Rotorcraft advocates believe that helicopters have a distinct advantage over any airplane in that when the engine stops providing enough power to keep the aircraft airborne, it can glide to a safe landing even in confined spaces and in almost any condition. However, this assumes the pilot has initiated the proper procedures for entry into the autorotation and does so in a timely manner. Achieving both actions is critical, as the accident statistics confirm. The Joint Helicopter Safety Analysis Team (JHSAT), a group of industry and government safety experts charged with reducing rotorcraft accidents worldwide, reviewed all NTSB data on helicopter accidents for the calendar years 2000, 2001 and 2006. Of the 523 accidents analyzed, nearly one-third (32%) involved autorotation.

An autorotation consists of four general phases: entry, established glide, flare and landing. Ray Prouty, author of “Helicopter Aerodynamics” and former chief of Flying Qualities Analysis for McDonnell Douglas Helicopter, emphasizes, “Failure to make a good entry into autorotation after the engine stops is one of the primary causes of helicopter accidents. The key to making a good entry is to keep the rotor speed up.”

Time is a critical element for several reasons. Not reacting quickly enough allows rotor rpm to drop below a recoverable level. All certified helicopters have an estimate of how much time a pilot has to react to power loss, lower the collective and begin entry procedures. Some aircraft may require action in less than a few seconds before losing minimal rpm for recovery.

This rapid loss of rotor rpm was tragically illustrated Aug. 26, 2011, near Mosby, Missouri, when a Eurocopter AS350 B2 on an emergency medical mission crashed, killing the pilot, a former military aviator, along with the flight nurse, flight paramedic and patient.

The revelations from the NTSB accident investigation clearly demonstrate systemic oversights in training, flight manual instructions, FAA educational materials and check ride standards on a critical emergency that is unforgiving of anything but accurate and instantaneous control inputs.

Here are some key facts in the accident chain. Even though the helicopter had only about 30 min. of fuel remaining and the closest fueling station along the route of flight was at an airport about 30 min. away, the pilot elected to continue the mission. The helicopter ran out of fuel and the engine lost power at 300 ft. AGL and within sight of the airport. The pilot failed to make the flight control inputs necessary to enter an autorotation, and the crash occurred just 10 sec. after the flameout. The helicopter struck the ground approximately 40 deg. nose low; the crash was not survivable.

The accident site was a wide open field, which should have been an ideal emergency landing site. A key observation by the initial responding investigators was that the rotor blades exhibited minimal rotational energy at impact. So, how did an autorotation to an open field end so tragically?

Let’s look at the FAA’s training information on autorotations. During cruise flight the main rotor is substantially tilted forward, so the flow of air to the rotor is coming down through the blades and above the direction of travel. (This is depicted in Figure 3-20 on page 3-9 of the FAA’s “Rotorcraft Flying Handbook.”) By contrast, during autorotation the air flows up through the main rotor of the rapidly descending helicopter. The FAA handbook notes that when reacting to an engine failure, the pilot should immediately lower the collective, thereby reducing lift and drag on the main rotor blades and initiating an immediate descent. The upward flow of air through the main rotor system provides sufficient thrust to maintain rotor rpm throughout the descent.

The AS350 Autorotative Procedure described in the aircraft’s flight manual instructs the pilot to set low collective pitch, monitor and control rotor rpm, and establish an airspeed of approximately 65 kt. Neither the FAA’s handbook nor the AS350 flight manual addresses the matter of simultaneous control inputs or use of aft cyclic to prevent rapid loss of rotor rpm.

Peter Gillies, chief pilot of Western Helicopters, gave several presentations at Heli-Expo 2013 on this topic. He emphasized the importance of bringing the cyclic back instantly since that positions the rotor tip path plane properly to drive the rotor for an autorotation. Gillies pointed out that the side effect of lowering the collective alone and abruptly during the entry into an autorotation actually tilts the rotor tip path plane forward/downward.

Prouty notes, “The pilot can take advantage of the kinetic energy associated with forward speed by doing a mild cyclic flare before lowering the collective pitch. This puts the rotor into a nose-up attitude that reduces the decelerating torque and maintains thrust and altitude until the forward speed is decreased to the best autorotational speed.”

The NTSB’s re-creation of the accident in a flight simulator correlates well with these experts’ opinions. The flameout occurred while the aircraft was flying at 115 kt. When the pilot reacted to the simulated flameout with simultaneous aft cyclic, down collective and left pedal (the AS350’s main rotor rotates clockwise), the helicopter successfully entered into a stabilized autorotation and landed in 27 sec.

In contrast, when the simulator scenario attempted an autorotation entry with just the collective and no cyclic under the same entry conditions, time to impact was less than 5 sec. And no, under those circumstances, the impact was not survivable.

Gillies urged his audience to ignore taking the time to troubleshoot under such circumstances because there is little to be gained and plenty to be lost if the rotor rpm is allowed to slip out below the green band even a small amount. “You’re going to die,” he warned emphatically. ”There is no way to recover the rotor rpm without an engine.”

The NTSB’s video and report on the accident provide a clear, important lesson about the proper control inputs during the entry into an autorotation under those conditions in that type of helicopter. Aft cyclic is critical to maintaining and regaining rpm at high cruise airspeeds. Simultaneous collective, cyclic and pedal inputs are needed. Additionally, these reactions are required within about 2 sec. to maintain rotor rpm.

