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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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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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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Moral Courage Award

November 30, 2012 by Tim McAdams

Moral Courage can be thought of as the willingness to make sound safety decisions even when they are difficult or unpopular. Many times the pilot that exercises the courage to turn a flight down is making a tough decision, and in many cases their actions go unnoticed. Although it is impossible to know about the accident that didn’t happen, it is a reasonable assumption that a certain percentage of no-go decisions have prevented accidents and possibly saved lives.  Mr. D Smith, Senior Air Safety Investigator with the DOT/Transportation Safety Institute, believes that although these types of decisions are ones without fanfare, they are no less heroic than the more visible actions of other aviators. As such, he is looking for nominations for a new Moral Courage Safety Award. The award is aimed at recognizing individuals and organizations in the helicopter industry that make operational decisions based on sound safety risk management principles.  

According to Mr. Smith, “The award was inspired from the true story of an EMS pilot who told of his decision to abort a critical neonatal transport after encountering un-forecasted bad weather.  It was a very tough call; he had to weigh the safety of the crew with the life of a patient.  In the end he aborted the transport knowing it was the right decision for the safety of everyone.  His organization supported the decision and even went so far as to recognize him for making that tough call.  Sometimes choosing the safest course of action can cost time or money, but in the long run it saves time, money, reputation, and possibly lives.  It takes moral courage to do the right thing.  We believe there are many individuals and organizations making these tough calls every day.  We want to recognize them for their contribution to promoting a positive safety culture in the rotorcraft community.” 

Helicopter crew members, maintenance personnel, managers, and their organizations are eligible for the award. If you are interested in nominating someone please draft a short narrative of the event(s) and send it to d.smith@dot.gov . The award will be presented during the annual HAI Heli-Expo.

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Flight controls and passengers

November 20, 2012 by Tim McAdams

I was at a state fair several years ago where a pilot was giving helicopter rides in a Bell 206 JetRanger (a 5-seat single engine turbine powered helicopter). What caught my attention was that the dual controls were installed and passengers were being loaded into the left front seat. Allowing strangers access to the flight controls, like when giving rides, is very risky. Even when a pilot knows the passenger, they need to be extremely cautious and give serious consideration as to whether someone is provided access to the flight controls. For example consider the following accident that happened February 14th, 2010.

According to the NTSB, a ranch foreman who observed the flight preparations saw the helicopter owner board the helicopter through the left forward cockpit door and occupy the left front cockpit seat. The helicopter owner’s 5-year old daughter also boarded the helicopter through the left forward cockpit door and sat on her father’s lap. The pilot, who had 11,045 hours of total flight time, all in rotorcraft-helicopters, 824 hours of which were in the EC135 T1, was already seated in the right front cockpit seat. Both the left and right front cockpit seats were equipped with dual flight controls. Operator personnel revealed that the helicopter owner’s daughter had sat on her father’s lap occasionally during flights, that the owner liked to fly the helicopter, and that it was common for him to fly. Although the owner held a certificate for airplane single-engine land, he was not a rated helicopter pilot. However, it could not be determined who was flying the helicopter at the time of the accident.

About 35 minutes after departing the ranch, radar data revealed that the helicopter was about 2,000 feet above ground level when witnesses on the ground stated they heard unusual popping or banging noises. Several witnesses also stated that they saw parts separate from the helicopter before it circled and dove to the ground. The helicopter impacted a river wash area north of the destination airport in a slightly nose-down and slightly left-bank attitude. The helicopter was subsequently consumed by a post crash fire. The accident was not survivable.

A post accident examination of the helicopter revealed that the yellow blade had impacted the left horizontal endplate and the tail rotor drive shaft in the area of the sixth hangar bearing, which resulted in the loss of control and subsequent impact with terrain. No pre-impact failures or material anomalies were found in the wreckage and component examinations that could explain the divergence of the yellow blade from the plane of main rotor rotation. Flight simulation indicated that the only way that this condition could have occurred was as a result of a sudden lowering of the collective to near the lower stop, followed by a simultaneous reaction of nearly full-up collective and near full-aft cyclic control inputs. A helicopter pilot would not intentionally make such control movements.

A biomechanical study determined that it was feasible that the child passenger was seated on the helicopter owner’s lap in the left front cockpit seat during the flight and that the child could fully depress the left-side collective control by stepping on it with her left foot. The study also found that the collective lever’s full range of motion was 9.5 inches from full up to full down and that the spacing between the left edge of the seat, the collective, and the door are sufficient such that a child’s foot could rest on the collective and depress it. The study noted that the cyclic control could be moved to the full-aft position even with a small child of this size seated on the lap of an adult male in various positions.

Considering that the child was sitting on the owner’s lap in the left front cockpit seat, it is highly likely that the child inadvertently stepped on the collective with her left foot and displaced it to the full down position. This condition would have then resulted in either the pilot or the helicopter owner raising the collective, followed by a full-aft input pull of the cyclic control and the subsequent main rotor departing the normal plane of rotation and striking the left endplate and the aft end of the tail rotor drive shaft.

The National Transportation Safety Board determined the probable causes of this accident are:

The sudden and inadvertent lowering of the collective to near the lower stop, followed by a simultaneous movement of the collective back up and the cyclic control to a nearly full-aft position, which resulted in the main rotor disc diverging from its normal plane of rotation and striking the tail rotor drive shaft and culminated in a loss of control and subsequent impact with terrain. Contributing to the accident was absence of proper cockpit discipline from the pilot.

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