Technique Archive

Bad ideas

Wednesday, October 28th, 2009

There are some things that helicopter pilots do that are just not smart.

For example the pilot of a Robinson R22 Beta landed in a field to pick up some equipment and while he was there he decided to hot refuel. The pilot’s father drove a pickup truck equipped with an auxiliary fuel tank under the rotor disk of the running helicopter to accomplish the refueling. The pilot said he stayed at the controls of the helicopter and a wind gust caused the main-rotor blades to flex down, striking the top of the truck. Although no one was injured, the helicopter rolled to the right and into the truck resulting in structural damage to the helicopter. At the time this happened winds were reported from 170 degees at 18 knots, gusting to 25.

Another bad idea is leaving the cockpit while the engine is running and the rotor system is spinning. That’s how a pilot damaged an Enstrom 280X after landing in a corn field and getting out of the helicopter. In an interview with the NTSB, the pilot stated a gust of wind appeared and the main rotor severed the tail boom.

Another pilot preparing to lift-off in an S76 noticed a “door unsecured” indication on the instrument panel for the left cabin door. He brought the engines to idle and exited the cockpit to check the door. He re-closed the door and returned to the cockpit. However, the door open annunciation came on again. He then left the cockpit two or three times to deal with the door. He did not recall retarding the engine power control levers to ground idle before leaving the cockpit the final time.

The wheel-equipped helicopter started to move as the pilot was returning to the cockpit. He told the NTSB it was moving toward the edge of the elevated helipad. He managed to climb into the cockpit, but before he could regain control, the helicopter was on its side.

I can remember several times getting ready to depart and then realizing that I needed to check or do something. It is very tempting to just friction down the flight controls and get out. However, every time I consider doing that I think of what has happened to other pilots.

Basic autopilots

Tuesday, October 20th, 2009

The majority of single-engine helicopters do not have autopilots installed. The few that do have autopilots (not counting experimental designs) use a series-parallel model. Even a simple two-axis system without a flight director can be somewhat complicated as it will have a series actuator and a parallel actuator for both pitch and roll.

In the case of the Sagem autopilot, the series actuator is known as a SEMA (Smart Electro Mechanical Actuator), the flight control tube is cut and the actuator inserted. SEMAs are fast moving with limited authority (plus or minus 3.5 mm). The parallel actuator is called a trim actuator and is normally attached to one end of the flight control tube.








When the pilot engages the pitch-and-roll switch, the two SEMA actuators (one for pitch, one for roll) provide a SAS (Stability Augmentation System) by making very small rapid movements that enhance stability through rate dampening. When the force trim switch is engaged, the two trim actuators will hold the cyclic control in that position. The trim actuator contains a spring-and-clutch mechanism that provides the force trim. If the pilot moves the cyclic control it will want to return to its original position.

The autopilot’s upper modes allow it to hold heading, a navigational course, altitude, and indicated airspeed. Heading and course are controlled by roll and only one of these can be active at a time. Altitude and airspeed are controlled by pitch and only one of these can be active at a time as well. In both of these modes it is normally the SEMA actuator that controls the rotor system while the trim actuator anchors the flight controls. When a SEMA is operating off its midpoint, the trim motor is activated to move the cyclic in the appropriate direction and amount to cause the SEMA to return to its center position, restoring full authority.

The autopilot computer receives data on airspeed, altitude, heading, and course and compares it to the value selected by the pilot. If there is a deviation, the autopilot computer sends the appropriate signal to the actuators which moves the rotor system in a direction to cancel the deviation. This allows the autopilot to maintain heading or course and altitude or airspeed.

This describes a very basic system. More advanced helicopter autopilots have flight directors, yaw servo actuators, and servo actuators that control the collective. There are also systems that will auto hover.





Due diligence

Monday, September 28th, 2009

In response to my previous blog, Jon S. brought up some very good points. He questioned whether an EMS pilot would climb into the clouds, autopilot or not, if he or she would face an FAA violation for doing so. He is absolutely right as declaring an emergency does not guarantee a pilot won’t be cited with a violation. The FAA has taken the position that if the emergency is caused by the pilot’s action or inaction, then a violation is appropriate. In many cases the NTSB has upheld the FAA’s decision.


