Uncategorized Archive

Uncommanded yaw

Sunday, July 7th, 2013

In cruise flight, should a helicopter experience an uncommanded yaw the cause in most cases would be an engine failure or tail rotor failure. The direction of the yaw indicates the type of failure.

In clockwise turning rotor system (like the Eurocopter AS350) the engine torque causes the fuselage wants to spin the opposite way (Newton’s third law – for every action, there is an equal and opposite reaction). In this case, from the pilot’s prospective, the nose of the helicopter wants to go to the left. The tail rotor applies a thrust that counters this reaction and pushes the nose back to the right. The pilot varies the amount of thrust with the pedals to control yaw. In powered flight, everything is in balance.

Should the engine fail, the engine torque that the tail rotor is opposing goes away. However, the tail rotor is still producing thrust that is trying to turn the nose right. In this case, the pilot will experience a right yaw and will use left pedal to neutralize it. With a tail rotor failure (loss of drive, producing a complete loss of thrust) the force opposing the engine torque ceases allowing the fuselage to spin the opposite of the rotor system. As such, the pilot will experience a yaw to the left. However, since the tail rotor is no longer effective applying opposite pedal does not work. Airflow over the vertical fin will prevent the helicopter from completely spinning and allow the pilot to fly the helicopter to a suitable area and perform an autorotation. Shutting the engine off eliminates the torque allowing the nose to come back to the right.

In a hover, it is a little different due to the reduced air flow over the vertical fin. Instead of just yawing, the fuselage will start spinning. In the case of an engine failure, opposite pedal will work. With a tail rotor failure, the pilot must immediately enter autorotation.

In a counter-clockwise turning rotor system (like the Bell 206) the theory is the same, but the nose moves in the opposite direction for each failure.

Ground effect

Sunday, June 30th, 2013

Hovering in ground effect results in a condition of improved performance that comes from operating near a firm surface. A helicopter is normally considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground. However, the amount of benefit varies as a function of height. A lower hover will generate more efficiency and as the helicopter climbs the advantage decreases reaching zero about one and one-quarter times the rotor diameter.

A helicopter requires less power to hover in ground effect for two reasons. The main reason is the reduced velocity of the induced airflow caused by the ground. (Induced flow is air flowing down through the rotor system and is also called downwash.) This reduced velocity results in less induced drag and a more vertical lift vector. As such, the lift needed to sustain a hover can now be generated with a lower angle of attack in rotor blades, which requires less power.

The second reason has to do with vortices generated at the rotor tips. The close proximity of the ground forces more air outward and restricts vortex generation. This reduces drag and increases the efficiency of the outer portion of the rotors.

The maximum benefit is achieved from hovering over a hard surface such as concrete. When a helicopter hovers over an area such as tall grass or water, energy is absorbed by displacing the surface, allowing the induced flow to increase, thus reducing the lift vector. This will require the pilot to add power to maintain that hover height.

When a helicopter is in a high hover, or out-of-ground-effect, it requires a lot more power because there is no obstruction to slow the induced flow or force it outward. This results in a more vertical downwash and also allows the formation of stronger rotor tip vortices, reducing efficiency.

The helicopter’s Pilot Operating Handbook (POH) has both In-Ground-Effect (IGE) and Out-of-Ground-Effect (OGE) hover charts. This allows the pilot to take the density altitude and gross weight into account to predict hover performance.











Ground resonance

Sunday, June 16th, 2013

A helicopter’s rotor system, engine(s), and other dynamic components generate vibrations in the airframe. These components will vibrate at a natural frequency which in turn causes other parts like the landing gear, tail boom, cabin etc… to vibrate as well. Each part’s frequency will vary according to its weight, stiffness, shape, etc…. As such, a helicopter contains a complex set of vibrations that add up to a resulting airframe vibration. Engineers attempt to reduce the overall vibration level by tuning the natural frequency of all the components. 

