Fit for flight

May 19, 2013 by Tim McAdams

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.

 

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Delta-3

May 6, 2013 by Tim McAdams

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

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Eurocopter’s AStar

April 25, 2013 by Tim McAdams

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.

CDPH0591-086

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Vibration analysis

April 12, 2013 by Tim McAdams

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

UNUSUAL VIBRATION CAN INDICATE A MAIN ROTOR BLADE CRACK

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.

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In-flight vibrations

April 3, 2013 by Tim McAdams

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.

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Boss weights

March 21, 2013 by Tim McAdams

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).

20130321_103824

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Power-off Vne

March 7, 2013 by Tim McAdams

Helicopters have a power-on never exceed airspeed (Vne) that can be an aerodynamic limitation, a structural issue or based on the onset of retreating blade stall. Some also have a power-off airspeed limitation which will be shown on the airspeed indicator as a red/white hatched line or sometimes a blue line.

During autorotation at high airspeeds it may not be possible to maintain sufficient main rotor RPM even with full down collective.  In an autorotative descent the incoming airflow goes up through the disk to maintain rotor RPM. As a helicopter’s speed increases the airflow becomes more horizontal causing the main rotor rpm to decay. As such, a power-off never exceed speed would prevent the main rotor RPM from dropping too low at high speeds.

However, a power-off never exceed speed could also be based on the vertical fin, as is the case with Eurocopter’s AS350 helicopter. The AS350’s rotor system spins clockwise (when viewed from above) – opposite of most helicopters. Therefore, the tail rotor produces thrust that pushes the tail to the left to counter the torque and hold the fuselage straight. To help reduce the power required by the tail rotor the upper part of the vertical fin is angled 6 degrees to the right to also apply a left force on the tail. The higher the airspeed, the more effective the vertical fin becomes. In autorotation the pilot can neutralize tail rotor thrust with the pedals, however, the vertical fin continues to push the nose right. Moreover, transmission drag wants to turn the fuselage in the same direction as the rotor system causing the nose to go to the right as well. At high airspeeds, the amount of left pedal needed to maintain trim increases and the power-off never exceed airspeed (125 knots vs. 155 knots power-on) insures adequate left pedal to maintain yaw control.

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EMS helicopter pilots

February 27, 2013 by Tim McAdams

Having been an EMS helicopter pilot, I believe it is some of the most demanding flying a civilian pilot can do. The accident rate certainly supports this notion. One would think that this type of job would be at the top of the career ladder. One of those jobs that the most experienced and successful pilots would go after. However, that is not always the case.

Air medical should be an industry where turnover is low and getting in would take patience and persistence.  This environment seems to be more prevalent in corporate helicopter operations. One reason for this might be higher pay and benefits. Despite the demanding work an EMS helicopter pilot is required to do, pay and benefits are comparatively low. Would higher pay help the industry? Is pay and benefits the only issue that needs to be addressed? The debate on this subject seems to crop up a lot, especially the idea of raising compensation levels to help the safety problem. Not that any one individual pilot will fly any safer with a bigger paycheck, but industry turnover will certainly decrease.

I have known many good EMS helicopter pilots who have transitioned to fixed-wing aircraft or left the air medical industry to seek better pay and benefits. Instead of a stepping-stone to a higher paying job, EMS flying could be the career that pilots work hard to achieve. Over time, a low turnover rate will build an experience base of pilots skilled at making the tough decisions uniquely required by EMS flying.

A survey of pilots conducted by the National Emergency Medical Services Pilots Association found the number one suggestion to increase safety was to increase the quality and frequency of training. A close second was improving pilots’ salaries and benefits. Unfortunately, all of these strategies require increased funding at a time when cost pressures are high. However, overcoming these challenges and moving toward increased training and compensation would bode well for the air medical industry.

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Rotor RPM

February 20, 2013 by Tim McAdams

Main rotor RPM is like airspeed to an airplane. It creates the airflow over the blades that produce lift. A rotor blade is a rotating airfoil that experiences a much higher airflow over the blade tips than the inboard areas. In order to improve the distribution of lift across the blades, engineers twist the blade so that the inboard part has a higher angle of attack for a given pitch angle. At a constant pitch angle, changing the RPM will vary the lift. However, in helicopter rotor design the main rotor RPM is a fixed value and lift is changed by varying the angle of attack by changing the blade’s pitch angle.

Main rotor RPM limits are established by the helicopter’s manufacturer. Normal operating RPM is shown on the RPM gauge as a green arc (the actual RPM will vary depending on rotor system design). Above the green arc is a yellow or caution arc that terminates at the rotor system’s maximum RPM red line. Rotor RPM that moves into in the yellow arc should be reduced by retarding the engine throttle or raising collective pitch to increase rotor drag. Allowing the rotor RPM to exceed the red line (an over speed) can increase the centrifugal forces to a level that can damage the rotor system. Depending on the severity of an over speed, an inspection or new part might be required.

Below the green arc is another yellow area with a minimum rotor RPM red line. Allowing the rotor RPM to decay into the yellow is recoverable, however going below the red line can become very dangerous. One way this can happen is if a pilot fails to lower the collective pitch (reducing the drag) quickly enough during an engine failure. FAA Part 27 certification requirements for autorotation require the manufacturer to demonstrate acceptable controllability and rotor RPM recovery at 5% below redline RPM. Rotor RPM allowed to drop more than 5% below red line might or might not be recoverable and will cause high coning and flapping angles coupled with significant vibrations. The rotor system can experience extreme stress levels which it was not designed for which will eventually lead to a failure of the hub or blade root. These types of accidents are always fatal.

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Gas turbines

February 7, 2013 by Tim McAdams

Gas turbine (jet) engines used in helicopters do not produce thrust. Instead, the air exiting the engine passes over a wheel (normally called the power turbine) with specially designed blades that turn a shaft. The shaft is geared down and connected to the transmission that drives the main rotor system. This design is called a turboshaft engine and its power is measured in shaft horsepower (shp). As in typical turbine engines, some of a turboshaft’s power is used to drive the inlet compressor or gas producer section. 

Eurocopter’s AS350 series helicopter uses Turbomeca’s Arriel line of engines. The Arriel 1B was certified in 1977 with 640shp. Throughout the years various upgrades have raised the output power, the most recent was in 2011 with the Arriel 2D at 951shp. The Arriel design uses a two stage compressor. The first stage is an axial compressor that draws in ambient air and increases its pressure and speed. It is then directed to the centrifugal compressor that further compresses the intake air to 118.9 PSI and raises its temperature to 335 degrees C before the air enters the combustion chamber. Because the centrifugal compressor is designed to be very efficient at high turbine speeds (high power demand) a bleed valve vents the excess pressure from the axial compressor at low turbine speeds (low power demand). 

The bleed valve is normally open when the engine is shutdown, during starting, and at low power settings. Unlike some compressor bleed valves the Arriel series engines’ are modulated, so as the pilot increases the power the bleed valve gradually starts closing. In the Eurocopter AS350 helicopter when the bleed valve is fully closed a green and white indicator in the cockpit disappears.  If the indicator does not disappear at high power settings, this tells the pilot that the bleed valve has failed to close and maximum engine power will not be available. If the indicator does not reappear at low power settings, the bleed valve has failed to open and the engine may experience compressor stall or surge.  In this case, the pilot should avoid abrupt power changes.

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