Technique Archive

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.

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.

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.

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.

Wire strike protection

Thursday, December 10th, 2009

I fly a Bell 206 JetRanger helicopter as a demonstration aircraft for my company’s autopilot and glass cockpit systems. It is equipped with a Wire Strike Protection System (WSPS) and many times I am asked what it is and how it works.

Bell 206 with Wire Strike Protection

Bell 206 with Wire Strike Protection

The system on the Bell 206 has three main components: an upper cutter, lower cutter, and deflectors. Each cutter has a deflector that forces the wire into sharp high-tensile steel blades (they are rubber coated to prevent inadvertent injury to service personnel). An additional deflector strip runs vertically between the pilot and copilot windscreens to guide the wire to the upper cutter. On different helicopters other deflectors are mounted as necessary to protect critical areas. For example, on the toes of the skids to force a wire to go under the helicopter and stop it from getting caught between the skid gear and the fuselage.

It is a passive protection system that reduces the chances of an accident in the event that the helicopter is flown into horizontally strung wires. The key phrase is “reduces the chances” as the system is not 100-percent effective. In order to work properly the helicopter needs forward speed; faster speeds increase the probability of cutting the wire. Also the level of effectiveness is a function of several other factors including where the wire impacts the fuselage, the cable tension, and the diameter of the wire.

The US Army evaluated the WSPS by performing pendulum swing tests using a Bell OH-58 (basically a military version of the Bell 206). The tests went well and they adopted the system for use on U.S. Army helicopters. Since then several Army helicopters have hit wires that were then cut by the system resulting in no injuries and minimal to no aircraft damage. Several civilian helicopters equipped with the WSPS have cut wires and avoided an accident as well.

Of course the best protection from wire strikes is prevention. Some things to consider are only flying below 500 agl when it’s necessary, looking for poles because they are easier to spot than wires and when you need to fly low over wires cross at the poles or supporting structures. Additionally, when landing in unapproved areas be sure to perform a complete aerial reconnaissance. If your helicopter is equipped with wire strike protection it should be viewed as a last line of defense.

Gross weight

Wednesday, December 2nd, 2009



I was in a pilot lounge at a heliport where an operator was giving sightseeing rides when a pilot returned to the loading area after getting fuel. The loaders brought out five passengers and I heard the pilot say over the radio that they put too much fuel onboard and he could only take four passengers. Right then another pilot who worked for the sightseeing operation jumped up and said, “No, don’t take the passenger back. I’ll do the flight.” He ran out and told the pilot he would take over so the pilot could take a break. As I watched the helicopter lift off, the guy standing next to me (who was not a pilot) said, “Now there goes a real pilot.” I looked at him waiting for him to crack a smile or give me some signal he was kidding. He was serious.


I have known helicopter pilots who don’t think too much about gross weight. If it can hover, it will fly fine they would say.


Case in point, according to the NTSB the pilot of a Bell 206L departing on a sightseeing flight on a hot summer day lifted the helicopter to a hover and started a takeoff run. The pilot said it felt like the helicopter did not have full power and it did not gain altitude as it neared the end of the heliport. The tail rotor struck the edge of the pier. The helicopter then hit the water, the pilot deployed the floats, and the helicopter rolled inverted.



When the pilot was questioned about the lack of engine power, he stated that sometimes dirt or dust could lodge in the fuel system and then dislodge from the impact. When asked if the helicopter was overweight, the pilot stated no, because he was able to hover with an indicated turbine outlet temperature (TOT) of 720C and 92-percent torque.


Regarding the helicopter’s weight and balance, the pilot stated that he did not ask passengers their weight and did not have a scale at the heliport. Rather, he estimated the weight and balance. For the accident flight, he estimated 150 lb. per person, as there were three male passengers and three female passengers. After the accident, an FAA inspector questioned the passengers about their weights. The passengers reported their weights as 132 lb., 176 lb., 187 lb., 207 lb., 210 lb., and 213 lb. In addition, the pilot weighed about 190 lb. Although the pilot estimated 150 lb. per passenger, the average weight of the passengers was approximately 188 lbs. Those passengers plus fuel made the helicopter about 250 lbs. over gross weight at the time of the accident, not including the weight of clothing, personal effects, and baggage.


The FAA puts the standard average weight for operators with a no-carry-on bag program to 184 lb. in the summer and 189 lb. in the winter.



Copter ILS

Monday, November 23rd, 2009

I remember in the mid-1990s Copter ILS approaches began showing up in the New York area. They came from an interpretation by the FAA’s Eastern Region of the Part 97 U.S. terminal instrument procedures (TERPS) that granted helicopters lower minimums. The prevailing thought was that because of a helicopter’s unique maneuvering capabilities the craft could safely operate with lower minimums. I had flown these approaches a couple of times and they seemed to work well.

