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What is Rotor Wash?
Rotor wash is comprised of downwash, which is the vertical component produced by the main rotor blades that supports the helicopter in flight, and sidewash, which is the horizontal component of rotor wash.
The key factors that determine the strength of rotor wash include:
• weight of the helicopter – a heavier helicopter generally produces higher downwash speeds
• main rotor size – a larger rotor disc area will lower the downwash speed, but a larger rotor disc is usually associated with heavier helicopters that produce higher downwash speeds
• disc loading – how much weight is being carried per unit area of the rotor blades
• wind – the relative wind influences the movement of the rotor wash
• flightpath – rotor wash is concentrated underneath the helicopter as illustrated below
Mathematical modelling of rotor wash by Airbus determined the highest velocity of this stream of air occurs from 1 to 3 rotor diameters below the rotor blades directly beneath the helicopter. Beyond this distance the high velocity airflow dissipates due to turbulence. This is demonstrated in the below image of a Sikorsky S92 helicopter.
As the airflow comes under the influence of the terrain or a structure, there is a directional change and acceleration of the airflow as it transitions into sidewash as illustrated below.
Due to helicopter power requirements and the influence from the ground, rotor wash effects are most pronounced during hover, take-off, and landing where it can produce localised wind strengths greater than 100 km/h.
What are the Hazards?
The effect of rotor wash on people has been the subject of several studies, with most agreeing that 80 km/h winds represent conditions that are ‘unsuitable for walking’, and 60 km/h winds are at the ‘threshold of danger’ for the average population. It has also been found that the effect of rotor wash on the more vulnerable groups of the population and in people over the age of 50, roughly half were displaced by a gust of 40 km/h.
In addition to the hazard posed to people, rotor wash can also damage vehicles and equipment by picking up loose articles such as stones, building equipment, signage etc and hurl them in every direction, often with significant force.
Effects at Hospitals
The Australian Safety Bureau released a report titled "Safety risks from rotor wash at hospital landing sites" and found between 2017-2022 there were 9 safety reports related to downwash at hospital landing sites. Of these, 6 resulted in injuries, 3 serious injuries and 3 minor injuries, and 3 resulted in damage to third party property caused by debris. In 5 out of the 6 injury-related events, the pedestrians age was reported to be 75 or greater, and in the 6th event, the pedestrian was reported to be using a walker. Where the locations of injured pedestrians were reported, they were outside the landing zone perimeter fence but within 30 m of the final approach and take-off area (FATO). It should be noted that most incident crews were unaware of the incident at the time of occurrence, and therefore unable to render immediate assistance.
Taupo Aerodrome - Westland Wasp Incident
On 20 November 1992 two Westland Wasp helicopters operated by the NZDF called into Taupo Aerodrome to refuel. Adjacent to the fuelling point were two civil helicopters and an 11m powerboat on a trailer. The aircraft commander was more concerned with the two helicopters than with the powerboat and had undone his harness to look down to provide better visibility of his tail rotor and provide directions to the pilot flying. At this point a large nylon cover, which had been stowed between the boat and its trailer was flung into the air by the rotor wash and was sucked into the main rotor from above. This resulted in the aircraft pitching forward and impacting the ground, causing the gearbox to be sent through the cockpit at seatback height. The commander was saved by having undone his harness and bending down. Other parts of the Wasp were thrown a considerable distance prior to the main fuselage catching fire, with bystanders being lucky to not be hit by any debris. Both pilots onboard escaped without serious injury. TAIC report No. 92-019 has further details of those interested.
Cyclone Gabrielle
During the Cyclone Gabrielle relief efforts in the Gisborne and Hawkes Bay region there were 1369 VFR aircraft movements during the response with 55 helicopters involved. Due to the large number of response personnel, some port-a-loos were set up nearby to the landing zone for ease of access. This was, until a large helicopter flew overhead on approach to landing and uprooted two of them (luckily they were unoccupied at the time) and blew them over.
On another occasion, during an approach to a remote area, the aircraft captain noted that the surface identified for the landing appeared to be loose gravel. The aircraft captain was worried about injuring the personnel he was due to pick up, or damaging their vehicles with loose stones. The pilot elected to conduct a faster and steeper approach to minimize the amount of rotor wash, however a large number of stones were kicked up by the wash, and subsequently shattered the rear window of a vehicle 40 metres away. For subsequent flights, a different pick up location was selected.
How can we mitigate its effects?
There are a number of ways to mitigate the risk of rotor wash, both from a design perspective and "on the ground on the day".
Design
Terrain - We can use terrain to our advantage, having a slope or raised helipad will push the rotor wash down the slope and away from the pad, however we do need to note the helicopters do need a flat area to land on.
Fencing - we can ensure that the safety area has adequate fencing - around hospitals that is not always possible, but temporary gates or security personnel can be used to create a safe perimeter for when helicopters are arriving and departing. Including signage to make people aware of helicopter operations.
Approach Sectors - We can manipulate the course the helicopter takes to get to the landing pad, to avoid flying over people and built-up areas, particularly as they get lower - again, this is not always possible though.
Surface - Long grass can be used to disperse and dampen the ground wash, and having a landing surface free of small rocks etc prevents debris from being kicked up and flung at high velocities into the surrounding area, or into the helicopter itself.
Daily mitigation
To avoid things being flung up by the rotor wash, it is always a good idea to have a check of the landing area, and ensure it is free of debris, and all light materials (such as boat covers) are removed or properly secured. Alerting people in the area of any impending arrivals, by lights, alarms, barriers or just with voice is an easy way to ensure the area is kept clear of unauthorized people and gives an opportunity for those who are more at risk to vacate the area.
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