When a helicopter is in flight, the airframe’s natural frequency (the sum of all the components’ frequencies) will vibrate without interference. However, when ground contact is made with the landing gear it can interfere with the airframe’s ability to vibrate at its natural frequency. Ground resonance happens when ground contact alters the natural frequency of the main rotor system. This unbalanced condition triggers vibrations that are augmented with every blade revolution causing a reflected impulse that increases in amplitude very quickly. The only rotor systems susceptible to ground resonance have three or more blades. This is due to each blade’s ability to lead and lag (speeding up and slowing down) independently. If something causes the blades to depart from their symmetry, the rotor system’s center of gravity shift causes it to become out of balance allowing divergent oscillations to rapidly become strong enough to cause serious damage to the helicopter. In some cases, complete destruction  can occur with many componets coming loose and being thrown from the helicopter. 

Engineers design damping systems for the main rotor and landing gear to absorb the energy and prevent the oscillations from accelerating. Still, a sudden shock to the airframe, like a hard landing, can unbalance the main rotor system beyond the damping system’s ability to absorb the energy and ground resonance can start. Improperly serviced or malfunctioning dampeners are usually the cause.  Ground resonance happens very fast, however, if the pilot recognizes it in time and there is enough power and rotor RPM available to lift the helicopter off the ground the divergent oscillations will stop. This is the quickest way to stop it and hopefully will result in little or no damage. If the situation is such that there is not enough power to lift into a hover, then a full power reduction is the only option. However, this approach will take time for the vibrations to fade and significant damage can occur.

Gemini ST

Monday, June 3rd, 2013

The problem with very light twin-engine helicopters is payload and range. The extra weight of the second engine, combining gearbox, extra fuel tanks and related systems severely limits the airframe’s payload.  Also, that smaller payload capacity will need to include the weight of more fuel. As such, carrying enough fuel for an acceptable range normally leaves little weight capacity for passengers. Operators of small helicopters who wanted the redundancy of a twin have to make some big trade-offs. 

In the late 1980s, Doug Daigle, a 14,000-hour helicopter pilot and owner of Tridair Helicopters, came up with an idea that tried to solve this problem.  He took a popular single-engine helicopter, the Bell LongRanger (206L3), and removed the 650-shaft-horsepower Allison 250-C30 engine (the 500-shp C28 for the older L1s) and added a pair of 450-shp Allison 250- C20R engines to create a twin with good single-engine performance. However, what was different about his design was it was the first twin to be certified for normal operation on one engine in all areas of flight. Certified by the FAA in 1994, he named it the Gemini ST conversion. 

When operating both engines, the Gemini consumes fuel at 45 gallons per hour, compared to the LongRanger at 38 gph. Because both helicopters have the same fuel capacity (he did not add extra fuel tanks), endurance drops from 2.9 hours for the LongRanger to 2.5 hours for the Gemini. However, the Gemini’s C20R engine burns only 28 gph in single-engine mode, and this increases the endurance to 3.9 hours. Daigle believed long trips will normally have some extended cruise flight, where the pilot could choose to fly on one engine. The other engine can be started for critical maneuvers that require the redundancy of a second engine. 

The extra weight of the engine and related systems does increase the empty weight, so there is some trade-off with the Gemini. It has a smaller useful load of 1,610 pounds, compared to 2,175 pounds for the LongRanger. The Gemini can carry 740 pounds of fuel (the same as the LongRanger), which leaves a payload capacity of 870 pounds. 

Bell Helicopter then entered into an agreement with the company that supported Tridair’s conversion of existing LongRangers while Bell would build a new production model, the 206LT TwinRanger, under Tridair’s STC. Bell only delivered 13 airframes and the concept of shutting one engine down in flight never caught on. This model was replaced by the Bell 427 which was under development for the EMS market. The 427’s cabin proved too small and Bell canceled the program and introduced the larger, single-pilot IFR certified 429 in 2009.

Fit for flight

Sunday, May 19th, 2013

Every so often I come across an accident that really makes me stop and think. Many of these can be a learning experience and some are just hard to understand. 

According to the NTSB, on July 22, 2010 a Eurocopter AS 350 B2 helicopter impacted trees near Kingfisher, Oklahoma. The commercial pilot and one flight nurse were fatally injured and one paramedic flight nurse was seriously injured. 

A Global Positioning System (GPS) device recovered from the accident scene revealed the helicopter was cruising at approximately 130 knots and about 200 to 300 feet above ground level. Seconds before impact, the helicopter descended at 385 feet per minute, followed by a descent rate of 1,890 feet per minute two seconds later. The location and altitude of the helicopter, as recorded by the GPS corresponded to the location rotor impact marks with the trees. 

