Flying Helicopters in Self-Induced IFR

?You know,? said my student, once we were inside our relatively warm helicopter, ?I?d never thought that it was possible to enter inadvertent instrument meteorological conditions just a couple of feet above the ground in sunshine, but surely, that?s what that guy just did.?

It was early winter in Chicago, the day after the first real snowfall. Typical of those Midwest days when the temperatures drop towards 0?F ( -18?C), the sky was blue and the visibility almost unlimited. Icing conditions were unlikely, and with more lift available than the Schweizer 300CB helicopter had a right to expect, conditions seemed perfect for flying.

With my student, a corporate airplane pilot learning to fly helicopters, I waited until the line crew pulled the helicopter out of the warm hangar, where we had performed our preflight inspection. We stood by the edge of the ramp watching a news helicopter make an approach to a nearby grassy area, now covered by almost two feet of snow.

As the helicopter descended below 60 feet or so, a small plume of snow danced across the surface, trailing the aircraft by about 25 yards. The plume of snow quickly grew and became so thick that we could no longer see through it, but it still trailed the helicopter until the final stages of the approach when the pilot pulled collective pitch to establish hover power.

Suddenly, a boiling white cloud enveloped the news copter from the rear ? so thick that, although we could hear the helicopter?s turbine engine, we could see nothing of the aircraft. ?First time I ever saw anybody get stuck in his own wake turbulence,? said my student. Moments later, the news copter rose vertically through the white cloud, which was by then, perhaps 50 feet tall and growing quickly. He proceeded forward, in a very high hover taxi, still trailing a white wake to a cleared taxiway for another attempt at touchdown.

The icy, foggy cloud caused by the news copter?s abortive attempt to land drifted over us. Minute ice crystals found their way into our eyes and down our necks, where they melted and trickled uncomfortably. Not only that, but the ice cloud also smothered our helicopter.

This was a clue. We inspected the tops of the Schweizer?s rotor blades and wiped off the water droplets. Our helicopter, fresh from the warm hanger, was ideally placed to melt the ice crystals blown by the incoming aircraft. At the temperatures on the ramp, though, it would not be long before they froze. We also performed a careful inspection of the control runs to ensure they hadn?t iced up.

?You know,? said my student, once we were inside our relatively warm helicopter, ?I?d never thought it was possible to enter inadvertent instrument meteorological conditions just a couple of feet above the ground in sunshine, but surely, that?s what that guy just did.?

My student had a point. Mention inadvertent entry into instrument meteorological conditions (IMC) to most pilots, and you will conjure up an image of flying into a cloud, perhaps at night or in very poor visibility, at an altitude at least several hundred feet above the ground. Not many pilots would think about flying into inadvertent IMC only a few feet from the ground on a sunny day. Even fewer pilots would believe they might be the cause of the IMC conditions they had just entered ? unless, that is ? they are helicopter pilots.

The rotor blades of a hovering helicopter cause air to start moving downward when it is as much as 100 feet above the ground. As air accelerates downward, it gathers speed until it passes through the rotor and it deflects outwards from proximity ot the ground and slows down. A helicopter hovering in ground effect ? that is ? within two rotor diameters of the ground, creates a large circulatory system. Much of the air that passes through the rotors, rises again to be drawn back down through the disc. To some extent, this circulation is the result of the vortices that exist around the tips of the rotors.

Not only air is drawn through the rotors. Given that the rotor wash from a hovering helicopter can exceed 100 mph, it tends to pick up whatever is on the surface. If the ground is covered with powdery snow or fine dust, this, too, will be carried around in the re-circulation and create the self-induced IMC conditions.

With our helicopter running and its engine temperatures snugly in the green, we made sure to hover taxi only over areas that had been cleared of loose snow as we prepared to depart our home field for the small nontower field where we were going to practice traffic patterns. We also had to watch out for the piles of hardened snow the plows had left at the sides of the runway and taxiways. Even hovering over cleared areas, we still had small amounts of recirculation, and this, coupled with the uniform bright whiteness of the scene, made these snow piles difficult to detect.

The powdery snow conditions gave our lesson a new emphasis. It also gave my student lots of questions. ?That TV pilot seemed to have problems landing,? he said. ?How are we going to land if he had that much trouble??

?Well,? I said ?he tried to land to a snowfield without a reference point. Also, it seemed that he didn?t anticipate the amount of recirculation he encountered.?

?Isn?t there always recirculation?? he asked.

?No,? I replied. ?Many days you?ll find a thin crust develops on top of the snow and prevents serious re-circulation from occurring. However, a wise helicopter pilot will always anticipate it. Also, it?s not unknown for slabs of frozen crust to break loose and become a hazard to others in the vicinity. When you shoot an approach in snow conditions, try to make the approach to a cleared area. If this isn?t possible, then choose a reference point.?

