Twirling Trouble

Helicopters are light and stay out of the pattern, so theyre harmless, right?


Pilots in the traffic pattern worry enough about looking for other airplanes that many ignore helicopters, even when tower controllers call out the traffic. The reasoning may be something like: They dont fly patterns that interfere with airplanes, so theyre not a collision threat, and theyre so much smaller than an airliner, how bad could a wake vortex really be?

The risk of collision is something only the pilot in the pattern can assess, but the threat of wake vortices from helicopters is actually much more ominous than most pilots realize.

Real-world research into the effect of helicopter rotor vortices on general aviation aircraft shows that some of the characteristics are the same as behind airliners, but real danger awaits the unsuspecting.

There are several similarities in the wake vortices that trail behind fixed-wing and rotary-wing aircraft. Just like on fixed-wing aircraft, helicopter vortices formed at low airspeeds are initially stronger than those formed at higher airspeeds. A heavier helicopter produces stronger wake vortices than a lighter helicopter. Larger helicopter size increases the vortex size. The strength of a vortex is also very dependent on its age.

But the differences are critical. The left and right vortices are distinctly different in helicopters. The retreating blade operates at a much lower relative airspeed, and therefore holds a higher angle of attack in order to produce as much lift as the advancing blade.

The vortex behind the advancing rotor blade is consistently smaller, tighter and more coherent, especially as the helicopters forward speed increases above 80 knots. The vortex behind the retreating blade is characterized by a larger diameter, less dense smoke marking and a greater cross section. Furthermore, researchers have found the wake reacts differently, depending on whether the helicopter is climbing or descending. The vortex cores move farther apart during descents and closer together during climbs.

Making Detailed Studies
Ground school instruction on wake vortices is usually very basic. It describes wake vortices produced by large airplanes as sinking below the generating aircraft in a predictable manner and supplies the usual caveats about flying above the larger aircraft when in trail and landing farther down the runway than the airliner. The only mention of other dynamics generally is the effect of the wind in blowing the vortices along the ground as they settle near the airport.

Several years ago, the FAA conducted a series of flight tests measuring the effect of various helicopters on general aviation airplanes. The FAA used a Bell UH-1H helicopter from the Army National Guard, a Sikorsky S-76A belonging to the FAA, a U.S. Army UH-60 Blackhawk, a U.S. Marine Corps CH-53E, and a U.S. Army Boeing Vertol CH-47D. The latter two helicopters have notable heavy lift capabilities. Each of the helicopters was outfitted with devices to generate smoke into the aircraft wake.

For test airplanes, the FAA used a Beechcraft T-34C and a Bellanca 8KCAB Super Decathlon to probe the helicopter wakes. The T-34C is a low-wing monoplane with a 33-foot wingspan and a maximum takeoff weight of 4,300 pounds. It has 715 shaft horsepower from its turboprop powerplant. The high-wing Decathlon has a maximum takeoff weight of 1,800 pounds and has a 180 horsepower piston engine.

As it turns out, the FAAs decision to use aerobatic airplanes for this testing was fortuitous, both because the airplanes could recover from the vortex-induced upsets and because the aerobatic planes are structurally stronger than other planes in the normal category.

The size, shape and weight of these aircraft are close enough to most light aircraft that the results apply quite well to most airplanes in the general aviation fleet. The Decathlons weight and configuration are close to lightly loaded single-engine Cessnas and the T-34Cs weight would approximate larger, cabin-class general aviation twins.

The research findings from actual in-flight testing are anything but simplistic. During the tests, the helicopter wakes did not descend in the same predictable manner. One possible explanation may be that the relatively large amount of power required to lift a helicopter creates a disproportionately large amount of hot exhaust, which is entrained in the wake and contributes to its buoyancy.

In addition, the area contaminated by the wake turbulence of a helicopter is larger than that of an airplane of comparable size and weight, especially at speeds below 70 to 80 knots.

The number of blades also appears to affect vortex size; more blades produce a bigger vortex. The 9,500-pound UH-1H, with two rotor blades and a rotor diameter of 44 feet, produced a smaller vortex than the 10,000-pound S-76A, which has four rotor blades and a 44-foot rotor diameter.