So how does this affect an EMS pilot’s decision making process? Well, in all the EMS Part 135 operations manuals I’ve read there is a defined procedure for inadvertent IMC. Basically, it is to climb, contact the nearest ATC, declare an emergency, and perform an instrument approach. In discussing this with other operators, I was told that the local FSDO has taken the position that if a pilot does the appropriate due diligence that they will not pursue a violation.


According to the NTSB, on June 8, 2008, an EMS pilot in Texas aborted a flight because of low clouds and fog encountered en route. The request was then made to a different operator. The second pilot was notified of the flight and performed a weather check for the route of flight. After his weather check, he contacted his company’s Enhanced Operational Control Center (EOCC) to discuss his weather observations and the previous turn down. Both the pilot and EOCC supervisor were observing 10 miles visibility and ceilings acceptable for the flight. At that time, the pilot or the supervisor did not understand the reason the other pilot turned down the flight. The pilot contacted EOCC a second time to discuss that the previous flight had been turned down because of fog. The pilot and the EOCC supervisor again discussed weather observations with the same conclusion, that the restriction to visibility reported by the previous flight was not observed by any official weather reporting station.


The Bell 407 crashed in densely forested terrain killing the pilot, flight nurse, and paramedic. Sheared treetops indicated initial impact occurred with the helicopter’s main rotor blade system in a straight nose-low attitude. It happened in the exact location where the other EMS pilot had encountered low clouds and lost reference to surface lights. The other pilot told the NTSB there were no traffic or weather concerns at the time of his departure. While en route, approximately five miles south of the hospital, at 1,400 feet he encountered wispy clouds. He descended to 1,200 feet and encountered more clouds, continued to descend to 1,000 feet and encountered even more clouds, and finally descended to 800 feet when the visibility decreased rapidly. He stated that he could see to the east but had lost his surface light reference. He turned immediately to the right, towards the freeway system, and was back in good weather. He stated that the low clouds and visibility were very sudden and dramatic.


Whether a potential FAA violation affected the accident pilot’s decisions that night will never be known. This kind of accident happens too often in EMS operations as some pilots obviously underestimate the potential for a CFIT accident. Better training would definitely help. I think climbing is normally the best option, however, Jon’s point is well taken and EMS pilots who could be put in an inadvertent IMC situation need to be sure they perform reasonable due diligence.


Another good question is whether all EMS operations should be flown under IFR. That’s coming up next.

Stuck pedal

Monday, September 14th, 2009

For a helicopter pilot, one of the more difficult anti-torque system failures to deal with is when the tail rotor thrust becomes fixed or limited to a certain amount. This could happen if something jams or blocks the pedals or the associated linkage.

In flight, the pilot needs to determine at what position the pedals became stuck. In a counterclockwise turning rotor the more power a pilot is using, the more left pedal input is required. In this case, the left pedal is often called the power pedal. Should something jam the pedals during a high power take off or at maximum cruise speed, the tail rotor will be producing a lot of thrust.

The following illustrates the challenges of performing this type of emergency landing. As the pilot slows the helicopter to attempt to land, the helicopter approaches its most efficient airspeed (normally about 60 knots). The pilot must reduce power to prevent the helicopter from climbing. This would normally require adding right pedal, but since this is not possible the nose will start to yaw left and if airspeed gets too slow the helicopter will start spinning. The only way to stop the left yaw or spin is to add power, but that makes the helicopter climb and that’s not good because the pilot needs to get close to ground to land.

Some instructors have different techniques to land with a stuck left pedal. One method is to approach the longest runway available at cruise speed. This keeps power high and the helicopter pretty much in trim. Since the nose is trying to turn left, the tail wants to move right so finding a runway with a right crosswind will help the vertical fin oppose the left turning motion. Once over the runway, slowly start to decelerate with aft cyclic. As the helicopter’s airspeed decreases the pilot will need to reduce power. Lowering the collective should be done as carefully as possible as the nose will begin to yaw left. As the airspeed continues to drop below 60 knots the airflow over the vertical fin will at some point no longer be able to prevent the helicopter from spinning. The good news is that as the helicopter continues slowing below 60 knots more power is needed. Timing is critical as the pilot needs to keep adding power to prevent the helicopter from spinning, but can’t add too much power or the helicopter will climb. If all goes well and the pilot is able to get the helicopter to a very low hover with little or no left spinning, he or she will have the best chance to put it on the ground without rolling over.

