Posts Tagged ‘Tim McAdams’

LTE

Friday, May 28th, 2010

The pilot of a Bell 206B helicopter approached a construction site located at Baltimore-Washington International Airport (BWI) and brought the helicopter to a 250-foot out-of-ground-effect hover with a quartering left tailwind. Once in a hover, the aircraft made a rapid right 180-degree pedal turn, stopped momentarily, and then began another rapid pedal turn to the right. The helicopter continued turning at a fast rate and entered a spinning vertical descent impacting Alpha taxiway abeam Runway 15R. The FAA’s examination of the helicopter found no mechanical anomalies.

The NTSB determined the probable cause was the pilot’s improper decision to maneuver in an environment conducive to loss of tail rotor effectiveness (LTE) and his inadequate recovery from the resulting unanticipated right yaw.

So what exactly is LTE? According to FAA Advisory Circular AC90-95, any maneuver that requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where an unanticipated right yaw may occur. It also advises of greater susceptibility for loss of tail rotor effectiveness in right turns and states the phenomena may occur to varying degrees in all single main rotor helicopters at airspeeds less than 30 knots.

Allowing a loss of translational lift results in a high-power demand with low airspeed and can set the helicopter up for LTE when certain wind conditions are present. Using the nose of the helicopter as a 0-degree reference, main rotor vortex interference can occur with a relative wind of 285 degrees to 315 degrees and cause erratic changes in tail rotor thrust. Moreover, be aware of tailwinds from a relative wind direction of 120 degrees to 240 degrees as this can cause the helicopter to accelerate a yaw into the wind. A tail rotor vortex ring state can also occur with a relative wind of 210 degrees to 330 degrees and cause tail rotor thrust variations.

To recover if a sudden unanticipated yaw occurs, apply full pedal to oppose the yaw while simultaneously moving the cyclic forward to increase speed. If altitude permits, power should be reduced.

Power source

Thursday, May 20th, 2010

What is the best power source for a helicopter? The two choices are a turboshaft or a reciprocating engine. A turboshaft engine has the same basic structure as a turbojet; however, the energy produced by the expanding gases is used to drive a turbine instead of producing thrust. The turbine is connected to a gearbox that drives the helicopter’s main rotor transmission. Likewise, the reciprocating engine’s output drives the main rotor transmission; however, these engines have traditionally been viewed as less reliable.

To understand where that reputation came from we need to look at early helicopter designs. Helicopter manufactures took piston engines used in airplanes and installed them in their helicopters. However, these engines didn’t quite have enough horsepower for hovering. So to increase the power, manufactures ran the engines at a higher rpm, and as a result reliability suffered. So much so that Lycoming reduced the TBO on the O-360 from 2,000 hours to 1,600 hours for engines installed in helicopters. This fueled the unreliable reputation of the piston engine.

In 1979 Frank Robinson introduced the two-seat R22. His idea was to reduce the helicopter’s weight to reduce the power required. For example, the T-bar cyclic system is simple and weighs less than the conventional dual control system. He then took the reliable Lycoming O-320 engine and reduced the rpm from 2,700 to 2,652 and de-rated the maximum horsepower from 160 to 124. Lycoming then approved the same 2,000-hour TBO it had for fixed-wing installations. He did the same thing with the R44’s Lycoming O-540 engine. The engine’s reliability proved so good that Lycoming increased the TBO to 2,200 hours for both airframes, giving these helicopter installations a higher TBO than the same engine installed in a fixed wing. NTSB accident data supports the higher reliability achieved by derating a reciprocating engine.

Even with the vast improvement in reliability, reciprocating engines suffer from a low power to weight ratio. So for helicopters above about 2,500 lbs gross weight, a turbine engine makes sense. It is compact, light weight, and has a simple design that gives it excellent reliability. However, perhaps the most important feature is its high power-to-weight ratio. This makes turboshaft engines the only choice for large single and all twin-engine helicopters. However, the downside to these engines is the high cost to acquire, maintain, and operate them.

