

In the second one, we're doing a sharp turn to change direction. This often is useful if surprised by a Zu-23 or similar anti-aircraft gun.

Earlier I showed situations like these that led to a crash. So what's different with these? It's the duration as well as the precautions you're taking.

Excursions are possible thanks to Arma's flight model making it so that a constant positive-pitch movement, aka pulling the nose up, will tend to either maintain rotor RPM, or lose it more slowly than otherwise. By choosing maneuvers that utilize a constant positive-pitch input, you're able to enter into otherwise dangerous attitudes with significantly minimized risk. By ensuring those maneuvers are strictly limited in duration, and that you enter into them with a reasonably high RPM to start with, you can further minimize risk. If turning, keeping your nose close to or above the horizon, and keeping your roll from becoming too extreme, can also help.

These examples work because of this positive-pitch element being maintained from the initial departure of level flight all the way until the return to level flight, and because the overall movement was kept deliberately short.

To conduct an excursion, apply full aft cylic to develop and hold positive pitch, conduct the maneuver, then immediately return to a level attitude. Monitor your rotor RPM both during and after the maneuver so that you can get a feel for how much RPM was spent and what that means for the remainder of your autorotation.

Remember that the greater of an altitude you have to work with, the more time you'll have to recover RPM during the descent phase, and the more you'll be able to make use of this technique. At lower altitudes you should keep things more horizontally oriented, while higher altitudes can allow for steeper diving turns or even split-S maneuvers. Also keep in mind that the more agile the helicopter, the more dramatic an excursion you can make. The less agile, the more you'll have to keep that in mind when choosing your maneuvers.

Dips/Nudges

A dip or a nudge is a subtle kind of excursion that's brief and is intended to change your velocity to avoid an obstacle or otherwise finesse your landing site as you're getting closer to the ground. Dipping is done by quickly angling in the direction you're trying to go, giving it a moment to build some speed, then quickly angling back to level or nearly level. This is to say that it's not necessarily only pitching, but rather that rolling can also be used if you're trying to move laterally, or a combination of the two. As with excursions, the most important part is making this brief and ensuring that you return to level attitude immediately after.

Dips and nudges are particularly helpful in cluttered terrain where you're needing to fit into a small area surrounded by obstacles, or when your path is taking you in a direction you don't want to go, and you have a nearby alternative you can attempt instead.

Loss of Tailrotor

Losing a tailrotor as part of your autorotation situation dramatically changes the options available to you. You have little control over your direction, can't do excursions without significant risk of spinning out of control, and must find a workable landing site in the general direction you're already headed, even if means going into a site you'd otherwise avoid. In this situation you simply do the best you can with what you have.

Since you have little practical control over your heading once the tailrotor is gone, being able to keep track of and visualize obstacles and open areas as they pass out of your view becomes important, and as long as you can keep a picture in your head of what's below you, you have a much better chance of being able to land safely, even blindly.

If you're in a hover when this happens, down collective will help to slow the spin, while a quick forward dip will help to ensure you have forward movement and thus can see the area you're flying into. Without this, it's possible to end up flying backwards, which can quickly become disastrous. You may end up in lateral flight from doing that forward dip, but you should no longer be spinning, making it easier to keep track of where you're trying to go. This dip should be very short in duration to avoid bleeding RPM.

As mentioned earlier, any sustained dive to try to weathervane will almost certainly result in a crash.

If you're already moving enough to have weathervane stability, you can adjust your path somewhat by slightly rolling in the direction you want to go. Be sparing with this, use shallow angles, and monitor your RPM as you do. Maintaining this weathervane stability by maintaining sufficient speed will give you many more options in how you approach your landing and influence how successful it will be.

Remember that because you're holding down collective throughout your descent, the torque effect from the main rotor will be minimal. However, when you release or raise collective towards the end, this torque will ramp up and you'll need to keep it in mind for your landing phase. Arma helicopters spin to the right when they lose tailrotor, so you should be looking for clearance on your left in order to give room for your tail to swing around without running into something.

No-tailrotor autorotations are the spiciest kind. Being familiar with all the different aspects and options availble can help you to salvage what otherwise seems unsalvageable.

Special Considerations

There are a few different aspects of terrain that you'll need to keep in mind as you're coming in for a landing. The less flat and open the terrain is, the more potential complications arise. Let's go over the main ones.

Obstacles

First let's talk about obstacles. In this context we're casting a broad net that includes things like buildings, poles, trees, and anything else that can cause damage to the aircraft or rotors before it gets to the ground.

There are two basic types of obstacles to consider in an autorotation - ones that are an influence on the way down, and ones that are an influence as you're touching down. The former are things like multi-story buildings, large trees, poles, towers, and similar, while the latter can be anything that would intersect your rotor disc when landed. Since the rotor disc is a different height above the ground in different helicopters, taller helicopters will have fewer obstacles to worry about at ground-level, while shorter ones will have more.

