Friday, May 6, 2011

Rough Landing

vrs1There’s an adage that “any landing you walk away from is a good landing.” That sounds cute, but no pilot really sees it that way if it involves leaving a broken aircraft behind. By now, we all know that the American forces who raided bin Laden’s compound experienced a rough landing and left the remains of a helicopter behind. You might have heard that the helicopter likely crashed due to a phenomenon known as “settling with power” while some other explanations have suggested that it might have been a “high, hot” landing scenario. I thought I would try to explain how that kind of thing happens in case anyone is interested.

You probably already realize that helicopters fly by producing lift with their rotors. The helicopter is controlled by the pilot’s manipulation of three controls: the cyclic which raises and lowers the nose of the aircraft and rolls it to the left and right, the pedals which move (or yaw) the nose (or tail) of the aircraft to the left or right, and the collective which generally has a mechanical linkage to the fuel control to increase power and to the rotor blades to change the angle at which they cut through the air. A higher angle on the blades increases lift with an increase in power, and vice versa.

As a helicopter flies, the rotor blades move the air downward, producing lift. When the aircraft is within one rotor diameter of the ground, it gains as much as a 10% boost in lift from that downward airflow against the ground. The helicopter I flew in the Marines, for instance, had a 51 foot rotor diameter, so we began to pick up some ground cushion within about 51 feet of the ground.

vrs2When the helicopter is close enough to the ground to gain some ground cushioning, the airflow presses against the ground and tends to dissipate out to the sides. However, when the aircraft is farther from the ground outside of that “ground effect,” the airflow actually creates a vortex that tends to circle back around and exert a downward force on the rotor blades. When that happens, the lift produced from the rotors is neutralized. The helicopter needs lift to stay in the air, so when rotor lift is neutralized, the result is that the helicopter “settles.”

In most flight profiles, the aircraft is able to either fly past that vortex by being past the vortex by the time it circles back around or it can descend faster than the vortex can circle back around and influence the rotors adversely. However, at certain airspeeds and rates of descent – in the helicopter I flew it was 700-1000 feet per second rate of descent and less than 40 knots of airspeed – that vortex can keep up with the helicopter and actually force the helicopter to the ground. This is called the “vortex ring state.” Since the aircraft settling is not caused by the aircraft being underpowered, the settling that occurs is known as “settling with power.”

Here’s the problem: When a pilot encounters the vortex ring state and the aircraft begins to settle, the intuitive thing to do is to increase power (since the aircraft still has power to spare) in order to slow or stop the rate of descent. However, since it is the rotor thrust that is forcing the helicopter down, increasing power actually makes the situation worse. The pilot should actually do what is not intuitive in that situation and lower the nose toward the ground – essentially dive toward the ground – and decrease power. The idea is to decrease the rotor thrust with the collective while lowering the nose with the cyclic to pick up some speed so you can fly out of the vortex. The result is normally an aborted landing or a “wave-off” because the landing zones where this is normally encountered are too confined to save the approach. The pilot has to do this right away when he enters this condition because if he doesn’t, the rate of descent will be too much to overcome and he will crash.

The way you keep from having this happen to you is by staying out of the vortex ring state in the first place. In certain circumstances, especially combat situations where the flying tends to be a bit more aggressive, the flight profile invites the vortex ring state and it’s more difficult to stay out of it. The pilots who were selected to fly that mission into the bin Laden compound were very likely quite experienced and knowledgeable about aerodynamic phenomena like the vortex ring state, so that shows you how insidious it is.

This is how a combat scenario can make it easier to enter the vortex ring state: As I wrote earlier, the vortex ring state relates to rates of descent and airspeed. When a pilot makes an approach into a confined area like a walled compound, he needs to take a relatively steep angle on the approach to the landing zone. The angle – or glideslope – is a product of vertical and horizontal distance covered over time. In other words, the faster the helicopter is traveling over the ground, the faster it needs to descend in order to hit the landing spot.