Disc loading

Friday, May 7th, 2010

Disc loading is defined as the ratio of a helicopter’s gross weight to its rotor system’s disc area. A large disc area allows the rotor system to work with more air creating a higher efficiency in a hover. A smaller rotor system compromises hover efficiency for speed and a compact rotor system.

An example of a production helicopter with low disc loading is the Robinson R22. This improves the R22’s hover performance using the relatively low power of its Lycoming piston engine. Taking the concept of low disc loading to an extreme is human-powered flight in a helicopter. The low power output of a human requires a very large rotor system. Students at California Polytechnic State University at San Luis Obispo designed a human powered helicopter that weighted 250 pounds including the pilot/power source. It had a rotor diameter of more than 100 feet and was only designed to hover. In December 1989 it flew for 7.1 seconds reaching a height of 20 cm. It was built to compete for the Sikorsky Prize offered in 1980 by the American Helicopter Society. The award is $250,000 to the team whose human-powered helicopter can stay airborne for 60 seconds and reach an altitude of 3 meters. To date, the prize is unclaimed.

In contrast, a helicopter with high-disc loading requires a lot of power to hover. For example, the Sikorsky CH-53E Sea Stallion uses three General Electric T64-GE-416/416A turboshaft engines producing 4,380 shp each. Its gross weight is 73,500 lbs and has a rotor diameter of 79 feet. The CH-53’s rotor downwash in a hover is so strong that standing near it is nearly impossible. In addition, high disc loaded helicopters have rapid descent rates making them more challenging to autorotate. Taking high disc loading even further is the V 22 Osprey tilt rotor. It has two 38 foot diameter rotors and a max gross weight of 60,500 lbs. In order to hover it uses two Rolls-Royce Allison T406/AE 1107C-Liberty turboshaft engines producing 6,150 hp each.

Aircranes

Monday, March 29th, 2010

While some helicopters are designed for speed, others are built simply to lift a lot of weight. Perhaps the best example is the Erickson S64 Aircrane. The S-64 was the first helicopter built as a flying crane with an aft-facing pilot station that allows the pilot to directly view the load being carried and fully control the aircraft during precision operations. This unique helicopter was certified in 1969 and originally manufactured by Sikorsky Aircraft as the S-64A Skycrane. In 1992, Erickson purchased the type certificate to the Sikorsky S-64E and S-64F models, and the aircraft designation was changed to the S-64 Aircrane. Today, Erickson owns and operates a fleet of 18 Aircranes throughout the world.

The Aircrane’s rotor system consists of a six-blade fully articulated main rotor and a four-blade tail rotor. The S-64E is powered by two Pratt and Whitney turbine engines generating a combined maximum takeoff rating of 9,000 shp, giving the S-64E model an external load lift capacity of 20,000 pounds (9,072 kg) at sea level. The S-64F features a strengthened airframe, a rotor system with longer chord length, and two Pratt and Whitney engines rated at 9,600 shp which gives the S-64F model an external load capacity of 25,000 pounds (11,340 kg) at sea level.

Initially, the Aircrane’s civilian mission centered on timber harvesting and power line construction; however it has been used in many areas of heavy lift construction. For example, installing ski lifts, air-conditioning systems, and delicate steel artwork.

One of the most publicized jobs involved removing and replacing the Statue of Freedom, which sits atop the United States Capitol dome in Washington D.C. Using its precision maneuvering capability the Aircrane lifted the statue off of its pedestal on May 9, 1993, and placed it back after much needed renovation on October 23, 1993. Another high-profile project was the construction of the CN Tower in Ontario, Canada. In 1975, the Aircrane transported and placed the seven-ton steel sections that made up the antenna and weather metering systems, on at that time what was the world’s tallest freestanding structure, at an altitude of more than 1,850 feet.

