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Helicopter Landing

As anyone with aviation experience can attest, landing an aircraft is often the most dangerous phase of flight. While the goal is the same, helicopter landing procedures are quite different from fixed wing aircraft. A helicopter is more flexible: it’s able to land from a variety of angles and on a variety of surfaces. In this article, we’ll discuss helicopter landings in detail.

Normal approach and landing

We start with a normal helicopter landing. By normal we mean the aircraft is operating normally and landing on a runway or other certified landing zone (LZ). We assume the terrain is flat and paved, like a runway, with no obstacles nearby.

The approach

The landing begins with an approach to the landing zone. The approach should follow a path that’s roughly a straight line from the LZ with an approximate 10 degree incline from the ground, as shown in the image below.

Intercepting a normal helicopter approach
Figure 1 - The path of a normal approach to a landing zone, which is inclined 10 degrees from horizontal.

Upon intercepting this line the pilot should lower collective and pull aft cyclic to decelerate and descend. Pedals should be adjusted to maintain alignment with the LZ.

Wind

If winds are significant, it’s better to approach the LZ from downwind—it’s best to fly against the wind when landing. This utilizes more translational lift closer to the LZ to reduce the chance of running out of power. It also reduces the probability of entering vortex ring state, a dangerous condition described below.

Approaching from downwind
Figure 2 - A bird's-eye view of the approach path. The LZ should be approached from downwind as much as practical.

Following the approach

Maintaining the proper approach angle requires practice. Pilots use visual cues to monitor their approach angle. For example, for a given helicopter, a pilot may know where the landing zone should appear in the front window. If he’s above the 10 deg approach, the LZ will appear too low in the window. If the approach is too shallow, the LZ will appear too high in the window.

A more subtle trick some pilots use is to look at the apparent shapes of objects. Let’s say the helipad is a circle. When viewed from an angle that circle will appear as an ellipse. The more the angle (relative to being directly above the circle), the more elongated the ellipse will appear. Thus, if you know what it should look like from the 10deg approach, you can use this fact to judge if you are approaching too steep or shallow.

Speed

Unlike a fixed wing, helicopters decelerate throughout the approach to reach 0 kt ground speed just before landing. Some pilots use the trick of looking at the ground and keeping a constant apparent ground speed, which naturally causes them to slow down proportional to their altitude. This is because the ground appears to move by slower at higher altitudes.

While power and collective were reduced in the initial phase of approach, once the airspeed drops below a threshold (translational lift, around 25 KTAS) the power required actually increases and collective must be increased. This can be tricky for the inexperienced pilot. Failure to increase collective will cause the helicopter to fall below the normal approach line, ending up with a shallow approach.

Around 100 feet above ground level (AGL), the pilot should be more aware of aligning the helicopter toward the LZ. Flying laterally toward the the LZ should be avoided as much as possible. At touchdown it will be of high importance—if a helicopter lands with lateral velocity it can easily rollover.

The helicopter should fly directly forward to the LZ, lateral flight should be minimized
Figure 3 - A bird's-eye view of the final approach, from 100 feet AGL. The helicopter should fly directly forward to the LZ. Lateral flight should be minimized.

Landing

The normal approach should end in a hover several feet above ground level. From here the pilot may need to hover-taxi to the target landing spot. Once above the landing spot the pilot should keep the helicopter in place with fine control movements. The pilot should lower the hover height slowly and repeatedly until the skids are on the ground, at which point the collective should be sharply reduced—it’s dangerous to keep the collective up with substantial thrust while the skids are in contect with the ground.

Why does one skid land first?

One skid will typically land before the other, this is normal. For a US helicopter, it will normally be the left skid. This is because the tail rotor is thrusting to the right. To prevent the tail rotor from pushing the helicopter right, the helicopter hovers with a small left roll (which causes the main rotor thrust to push left slightly).

Ground effect

Another phenomenon that manifests before landing is ground effect. The presence of the ground alters the flow of downwash from the main rotor. The rotor becomes more efficient and provides more thrust closer to the ground. Some pilots describe this as hitting a cushion—if the collective is held fixed with a small descent rate the helicopter will naturally stop descending before reaching the ground.

Ground resonance

Yet another phenomenon to be aware of is ground resonance. Ground resonance can occur when some helicopters are contacting the ground with the main rotor spinning. It’s caused by a resonance between the oscillations of the landing gear and main rotor. If a pilot detects such extreme vibrations / wobbling around on the gear, the best course of action is to lift off the ground into a hover—ground resonance can’t occur without ground contact.

Steep approach

Obstacles may prevent the pilot from flying a normal 10 degree approach. For example, avoidance of a building, powerlines, or even areas of high turbulence may necessitate this. It’s important that the approach not be too steep as this can present dangers such as vortex ring state (described later). According to the FAA, a steep approach should be in the range of 13-15deg. This will require a lower collective and more aft cyclic than a normal approach. The pilot should be looking out for changing winds during the descent—the obstacles/conditions that necessitate the steep approaches often induce variable winds. The steep approach should end with a hover about 3 feet over the landing spot followed by a normal landing.

