Article paru sur Vertical Mag :
Helicopters and thunderstorms do not mix.
Even Airwolf, the invincible helicopter from the eponymous 1980s television show, met its demise in part because of a thunderstorm. After the show ended, the modified Bell 222 was sold to a German company and performed helicopter emergency medical services (HEMS) missions. On June 6, 1992, after completing a patient transport, the helicopter was returning to base and crashed during a major thunderstorm, killing its pilot and the two medical personnel on board.
Now, flash ahead to July 27, 2013, and an incident involving a newer helicopter model. In this instance, a Robinson R66 crashed near Noxen, in northeastern Pennsylvania, killing all five people onboard. Thunderstorms are being considered as a major causal factor in this accident.
Widely Known Thunderstorm Threats
To the general public, lightning would seem to present the biggest thunderstorm- related threat to aircraft. In reality, today’s helicopters are designed to withstand lightning strikes while in the air; so, crashes due to lightning alone have largely been eliminated.
However, lightning strikes can still cause significant damage, as seen last August when a Royal New Zealand Air Force, twin-engine, NHIndustries NH90 was hit by lightning. While no one was injured and the crew landed the helicopter safely, all the main and tail rotor blades were badly damaged, as were various structural components. The reported repair/replacement cost was $10 million NZ (approx. $8.5 million US).
It seems that helicopters can even induce lightning strikes. This phenomenon is fairly common in the North Sea in winter. It appears that major winter storms there often generate lightning when helicopters are present: the aircraft collect negative electrical charges on their exterior while flying, thus initiating a lightning discharge when they fly near an area of positively charged clouds.
To maximize flight safety, there are a few points about lightning that pilots should be aware of. First, lightning varies in intensity; low-intensity “bolts” pose little risk, but higher-voltage ones are capable of producing damage. Second, lightning bolts can be several miles long, and can strike outside a thunderstorm cloud at distances of 10 miles (16 kilometers) or more.
The dangers from hail, meanwhile, are more obvious, considering that hailstones can grow to the size of softballs, and potentially fall at rates up to 106 miles per hour. Fortunately, helicopter pilots know that while hailstones are still in the thunderstorm cloud, especially large hailstones, the region they inhabit features winds and turbulence that could destroy an aircraft. As such, accidents due solely to hail are rare.
Of course, unsheltered helicopters on the ground are “sitting ducks.” One hailstorm at Kandahar Airfield in southern Afghanistan on April 23, 2013, badly damaged more than 80 military helicopters.
By far the greatest danger from thunderstorms, though, comes from the winds and turbulence they generate. Thunderstorms are composed of updrafts and downdrafts. Even in moderate storms, vertical velocities can reach 50 m.p.h. Severe thunderstorms can have updrafts and downdrafts exceeding 100 m.p.h. Such turbulent flow can easily cause loss of aircraft control and do structural damage. Even if the aircraft stays intact and remains under control, controlled flight into terrain (the dreaded CFIT) becomes a real possibility.
And, this thunderstorm threat isn’t confined to within the cloud. Strong downdrafts can exist below the cloud base, and when they encounter the earth’s surface, these vertical winds spread out horizontally. Microbursts, normally the strongest downbursts, can produce horizontal wind speeds over 100 m.p.h. (potentially as high as 168 m.p.h.). Worse, the horizontal winds from downbursts can switch direction in an instant, creating dangerous wind shear conditions. And, these outflows can fan out for tens of miles from the parent thunderstorm.
These winds pose an even greater threat to helicopters during takeoffs and landings, including increasing the risk of CFIT accidents.
One such accident occurred on April 26, 2012; it involved a Bell 206B-3 that was attempting to land at Rick Husband Amarillo International Airport in northern Texas. A thunderstorm-induced wind gust, measured at 25 knots (29 m.p.h.), spun the helicopter around 360 degrees, causing it to drop quickly to the ground. The pilot and lone passenger suffered minor injuries, but the helicopter sustained “substantial” damage.
An even worse event occurred on May 25, 2008, when the pilot of an MD 500E was coming in to land at his personal heliport at Sunrise Beach in central Missouri. A thunderstorm downburst resulting in wind gusts estimated at 45 knots caused the helicopter to spin uncontrollably and plummet into a nearby lake. The pilot and two passengers escaped unharmed, but the third passenger suffered serious injuries and the fourth drowned.
Another fatal accident occurred on July 3, 2006. In this instance, a Bell 206B had been ferrying three firefighters from the Nose Mountain tower (about 60 miles southwest of Grande Prairie, Alta.) to a fire site. During departure, the pilot encountered an unexpectedly strong and shifting tailwind caused by thunderstorms that had passed through the area. He corrected the resulting loss of tail rotor effectiveness, but the helicopter did not have enough height to allow for a recovery. One firefighter suffered minor injuries, one was seriously injured and the third was killed.
Lesser-Known Thunderstorm Threats
A less-obvious risk associated with thunderstorms is the rapid change from visual flight rules (VFR) to instrument flight rules (IFR) conditions. Thunderstorms often develop when conditions are initially VFR; but, very quickly, the low clouds and precipitation formed can produce an IFR situation. Even after the thunderstorms die out, the residual low clouds and fog left behind can create hazardous flying conditions.
The Noxen crash mentioned earlier was an example of a VFR flight that suddenly turned dangerous: the pilot encountered inadvertent instrument meteorological conditions, seemingly got confused, then had trouble maintaining control and altitude. The fog in the area delayed search parties for hours afterwards.
