New insights into how to avoid Vortex Ring State in its earliest stages.
AIRCRAFT
Helicopters are particularly vulnerable to Vortex Ring State (VRS) during low-speed operations out-of-ground-effect. This could include operations such as external load work, power line inspection, rescue hoist operations and mountain approaches. Yet until recently, effective training to show pilots how to avoid VRS has been lacking.
The Vuichard Recovery Aviation Safety Foundation has released a new safety video on "How to avoid a Vortex Ring State in helicopter operations." This is a “must see” educational video for helicopter pilots that reveals new information for avoiding VRS as well as detecting a VRS in its earliest stages. There is a wealth of information in this video that is necessary for pilots, flight instructors, training program designers, human factors specialists and accident investigators to understand.
How good is this video? I watched it five times, vigorously taking notes each time. Throughout this learning process I called friends who have flown helicopters dating back to the Vietnam Era, pondering how this information eluded the industry for 50+ years. Nearly every aspect of this video contains information that has not been disseminated (nor properly explained) in helicopter training materials.
Loft Dynamics (formerly VRM Switzerland) arranged an in-person visit with Capt. Claude Vuichard in Zürich and generously provided usage of the H-125 flight training device. Vuichard demonstrated many of the lesson points while executing landings to sharp pinnacles and hovering in front of the famous North Face of the Eiger. Vuichard’s recovery resulted in a rapid recovery from the VRS with a minimal loss of altitude.
Trim StringArguably the most informative feature of this educational video is the role of the trim string or “woolometer.” It is an important early warning sensor to avoid VRS. The behavior of the trim string is directly indicative of the airflow condition over the nose. It hangs straight down during hover and low speed flight due to the rotorwash’s prevailing downward motion. As a helicopter begins to pick up speed, the trim string rises. At 10 kts. of headwind, the trim string will deflect stably straight in the low disk loading category. It takes 20 kts. of headwind in a medium disk loading because the induced velocity from the rotor disk is stronger. Once the trim string rises steadily, the helicopter is outside of the VRS envelope.
The video has excellent illustrations of the trim string during normal operations and as a warning device to remain outside of the VRS envelope. Clearly this simple device has been under-appreciated, and yet it reveals accurate information of a helicopter’s airflow condition, especially at slow speeds when the pitot tube is inherently unreliable. During an approach it can indicate the influence of a crosswind.
As the helicopter slows perilously close to the edge of the VRS envelope, the trim string transitions from standing straight out to a random set of “nervous twitches.” This indication occurs prior to the other classic warning signs from the helicopter. This advanced warning can be invaluable, especially when the helicopter is operating close to the ground.
Several useful aspects of the trim string became apparent during our hands-on experience in Loft Dynamic’s H-125 flight training device in Zürich. The trim indicator is ideally situated in a pilot’s visual scan during approach and landing. It is located right on the nose and is easily viewed without the need for the pilot to rotate his or her eyes or head to view it. It is also far enough away from the eyes that an aging pilot who is slower at adjusting to far versus near images won’t have difficulty keeping the trim indicator in the visual scan.
It was also interesting to observe during our hands-on session that the VR technology simulated the changes in the trim string’s behavior throughout the flight envelope. This was evident while Vuichard demonstrated approaches to pinnacles, hovering in front of cliff faces and encounters with sudden downdrafts. This is a milestone in simulation technology, which enables systematic training to avoid and recover from VRS in different flight phases.
Vuichard strongly recommends that all rotorcraft manufacturers equip their helicopters with trim strings.
The trim string hangs straight down during hover and low speed flight due to the rotorwash’s prevailing downward motion. This provides vital information to a pilot of the proximity to the VRS envelope. Credit: Vuichard Recovery Aviation Safety Foundation
Other Warning SignsAnother significant revelation is the sudden low-g flight condition (0.2-0.8 g) at the initial entry into VRS. Under the mildest conditions, a pilot will feel a lightness in the seat. This is likely the downward acceleration that a pilot encounters when practicing VRS recoveries under the carefully controlled conditions during training.
In contrast, the real world often doesn’t mimic the carefully controlled conditions of the training environment. An inadvertent entry into VRS often results in an abrupt downward acceleration. I had not been trained in the Vuichard Recovery so I sought out a flight school that said it was qualified to teach this technique. The flight instructor in a Robinson R22 stated, “Let me show you a REAL vortex ring state.” Suddenly it felt as if we were in an elevator whose suspension cables had been cut. The abrupt downward acceleration was unlike any sensation I had ever experienced in an aircraft before. None of the ground training devices used for spatial disorientation training such as a Barony Chair simulate this abrupt downward acceleration.