?What?s a reference point?? He asked.

?A reference point is any area of visual contrast to which you can shoot an approach, but not like a house,? I said. ?A reference point has to be able to fit under the rotor disc, because when recirculation starts, you won?t be able to see any farther than the tips of your rotor blades. You plan the approach to the ground right down to the reference point, and if you lose sight of it, even for one second, you should immediately pull climb power and make a vertical ascent. If your helicopter is instrument equipped, so much the better, because the attitude indicator will assist you in keeping the aircraft level during the vertical climb. Vehicle tracks or vegetation can make good reference points, but be careful because the snow makes it difficult to judge scale. I once made an approach to what I thought was a small bush only to find it was the top of a good sized fir tree.

?It?s especially important,? I said ?to make the approach into the wind, because you want any recirculation to stay behind the helicopter for the longest possible time. If you approach downwind, the cloud of recirculating snow gets ahead of the aircraft and blinds you that much earlier.?

?But how bad can it really be?? my student asked.

?A friend of mine during my army days,? I told him ?tried to make an approach to a snowfield in the Alps. It was a clear sunny day, but in the final stages of the approach, he encountered fierce recirculation, what we call a ?whiteout.? He thought he had his landing point in sight, but in fact, he didn?t. The next thing he saw, coming out of the whirling snow from the side of his helicopter at about 40 miles an hour, was a good sized wooden structure that had been about 50 yards from his intended touchdown point. I wish I still had the photographs, but I can tell you that the picture of the helicopter lying on its side next to the hut didn?t look too pretty. No, neither he nor his passengers were hurt, but I bet his pride took as many hits as the hut did from the rotor blades.?

?Sounds like it can get pretty bad then,? he said.

?You bet it can. Don?t be afraid to make a go-around anytime it looks bad. Although you might have to wait for the snow cloud to subside, you?ll have blown some of the snow away, so the next approach may not be so bad. But if you don?t make it after a couple of attempts, go and find another place to land.? I told him.

?Anyway, here we are at the practice field,? I said. ?Let?s set up for a normal approach to the marked runway, and then we?ll try for the snow areas later. Be careful to monitor your speed and angle during the approach,? I told him. His approach angle was fine, but in the final stages, he realized that he was going much faster than normal.

?Wow,? he said. ?I know you told me to watch my speed, but I didn?t realize I was too fast until right at the end.?

?I knew that would happen,? I said. ?The snow removes all the usual movement cues and you don?t pick up rate of closure until too late. Your approach angle was good, but for the same reason, it?s common for approaches in snow to be too shallow. The lack of visual cues makes depth perception and altitude judgment difficult. During Operation Desert Storm, several helicopters were flown into the top of sand dunes.?

?Sand isn?t snow,? my student said.

?No, it isn?t,? I agreed, ?but it causes many of the same problems, lack of visual contrast and difficulty judging altitude. Actually, sand and powdery dirt will give all the same re-circulation problems as snow, only in this case, it?s called a ?brown out.??

We flew some approaches to the runway, and then we set up an approach to a large snow-covered area. ?What will you use as your reference point?? I asked.

?I?ll use that place where the snowmobile tracks cross,? he said. ?That?s good,? I said, ?the snowmobiles will have flattened down some of the snow which will reduce the amount of re-circulation.?

As we approached the area, I told my student to glance at the ground behind to see the white cloud closing on the helicopter from the rear. ?Now continue right to the surface, but once you get there, keep the collective raised so the helicopter?s skids rest lightly on the snow.? The miniature snow storm caught up with us just prior to touchdown, and for a few seconds, it was hard to see anything outside the immediate vicinity of the helicopter. If the tracks had not been there, it?s likely that we would have lost all positional awareness. The reason I had him to hold the helicopter lightly on the surface was that I didn?t know for sure what lay under the surface or how well the snow would support the helicopter?s weight.

?Easy does it,? I told him. ?Let the weight onto the skids gently.? Landing on snow is always nerve wracking because a hard crust that seems to support the helicopter?s weight can suddenly give way. However, in this case, both skids went straight into the snow, but not so deep that getting out would be a problem. ?We need to be careful when we apply power to lift off,? I said. ?It?s not unknown for an unseen obstruction under the snow to hook a skid, or for one skid to freeze to the surface. Either condition could lead to dynamic rollover on lift off.?

I showed him how to raise the collective to the point of lift off, and then move the pedals enough to ensure that both skids were free of the surface. Without further delay, I then raised the collective to climb power to initiate a vertical climb from the surface. As soon as we began to climb, the white out began to form again, but by lifting vertically, we were soon out of it. Once we were in the clear, we lowered the nose to gain forward speed and reverted to a normal climb.

?Wow, pretty interesting stuff,? he said. ?It seems like there?s quite a bit to learn about operating in snow conditions.?

?Snow question about that.?