AOPA Hover Power

Helicopter pilots refer to certain operations as hot or cold. A hot operation is one where the engines are kept running during the procedure and cold is with everything shut down. The arguments for and against hot operations center around safety vs. time savings.

For EMS operators, one of the more common hot operations is loading the helicopter. Many times at an accident scene the pilot will keep the helicopter running while the medical crew gets out to retrieve the patient. Since the idea of helicopter EMS, especially with trauma patients, is to save time, the hot loading of patients is performed routinely. However, there have been studies that have shown very little time difference between hot loading, shutting down the helicopter, and then restarting it to depart. The argument for shutting down is that maneuvering around a running helicopter can be hazardous. For example, people have walked into tail rotors and objects have come in contact with the main rotor system. On the other hand, helicopters are mechanical machines and there have been cases where the helicopter failed to start. On an accident scene, this could shut down a highway for a much longer period of time and delay getting the patient to a trauma center.

Another hot operation that is performed is refueling. Pilots trying to save time or an engine start (turbine engines have start cycle TBOs) will ask to be hot refueled. For trained personnel this can be preformed safely on most helicopters especially when the fueling point is low and below the engine. A fueling port high up on the fuselage and above the engine increases the possibility of a fire if fuel spills. Also, climbing on a ladder or other object to reach the fuel port can place personnel dangerously close to a spinning rotor system.

The case for proper training was apparent several years ago when I was watching a Bell JetRanger giving rides at an air show. When the pilot needed fuel, I watched someone drive a pickup truck, with a fuel tank in the bed, up close to the helicopter. The driver climbed out, ran around to the back, and jumped up into the bed. He stood completely up and then quickly ducked. He obviously felted how close his head came to the spinning rotor system. I turned away because I thought he was going to get hit. I remember thinking, wow, he was lucky!

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.

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 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.

On some helicopters, running the length of the tail boom are “L” shape (or something similar) brackets that protrude about an inch. These are known as tail boom strakes and they act like spoilers.

Because the tail boom is underneath the rotor system, at a hover, very low airspeeds or sideways flight rotor down wash passes around the boom. Like an airfoil, this produces high and low air pressure areas that exert a force along the tail boom. This force decreases the tail rotor’s capability during hover and slow flight. At higher speeds the down wash moves to the rear and passes above the tail boom. Strakes control the airflow around the tail boom, thus increasing the tail rotor’s efficiency and decreasing the turbulent air, which improves yaw control.

During the late 1980s, NASA and the U.S. Army performed wind tunnel and flight tests to analyze the performance gain from adding a tail boom strake. The tests were performed using a Bell 204B helicopter. Published in 1993 the NASA Technical Report 3278 stated a 5-percent improvement in pedal control margin will provide an additional 2,000 feet of altitude capability or 500 lbs. of payload. The report concluded that the strakes improves handling qualities, reduces tail boom fatigue, improves climb and cruise performance, and increases yaw control safety margins for all single rotor helicopters with enclosed tail booms.

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.

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.

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.

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.

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.