One positive aspect of this horribly botched “training maneuver” was that it helped me realize the significant human factors with VRS. In a terrestrial environment under 1g, the human body is well adapted to maintain the perception of the correct orientation provided by the body’s visual, vestibular and proprioceptive systems. Conversely, during flight, the sensory systems are poorly suited to these abnormal accelerations and can easily trick the body’s balance and motor mechanisms. This abrupt downward acceleration opens up a Pandora’s Box of visual, vestibular and proprioceptive illusions that can hinder a pilot’s ability to make a timely recovery.
Other symptoms of a VRS are the lack of response when increasing power by pulling the collective, and the helicopter exhibits random uncontrolled pitch, roll and yaw oscillations. As soon as the helicopter transition from a hover into a descent, the blades will hit the vortices of the preceding blades, which causes vibration that can be felt and heard in the helicopter. The cyclic will shake and has less control authority.
A fully developed VRS is signified by a reduction in the vibration level of the main rotor when the induced flow circulates in a closed circuit and the rotor blades encounter less turbulent air. This will also be accompanied by a low frequency of random buffeting.
A helicopter is susceptible to VRS when the airspeed range occurs from any direction. For helicopters in the low-disk loading category, the airspeed range of susceptibility to VRS is between 0-10 kts. in combination with a descent rate exceeding 500 fpm.Credit: Vuichard Recovery Aviation Safety Foundation
Descent RateThe combination of airspeed and descent rates that form the VRS envelope are specific to each helicopter make/model. In general, helicopters with higher disk loading will produce higher induced velocities. Therefore, the envelope of airspeed versus rate of descent (ROD) will generally be larger, and secondly, the descent rate envelope for VRS will begin at a faster rate.
The video compares and contrasts the VRS entry parameters for three categories of disk loading. The R-22, which has a disk load of approximately 4 psf (pounds per square ft.), is in the low disk loading category. The H125, with a disk loading between 4-6 psf, falls within the medium category, and the AW139 fits the high disk load category with 6-10 psf.
For example, at sea level at the maximum takeoff weight, the range of descent rates for entry into the VRS envelope in the R-22 are 500-2,000 fpm. The H125’s range is 800-3,000 fpm. The heavily disk loaded AW139’s range is 1,200-4,000 fpm.
A vital revelation is the immense descent rate that occurs when a helicopter enters into a full VRS. At sea level and the maximum takeoff weight, an R22’s descent rate in a fully enveloped VRS would be 3,600 fpm. This equates with losing six building floors in a second.
By the way, the R22 flight instructor’s “recovery” wasn’t effective until we were less than 300 ft. above the ground. Ironically I had asked this flight school to teach me the Vuichard Recovery because I had been trained long ago in the conventional recovery technique. That experience certainly calls into question how a flight school can deem itself “specially qualified” to teach this recovery technique. That demonstration placed us in a dangerous situation and resulted in negative training.
An R22’s descent rate in a fully enveloped VRS would be 3,600 fpm. This equates with losing 6 building floors in a second.Credit: Vuichard Recovery Aviation Safety Foundation
This is yet another point that illustrates the safety and efficiency afforded by proper training in a flight training device whose software models the aerodynamics of the specific helicopter model. It allows students to safely practice these reflexes in repetitions to formulate the correct and nearly instantaneous control inputs in the incipient stage. The simulator can fly the helicopter into a VRS condition automatically to enable the pilot to practice recovering from VRS in a more efficient way.
The heavier disk loading in an H125 results in a final descent rate of 4,900 fpm, which is the equivalent of losing eight floors per second. This equates to 53 mph. An AW139 with an even heavier disk loading would plummet out of the sky at 6,600 fpm, which is the equivalent of losing 11 floors per second or a speed of 74 mph. In basic terms, impacting the ground at these speeds is not likely survivable.
The descent rates worsen with altitude. At a density altitude of 13,000 feet, the H125 plummets at a speed of 67 mph (equivalent of 10 floors in a second). The AW139 would plummet at 94 mph (14 floors in a second).
The emphasis in this section of the video is absolutely clear. Due to the tremendous sink rates in the VRS, it is essential to reflexively apply a recovery technique because every second lost results in an enormous loss of altitude. When practicing, recovery should be initiated at the first sign (lightness in the seat) in the incipient stage of the vortex ring.
