During flight training every pilot learns the basics of mechanical turbulence, vortices and the inherent dangers of rooftop and pinnacle landings. Yet, accidents and incidents continue to happen on rooftop heliports. While artificially induced wind flow patterns and turbulence are contributing factors, almost all accidents are attributed to pilot error. But how often is the pilot really to blame?
Recent studies conducted in preparation for new rooftop heliports illustrate the significant dangers due to wind flow and turbulence, some of which can be so substantial that even the most experienced pilot could encounter danger.
Chet Wisner, president of Ambient Air Technologies in Fort Collins, Colo., conducts wind tunnel tests for laboratory and healthcare facilities in the United States. His work includes tests for rooftop heliports.
In my work Ive learned one of the major contributing factors to heliport accidents is a failure to properly study the airflow patterns around a building, including airflow on and around a rooftop heliport, Wisner said. There are significantly more forces at play than prevailing wind direction. Yet architects who are not heliport experts may inadvertently design hazards to flight.
A significant example is building parapets. Architects and city planners frequently want parapets in place to mask items on rooftops from view below, making the buildings themselves more visually appealing. These parapets, often designed to the same level of a heliport or higher, create their own turbulence and, depending on the speed of prevailing winds, severe danger.
Wind approaching one face of a building needs to go somewhere, Wisner explains, It goes up, then comes over the parapet, and effectively becomes an eddy with updrafts and downdrafts right on the heliport. And thats just a flat roof with no other obstacles. In most cases when there is a rooftop heliport, there is also an elevator, so you have an elevator shaft itself creating mechanical turbulence in the form of eddies around that structure. Depending on the size of these vortices, you can get your aircraft in a position where one side of the rotor disk is in an updraft and the other in a downdraft.Another issue arises when the rooftop heliport is located in an urban environment with several high rises in the vicinity. The buildings themselves create a venturi, lowering pressure and speeding up prevailing wind as it moves down the street and between buildings. Increased air speed increases eddies and turbulence on and around the pad.
As if that were not enough, add the increased dangers of prevailing winds hitting the building at a 45 degree angle, basically impacting the building on the structures corner, Wisner explains. The turbulence created by this air flow is some of the most insidious.
If youve ever been on a rooftop that has a gravel finish to it, you may have noticed that the gravel has been swept away from the corners, Wisner said. This is the effect of corner vortices. Basically, they are very similar to wingtip vortices and carry a significant downdraft component.
While vortices cant be changed, there are steps architects and heliport designers can take to reduce their effect on a rooftop heliport. Wisner said the most common step is to raise the heliport several feet above the parapet using a steel structure that allows airflow to tumble over the parapet and pass under the heliport.
However, before designing a fix, he suggests the best way to determine hidden inherent dangers and design around them is to conduct a wind tunnel test. At Wisners Colorado facility, he builds a 1:200 scale replica of the building and the surrounding half mile of buildings and terrain (resulting in about 12-by-12-feet of scaled model) and puts it through a series of wind tests, where prevailing winds and wind speeds are simulated with visible smoke added.
Weve tried determining what wind will do with computer simulations, but nothing has been as accurate as the wind tunnel test, Wisner explains. Here you can see exactly what is happening. You can then make adjustments, like raising or relocating the heliport, or repositioning obstacles on the roof, and test again, seeing right there how to reduce those dangerous eddies.
Rex Alexander, a heliport consultant with HeliExperts International and a high-time EMS pilot, agrees with Wisner, he himself having been involved in extensive wind studies regarding other heliports, including the new Lurie Childrens Hospital of Chicago.
There is no data out there on accidents caused by turbulence at heliports due to poor design because over the years these accidents have generally been attributed to pilot error, Alexander said. Unfortunately, there is no requirement for a wind study to be conducted or for any site specific pilot orientation training to be developed to point out inherent wind dangers at a specific location when an organization installs a rooftop heliport. In my opinion, that is often the real cause.
Wisner said the FAAs Heliport Design Guide mentions a wind study should be done, but there is no requirement to do so. In the end, a good wind study will save money when you consider the cost of just one tail boom strike, let alone a fatal accident. Just as importantly, though, is how to educate pilots on the inherent wind at a site. There is also no universal way to share this information, which adds to the problem.