Parallel and Cross-Track Encounters
The FAA tested vortex encounters in both parallel and cross-track flight paths. A parallel encounter is the most obvious and occurs when the trailing aircraft is flying in roughly the same direction, behind and below the generating aircraft. Most pilots anticipate a very strong rolling reaction when their aircraft encounters a vortex in this way.

A cross-track encounter occurs when you fly through the wake vortex at a large angle, perhaps even directly across it. Cross-track encounters usually resulted in very short sharp vertical jolts with little roll or yaw motion. The main concern from the vertical jolts was primarily the structural integrity of the aircraft, rather than loss of control considerations.

Because helicopters tend to fly directly to the ramp instead of following the traffic pattern fixed-wing aircraft do, a light plane is quite likely to fly cross-track to a helicopters path on many occasions, especially close-in, after the helicopter has broken off toward its landing area on the ramp.

The FAAs test pilots entered the helicopters wake vortices during parallel encounters by flying above, below, left and right of the vortex. In general, at small separation distances, the fixed wing aircraft experienced far more severe upsets when the helicopter was flying at slower speeds.

Instead of the anticipated rolling motion as the primary reaction of the airplane to a vortex encounter, the structure of a helicopter wake vortex is much more complex, and typically resulted in pitch and/or yaw excursions of the probe airplane. Closer encounters resulted in loss of control.

At larger separation distances, upsets tended to be more severe at higher airspeeds. The advancing blade vortex is generally more solid and generates more abrupt roll and yaw excursions than the retreating blade vortex. During parallel encounters, the trailing aircraft experienced abrupt roll, yaw or pitch excursions. In documenting the tests, the FAA broke down the results by helicopter type.

The UH-60 Blackhawk is the U.S. Armys most common rotary wing aircraft and is also used by many of the other branches of the armed forces. It is based at many locations throughout the nation, therefore encounters behind the UH-60 are not uncommon.

The Blackhawk has a maximum takeoff weight of 20,250 pounds and has four rotor blades, with a rotor diameter of just over 53 feet. At distances of a mile behind a UH-60 helicopter flying at 70 to 80 knots, the T-34C experienced upset bank angles of 45 degrees, accompanied by turbulence the pilots characterized as hard chops.

When the distance was reduced to a half mile, the upset bank angles increased to 75 to 90 degrees. Thats bad enough, but then consider that a helicopter is usually slowing down as it approaches the airport, perhaps to 40 knots or less. With the helicopter at a slower speed – thus producing a stronger wake vortex – the T-34C was rolled beyond 90 degrees when it flew a mile behind the UH-60. When the T-34C flew a half mile behind the UH-60, it was rolled beyond 180 degrees of bank.

The UH-1 Huey series of helicopters is still very common. Some versions remain in service with the Army National Guard, while many have been sold as surplus into the public sector and are now active as fire-fighting and heli-logging aircraft. The Huey series is a relatively small helicopter, which might lead you to believe that its wake isnt that potent. So what happened to the T-34C behind the Huey? At distances from 0.3 to 0.5 nautical miles, the T-34C experienced rolls between 30 to 75 degrees. However, several of the test points caused much more pronounced roll excursions and led to loss of control and spins.

The U.S. Armys CH-47D Chinook helicopter is a definite heavy-lift helicopter, recognizable by its tandem rotor configuration. Its maximum takeoff weight is 50,000 pounds, and it has a 60-foot rotor diameter. The CH-47Ds wakes led the T-34C to make roll angles of 90 degrees when the helicopter was flying at 120 knots. At distances of less than 0.8 miles, the roll excursions varied from 90 to 210 degrees of bank, and many resulted in loss of control and spins.

The Sikorsky CH-53E Super Stallion is even bigger. It has a maximum takeoff weight of 69,750 pounds and a seven-bladed rotor system with a 79-foot diameter. The CH-53E also produced strong roll excursions. At a mile separation, the trailing aircraft was rolled beyond 90 degrees. At a half mile, the trailing aircraft was rolled to nearly 180 degrees and also put into spins.