Vortex Ring State

Vortex ring state is a particular concern in steep descents. We won’t attempt to describe the physics in detail—it’s a very debated subject. What everyone agrees on is that the phenomenon occurs in a combination of very low airspeed and high descent rate.

Vortex ring state causes poor, apparently inconsistent, aerodynamic performance of the main rotor. For one, this causes a lack of thrust leading the helicopter to descend even faster. The key characteristic is that increasing collective doesn’t help—pulling collective will not arrest the descent!

The inconsistent aerodynamic forces cause apparently random yaw, pitch, and roll excursions as well. When in this scenario, the best way out is typically to accelerate forward. Pushing the cyclic forward should eventually increase airspeed and alleviate the vortex ring phenomena.

Shallow approach / running landing

On the flip side of a steep approach is a shallow approach. A shallow approach is used when there is little power margin—the helicopter is heavy, OEI, and/or operating at high density altitude. In these conditions, the helicopter may not have enough power to hover (recall hovering requires more power forward flight at moderate speeds). To avoid hovering, a running landing will follow the shallow approach. The standard approach angle used here is 3 to 5 degrees.

Running landing

Landing should occur with translational lift, over 15kt airspeed, which requires less power than hover. Unless there’s a large headwind, this means the helicopter will land with significant ground speed and need to roll or slide to a stop on the ground. The landing area must be large enough to allow for this, with margin for error.

It is critical to ensure the helicopter moves directly forward on the ground. If the helicopter has much lateral speed and a skid contacts something unexpected a dynamic rollover can occur, leaving the helicopter in the condition shown below.

The result of dynamic rollover
Figure 4 - The aftermath of a dynamic rollover. If a helicopter slides laterally on the ground, dynamic rollover can occur if a skid contacts an unexpected object, even a rock.

The length of the slide will depend on winds. With a headwind, less ground speed is needed at touchdown, and hence the slide will be shorter. Lowering the collective during the slide puts more weight on the skids, increases friction, and hence has a braking effect. If there’s little room for sliding, the collective should be moved all the way down as soon as practical. Of course, as the collective moves torque changes and pedal movements are needed to maintain heading.

Landing in a non standard location

A helicopter may need to land in a non standard location such as in a field. Perhaps a passenger has a medical problem, or the helicopter has a malfunction like a hydraulic leak or engine fire. Before landing in a non standard location, the pilot should fly a low altitude circuit around the planned LZ if feasible. The aim is to check the ground for any large plants, rocks, slopes, powerlines or other obstacles that could pose danger to the helicopter.

Once a suitable LZ is selected, the pilot should approach the LZ from downwind with a steep, normal, or shallow approach as suitable.

Slope landing

While slope landings are ideally avoided, they may be necessary. Before considering a slope landing, the pilot should be familiar with the flight manual and operating handbook for the helicopter. These documents should have specific instructions as to the maximum landable slope and any special considerations specific to the helicopter being flown. The maximum slope is normally around 6 degrees.

Slope landings are typically safest with the helicopter perpendicular to the slope as shown in the image below (some people call this “across the slope”). The helicopter should not land or even hover low facing downslope as the risk of the tail rotor striking terrain is high.

When landing on a slope, a helicopter should land accross the slope
Figure 5 - Landing on a slope. When landing on uneven ground, the helicopter should land with the right or left skid going up the incline. The tail absolutely should not be directed up the incline, as the possibility of the tai striking the ground is high.

If there’s significant wind, the landing direction should be chosen to maximize headwind, as usual. Be aware that helicopters are inherently not symmetric, so landing with the right skid downhill vs. the left skid downhill will be different. In fact, the flight manual may cite reasons to prefer one of these two directions.

Once the location and direction of the landing are determined, the pilot may approach the location. The approach typically comes from downhill at 45 degrees to the target landing orientation (45 degrees from flying directly uphill). This allows the pilot to get a good look at the landing spot and keep the tail rotor from facing upslope, where risk of ground strike is high.

Once the helicopter is hovering over the landing spot it should be slowly and carefully yawed 45 degrees to the target orientation. After that, the collective may be lowered slowly until the upslope skid touches the ground. Lateral cyclic is then applied upslope to more firmly plant that skid on the ground. The collective is then decreased further until the other skid contacts the ground and then until full weight is on the gear.

Before shutting down the engine, it’s often recommended that the pilot perturb the cyclic and pedals to ensure the helicopter does not move. If the helicopter is on too steep of a slope or on slippery ground then movement or vibrations may occur. If this happens, the pilot should take off and select a new landing spot.

Extreme slopes

In emergency situations, a helicopter may need to “dock” against a more extreme slope on the side of a mountain. Below are a couple videos of this being performed in real life.



Autorotation landing

This one of the most difficult maneuvers to perform in a helicopter. When power is lost, e.g. due to an engine or drive system failure, a helicopter may glide to a landing like a fixed wing airplane. The helicopter necessarily flies at a high descent rate so that the upward flow of air powers the main rotor like a wind turbine. Shortly before touching down, the pilot must use impeccable timing to arrest both the forward speed and descent rate just as the helicopter touches down. This last part of the maneuver is called the flare landing. For more information about autorotation and the flare landing, see our autorotation article.

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