All aircraft classifications have operating limitations subject to the complex effects of weather phenomena, but helicopters are particularly susceptible to certain operating risks that arise from aerodynamic characteristics, flight environments, mission profiles and other factors different from those in the fixed-wing community.
Air movement (wind and turbulence), air density and degraded visibility are of critical concern for helicopter pilots. Even the airflow generated by the helicopters themselves can be problematic. In addition, many helicopter missions require flying at low levels, which increases the risk of collision with objects or the ground.
According to the U.S. Federal Aviation Administration’s (FAA’s) Helicopter Flying Handbook,1 “Wind direction and velocity affect hovering, takeoff and climb performance.” FAA characterizes wind in this handbook as one of “the three major factors that affect performance.” The other factors are air density and weight of the aircraft.
Translational lift, for example, occurs when there is a secondary relative airflow over the rotor disk. In addition to the forward motion of a helicopter in flight, translational lift can be produced by a surface head wind, and the efficiency of a hovering rotor system increases with the speed of the incoming wind. Crosswinds and tail winds, however, can decrease lift, meaning that more power is needed for takeoff or hovering. In these situations, takeoffs involving horizontal flight occur at a shallower angle than when the wind conditions are absent, which could be dangerous if obstacles are in the flight path. This means that, in some situations, wind direction can be a critical safety factor.
Another problem — one unique to flying helicopters — is loss of tail rotor effectiveness (LTE). The purpose of a tail rotor in a helicopter with one main rotor is to neutralize the torque created by the main rotor. If the tail rotor’s performance is impeded, it can lead to an uncontrollable spin (rotation of the fuselage). The magnitude of the LTE and difficulty of landing can be significantly affected by the speed and direction of the wind.
FAA notes in the handbook, “An effective tail rotor relies on a stable and relatively undisturbed airflow.” Anything that can disrupt airflow into the tail rotor can cause problems. This would include variable, gusty winds and turbulence.
Wind direction also is important. For example, winds from approximately 60 degrees left of the nose (10 o’clock) can blow the main rotor vortex (swirling air generated by the helicopter downwash) into the tail rotor, greatly increasing the turbulence of the airflow. Any significant tail wind can decrease tail rotor effectiveness. This decrease may be due to a loss of translational lift, requiring more main rotor thrust and generating more torque than the tail rotor can counteract. The aircraft may also begin to weathervane with its nose into the relative wind. This can also cause an accelerating spin.
A typical LTE accident occurred on May 29, 2013, north of Fort McMurray, Alberta, Canada. The pilot of a Bell 206B was conducting wildlife survey work with two biologists aboard the helicopter. While slowing for an attempted landing, the helicopter began to spin to the right. The pilot lost control of the aircraft, and it crashed into the woods, killing the pilot and one passenger and seriously injuring the other passenger. The Transportation Safety Board of Canada concluded, “The helicopter was operating in a flight regime where it was exposed to the left crosswind and tail wind, which would have placed the relative wind into the critical azimuth zone. The helicopter experienced LTE, causing a loss of directional control at a height above the trees that precluded an effective recovery.”2
Another high-risk, potentially weather-related situation for a helicopter in flight is mast bumping (i.e., contact between the inboard end of a main rotor blade or the rotor hub and the main rotor drive shaft), which is caused by excessive flapping of the rotor blades. Mast bumping can cause significant damage to the main rotor assembly and may cause it to detach from the aircraft. The rotor may strike the fuselage. A number of fatal accidents have been attributed to mast bumping — which some data indicate may be more likely in some two-bladed–rotor designs. This occurs under “low-g” flight conditions (relative to 1 g standard gravitational acceleration), such as in a weightless state. Besides mechanically or pilot-induced situations, low g also occurs with vertical turbulence.
For example, on March 9, 2013, a Robinson R66 crashed in the Kaweka Range on New Zealand’s North Island. The New Zealand Transport Accident Investigation Commission (TAIC) determined that the main rotor blade struck the fuselage, causing a midair breakup. The pilot was killed in the crash. According to the TAIC’s final accident report, “The mast bump very likely occurred when the helicopter encountered moderate or greater turbulence, which likely resulted in a condition of low g. The effect of any turbulence would have been exacerbated by the helicopter’s light weight and estimated airspeed of 115 kt.”3
Low-level flying means that helicopters are likely to encounter what meteorologists call mechanical turbulence, which is generated when the prevailing wind encounters obstacles. An urban area with tall buildings is such a place.
Turbulence also is common in mountainous regions, and helicopters being flown in firefighting, search and rescue, and sightseeing are especially at risk.
