AOPA Hover Power

On a sectional map, many large airports have “NO SVFR” printed near the airport information. SVFR refers to Special VFR, which allows a pilot to fly in lower visibility in controlled airspace. When giving flight reviews to helicopter pilots, I ask what that means. Occasionally, I am told that SVFR is not permitted at that airport. The correct answer is SVFR is not permitted for fixed-wing aircraft. FAR 91.157 states the requirements for SVFR, and appendix D, section 3 contains the verbiage that prohibits SVFR for fixed-wing only.

In fact, many of the requirements for SVFR are different for helicopters. A helicopter pilot still needs an ATC clearance and must remain clear of clouds; however they are exempted from the 1 statute mile restriction and the requirements for night time operations. Additionally, helicopters are excluded from the takeoff and landing requirements outlined in 91.157 (c). However, as with fixed-wing aircraft, the controller cannot suggest SVFR, a helicopter pilot must still request it.

In Canada the requirements for helicopters are more stringent than in the US. For example, a pilot must have at least 500 hours as pilot-in-command, have completed an approved pilot decision making course and received ground/flight instruction on issues related to reduced visibility. More details on additional requirements can be found in Canadian Aviation Regulation 602.117.

Many helicopter photo flights are performed in small helicopters like the Robinson R22 or Schweitzer 300. As a result, pilots tend to be less experienced. This coupled with the need to perform some demanding maneuvers, photo flights can be dangerous. In fact, Robinson Helicopter issued Safety Notice SN-34 in March 1999, titled “Photo Flights – Very High Risk.” It describes the problems encountered when the pilot slows the helicopter below 30 KIAS and then attempts to maneuver the helicopter.

“The helicopter can rapidly lose transitional lift and begin to settle,” it states. “An inexperienced pilot may raise the collective to stop the descent. This can reduce rpm, thereby reducing power available and causing an even greater descent rate and further loss of rpm. Because tail rotor thrust is proportional to the square of rpm, if the rpm drops below 80 percent nearly half of the tail rotor thrust is lost and the helicopter will rotate nose over. Suddenly, the decreasing rpm also causes the main rotor to stall and the helicopter falls rapidly while continuing to rotate.” The safety notice recommends photo flights only be conducted by well-trained, experienced pilots.

The following accident supports Robinson’s recommendation.

According to the NTSB, on May 28, 2005, about 1150 Pacific Daylight Time, a Robinson R44 impacted terrain while maneuvering during a low-level photo flight near Lucerne Valley, California. The accident site was located at 4,266 ft msl and the temperature was about 90F, creating a density altitude of 7,350 ft. The owner, a private pilot, was seriously injured as was a safety pilot (who was also a CFI) and one passenger.

A witness reported that shortly after crossing the racecourse southbound at a low altitude it appeared that the helicopter was attempting to reverse course back toward the north. The helicopter pitched nose down and leveled off just before it impacted a dry streambed. Upon impact, the helicopter burst into flames. All three people on board sustained burns while exiting the burning helicopter.

The CFI reported he was the safety pilot for the flight and not pilot-in-command. He explained that while southbound and crossing the racecourse the private pilot started to turn the helicopter to the right when the helicopter began spinning to the right. The private pilot told him he had lost control and asked for help. The CFI took over the flight controls and tried to keep the helicopter in a level attitude. The helicopter was descending and the CFI realized the rotor rpm was decaying. He knew he was too low to try to recover the rpm so he tried to cushion the impact with the collective. The helicopter impacted the ground and rolled onto its left side.

The private pilot also stated he was flying southbound along the racecourse then made a hard right 180-deg turn and lost control of the helicopter. He indicated he used to fly off road races in his airplane and this was his second flight using a helicopter. He added the accident flight was the first time he had flown this type of operation with his own helicopter and as pilot-in-command. The pilot had just completed the Robinson Helicopter safety course, but he stated he did not know about Robinson Safety Notice SN-34.

The pilot held a private pilot certificate with ratings for airplane single-engine land and multi-engine land. An additional rating for rotorcraft-helicopter was added seven days prior to the accident. At that time, the pilot reported a total airplane time of 1,550 hours and total helicopter time of 50 hours. The CFI held a commercial pilot certificate with a rating for rotorcraft-helicopter and a certified flight instructor rating for rotorcraft-helicopter. According to the CFI, he had 520 hours total flight time in rotorcraft, including 130 hours of flight instruction given. The CFI had received his endorsement for the R44 seven days before the accident.