The Copter ILS approach used the existing ILS, but allowed helicopters a DH of 100 feet and an RVR of not less than 1,200 feet. Although this was basically CAT 2 minimums there was no aircrew qualification required. Moreover, pilots were flying below 200 feet without visual reference to runways that did not have CAT 2 certification. So in 2000, (Copter ILS approaches had been flown for years without incident) citing concerns over technical issues such as signal strength and reliability below 200 feet, threshold clearances and lighting, the FAA issued a Notice to Airmen (NOTAM) that terminated the Copter ILS approaches.

Various industry groups worked with the FAA to help re-establish the lower minimums. Today there are Copter ILS approaches; however, they overlay CAT 2 approaches as this solves the technical TERPS issues. They also require special aircrew and aircraft qualification. An example is Ronald Reagan Washington National Airport (DCA) Copter ILS or LOC RWY 1 approach (

Rotor downwash

Friday, November 6th, 2009

Jim Thomas asked, “Are the hazards of a helicopter rotor blast taught to new students?” The answer is sometimes and sometimes not. Being dual rated, I understand the affect the rotor down wash can have on an airplane. As an instructor, it is always something I teach to students.

Wind direction and strength must be considered when hovering near other aircraft. For example, when hovering with a strong right crosswind, leave more room between other objects and left side of the helicopter. The rotor downwash will be very strong on the left side and very weak on the right side. I have had line personnel direct me to park too close to another aircraft that was downwind of the rotor wash. I would just touchdown in a better suited area and explain why once they approach the helicopter.  Some understood, some did not.

However, it’s not just parked airplanes that can get tossed around. I was on a hospital helipad with another helicopter when the other pilot needed to depart. While he was starting up I was busy finishing some paperwork. There were no obstacles and the wind was calm, so the last thing I expected was for him to depart directly over top of my parked helicopter. That’s exactly what he did causing the blades to flex down, and the fuselage to shake. I was sitting in the helicopter with the door open and fortunately was able to get it closed. If I were to guess, he couldn’t have been more than 10 feet above me. I never understood why he did that.

On another occasion, I was hover taxing along the edge of a ramp to parking when I noticed some smaller airplanes rocking a little and in front of me was a pilot with an open door. I moved out toward an empty taxiway and ATC immediately told me I was too close to an active taxiway. I explained why and he said, “Understand, but you can’t say there.”

It’s unfortunate that students are not taught more about this subject. However, many pilots are aware of their prop or rotor blast and act courteously and try to minimize the impact on others. However, some either don’t understand or care. To protect yourself and your aircraft around an airport or heliport, I think the best advice is to always assume you could be subject to a prop blast or rotor downwash.

Bad ideas

Wednesday, October 28th, 2009

There are some things that helicopter pilots do that are just not smart.

For example the pilot of a Robinson R22 Beta landed in a field to pick up some equipment and while he was there he decided to hot refuel. The pilot’s father drove a pickup truck equipped with an auxiliary fuel tank under the rotor disk of the running helicopter to accomplish the refueling. The pilot said he stayed at the controls of the helicopter and a wind gust caused the main-rotor blades to flex down, striking the top of the truck. Although no one was injured, the helicopter rolled to the right and into the truck resulting in structural damage to the helicopter. At the time this happened winds were reported from 170 degees at 18 knots, gusting to 25.

Another bad idea is leaving the cockpit while the engine is running and the rotor system is spinning. That’s how a pilot damaged an Enstrom 280X after landing in a corn field and getting out of the helicopter. In an interview with the NTSB, the pilot stated a gust of wind appeared and the main rotor severed the tail boom.

Another pilot preparing to lift-off in an S76 noticed a “door unsecured” indication on the instrument panel for the left cabin door. He brought the engines to idle and exited the cockpit to check the door. He re-closed the door and returned to the cockpit. However, the door open annunciation came on again. He then left the cockpit two or three times to deal with the door. He did not recall retarding the engine power control levers to ground idle before leaving the cockpit the final time.

The wheel-equipped helicopter started to move as the pilot was returning to the cockpit. He told the NTSB it was moving toward the edge of the elevated helipad. He managed to climb into the cockpit, but before he could regain control, the helicopter was on its side.

I can remember several times getting ready to depart and then realizing that I needed to check or do something. It is very tempting to just friction down the flight controls and get out. However, every time I consider doing that I think of what has happened to other pilots.