In 1992 Erickson created the Helitanker firefighting system with a 2,650-gallon tank that can spray water, foam mix, or fire retardant. Two snorkel attachments take 45 seconds or less to fill up from any freshwater or saltwater source at least 18 inches deep. In 1997 the FAA certified a horizontal monitor water cannon attachment to fight high rise structure fires in congested urban areas. The cannon uses aircraft hydraulic power to propel a focused stream of water or foam mix up to 150 feet at a rate of up to 300 gallons per minute. The helicopter has now become a valuable firefighting tool in California and other parts of the world.

See the AOPA Pilot story on the Sikorsky Skycrane, “Dancing with Lucille.”

 

Thoughts on EMS training

Thursday, March 4th, 2010

The helicopter EMS industry is struggling with a high accident rate. Several months ago the NTSB published recommendations ranging from equipment requirements to increased training. There seems to be no doubt in the helicopter industry that the FAA will mandate one or more of the NTSB recommendations this year. In the past the FAA has been reluctant to act; however, the feeling now is if the FAA does not come out with something strong to stop the accidents, Congress will.

In my opinion, increasing the amount and type of training will do the most good. Using technologies such as HTAWS and NVGs are helpful as well, but I think the most benefit will come from better training.

EMS is a tough business with lots of cost pressures, and spending more money on training can be hard to justify sometimes. I was told by one EMS vendor that watching costs was paramount to survival, if he couldn’t bid a competitive price and lost contracts they’d be out of business.

An interesting dichotomy was when I flew a corporate helicopter. I was trained at FlightSafety every six months and could take the helicopter (a Bell 430) out once a month to practice. The corporate mission was nowhere near as demanding as EMS flying, yet there was considerably more emphasis placed on training. Sometimes I wonder if the difference was because the person who ultimately approved the training budget also rode in the back of the helicopter. Those passengers certainly had a vested interest in the proficiency of the pilots.

It will be interesting to see what the FAA does. If operators can afford the technology and the increased training then that’s the best scenario. However, if it’s one or the other I believe the best improvement in the accident rate will come from enhanced training.

Drive link

Monday, February 15th, 2010

Connecting the rotating swash plate to the rotor shaft is an assembly known as the drive link. Because the swash plate needs to move up and down and pivot, the drive link has a joint that acts like a scissor – as such it is sometimes referred to as a scissors link. I have had several students ask me why it is needed.

The swash plate has a rotating and non-rotating side. The non-rotating side is on the bottom and is connected to the flight controls. The rotating side is on the top and is connected via pitch links to each rotor blade. The collective control moves the entire swash plate assembly up and down to change the pitch on each blade equally. The cyclic control tilts the swash plate, changing each blade’s pitch independently depending on its position around the rotor disk. This tilts the rotor disk in the desired direction.

Since the rotor mast runs from the transmission up through a sleeve that the swash plate moves around, there needs to be a method of turning the rotating part of the swash plate. This is the function of the drive link as it connects the mast directly to the swash plate. It is critical that this part be functioning correctly.

During preflight it should be examined closely as the failure of the drive link has caused several accidents. On the Bell 222 an improperly sized bolt that attached the drive link to the swash plate allowed play which caused the bolt to fail. As you can imagine without the drive link the blades will continue turning the swash plate through the pitch links. This stresses the pitch links in a manner they were not designed to handle and can result in a pitch link failure. In this case with the Bell 222 it caused an in-flight break up.

In 1988 the pilot of a Bell 47 spraying a field reported an extreme vibration followed by a loss of control and hard landing. Then in 1992 a CFI and student flying another Bell 47 also felt a sudden and severe vibration and managed to successfully autorotoate to a field. In both cases the center bolt connecting the drive link was missing and disconnected drive to the swash plate.

Low-G pushovers

Friday, January 29th, 2010

A two-blade or semi-rigid rotor system (such as the Robinson or some Bell series helicopters) is susceptible to a phenomenon called mast bumping. To avoid mast bumping it is important to fully understand the limitations and performance capability of this type of rotor system.