Helicopter manufactures publish a chart in the flight manual that depicts combinations of airspeed and altitude that should be avoided. It is commonly referred to as the H-V curve or, technically, the height-velocity diagram. Typically it is located in the performance section of the flight manual, not the limitations section, so the pilot is not prohibited from flying in these areas. The chart shows shaded areas that should be avoided because in the event of a power failure the helicopter might not be able to perform a successful autorotation.

The instant that a helicopter’s engine quits, it has stored energy in the form of altitude, airspeed and rotor rpm. A successful autorotation is the effective use of that energy to safely land the helicopter. It is worth noting that this same energy, if not used properly, can destroy the helicopter and its occupants. The shaded area on the left side of the chart shows low airspeeds and altitudes where the helicopter does not contain enough stored energy to perform a successful autorotation. The bottom of the graph also shows a shaded area. This area of low altitude, high speed flight should also be avoided because it does not allow the pilot sufficient reaction time to establish a level attitude and may require an aggressive flare that could result in the tail rotor striking the ground.

The chart shown here is from an R44 and depicts a shaded area for sea level and 8,500 feet density altitude. It also shows a recommended take off profile that favors airspeed over altitude until about 50 kts. Other factors such as high power settings (more pitch in the main rotor blades will cause a faster decay of rotor rpm due to drag), high gross weight and pilot experience (the chart is based on the reactions of an experienced pilot) can affect the outcome as well. Due to the nature of helicopter operations like confined area take offs, sometimes pilots need to operate in the shaded area. Knowing the H-V diagram for the model helicopter you are flying is important for understanding when recovering from an engine failure might be difficult or even impossible.

In 1979 Bell Helicopter certified the Model 222 helicopter to target the corporate market. Although it had a sleek corporate look, the helicopter struggled to find acceptance in the business world. This was due to reliability problems with the Lycoming LTS 101 engines and the two-blade rotor design that could not achieve an acceptable level of smoothness.

As a result, Bell began working on a new helicopter that would use advanced technologies to improve the engines, rotor system, and cockpit. In 1994, with the new design not quite ready, Bell introduced the model 230 with the more powerful Allison 250-C30 engines and numerous small refinements as an interim fix for many of the 222’s problems.

Finally, in early 1996, Bell certified its next generation helicopter, the model 430, and stopped production of the 230. The Bell 430 has a bearingless four-blade composite rotor system combined with Liquid Inertia Vibration Eliminators (LIVE) mounts on the transmission that give it a smooth ride. The engines were upgraded to the more powerful FADEC controlled Allison 250-C40B and a glass cockpit was available. The airframe was stretched 18 inches making for a larger cabin and the gross weight went from 8400 lbs. to 9300 lbs. One of my favorite features is the pilot’s side (right seat) collective control that moves forward and aft in a horizontal arc instead of up and down. To me it felt more natural.

Although, a little underpowered Bell did a good job with this helicopter, it was smooth, stable and fun to fly and had gained acceptance with corporate and EMS operators. Unfortunately, in January 2008 after building 136 helicopters, Bell announced they are stopping production of the 430 citing that it is optimizing its commercial product line to better serve its customer base and accelerate deliveries of its high-demand aircraft.

Bell 430

Bell 222

When a critical component in a helicopter’s main rotor system fails in flight, how much warning, if any, does a pilot get with these kinds of failures? Unfortunately, helicopters typically do not have cockpit voice recorders (CVR) so it can be hard to understand exactly what happened. Consequently, the following accident is unique in that it provides some insight as to what the flight crew knew. 

On Nov. 27, 1999, a CVR equipped Bell 212 crashed near Philadelphia, Miss. The transcript of communications recorded on the cockpit voice recorder showed that about 18 min. before the accident, the passenger (who was also the aircraft’s mechanic) stated to the pilot, “Boy, those catfish are going crazy down there, aren’t they?” 

“Yep,” the pilot responded, “must have been the vibrations from the helicopter.” 

About 2 min. later, the passenger and pilot discuss sighting deer in a field. About 1 min., 30 sec. before the accident, the pilot asked the passenger, “Has this vertical (a term used to describe a vibration that moves up and down) just gotten in here or has it been here for a while?” 

“We haven’t had any verticals at all,” the passenger replied. 

“We do now,” the pilot said. 

“Yeah, well it started right after we left back there,” the passenger said. “I think it maybe, ah, that’s why I was thinking it was the air.” 

