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).

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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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Tip driven rotors II

January 26, 2013 by Tim McAdams

Placing a small jet engine (such as a pulsejet, turbojet and ramjet) on a helicopter’s main rotor blade tips for propulsion never developed into a major commercial success. Despite the advantages of no heavy transmission or anti-torque rotor, several major problems could not be solved. One was the high centrifugal loads acting on the engines. However, from a design standpoint, driving the rotor system from the blade tips was so attractive that during the 1940s engineers came up with another concept called the pressure-jet rotor. 

The pressure-jet rotor operates by forcing compressed air out aft facing nozzles at the blade tips. An engine driven air compressor located in the fuselage pumps air through a rotating seal and into hollow rotor blades. Initially, this solved the noise issue that was associated with engines at the blade tips. However, compressed air alone did not provide enough thrust for flight, so fuel was added and then ignited at the blade tips. This added more thrust, but resulted in higher noise levels. Several prototypes were built as compound aircraft. This design uses the noisy rotor for take-off and landing, and then a propeller system with a small wing for forward flight. In an attempt to solve the noise issue without adding complexity, the French built a small helicopter with a large compressor that was successful without burning fuel at the rotor tips. It was quieter and worked well enough that the French Army ordered about 200 of them. However, using just compressed air was not powerful enough for a larger design. 

The final attempt at a tip driven design directed exhaust gasses from a turbine engine through the rotor blades. Hughes Helicopters built a working prototype during the 1960s; however, pumping 800 plus degrees (F) air through a seal in the rotor was problematic. Eventually, the tip driven concept was abandoned in favor of the engine driven main and tail rotor design that is used on the vast majority of helicopters flying today.

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Ramjet powered helicopters

January 15, 2013 by Tim McAdams

Early helicopter engineers were looking for ways to increase power available and decrease weight. Placing the propulsion system on the tips of the rotor blades eliminated the need for a power consuming anti-torque rotor (tail rotor) and a heavy transmission. Although, propellers mounted on the blade were tried it was the ramjet developed during World War II that launched a major effort to build a successful rotor tip driven helicopter. 

In 1946, McDonnell developed a single-seat, 285 pound (empty) helicopter for the U.S. Air Force. Called Little Henry, it used two ramjets (one on each blade tip) producing about 10 pounds of thrust each. A fuselage mounted tank supplies fuel to each engine through a rotating seal in the rotor hub. After a long flight test program, the Air Force decided not to purchase the helicopter. 

Hiller Aircraft also developed a ramjet powered helicopter. The XHJ-1 Hornet’s larger engines developed 40 pounds of thrust each. Flight test began in 1950 and the U.S. Military expressed interest provided the helicopter and its engines received FAA certification. Hiller worked hard to overcome several design challenges like excessive centrifugal loads acting on the engines and high fuel consumption. Ramjets are also noisy and need a substantial airflow to start, requiring a small ground engine to spin the rotor system. Although Hiller eventually received certification for the engines, the FAA would not sign off on the high level of drag caused by the engines during autorotation. Without the Military, Hiller ended the project.   

A Dutch company solved the drag issue during autorotation with a larger high inertia rotor system. The company’s NHI H-3 Kolibrie (Hummingbird) received certification in 1958.

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CFIT

January 1, 2013 by Tim McAdams

Pilots can learn a lot from reviewing accident reports. The idea is to understand the mistakes that led up to a particular accident so as not to repeat them. Yet, several types of preventable accidents occur over and over. One of these is Controlled Flight Into Terrain (CFIT) in IMC conditions. According to the NTSB, October 2012 was a bad month with three fatal helicopter CFIT accidents in poor weather. 

On October 5, 2012, about 0758 central daylight time, a Bell407 helicopter was substantially damaged when it collided with terrain shortly after takeoff from CentralIndustriesAirport(2LA0), near Intracoastal City, Louisiana. The commercial pilot, who was the sole occupant, was fatally injured. Day instrument meteorological conditions prevailed for the post-maintenance flight. One witness reported that she saw the helicopter depart on the runway heading and disappear into fog or a low cloud ceiling. Several witnesses reported hearing a sound consistent with a ground impact shortly after the helicopter had departed toward the southwest.