A stuck right pedal makes it a little easier to land because in this case the pilot needs to keep power low. A common low-power landing maneuver in a helicopter is called a running landing. Since hovering requires more power, the pilot would touchdown on a flat smooth surface (a runway for example) with forward speed allowing the helicopter to slide to a stop. It must be performed carefully and is a maneuver that student pilots practice.

Different helicopters and situations will require different procedures. For example, in a clockwise turning rotor the same concepts apply, however the yawing direction and pedal inputs are reversed, as the right pedal is the power pedal. When provided, the manufacture’s recommended procedure should be followed.

Loss of tail rotor thrust

Thursday, August 27th, 2009

In a conventional tail rotor system, a complete loss of tail rotor thrust can happen from an internal drive system failure or if an object contacts the tail rotor and damages the blades or gearbox.

A complete loss of thrust from a drive failure is the easier of the two emergencies for the pilot to handle. In flight (airspeed at least about 60 knots) the pilot will experience a yaw to the left or right (it depends of which direction the rotor turns) that is not correctable with pedal input. The airflow passing over the vertical fin will prevent the helicopter from spinning and in this situation the helicopter can most likely be flown to a suitable landing area. Landing without a tail rotor thrust requires an autorotation. When the throttle is closed and the engine stops applying torque, the need for tail rotor thrust goes away. It’s important to keep the helicopter into the wind to prevent sideways movement during touchdown. Collective pitch should be added carefully because friction in the transmission can have a tendency to turn the fuselage. If the helicopter starts sliding sideways it could easily roll over.

In a hover or with low airspeed a tail rotor drive failure requires quick action. The helicopter will immediately begin spinning and the pilot will need to close the throttle and perform a hovering autorotation. A failure low to the ground is normally recoverable; however, for pilots performing high hovers (utility helicopters doing lift work for example) it is much more dangerous. In cases where this has happened some pilots have survived some have not.

Loss of tail rotor thrust resulting from an object striking the tail rotor is very serious. Many times the damage causes such an imbalance that the tail rotor assembly and gearbox will break free from the tail boom. The loss of weight at that long of a moment arm will cause the CG to shift too far forward. In addition to issues resulting from the loss of tail rotor thrust, the helicopter will pitch down and the pilot most likely will not have sufficient aft cyclic movement to recover. When this happens in cruise flight or a high hover the results are normally severe aircraft damage with a high potential for serious or fatal injuries. When pilots in a low hover (EMS accident scenes for example) hit something with the tail rotor the damage to the helicopter can be severe as well, but the potential for human injury is low.



Monday, August 10th, 2009

Newton’s third law of motion says, for every action, there is an equal and opposite reaction. So, when a helicopter’s rotor system spins in one direction, the fuselage wants to spin in the opposite direction (since this is a rotational force it is called torque). To prevent this engineers put a small thrust-producing rotor on a moment arm (the tail boom) to create a rotational force (torque) that is equal, but opposite, to the force trying to spin the fuselage. Its technical name is an anti-torque rotor, however it is often referred to as a tail rotor.

A set of pedals in the cockpit change the pitch of the tail rotor to vary the amount of thrust produced. Although they control yaw, they function differently than rudder pedals in an airplane.

As long as the main rotor rpm stays constant, so will the tail rotor’s. In fact, if you turn the main rotor by hand the tail rotor will also turn. This is because a system of drive shafts and gearboxes directly connect it to the main rotor transmission. Depending on the helicopter’s design, the tail rotor will spin 3 to 6 times faster than the main rotor.