VRS EnvelopeThis video highlights a helicopter’s susceptibility to VRS when the airspeed range occurs from any direction. For helicopters in the low disk loading category, the airspeed range of susceptibility to VRS is 0-10 kts. For medium disk loaded helicopters, the range is between 0-20 kts. For high disk loaded helicopters, the range expands from 0-30 kts.
Remaining out of the VRS envelope depends on a pilot’s accurate assessment of the wind direction. There are many reasons why this isn’t as simple as it sounds. This author has cited in previous articles the potentially misleading indications from a wind sock, especially when the wind sock is located where a prevailing wind can cause recirculation zones and shed vortices. Both of these phenomena can cause the wind sock to deflect in a different direction from the prevailing wind flow. Landing approaches in the vicinity of obstacles (buildings, trees, mountain ridges, etc) can expose a helicopter to a micro-climate of vastly changing winds in a short distance.
Capt. Vuichard provided a short segment from the educational video illustrating this behavior which is available only to our readers. It can be access through this link: https://www.youtube.com/watch?v=zrrvkfPtyCs Credit: Vuichard Recovery Aviation Safety Foundation
Is there a sensor that can provide immediate information to a pilot regarding the airflow condition over the nose of the helicopter in a tailwind? This is yet another reason in which the trim string appears to be invaluable.
Vuichard illustrates three ways to enter the VRS envelope. One method is by reducing airspeed at a constant rate of descent, similar to a landing approach. A normal powered descend with a high rate of descent and low indicated airspeed is one of the classic situations leading to VRS.
Another entry begins from above, by increasing the rate of descent into the envelope. This can occur when a helicopter can’t maintain a hover out of ground effect (HOGE) due to lack of power, or during wildfire suppression efforts when firefighting helicopters are forced to dip their bambi buckets within the tight confines of steep gorges and nearby electrical power cables that are suspended across the canyon.
A third method occurs during a decrease in the rate of descent from a very low power approach or a low-speed autorotation. The final phase of an autorotative landing to power recovery is a classic scenario in which a helicopter is especially close to the VRS envelope. VRS also is a big risk during a downwind approach or downwind quickstop, and a flare in combination with height loss.
After watching this video, the warnings in the Robinson flight manual make much more sense. Rather than merely memorizing the limitation, this video helps a pilot understand how to apply this limitation to real world flying. It is now abundantly clear why it is important to keep descent rate less than 300 fpm when slower than 30 kts.
Next StepsMany of helicopter aviation’s elite have fallen victim to VRS, the invisible phenomenon. Clearly much more needs to be understood about the human factors involved with it. The prevention and recovery from VRS depends heavily on our sensory, perceptual, memory and motor systems. Unfortunately the role of these human systems haven’t been addressed in any meaningful way in the training literature on VRS. Even though Vuichard’s recovery demonstrations look simple and elegant, there is plenty of potential for negative habit transfer by unqualified instructors or poor instructional methods, such as what I experienced in the R22.
Simply tossing a student into the seat and saying, “Just apply more collective, apply right cycle (for a helicopter with a counter-rotating main rotor) and left pedal” is a woefully inadequate and uninformed teaching approach. The instruction for upset recovery requires a carefully designed step-by-step approach. This will be especially challenging for those of us “old dogs” who were taught the conventional recovery technique long ago. There is insufficient science-based data to reveal how much practice is necessary to un-do habit patterns learned long ago.
In the interim, EASA is contracting with France’s Office National d'Etudes et de Recherches Aérospatiales (ONERA) to conduct an in-depth study to determine the flight conditions at which the VRS starts to develop, as well as to evaluate the effectiveness of the Vuichard recovery technique for at least two different types of helicopters. In addition, the FAA’s Flight Test Center is conducting a study on the efficiency of the different vortex recovery methods.
Vuichard emphasizes that “At the end of these studies, the industry must be advised to instruct only one method that really works under all operating conditions. It is absolutely impossible for a human to reflexively apply more than one procedure when feeling a low g at low speed mostly in a critical flight phase.”
—Upon his retirement as a non-routine flight operations captain from a fractional operator in 2015, Dr. Veillette had accumulated more than 20,000 hours of flight experience in 240 types of aircraft, from balloons, rotorcraft, sea planes, gliders, war birds, supersonic jets and large commercial transports. He is an adjunct professor at Utah Valley University.