Prouty also notes different rotor systems react differently to sudden power loss. “If the rotor has high inertia because of heavy blades or tip weights,” he stated, “the rpm will fall off slower than if it were a lightweight rotor. The flight condition at the time of engine failure will also affect the rate of decay.”

The NTSB’s review of helicopter training resources suggested that the accident pilot may not have been aware of the specific control inputs needed to successfully enter an autorotation at cruise speed. Without such guidance, pilots of helicopters with low inertia rotor systems may not know that aft cyclic must be applied when collective is lowered to maintain control of the helicopter and perform a successful autorotation.

When previously learned habit patterns are inappropriate for the current aircraft, environment or procedures, the condition is “negative habit transfer.” Research by the University of Manchester’s Dr. James Reason, world renowned expert on human error, found that negative habit transfer increases the chance of human error fivefold. Unfortunately, the limited reaction time available in the event of an engine failure means there’s none left to correct an error if the pilot screws up the entry into an autorotation.

Fatigue clearly was a factor in the accident as well. According to the NTSB’s human performance analysis by William Bramble, Ph.D., the pilot had slept five or fewer hours the previous night and felt tired. Moreover, he had a cumulative duty time of over 12 hr. and time-since-waking of over 13 hr. at the time of the accident. Bramble notes that this type of fatigue could explain attention lapses, continuing ineffective solutions and delayed reaction times.

There is, however, another important trait of fatigue that is especially pertinent to this discussion. A fatigued pilot when confronted with a startling situation is more likely to automatically revert to previous habits. Since the accident pilot had been trained as a U.S. Army aviator and was an Iraq veteran, it’s enlightening to review the emergency procedures for the twin-engine, two-pilot, 22,000 lb. (max gross weight) UH-60 Black Hawk, that service’s most common rotorcraft, and one whose rotor system turned counter-clockwise, as do most American-made helicopters.

Section 9.12 of the aircraft handbook states, “If both engines fail, immediate action is required to make a safe autorotative descent. The altitude and airspeed at which a two-engine failure occurs will dictate the action to be taken. After the failure, main rotor rpm will decay rapidly and the aircraft will yaw to the left. Unless a two-engine failure occurs near the ground, it is mandatory that autorotation be established immediately. During cruise, reduce collective immediately to regain main rotor rpm and then adjust as required to maintain main rotor rpm within power-off rotor speed limits. The cyclic should be added as necessary to attain and maintain the desired airspeed.”

When you think back to the rigors of military training and the frequent check rides, these responses would have become ingrained in the accident pilot’s reflexes. Now consider that as a civilian he was flying as a single pilot in a single-engine helicopter with a 4,960-lb. gross weight and low inertia rotor system that turned clockwise. The differences between the two helicopters are vast, and that’s where a company’s training program must be sufficient to help transition a pilot.

This is where another major link in the error chain occurred. According to the NTSB’s findings, the autorotation training that the pilot received in the AS350 B2 was not representative of an actual engine failure at cruise airspeed. Rather the autorotation training was done at airspeeds below cruise where less aft cyclic is needed to enter an autorotation. The NTSB believes this likely contributed to the pilot’s failure to execute a successful autorotation.

The Safety Board also stated if the pilot had received autorotation training in a simulator rather than in a helicopter he would have been better prepared and might have effectively responded to the engine failure during the accident flight. Accordingly, the NTSB is recommending realistic autorotation training in all environmental conditions.

Presumably, the principal operations inspectors overseeing these certificate carriers have rotorcraft backgrounds, and yet in reviewing this accident, the NTSB exposed incomplete autorotation training, training programs delivering wrong information and a flight manual with inadequate autorotation information, among other things.

The NTSB concluded that because of a lack of specific guidance in FAA training materials, many other helicopter pilots may also be unaware of the specific
actions required within seconds of losing engine power and recommended that the FAA revise its training materials
accordingly.

The Safety Board’s analysis of this accident raised concerns within the industry since many civilian helicopter pilots have military operational backgrounds similar to that of the accident pilot, that is, flying two-pilot, two-engine, high-inertia machines with rotors turning counter-clockwise. How well does that prepare them for flying solo in relatively light aircraft like the AS350?

And there are circumstances unrelated to that accident that also give one pause. Consider that quite a few business aviation pilots are rated for fixed-wing and rotary-wing aircraft. Then consider that Robinson Helicopter Co. found that highly experienced airplane pilots flying helicopters habitually apply fixed-wing control inputs to disturbances that can result in mast-bumping, even though such actions are contrary to their rotary-wing training. Is it possible to “un-train” such primal responses learned over decades in one type of aircraft when operating another type?

Such questions beget more. How many pilots lack sufficient training on the proper control input reflexes for an autorotation in their particular rotorcraft and from any phase of flight? How much should they practice and how frequently? And do dual-rated pilots need more?

Although a terrific safety feature, autorotations occur in the extreme — usually after an unexpected and sudden failure — and demand immediate and correct pilot response. Ensuring the latter involves well-considered action geared to a specific aircraft type and for which the pilot has trained so thoroughly and regularly that the right response is automatic. 

Rotorcraft Flying Handbook:
Pilot Reaction to
an Engine Failure

• Immediately lower the collective

 Lift and drag are reduced

 Helicopter begins
     immediate descent

 Produces upward flow of air        
     through the main rotor system

 Provides sufficient thrust
      to maintain rotor rpm
      throughout the descent

Source NTSB/FAA