Rapid changes in conditions are even more dangerous when they occur during takeoff or landing. In the case of a Eurocopter AS350 B2 near Chibougamau in central Quebec on Sept. 1, 2010, when light rain switched to moderate rain, the pilot decided to make a precautionary landing. Upon approach, the rain began to intensify to the point where, at 70 feet (21 meters) above ground level, the pilot lost all visual reference to the ground. Worse, the pilot didn’t realize the helicopter was descending; it hit trees and then impacted the ground. The pilot and one passenger received serious injuries; two other passengers received minor injuries. A line of thunderstorms moving slower than expected had impacted the crash site.
Acknowledging the dangers of adverse weather, the United States Federal Aviation Administration, on Feb. 21, 2014, issued a final rule requiring EMS, commercial and general helicopter operators to have stricter flight rules and procedures, as well as additional training and equipment (particularly for HEMS operators), for adverse-weather conditions. An avowed goal with the new rule is to reduce the possibility of CFIT accidents. And, one situation specifically mentioned was ensuring Part 135 pilots could properly recover from an “inadvertent encounter with instrument meteorological conditions.”
To truly judge the danger thunderstorms present, it helps to know more about them.
Meteorologists call the common summer-afternoon thunderstorms “air mass thunderstorms.” They’re produced by the ground being heated by the strong summer sun and tend to form randomly. They’re even common throughout the boreal regions of Canada and Alaska (and can be a cause of forest fires). Unfortunately, they’re even more frequent in mountainous terrain, where differential heating and topographically induced winds aid their development.
Although air mass thunderstorms are usually weak, often short-lived and tend to move slowly, or are even stationary, they still pose a serious aviation hazard. Individual storms are impossible to forecast; the best meteorologists can do is to say that “thunderstorms are possible” in an area. These storms can also develop rapidly, in tens of minutes.
The strongest of these disturbances are called “pulse storms.” These not only produce turbulence within the cloud, but intense downdrafts below the cloud base, which are a real threat to low-flying aircraft and often produce strong surface winds that can generate significant wind shear.
At other times, convection can become organized into “mesoscale convective systems.” The most common of these is the squall line, a solid line of thunderstorms often preceding a cold front. In these situations, ambient winds are stronger, hence the thunderstorms move faster. Forward speeds in excess of 50 knots have been noted. These storms magnify the risks to aircraft and can last for hours. Plus, it is very difficult to go around such a system, which can stretch for several hundred miles; even as individual storms die out, new ones are constantly forming.
One special concern is when a section of a squall line starts to bow outward. This is called a “bow echo” and is usually associated with very strong winds. The bow effect is produced when individual thunderstorms in the line begin to move very quickly.
This was the case on March 25, 2010, when a HEMS pilot thought he could beat an approaching squall line and fly back to home base in Brownsville, Tenn., from nearby Jackson. Unfortunately, the squall line began to bow and approached Brownsville at a speed estimated at 61 knots. The helicopter, an AS350 B3, was overtaken by the storms and crashed, killing all three crewmembers onboard.
Although we think of thunderstorms occurring on warm summer afternoons, it’s important to remember that convection can occur at any time of day or year. The famed “lake effect snows” of the Great Lakes region are convective in nature. And, the phenomenon of thundersnow can produce blinding snowfall rates in excess of four inches an hour.
What can helicopter pilots do to lessen the risk from thunderstorms?
Both the U.S. and Canada have aviation weather websites: the Aviation Weather Center (www.aviationweather.gov) and the Aviation Weather website (flightplanning.navcanada.ca), respectively. These sites include not only the standard aviation weather data and forecasts, but also a wide variety of newly developed, computer-generated convective products. Keep in mind, though, science still can’t predict exactly where individual thunderstorms will form.
Once convection has started, it may be observed and reported in Metars or Pireps. And, the best tool we have for locating thunderstorms and making short-term forecasts is weather radar.
How do we use radar to locate thunderstorms? Strong convection produces heavy rainfall with large water droplets. These large droplets reflect more of the radar’s microwaves and show up as a stronger return on the radar screen. On a color display, yellow and (especially) red indicate significant convection. And, since large droplets are formed by strong updrafts, this means severe turbulence is present. Hail reflects even more of the radar beam and may show up as purple, which means even stronger updrafts and turbulence.
Another thing weather radar can do is to scan thunderstorms vertically. This can give pilots an idea of cloud-top height, which is also related to storm intensity.
Short-term forecasting of thunderstorm movement using radar is simply continuity: observe a storm long enough to determine its speed and direction, then extrapolate that into the future.
In closing, there is one more incident we should examine. On Sept. 25, 2009, the pilot of an AS350 B2 being used for HEMS missions was attempting to fly into Conway-Horry County Airport (near Myrtle Beach, S.C.). Although he knew thunderstorms were expected, he elected to make the trip anyway. The helicopter crashed, killing the pilot and two medical personnel onboard. Weather radar indicated thunderstorms were close to the flight route at the time of the crash. And, the flight recorder showed the helicopter had been flying at 1,000 feet or lower for quite some time, suggesting the pilot was trying to stay under the clouds.
While other factors played a role in many of the accidents cited, the tragic fact seems to be that each pilot might not have fully understood the dangers thunderstorms presented — and likely underestimated the risk associated with them.