Remember the vortex ring state occurs at airspeeds below a certain speed, say 40 knots. If I’m flying into a 20 knot headwind, my airspeed indicator will read 20 knots faster than I’m traveling over the ground. So, if I’m flying at a groundspeed of 40 knots with a 20 knot headwind, my airspeed indicator will show 60 knots. A headwind enables me to keep my airspeed up even with my groundspeed – the speed I’m traveling over the ground – relatively low. Remember, the pilot controls his glideslope by managing his groundspeed. Normally, the pilot can count on a little bit of headwind to enable him to stay above the airspeed where the vortex ring state occurs while he manages his groundspeed to control the glideslope until he gets close enough to the ground to break up the vortex with ground effect.

With many highly “choreographed” raids where surprise and speed are important, the pilot’s wind is not always favorable. If the door or ramp of the helicopter needs to be right at the main ingress point of the building they’re attacking, he might need to orient his approach to put that door or ramp in just the right location. That might mean accepting a tailwind that makes it more likely he’ll enter the vortex ring state if he’s making a slow steep approach to the landing zone.

Sometimes, the terrain and wind can funnel the sound of the aircraft to the enemy so you might end up approaching the landing zone with a crosswind. With a true cross wind the effect of the wind is negligible on the main rotor, except that what you see on the airspeed indicator is also the speed you’re traveling over the ground.

Now, let’s say the pilot is lucky and has the right wind all the way, including a nice headwind on the glideslope. He carries a steep profile into the landing zone so he doesn’t hit the walls surrounding the compound, but as he nears the ground, the wind that he’s had this whole time suddenly disappears because it’s blocked by the walls. He immediately goes from a very favorable flight profile to a very dangerous one just as he’s trying to land.

To make matters worse, depending on the strength of the wind, the wind that gets deflected by the wall can create its own vortex. A moderate wind tends to rise up just in front of a vertical obstacle, then come back down as it seeks the wind stream again, sort of like water rushing around a rock in a stream. This can result in a down force wind acting on the helicopter right as it comes into the landing zone. This is sort of what happens in a thunderstorm microburst.

I hope this explanation has been clear enough.

Let me say one more thing about the vortex ring state. When it comes to combat assaults where surprise is key, you only have one shot at the landing zone or you risk losing the element of surprise by taking a wave off. When this pilot started feeling himself getting caught in the vortex ring state (if that’s actually what happened), the only way to have gotten out of it would have been to wave off and try the landing a second time. Even if he could have executed the wave off without hitting the wall on the opposite side of the compound, he could have ruined the entire mission by not staying with that approach that was forcing him down. It’s ironic, perhaps, that he might have endangered American lives and the mission by preventing the crash with a wave-off since the element of surprise would have been lost with only half of the ground force in place by the time he circled back around to land. That is always an important element.

Very quickly, the “high, hot” scenario was also mentioned. I don’t know if the news source intended to pair the “settling with power” potential cause factor with the “high, hot” explanation, but they are often confused as being related or the same. They’re not the same.

The high, hot scenario relates to the fact that at higher altitudes and hotter temperatures, the air density is lower. Since lift is produced by moving a volume of air with the rotors, a lower air density in “high, hot” situations means that more power might be required to produce that lift than normal. It is possible to require more power to land than the aircraft is capable of producing, especially if the aircraft is also heavily loaded with troops and equipment.

On a raid like this, it is possible for the air density at your origin to be considerably less than at your destination. In planning missions, you typically make an educated guess about the destination air density. When operating in the field, you might even be making educated guesses at both ends of the trip because of a lack of meteorological equipment. The difference between the point of origin and destination in that case is that you know right away whether you have enough power to take off at your origin before you do it, while you don’t know for sure that you don’t have enough power to land at your destination until you try it.

I don’t know which, if any, of these factors played a role in the helicopter mishap at the bin Laden compound, but I do know that any experienced helicopter pilot will admit that “there but by the Grace of God go I” when it comes to things like this. These phenomena can reach out and grab even the best and luckiest of pilots, but when being good and lucky aren’t enough, you get to make the news.