In an interview with the surviving paramedic flight nurse, he recalled that during the flight, the left side door had come unlatched and was slightly ajar. The paramedic informed the pilot that he was getting out of his seat to close the door and secure the handle. The pilot acknowledged the paramedic. After securing the handle, the paramedic stated that he had sat back down and begun to gather his seatbelt when a conversation began about another pilot flying on a coyote hunt. The paramedic reported that the pilot made a statement similar to “like this… (with some laughter)” and made a nose down control input. He reported that the pilot pulled up on the collective and the helicopter struck a tree. During the ground impact, the paramedic, who was not secured in his seat, was thrown through the windscreen; the paramedic crawled away from the wreckage and dialed 911 on his cell phone. 

The pilot, age 56, held a commercial pilot certificate for airplane single-engine land, instrument airplane, rotorcraft-helicopter, and instrument helicopter. He held a second class medical certificate issued February 8, 2010. On the pilot’s last application for a medical certificate he reported having accumulated 12,241 hours, with 119 hours logged with the preceding six months. Of note, the pilot reported that he was not currently using any medications. 

An autopsy was performed on the pilot and toxicology noted the following: 

  • 39.31 (ug/ml, ug/g) Acetaminophen detected in Urine
  • Chlorpheniramine detected in Blood
  • Chlorpheniramine detected in Urine
  • 0.198 (ug/ml, ug/g) Diazepam detected in Blood
  • 0.026 (ug/mL, ug/g) Dihydrocodeine detected in Blood
  • 1.026 (ug/mL, ug/g) Dihydrocodeine detected in Urine
  • 0.15 (ug/ml, ug/g) Hydrocodone detected in Blood
  • 4.112 (ug/ml, ug/g) Hydrocodone detected in Urine
  • 0.302 (ug/mL, ug/g) Hydromorphone detected in Urine
  • 0.322 (ug/ml, ug/g) Nordiazepam detected in Blood
  • 0.629 (ug/ml, ug/g) Nordiazepam detected in Urine
  • 0.011 (ug/ml, ug/g) Oxazepam detected in Blood
  • 2.169 (ug/ml, ug/g) Oxazepam detected in Urine
  • 1.569 (ug/ml, ug/g) Temazepam detected in Urine 

A review of the pilot’s medical history found that the pilot was being treated for several medical conditions and had been prescribed multiple medications since at least 2007. In April 23, 2007, the pilot reported to his personal physician that he had bronchitis, hypertension, and sleep apnea, and after his visit, he was prescribed the following medications: Nexium (for gastroesophageal reflux), Caduet (for hypertension), Flexeril (sedating muscle relaxant), Lortab (hydrocodone and acetaminophen; narcotic pain medication), Lunesta (for sleep disturbance), and Requip (for restless leg syndrome). The pilot continued to report to his personal physician that he experienced increased pain and was prescribed stronger pain medications, to include prescription narcotics and benzodiazepines. In addition, steroid joint injections were applied to his right knee and shoulder to treat persistent pain. The last documented visit, February 25, 2010, the pilot was prescribed the following: Caduet (for hypertension), omeprazole (for gastroesophageal reflux); Meloxicam (a non-steroidal anti-inflammatory); Lunesta (sleep aid); Norco (10/325 hydrocodone/acetaminophen combination two tablets three times a day); baclofen (a muscle relaxant, 10 mg three times a day) and Valium (diazepam, a benzodiazepine, 10 mg three times a day). In addition to his prescribed medications, chlorpheniramine, an over-the-counter sedating antihistamine medication was also detected in the toxicology. There was no evidence that the pilot’s sleep apnea had been treated prior to the accident. In addition, the pilot did not report any of his conditions and prescription medications to the FAA, to the certificate holder, or to the operator.



Monday, May 6th, 2013

Dissymmetry of lift occurs when a rotor system is flown edge-wise through the air. With helicopters, many times these discussions center on the main rotor system. However, this aerodynamic condition also affects the tail rotor.