Its a Bird, Its a Plane
Several of the flight test runs in the Super Decathlon were abandoned behind the Super Stallion when the Decathlon experienced an unexpected shudder, which the test pilot characterized as flapping of the wings. The tests were immediately abandoned because of concerns about creating a catastrophic wing flutter mode.

Engineers concluded that the vortices of the individual rotor blades were present in the overall wake pattern downstream from the helicopter. Such a pattern of individual vortices was often seen in the smoke patterns. Apparently the individual vortices created the rhythmic pattern.

When you consider that the Super Decathlon is a very capable aerobatic aircraft and that the vortices excited such a reaction in the wing, it should warn those flying average general aviation aircraft to stay away from helicopter wake vortices.

Airport Operations
A typical mid-sized general aviation airport will show the number of light aircraft and helicopters that share the same airspace. There are also a number of general aviation airports that support National Guard helicopter units, and some that are dual civil-military use.

Airports near scenic areas, such as Hawaii or the Grand Canyon, have a large number of helicopters, increasing the probability of helicopters and airplanes operating near each other.

The helicopters at even small airports may be engaged in a variety of missions, which include fire-fighting, EMS, heli-logging, sightseeing, flight training, construction, law enforcement and television news. The military units have their own unique missions, including EMS, transport or attack.

Needless to say, some of these operators can be very busy during certain circumstances, and some of those situations are unpredictable. The bottom line is that the chance of flying near a helicopter operation is fairly high.

Since the wake vortices from a slow helicopter are stronger (initially) and create a larger hazard volume, general aviation pilots would be well-advised to avoid flying close to slow flying helicopters. Unfortunately, the most likely place to encounter slow flying helicopters is when the airplane is also flying slow and close to the ground in the traffic pattern. This is particularly troublesome because helicopter wake vortices tend to contaminate more area when the chopper is flying slowly, especially below 70 to 80 knots.

There are several locations where you should be especially cognizant of helicopter wake vortices. At some airports, the helicopters conduct their takeoff and landing operations to the grass adjacent to the runway. Remember that wake vortices drift with the wind, and helicopter pilots also prefer to land and take off facing into the wind. Its common for a helicopter to land and take off upwind of the landing zone for the runway.

Even a slight crosswind component to the wind can easily blow the helicopters vortex toward the runway, and Murphys Law would say thats probably going to occur somewhere very close to the touchdown zone. The vortices become very problematic for the light fixed-wing pilots doing touch and goes.

Because the airplane is slow and low to the ground, it is very susceptible to upsets. Its low airspeed means the controls are less effective and the low altitude means less time to recover. Its very plausible that some accidents attributed to the pilot failing to maintain control during the landing flare were actually caused by an encounter with a helicopters wake vortex.

Another situation to consider concerns military training at night. Night vision goggles allow military pilots to conduct their missions at night. Its a unique and highly demanding form of flying that requires specialized equipment, training and proficiency. Some units of the military are trained and equipped to conduct this type of operation, and hence must regularly practice.

Commonly they will operate without any external lights on the helicopter. At the airport where Im based, there have been several times when I wouldnt have known that helicopters were operating at the airport if I hadnt been standing on the ramp listening to their rotor blades and watching the blackened silhouettes moving along the taxiway.

Military pilots may be communicating on a different frequency and not be making position reports on the CTAF. Therefore, incoming pilots may have no idea that there are unlit helicopters conducting operations at the airport.

More than a few of my students have been surprised over the years by encountering an unexpected gust on a windless night, only to get out of the aircraft on the ramp and discover that the Guard unit is practicing night flying without lights. Just because it looks dark at the airport and just because no one answers on CTAF doesnt mean that youre the only one in the traffic pattern.

The FAA study suggests that the high hazard of wake vortices from helicopters demands large separation distances when flying behind helicopters. ATC may allow you to give them a wide berth when flying IFR, but when flying VFR the ball is clearly in your court to avoid the unseen danger.

Also With This Article
Click here to view “Trailing a Chopper.”

-by Patrick R. Veillette

Patrick R. Veillette has flown and instructed in airplanes from J-3s to 727s and aerial fire-fighters.


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