When viewed from above, most main rotor systems spin counterclockwise (CCW). Sometimes people refer to this as the American direction and clockwise (CW) as the European direction. This is not really accurate as some models built in Europe also turn counterclockwise. For example, Augusta (based in Italy) manufactures models that spin CCW and several Eurocopter models (EC135, EC145) do as well. However, the most popular helicopter with a CW turning rotor system is the Eurocopter Astar.

The rotor system’s rotational direction makes very little difference to gravity or air, but it does change things a little for the pilot. When a pilot increases power (raising the collective control) the torque applied to the fuselage increases. In a CCW turning rotor the pilot must add left pedal to increase the tail rotor’s pitch, and therefore thrust, to keep the nose straight. Likewise, decreasing power requires right pedal input. Right pedal reduces the pitch and thrust allowing excess engine torque to turn the fuselage. In a CW turning rotor just the opposite is true.

Pilots who routinely switch between airframes with different rotor directions, have to remember which one they are in as over time collective movement and the associated pedal movement become automatic. Even if they forget, it is not that big of a problem as it is fairly easy to just react to yaw direction with the necessary pedal movement. Spend enough time switching airframes and eventually it becomes an automatic response again for each airframe.

When an engine fails the torque goes away. As part of the entry into autorotation the pilot must neutralize the tail rotor thrust. With a CCW turning rotor this means pushing almost full right pedal and for a CW turning rotor it’s left pedal. In a hover this must be done quickly as the unnecessary tail rotor thrust will start spinning the helicopter. In forward flight, the pilot will experience a yaw to the left as airflow over the vertical fin helps hold the tail straight.

Coming next is more on tail rotor emergency maneuvers and different types of anti-torque designs.

Main rotor systems

Monday, June 29th, 2009

There are several different main rotor system designs that are used on modern helicopters. The three basic designs that have traditionally been taught to students are semi-rigid, fully articulated, and rigid. Today there are versions that make extensive use of composite materials and are known as hinge less systems.

A fully articulated system normally has more than two blades. In this design each blade is attached to a hub with hinges that allow it to move independently of the others. A feathering hinge is used to change the pitch of each blade. A flapping hinge allows each blade to move up and down to compensate for dissymmetry of lift. Blades are able to move fore and aft or lead-lag, (called hunting) by use of a drag hinge. Normally a damper is attached to the blade and hub to restrict excessive movement. The drag hinge is used because, when a rotor blade flaps up, its center of mass moves closer to the axis of rotation. This causes the rotor system to spin faster, much like a spinning ice skater speeds up when pulling her arms in closer. Allowing the blades to lead-lag reduces this tendency.

A semi-rigid system refers to a two-blade system where each blade is mounted to a hub that has a center teetering hinge. In this configuration, when one blade flaps up the other one flaps down – like a see saw. As with the fully articulated system, each blade has a feathering hinge. The two blades are mounted in an under-slung position, that is where the teetering hinge is mounted above the plane of rotation. The geometry of this arrangement minimizes the change in distance between the center of mass and the axis of rotation during flapping. This allows a semi-rigid system to not need a drag hinge.

In a slight departure from the traditional semi-rigid design, Frank Robinson used a coning hinge on each blade (some refer to this as a flapping hinge, but it is used for blade coning). When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). This places a high stress load at the blade’s root, so in order to relieve this stress Robinson’s design allows the blade root to cone about a hinge. This reduced the amount of reinforcing required at the blade root making for a lighter easier to manufacture rotor blade.

Rigid rotor systems do not use hinges and limited movement is absorbed through the hub and rotor blades. Many of the modern composite rotor systems also do not use traditional hinges, but have elastomeric and specially designed composites structures (flextures) that allow the blades to flap, feather, and hunt. Manufactures do not use the term rigid rotor system, opting instead to describe these systems as a fully articulated hinge less rotor system. These systems do not require lubrication and are less maintenance intensive. The extensive use of composite materials also increases reliability and helps absorb vibration.