For example, the pilot of a Robinson R44 II was flying close to a ridge line near Carcross, Yukon, Canada, on July 10, 2012. A strong wind gust pushed the helicopter into a leeside mountain wave. A rapid, uncontrollable descent resulted in a crash that killed the pilot and injured the two passengers.4
When a helicopter is in contact with the surface during takeoff or landing, it is susceptible to dynamic rollover, a lateral roll of the aircraft. An initial tilt angle of five to eight degrees can be enough to initiate the sequence of events that will lead to the rollover. At this range of tilt angle or greater, the main rotor thrust produces a strong moment rolling the aircraft, and the pilot often cannot overcome that force with the flight controls. One cause of dynamic rollover is a strong crosswind, which can give the initial sideways push. Takeoffs and landings on sloping terrain are even more problematic — with upslope winds and tail winds increasing the risk of rollover.
Adequate visibility is essential to unaided visual helicopter operations. A degraded visual environment increases the risk of collisions with buildings, towers, power lines and the terrain itself, especially in mountainous regions. Instrument meteorological conditions (IMC) due to fog or clouds are a major cause of a degraded visual environment. For example, an Agusta Westland AW109 struck a construction crane in dense fog on Jan. 16, 2013, in Vauxhall, London, England. The aircraft crashed into a building below, killing the pilot and one person on the ground (ASW, 11/14, p. 32).
It is difficult sometimes to avoid, in a discussion of weather-related rotorcraft flying risks, issues that also occur in airplane flight operations. Spatial disorientation, as one case, counts as a very real possibility for helicopter pilots and is often followed by loss of control–in flight (LOC-I). The crash of a Louisiana Army National Guard Sikorsky UH-60 Black Hawk helicopter, which killed 11 people on March 10, 2015, in Santa Rosa Sound, Florida, U.S., was attributed to spatial disorientation of the pilots due to thick fog.5
So was the crash of an Agusta Westland AW139 on March 13, 2014, in Norfolk, England, which killed four occupants.6 Five occupants were killed when a Robinson R66 crashed in Noxen, Pennsylvania, U.S., on July 27, 2013. The pilot of the visual flight rules (VFR) flight inadvertently entered IMC. Spatial disorientation was reported as the probable cause of the LOC-I accident.7
According to a report published in June 2015 and prepared through cooperation of the Helicopter Association International, American Helicopter Society International, the General Aviation Manufacturers Association and the Aircraft Electronics Association, “Over the period of 2001 to 2013, for single-engine helicopters worldwide, there were 194 accidents related to IMC or CFIT [controlled flight into terrain] due to low-level flight to avoid weather, with 133 of these accidents involving fatalities and 326 people [who] lost their lives. None of these rotorcraft were [equipped with instruments required for instrument flight rules (IFR) flight]. In fact, IFR-certified single-engine rotorcraft are virtually nonexistent in the current fielded fleet. For multi-engine [rotorcraft] (over the same period), 54 accidents were related to IFR [operation], IMC or CFIT due to low-level flight to avoid weather, with 46 of these accidents involving fatalities.”8
Besides operation in clouds and fog during landing or other maneuvers close to the ground, the helicopter accident record shows that heavy rainfall can significantly reduce visibility. Even worse is the risk during heavy snowfall, which can produce whiteout conditions with visibility near zero. In drier climates, dust or sand storms also can reduce visibility, producing so-called brownout conditions.
As noted, FAA calls air density a “major factor” in determining safety margins in helicopter performance. High density altitude (HDA) — unusually “thin” (low-density) air due to the combination of hot temperatures and/or high altitude — can affect all aircraft, but once again, helicopters are even more susceptible than airplanes to severe problems.
The lift generated by the main rotor and the torque generated by the tail rotor (in single-rotor helicopters) are decreased. Takeoffs can be difficult. Risk of LOC-I is a strong possibility. Checking the actual HDA values in advance is critical for pilots in these situations. Loads must be carefully monitored and managed.
The normal ability to autorotate and to land safely in emergency situations also is compromised. Depending on the gross weight of the aircraft, the vertical descent rate reduction of an autorotation may not be enough to safely land. Another problem with HDA can be low rotor rpm that leads to main rotor blade stall. With HDA, the power output of the engine can be significantly reduced. This can lead to low rotor rpm even with maximum throttle being applied by the pilot.