Just like the main rotor, a tail rotor will equalize lift by flapping. However, most tail rotor flapping takes advantage of the Delta-3 effect. Also known as pitch-flap coupling or K-Link (French term). This effect is achieved by having the pitch horn on a different plane than the flapping hinge, which mechanically changes the pitch angle of the blade as it flaps. The amount of the delta-3 offset is measured in degrees and determined by design engineers after considering many factors. This offset can be also be accomplished by using a Delta-3 hinge (setting the hinge at an angle to the chord of the blade). In either case, when the advancing blade (the blade that experiences a higher relative wind) starts to flap the offset lengthens the distance between the blade’s pitch horn and the pitch link’s attach point. This forces the pitch link to pull the blade’s pitch horn closer, thereby reducing its pitch angle. On the retreating side, the distance is shortened and the pitch link forces the pitch horn further away, increasing the blade’s pitch angle. This effect minimizes flapping in order to control dissymmetry of lift on the tail rotor.

This can be demonstrated by moving a tail rotor blade with a delta-3 hinge through its flapping range and observing the pitch angle changes as you manually flap the blade.


AS350 tail rotor

AS350 tail rotor

Eurocopter’s AStar

Thursday, April 25th, 2013

In the late 1970s, Aerospatiale introduced the AS350B as a replacement for the company’s Alouette II helicopter. Named Ecureuil (Squirrel) it used a Turbomeca Arriel 1B engine rated at 641 shp. For the U.S. market Aerospatiale gave it the name AStar and the model number AS350C. Both models had a maximum gross weight of 4,300 pounds, however, the C model used the Lycoming LTS 101-600A engine rated at 592 shaft horsepower (shp) for takeoff. Six months later the helicopter was upgraded to the D model with the installation of the Lycoming LTS 101-600A-2 engine, boosting its takeoff rating to 615 shp.

In 1987, Aerospatiale discontinued the D model and upgraded the B to a B1. This new version had a more powerful Arriel 1D (684 shp) engine and a higher gross weight of 4,850 pounds. The rotor system was upgraded with larger (more inertia) asymmetrical rotor blades and rotor rpm was increased slightly. This resulted in increased performance and better autorotation characteristics. The company also produced the BA model that had the larger blades, but retained the B model’s Arriel 1B engine. Three years later the B1 was replaced with the B2, using a more powerful Arriel 1D1 (712 shp) engine and gross weight jumped to 4,961 pounds. The B2’s cruise speed at MCP (maximum continuous power) is 133 knots. In 1992, Aerospatiale merged with MBB to form Eurocopter.

In the late 1990s Eurocopter introduced the B3, a high altitude version.  It was powered by an Arriel 2B engine equipped with a single channel DECU (Digital Engine Control Unit) with a mechanical backup system. Although the gross weight remained the same, take off power increased to 747 shp. This was followed by a variant using the 2B1 engine with a dual channel FADEC (Full Authority Digital Engine Control). This version had a dual hydraulic system available as an option which when combined with high skid gear allows a gross weight increase to 5,225 pounds. The latest version, B3e, was introduced in late 2011 and is outfitted with a dual channel FADEC equipped Arriel 2D engine. Take off power rating jumped to 847 shp boosting its MCP cruise speed up to 137 knots and adding extra lifting capability.


Vibration analysis

Friday, April 12th, 2013

All helicopters have an inherent vibration. The type and intensity varies as a function of rotor design and isolation systems. Understanding basic vibration levels and being alert to changes can be an important tool for preventing fatal accidents. Difficulty with tracking and balancing the main rotor system is a condition that should raise concern with pilots and mechanics.

Two accidents involving Robinson R22 helicopters, one in Australia in June, 2003 and the other in Israel in February, 2004, involved increasing vibration levels in the main rotor system. In both aircraft, the vibrations were corrected with track and balance only to reappear a short time later. In fact, the accident in Israel happened during one of the track and balance flights. In both cases, investigations revealed that corrosion from water penetration initiated a fatigue crack in the main rotor blades.

More than a year prior to the first accident, Robinson Helicopter released a Service Letter (SL-53) regarding potential development of main rotor blade fatigue cracks when the helicopter is operated under conditions where the loads on the main rotor exceed the design limits. In part the letter stated, “The first indication of a fatigue crack in progress may be a rotor that will not stay balanced after being adjusted.”

Then in July of 2003 Robinson Helicopter issued a R22 Safety Notice again stating that vibrations that reappear after tracking and balancing the main rotor system should be consider suspect.