Speed limits – Part 2

Monday, June 8th, 2009

How exactly does flapping change a rotor blade’s angle of attack? That was a great question with many good explanations provided by readers. I think to fully understand it is important to know the difference between pitch angle and angle-of-attack. Pitch angle is the angle between the rotor blade’s chord line (a straight line intersecting the leading and trailing edges of an airfoil) and a reference plane of rotation. Angle-of-attack is the angle between the rotor blade’s chord line and the relative wind (the airflow that results from, and is opposite of, the velocity of an airfoil. Velocity is used here as a vector to mean speed and direction.)

When the rotor blades stay in the reference plane of rotation the pitch angle and angle-of-attack are the same. The pilot controls the pitch angle with the collective control and thus the angle-of-attack as well. However, when a rotor blade leaves the plane of rotation (flapping causes this to happen) the direction component of its velocity changes. Since relative wind is a function of velocity, it changes as well. In the case of a blade that flaps up the relative wind moves opposite the blade’s new direction. This change in relative wind direction reduces the blade’s angle-of-attack. The opposite is true for the blade that flaps down on the retreating side.

As the helicopter’s forward speed continues to increase, the retreating, or down flapping, side encounters higher angles of attack. Eventually, the rotor system encounters retreating blade stall.

From the pilot’s perspective, when this happens an abnormal vibration will be felt, the nose can pitch up, and the helicopter can have a tendency to roll in the direction of the stalled side. The amount and severity of pitch and roll will vary depending on the rotor system design.

The tendency for the nose to pitch up is because the spinning rotor system acts like a gyroscope and therefore experiences gyroscopic precession (a physical property that states when an external force is applied to a rotating body the effect will happen approximately 90 degrees later in the direction of rotation). As such, when the retreating blade stalls and stops producing lift, the effect of this happens toward the rear of the rotor disc. This causes the disc to tilt back, and the nose to pitch up.

Conditions like high density altitude, steep or abrupt turns, high blade loading (caused by high gross weight), turbulent air and low rotor rpm will increase the likelihood of encountering retreating blade stall when operating close to a helicopter’s Vne (never exceed speed). Helicopter flight manuals contain a chart or textual description in the limitations section that reduce the helicopter’s Vne at higher altitudes and temperatures. This is the airspeed limitation chart from a Bell 407.

Should a pilot encounter retreating blade stall, lower the collective and reduce airspeed. Other actions that will help are increasing rotor rpm and decreasing the severity of any roll or pitch maneuvers. Taking immediate action at the first sign will normally result in a quick recovery. However, if a pilot attempts to increase speed a severe stall would develop with possible loss of control.

Thoughts on EMS safety

Wednesday, May 13th, 2009

Over the past year 28 people have died in EMS (emergency medical services) aircraft crashes. The industry is experiencing one of the worst accident rates in its history. Solving this problem is a complicated issue for sure, however I have some very basic thoughts on how this problem can be fixed.

Flying an EMS helicopter was some of the most demanding flying I have done. Flying at night and landing on streets or other confined areas, having to make quick weather decisions sometimes with little information available, and having to block out the pressure to fly. Yet many EMS helicopter pilots receive the minimum amount of required training.

Conversely, when I flew a corporate helicopter it was normally airport to airport or heliport. The occasional off-airport landing was performed, however it was planned and I had plenty of time to assess the area. This was far less demanding and risky than flying an EMS helicopter. Yet, it was also where I received the best and most consistent training. We had the time and resources available to practice our skills and FlightSafety training every six months.

Corporate helicopters are not expected to make money and the person who has the authority to cut training expenses normally rides in the back. That’s a strong motivator to ensure that the pilots know what they’re doing. EMS helicopter operations by contrast need to make money and that means keeping a close eye on costs. Also, because of the competitive bid process hospitals use when selecting vendors, margins are thin. Training costs come right off the bottom line. If a vendor increases its training costs and the others do not, then that vendor is at a competitive disadvantage. Hospital-owned programs are also in business to get patients to their hospital and make money.

To level the playing field, I think two requirements are needed. The first is more frequent and comprehensive training. Not just training in maneuvers but scenario-based training that addresses issues such as crew coordination, judgment, and accident chains to name a few. Additionally, more IFR and inadvertent IMC training, even for VFR-only programs, is needed. Pilots need to be very comfortable initiating a climb and not descending if they get caught in bad weather.