Lift generation can be greatly reduced, and the risk of rotor blade stall may increase greatly. HDA can also affect tail rotor aerodynamic performance. Thrust and efficiency are reduced by the thin air. LTE can occur especially with heavy loads. For example, a Bell 407 entered an uncontrollable spin and crashed on Feb. 15, 2012, in Moran Junction, Wyoming, U.S., while on a search and rescue mission. The crash resulted in one fatality and two serious injuries. The U.S. National Transportation Safety Board’s (NTSB’s) final accident report cited as the probable cause “the pilot’s failure to maintain yaw control while hovering at high density altitude, which resulted in a loss of tail rotor effectiveness.”9
With the main rotor of a helicopter displacing so much air, this factor alone can produce safety problems with weather-like characteristics. Since helicopters often land vertically, they can be exposed to risks from their own downwash. Descending to a surface area composed of fine dirt or sand can produce a cloud of dust that in seconds can reduce visibility to near zero. A similar situation can occur if the landing area has a loose snow cover.
Another problem discussed in AeroSafety World articles through the years has been helicopter self-generated airflow that can produce vortex ring state, sometimes called settling with power. When a helicopter descends into its own downwash, unusually large vortices can develop around the main rotor blades. These vortices dissipate the lifting capacity of the main rotor. The aircraft will continue to descend regardless of power applied, often to a rate that can approach 6,000 fpm/1,829 mpm. The low airspeed called translational velocity often initiates the cycle. Vortex ring state was cited as a probable cause in the crash into the North Sea of an Airbus Helicopters AS332 L2 Super Puma, which resulted in four fatalities and four serious injuries on Aug. 23, 2013 (ASW, 7/16, p. 24).
One sector of the aviation industry that has responded to accident rates with many new risk mitigations in recent years, as reported in ASW, is helicopter emergency medical services (HEMS). In terms of weather-related HEMS accidents, the FAA and the University Corporation for Atmospheric Research (UCAR) held a weather summit in Boulder, Colorado, U.S., in March 2006. Two major results of this initiative were analysis of the related weather concerns and new mitigations specifically dealing with HEMS operations (most recently addressed in FAA Advisory Circular (AC) 135-14B, “Helicopter Air Ambulance Operations”)10 and the Helicopter Emergency Medical Services Tool <www.aviationweather.gov/hemst)>.
The AC highlights the threats of IMC — especially inadvertent VFR flight into IMC and other degraded-visibility issues, including whiteouts and brownouts. It also emphasizes the benefits of a preflight risk analysis, which specifically includes all relevant factors from appropriate sources of current and forecast weather.
Critical elements in the AC include awareness of “ceiling, visibility, precipitation, surface winds, winds aloft, potential for ground fog (especially for off-airport operations), and severe weather such as thunderstorms and icing. These factors should be considered for the departure point, en route, and primary destination and contingency routes/diversion landing facilities.” The HEMS Tool, as described by the U.S. National Weather Service is “a graphical flight planning tool for ceiling and visibility assessment along direct flights in areas with limited available surface observations capability. It improves the quality of go/no-go decisions for air ambulance operators.”
Edward Brotak, Ph.D., retired in May 2007 after 25 years as a professor and program director in the Department of Atmospheric Sciences at the University of North Carolina Asheville.
- FAA. Helicopter Flying Handbook. FAA-H-8083-21A, 2012.
- TSB. Aviation Investigation Report A13W0070, Loss of Tail Rotor Effectiveness and Collision With Terrain, Aurora Helicopters Ltd. (dba Wood Buffalo Helicopters), Bell 206 C-FZWB, Fort McMurray, Alberta, 75 nm N, 29 May 2013. 2013.
- TAIC. Investigation 13-003, Occurrence Report Details, Robinson R66, ZK-IHU, Mast Bump and In-Flight Break-up, Kaweka Range, 9 March 2013.
- TSB. Aviation Investigation Report A12W0088, Loss of Control and Collision With Terrain; Horizon Helicopters Ltd., Robinson Helicopter R44 II, C-GHZN; Carcross, Yukon, 5NM E; 10 July 2012.
- Aviation Safety Network. ASN Wikibase Occurrence #174422. <https://aviation-safety.net>.
- U.K. Air Accidents Investigation Branch. AAIB Investigation to Agusta Westland AW139, G-LBAL. < www.gov.uk/aaib-reports>.
- NTSB. Accident Report No. ERA13FA336, July 27, 2013. <www.ntsb.gov>.
- International Helicopter Safety Team. “14 CFR 27 Single-Engine IFR Certification Proposal.” Helicopter Association and Industry Whitepaper, June 2015.
- NTSB. Accident Report No. WPR12GA106. Feb. 15, 2012. <www.ntsb.gov>.
- FAA. AC 135-14B, “Helicopter Air Ambulance Operations.” March 26, 2015.