In order to produce thrust a helicopter’s rotor system must be loaded. Controlled by the cyclic, the swash plate changes the pitch angle on each blade separately. This creates an imbalance of thrust across the rotor disc forcing the disc to tilt, which causes the helicopter to roll or pitch in the desired direction.

Pushing the cyclic forward following a rapid climb or even in level flight places the helicopter in a low G (feeling of weightlessness) flight condition. In this unloaded condition rotor thrust is reduced and the helicopter is nose low and tail high. With the tail rotor now above the helicopter’s center of mass, the tail rotor thrust applies a right rolling moment to the fuselage (in a counter-clockwise turning rotor system). This moment causes the fuselage to roll right and the instinctive reaction is to counter it with left cyclic. However, with no rotor thrust there is no lateral control available to stop the right roll and the rotor hub can contact the mast. If contact is severe enough it will result in a mast failure and/or blade contact with the fuselage.

In order to recover the rotor must be reloaded before left cyclic will stop the right roll. To reload the rotor immediately apply gentle aft cyclic and when the weightless feeling stops, use lateral cyclic to correct the right roll.

The best practice is to exercise caution when in turbulent air and always use great care to avoid putting the helicopter in a low-G condition.

Safer night ops

Tuesday, January 19th, 2010

Threats, clearly visible during the day, are masked by darkness. In fact, controlled flight into terrain (CFIT) at night is a major problem for rotor-wing operations. CFIT is defined as colliding with the Earth or a man-made object under the command of a qualified flight crew with an airworthy aircraft.

During the 1970s, CFIT became a major problem for commercial aviation. In response the FAA mandated the installation of ground proximity warning systems (GPWS) in commercial airliners. Although this resulted in a drop in CFIT accidents, these earlier systems were plagued with false and late warnings. Improved versions, called enhanced ground proximity warning systems (EGPWS), were introduced. These systems have made a valuable contribution to the reduction of fixed-wing CFIT accidents.

CFIT at night during VMC has been especially troublesome for helicopters in the air medical industry. According to the Air Medical Physician Association, half of all EMS accidents happen at night. EGPWS have been discussed as a solution to reduce the air medical helicopter accident rate. However, because of the unique low-flying operation of helicopters the effectiveness of current EGPWS is unclear. This prompted Honeywell to introduce the Mark XXII EGPWS, specifically designed to address the needs of helicopters. Moreover, the company is developing a database of power lines to add to the system. As computer memory capability grows, databases will be able to contain more detailed maps.

However, by the time the EGPWS activates, the pilot has probably already lost situational awareness. A method to help with situational awareness is improving the pilot’s ability to see obstructions at night. That’s the technology behind night vision goggles (NVG). They work by detecting and amplifying existing visible light, so there must be at least some light available for them to work. Originally NVG were only for military use, but recently they have been allowed in the air medical industry, and more than half of the EMS helicopters are flying with them.

Another technology that holds promise is enhanced vision systems (EVS) which detects and displays thermal energy not visible to the naked eye. In this arrangement a camera is mounted in the nose and feeds the image to a monitor in the cockpit. Some glass cockpit systems will project the image behind the attitude indicator for better situational awareness. These systems are effective in smog, smoke, duststorms, and other limited visibility situations. Likewise, they can help in brownout and whiteout conditions. The U.S. military uses thermal imaging systems in combination with NVGs.

The air medical industry is expecting the FAA to possibly mandate additional equipment requirements like they did with earlier with commercial aviation. With the different technologies available it will be interesting to see what happens.

Servo transparency

Friday, January 8th, 2010

Pilots who learn to fly in smaller helicopters probably hear very little about servo transparency, yet this phenomenon has caused or played a role in several accidents. When giving flight reviews I have found some helicopter pilots who totally misunderstand why and how it happens. However, the concept is not too difficult to understand.