If it looks like your path is going to take you into a tree or building, but only just barely, and you've maintained your rotor RPM well, you can briefly raise collective to pass over such an obstacle without compromising your landing. You don't have much margin for error with this, but it's better to lift over some trees or a building edge and land a bit harder later than to slam into the obstacle at speed.

Don't stress bushes. If it looks like you're going to land on or run into a bush, that's probably ok. There are even situations where small trees and bushes can be used to cushion your fall, like this one where an Mi-8 is able to use a cluster of trees to prevent sliding off the ridgeline.

In urban areas, wires and poles are a concern, and streets can often have wires and poles along their edges, or even wires crossing over the street. Due to Arma physics, hitting a wire is akin to hitting a solid bar, so don't expect them to break under the weight of the helicopter.

Outside of urban areas, wires and poles can often be found beside roads, while larger powerlines will take their own path through terrain.

Since poles tend to be spaced at regular intervals, seeing several in one direction can give you an idea of where they might be below or behind you.

When operating in forested or jungle environments, roads and rivers will tend to have fewer obstructions than otherwise and are often viable landing sites.

Rotors

Your rotors in an autorotation are only useful while they have energy in them and are still attached to your helo.

If you know that you're about to lose them from a collision, such as when you're forced to ditch in trees, the best thing to do is to use all that energy while you still have it.

Treat the top of the trees as your landing site and come as close and as low of a hover as possible over them before your rotors lose their energy. By doing this, you'll have slowed yourself as much as possible, and when you lose lift and fall into the trees or the rotors collide and are torn off, you'll start falling from that hover instead of at your descent velocity.

While you might not survive a fall through a tall jungle this way, it gives you a much better chance than otherwise. Remember to try as best you can to be level when you start to fall, as inverting and hitting the top of the helo on the ground will cause it to explode. If you're playing vanilla Arma, you're probably dead regardless, but mods or modded content might allow survival.

If descending into trees, it's possible to get hung up or even land on their crowns. There's no special handling in Arma for this kind of situation. You may be close enough to the ground to jump out safely, but if not, I'd suggest radioing up what happened, then waiting in your helo (if it's lightly damaged and stable, and the tactical situation permits) until a friendly medic can arrive. Once they're there, take a leap of faith out into the trees and hope that you survive the fall - this of course being much more likely in a modded damage system than vanilla Arma.

Having your rotors break from a last-second collision as you're close to touching down is generally not a bad thing unless the aircraft isn't configured to Arma 3's standards. It's often better to let them break from a slow collision than wildly try to flare to avoid the collision, and in doing so, compromise your landing.

Sometimes you might find that the only good landing site in your descent direction is one that won't fit your rotors, which can force you into deliberately breaking them off. Just make sure you're at or near a hover first, unless the distance is very short and you're already slow. In this example, autorotating in Clafghan's mountains is a tricky affair, and the Mi-8 makes everything more difficult. The road is a good landing site, and while there are buildings and trees closely bordering it, breaking off the rotors to be able to land on the road is worth a shot.

Water

If your area includes rivers, lakes, or the ocean, one option you have is to ditch in the water. This is done with the exact same approach as a normal autorotation, with "touchdown" being the same aside from the fact that you may or may not float depending on the helo configuration. Water landings are generally safe in Arma and can be a much better option than landing on a forested side of a hill or similar.

This is particularly useful in terrains where the coast is steep or rocky, or in jungles where rivers might be the only open area for some distance.

Steep terrain

If landing on terrain where you expect to slide, try to land facing uphill or sideways, and if possible, pick a location where your slide can be stopped by trees, rocky outcroppings, or similar. This isn't a good situation to be in in the first place, but you may be able to salvage the landing this way.

In this example, an excursion reversal is used to get out of the threat's view, then a hasty assessment makes it possible to land near the crest and offload troops.

Keep in mind that matching the slope of steep terrain while trying to extend away and mask from a threat will put you in a nose-down attitude that will quickly drop your rotor RPM to fatal levels. Avoid the temptation to dive down along the contour as you might when evading with a working engine, as this will almost certainly prove fatal.

Damage Model

Keep in mind that any of the techniques talked about here will be influenced by how much damage your aircraft has taken. If you just lost engine, you have more margin to work with, but if you're flying a block of swiss cheese that's also on fire, you'll probably want to be a bit more gentle with your landing.

If you're using unmodded vanilla Arma 3, the damage model in it handles crashing pretty poorly. There are mods that help with this, to include some mods like RHS having built-in support for tweaking how hard landing damage is handled. It's highly recommended to use something to tweak the vanilla damage model in this regard, as the default settings are... bad. You'll have a lot more fun when crashing leads to wounded players that need rescue, instead of it always being a funeral pyre.

Closing

I doubt you'll be surprised to hear me say that the key to becoming proficient with these techniques is extensive practice. You're not paying for fuel or airframes, and crashing isn't going to kill you, so nothing's stopping you from spending time to develop your proficiency at the art of successfully crashing. Practice, practice, and practice some more, so that when the time comes to do it for real, you'll be ready for it.