Safety Notice SN-39


A catastrophic rotor blade fatigue failure can be averted if pilots and mechanics are alert to early indications of a fatigue crack. Although a crack may be internal to blade structure and not visible, it will likely cause a significant increase in rotor vibration several flight hours prior to final failure. If a rotor is smooth after balancing but then goes out of balance again within a few flights it should be considered suspect.

Knowing this information is important to help pilots and mechanics prevent future accidents.

In-flight vibrations

Wednesday, April 3rd, 2013

When a critical component in a helicopter’s main rotor system fails in flight, the resulting accident is almost always fatal. How much warning, if any, does a pilot get with these kinds of failures? Unfortunately, the vast majority of helicopters do not have cockpit voice recorders and unless the pilot can provide ATC with details, it can be hard to understand exactly what happened. Even if the pilot is in contact with an air traffic controller, an emergency situation leaves little time to completely explain a problem. Consequently, the crash of a Bell 212 equipped with a cockpit voice recorder near Philadelphia, Mississippi is unique in that it provides some insight as to what the flight crew knew. The helicopter was destroyed and the airline transport-rated pilot and one passenger (who was employed by the owner as a mechanic) were fatally injured.

The transcript of communications recorded on the cockpit voice recorder showed that about 18 minutes before the accident, the passenger stated to the pilot, “Boy, those catfish are going crazy down there, aren’t they?”

“Yep,” the pilot responded, “must have been the vibrations from the helicopter.”

About 1 minute, 30 seconds before the accident, the pilot asked the passenger, “Has this vertical just gotten in here or has it been here for a while?”

“We haven’t had any verticals at all,” the passenger replied.

“We do now,” the pilot said.

“Yeah, well it started right after we left back there,” the passenger said. “I think it maybe, ah, that’s why I was thinking it was the air.”

About 20 seconds later, the passenger stated that another person had tracked the helicopter’s blades before they left and that he was commenting on how smooth it was. Forty seconds after that, the pilot said, “This stuff is getting worse.”

The recording then ended.

The National Transportation Safety Board determined the probable cause of this accident was the failure of the pilot and company maintenance personnel during preflight and periodic inspections to identify the signs of fretting and looseness in the red main-rotor blade pitch-change horn to main-rotor blade grip attachment. As a result, the NTSB found, the helicopter was allowed to continue in service with a loose pitch-change horn, which led to separation of the pitch-change horn from the blade grip and the in-flight breakup of the helicopter after the main rotor struck the tail boom. Contributing to the accident, the safety board said, was the pilot’s failure to respond to increased vibration in the main rotor system and land immediately.

The lesson in this accident is that any unexplained vibration should be investigated on the ground until the source is found and corrected. Some parts and bearings that become loose can experience exponential wearing and fretting and quickly reach a failure point.

Boss weights

Thursday, March 21st, 2013

The tail rotor on Eurocopter’s AS350 AStar helicopter uses weights to generate a Centrifugal force to help balance the forces that exist when changing the blades’ pitch angle. Known as boss weights, exactly how they work is sometimes misunderstood.

Eurocopter uses composite technology in the AStar’s main and tail rotor systems. The helicopter’s two-blade tail rotor uses a single composite spar that runs through both blades. It is clamped in the middle at the hub and pitch changes are accomplished by twisting the composite material. The spar resists the twisting and tries to return to its natural state (it has a 10 degree pre-twist). This force is referred to as a zero-pitch-return-force and is fairly strong. Making the spar thick enough to have the necessary strength also makes it hard to twist. In normal operation with hydraulic boost, the tail rotor servo delivers enough force to overpower the zero-pitch-return-force and twists the spar as necessary changing the blades’ pitch angle. Thus, producing the amount of tail rotor thrust the pilot requires.

The boss weights assist by generating a centrifugal force that opposes the stronger zero-pitch-return-force. Essentially, they help hold twist in the spar reducing the workload on the tail rotor servo. During a hydraulic system failure the pilot must change tail rotor pitch by manually twisting the spar. The centrifugal force generated from the boss weights reduces the amount of pedal pressure required by the pilot to maintain yaw control. To further assist the pilot during hydraulic failures Eurocopter added a yaw load compensator to the tail rotor control linkage in the higher gross weight variants (B1, B2 and B3).