This type of training can be done in simulators. Simulators are not only good for showing pilots how to do things correctly, but can also show how quickly a bad decision can degenerate into a serious problem. That’s a powerful learning tool.

Second is better equipment, such as terrain avoidance and warning systems and night vision goggles. In addition, important in adding new equipment is providing the appropriate level of training on how to use it effectively.

Another issue that should be addressed by the industry is pilot salaries. I have known many very good pilots that have left EMS for better paying jobs. This has made EMS a steppingstone for pilots to get to something better. EMS flying requires a very specific skill set and experience level. It should be the job that pilots aspire to get. Higher salaries will keep turnover down and keep experienced pilots in the industry.

I realize that all of my solutions cost money and that some operators will claim they cannot afford these programs. That is why training and equipment should be mandated for everyone who wants to operate an EMS helicopter. The difficult part is figuring out how the industry will get there.

The FAA has tried the quick and inexpensive solutions and they do not work. Case in point is the risk assessment matrix. Three years ago EMS pilots began filling out a questionnaire before each flight to determine a score that related to a risk level. The accident rate has gotten worse in the last three years.

As with most things in life, to get the best results one needs to spend the effort and money required. Cheap solutions are just that.

Protecting your tail

Friday, May 1st, 2009

Not visible from the cockpit, a helicopter’s tail rotor is perhaps the most vulnerable component to striking objects in a hover. EMS pilots are especially at risk, as their job involves routinely landing in obstacle rich environments.

In 2003, a Bell 430 was substantially damaged when its tail rotor hit a roadway sign during an off-airport landing at night. Prior to touchdown, the pilot said he rotated the aircraft and landed on an easterly heading, at which point the medical crew departed the helicopter. Then, the pilot decided to reposition the aircraft to face west for departure. During the hovering turn the tail rotor hit a steel reflector post. The aircraft touched down on the left rear skid first and came to rest 180 degrees from its initial heading. The tail rotor and gearbox assembly had come apart and departed the helicopter.

Darkness certainly makes objects harder to see. However, two years prior to this accident, during daylight conditions, a Bell 222UT was substantially damaged when its tail rotor hit a barrel while landing on a paved traffic turn-around area. The pilot said that while hovering, he decided to reorient the aircraft to help load the patient easier. During the right pedal turn, the tail rotor struck a 55-gallon trash barrel. The helicopter yawed to the right and the pilot brought the throttles to flight idle and landed the helicopter. The tail boom was twisted, the tail rotor blades were damaged, and the tail rotor gearbox was nearly separated from the airframe.

Although a tail rotor strike in a hover can cause serious damage, the potential for personal injury is low compared to what can happen in flight.

In 1999, a Bell OH-58A, on a photo flight with doors removed, was destroyed on impact with the terrain and the private pilot and passenger sustained fatal injuries. A witness reported that he saw the helicopter flying at an altitude of approximately 350 to 400 feet. He saw what was possibly a large bird hit the rear rotor of the helicopter. The helicopter made three to four rotations during its descent.

Examination of the tail assembly revealed an elastic material with navy blue yarns wrapped around the tail rotor. The material, along with a sample of a navy blue warm-up jacket found along the reported flight path, was sent to the NTSB’s Materials Laboratory for examination. The color, size, and texture of the navy-blue yarns in the elastic material were consistent with those found in the navy blue warm-up jacket. The NTSB concluded that the jacket exited the helicopter and became entangled in the tail rotor.

Removing a helicopter’s doors places the tail rotor at increased risk. In 1993, an R22 helicopter flying with its left door removed crashed after an aluminum kneeboard exited the helicopter and struck the tail rotor.

There have been numerous cases where objects have come out of the cabin or an unsecured baggage compartment and struck the tail rotor. In some cases the pilots have been able to enter autorotation or otherwise land with minor damage or injury. However, as with the two preceding accidents, the tail strike inflicted enough damage to cause the tail rotor assembly to come apart. In these cases, the resulting center of gravity shift will make recovery nearly impossible. The importance of protecting the tail rotor cannot be emphasized enough.