Because of the higher control forces in larger helicopters, hydraulically boosted servo actuators are used to assist the flight controls. The maximum force that these servo actuators can produce is constant and is a function of hydraulic pressure and servo characteristics. Engineers design the hydraulic system to adequately handle all aerodynamic forces required during approved maneuvers. Even so, with certain aggressive maneuvering it is possible for the aerodynamic forces in the rotor system to exceed the maximum force produced by the servo actuators. At this point, the force required to move the flight controls becomes relatively high and could give an unaware pilot the impression that the controls are jammed. To prevent servo transparency, pilots should avoid abrupt and aggressive maneuvering with combinations of high airspeed, high collective pitch, high gross weight, and high-density altitude.

The good news is that this phenomenon occurs smoothly, and can be managed properly if the pilot anticipates it during an abrupt or high-G load maneuver. On clockwise-turning main rotor systems the right servo receives the highest load, so servo transparency produces an un-commanded right and aft cyclic movement accompanied by down collective. The pilot should follow (not fight) the control movement and allow the collective pitch to decrease while monitoring rotor rpm, especially at very low collective pitch settings. The objective is to reduce the overall load on the main rotor system. It normally takes about two seconds for the load to ease and hydraulic assistance to be restored. However, be aware that if the pilot is fighting the controls when this happens, the force being applied to the controls could result in an abrupt undesired opposite control movement.

Many of these accidents have happened while aggressively flying the helicopter at low altitudes, leaving very little time to recover. Most important for avoiding this kind of accident is to follow the aircraft limitations published in the helicopter’s flight manual.

Above reproach?

Wednesday, December 30th, 2009

Commenting on my gross weight blog, Harold wrote:

“Leave the flying to he who is in the cockpit and the finger-pointing blogs to another publication please.”

That got me thinking, when is it (if at all) appropriate to comment, criticize, or even intervene on another pilots actions or behavior? I understand and agree with Harold to a point, but I don’t believe the complete answer is all that clear.

I have studied and written about helicopter accidents for many years. I think most of them have a lesson that can help us all be better pilots. I try to write about these in a way that states the facts without expressly passing judgment (gross weight included) and let the readers draw what they want from the situation. Believe me, I have made my share of mistakes but I have been lucky because they didn’t result in an accident. I have viewed them as learning experiences, because had something been just a little different I might not have been so lucky. I like to tell people that I can’t promise I won’t make a mistake, but I can promise I won’t make the same one twice. Having studied many accidents it is clear that there are no new accidents only the same ones repeated over and over, just in a different manner.

I also believe that simply being a licensed pilot does not make you above reproach. Listed below are three examples of pilot behavior that other people knew was dangerous. A link to the complete NTSB report is included because all the details can’t be listed here.

A pilot flying a news helicopter was well known as a hotdog and the photographer riding with him had expressed concern. His last radio transmission was “watch this” as he pulled the helicopter vertical and severed the tail boom killing himself and the photographer.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001212X20685&key=1

A very experienced tour pilot flying in the Grand Canyon was well known for being a skilled pilot and for his aggressive flying. He had earned the nickname “Kamikaze.” At high density altitude he slammed into a canyon wall killing himself and six passengers.

http://www.ntsb.gov/publictn/2007/AAB0703.pdf

A pilot continued to fail phase checks, check rides, and pre-employment rides. He eventually got a job where his flight skills were not evaluated prior to being hired. He crashed an R22 killing himself and a passenger on an introductory flight.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20060228X00255&key=1

I really appreciate all the professional comments that people post. So if this subject interests you please take the time to read all the details and let us all know your thoughts. I believe that approaching this topic in the correct way can be a powerful learning tool for those so inclined to listen.

My intent is not to point fingers but to get pilots thinking about how easily an accident can happen. I know that reviewing accidents has helped me be a better pilot. However, I am very curious if other pilots find this helpful.

One final thought. I have been involved as an expert witness for helicopter accident cases in court and believe me the intense scrutiny pilots endure is not pleasant. Seeing that has given me another reason to believe that being ultra conservative to avoid an accident is well worth it.