The closest weather observing station was located at the Abbeville Chris Crusta Memorial Airport (KIYA), about 13.6 miles north-northeast of the departure airstrip. At 0755, the KIYA automated surface observing system reported the following weather conditions: calm wind, visibility 1/4 mile with fog, overcast ceiling 200 feet, temperature 20 degrees Celsius, dew point 20 degrees Celsius, and altimeter setting 30.14 inches of mercury. 

OnOctober 9, 2012, about 2000 eastern daylight time, aBell407, was substantially damaged when it impacted trees and terrain inCoolbaugh Township,Pennsylvania. The airline transport pilot and one passenger were fatally injured, and one passenger was seriously injured. 

According to a limousine driver who was supposed to pick up one of the passengers at HPN, at 1938 he received a text from the passenger stating that they were running late. Then at 1953, he received another text instructing him to go back to MMU to pick up the passenger. After arriving at MMU, the driver waited but the helicopter never arrived.

A search by Federal, State, and Local authorities was initiated. On October 10, 2012 at approximately 0230 the helicopter was discovered in a heavily wooded area approximately 1.3 miles northwest of Pocono Mountains Municipal Airport (MPO), Mount Pocono, Pennsylvania. The recorded weather at MPO, at 2003, included: wind 100 degrees at 6 knots, visibility 1 1/4 miles, light rain, mist, overcast ceiling of 200 feet, temperature 09 degrees C, dew point 09 degrees C, and an altimeter setting of 30.10 inches of mercury.

According to Federal Aviation Administration (FAA) records, the pilot held an airline transport pilot certificate with ratings for airplane multi-engine land, with commercial privileges for airplane single-engine land, and rotorcraft-helicopter. His most recent application for an FAA first-class medical certificate was dated June 1, 2012. On that date, he reported 19,000 hours of flight time.

 On October 17, 2012, about 0640 eastern daylight time, an Aerospatiale AS 355 F2, was substantially damaged when it impacted trees and terrain shortly after takeoff from Brigham Heliport (4PN5), Erwinna, Pennsylvania. The certificated airline transport pilot was fatally injured. Dark night instrument meteorological conditions prevailed, and no flight plan was filed. Several witnesses reported hearing the helicopter as it over flew the residential neighborhood about the time of the accident, and one witness observed two lights that she presumed to be the accident helicopter as they descended into trees behind her home. Each of the witnesses described the lighting conditions at the time of the accident as dark, and reported that visibility in the immediate area was restricted due to fog.

No pilot takes off thinking that it’s highly likely they will have an accident. In these kinds of accidents most pilots know it is not ideal conditions, but probably believe it is manageable. However, what is missing is the realization that an unpleasant condition can quickly escalate into an unrecoverable and fatal situation. These kinds of accidents are worth thinking about every time you are faced with a decision regarding continued VFR flight in IMC.

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Flapping

December 13, 2012 by Tim McAdams

In the early 1900s, Juan de la Cierva, a Spanish aviator who built airplanes and gliders, unknowingly helped with the development of the helicopter. When one of his airplane prototypes crashed on its second flight during a low speed stall he decided to try to find a way to allow airplanes to fly slower. Windmills got him thinking that a rotating wing could produce lift without the need for forward airspeed. This led him to build the first autogyro (an aircraft that uses a propeller for thrust, but replaces the wing with a free-wheeling rotor for lift).

His first design lifted off the ground and immediately rolled over and crashed. He rebuilt the aircraft and tried again only to see the same result. This perplexed Cierva because the small model he built first as a proof-of-concept did not roll over. What was becoming clear to him was the concept of dissymmetry of lift – that is the difference in relative wind (and as a consequence lift) seen by the advancing and retreating sides of a rotor flown edgewise through the air. After much thought, the difference between his model and the full scale aircraft became clear. The model’s rotors were small and did not need supports which allowed them to flex, while the full scale rotors were heavy and required wire bracing making them stiff. The flexible rotors on the model could flap up and down which compensates for dissymmetry of lift. He then added hinges to his full scale aircraft to allow flapping and was able to proceed with development. The autogyro could not hover, but did meet his goal of slower flight. Over the next several years, various manufactures developed and sold autogyros.

The autogyro went on to achieve limited success until the 1930s, which saw the development of helicopters that could hover. As helicopter designs continued to mature, the autogyro faded out as a commercial aircraft. However, it was the autogyro that solved one of the biggest aerodynamic problems for rotary wing flight.

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