About 20 sec. later, the passenger stated that another person had tracked the helicopter’s blades before they left and that he was commenting on how smooth it was. Forty seconds after that, the pilot said, “This stuff is getting worse.” 

The recording then ended. 

The National Transportation Safety Board determined the probable cause of this accident was the failure of the pilot and company maintenance personnel during preflight and periodic inspections to identify the signs of fretting and looseness in the red main rotor blade pitch change horn to main rotor blade grip attachment. As a result, the NTSB found, the helicopter was allowed to continue in service with a loose pitch change horn, which led to separation of the pitch change horn from the blade grip and the in-flight breakup of the helicopter after the main rotor struck the tail boom. Contributing to the accident, the safety board said was the pilot’s failure to respond to increased vibration in the main rotor system and land immediately. 

Interesting to note is that the pilot and mechanic were aware of the vibration, but apparently never considered a precautionary landing. Any pilot would land immediately when a sudden and severe vibration occurs. But any unexplained vibration should warrant a precautionary landing. Some parts and bearings that become loose can experience exponential wearing and fretting and quickly reach a failure point. 

Many components on a helicopter can fail and still allow the pilot to make a safe landing. The main rotor system is normally not one of them. Thus, any abnormal low-frequency vibration felt in the airframe or through the flight controls should be treated with extreme caution.

One of the early pioneers of helicopter flying was Carl Brady. In early 1947 he was crop dusting in a Stearman airplane when he saw a Bell 47B-3 spraying a field. Intrigued, he approached the owner and worked a deal out to get his helicopter pilot license. That same year he and two partners leased a couple of Bell 47B-3 helicopters and started their own operation.

Early helicopters, including the Bell 47B-3, used wheels for landing gear – probably a design borrowed from airplanes. Brady discovered that this was a bad idea for helicopters. He was known to tell a story that many consider the birth of skid type landing gear. It was 1948 and he and a former Bell mechanic were flying for the first time in Alaska. They discovered that the wheels would caster on rocky mountain tops or slopes causing the helicopter to roll downhill. To solve this problem they had a local sawmill cut two two-by-fours out of hardwood and using clothes line tied them to each wheel. It kept the wheels from castering and made landing on soft terrain much easier. Because there was no STC, they would fly their missions during the day and then remove the two-by-fours and fly back to town.

I have never read anything regarding the former Bell mechanic’s comments on the Alaskan adventure; however, two years later Bell introduced the Model 47D-1 with metal tube skid gear instead of wheels. This design became the standard for light helicopters for decades.

On May 12, 2009, a Robinson R-44 helicopter was damaged during a hard landing about 57 miles northwest of Iliamna, Alaska. The purpose of the flight was game management patrol for the Alaska State Troopers, Fish and Wildlife Service. After take-off from a ridge, about 300 feet above the ground, the helicopter was flying about 90 knots when the pilot felt an unusual medium-frequency vibration in the controls. The pilot told the NTSB that the vibrations turned to oscillations in both yaw and pitch to the point he felt the helicopter was going to come apart. He decided to make an immediate precautionary landing. During the descent the vibration increased and the helicopter landed hard causing the main rotor blades struck the tail boom.

The NTSB discovered that operators of the Robinson R44 helicopter were aware of similar events and that the condition had been dubbed “chugging.” According to Robinson Helicopter, tests determined that a mast rocking oscillation may develop during operation of the helicopter at high gross weight and about 90 to 100 knots. The oscillation was more of a “bucking” motion due to the fore-and-aft movement of the rotor mast. Tests also showed the tendency to enter the oscillation was exacerbated by a forward CG (within the CG envelope) and a 30 degree banked turn to the left. The oscillation is not divergent (that is, the main rotor blades do not diverge from their normal plane of rotation) and can be reduced by adding power. The oscillation is due to the firmness, or lack of firmness, of the transmission mounts. At the time there were no service alerts/bulletins referencing the phenomena or the remedies to resolve it.

On August 22, 2011 the NTSB issued the following safety recommendations to the FAA:

• Require Robinson Helicopter to resolve the root cause of the mast-rocking vibration in the main rotor assembly to ensure that all applicable R44 helicopters are free of excessive vibrations in all flight regimes, as required by 14 Code of Federal Regulations Section 27.251, “Vibration.” (A-11-82)
• Require Robinson Helicopter to maintain a database of all reported incidents of mast rocking in the main rotor assembly of R44 helicopters. (A-11-83)
• Require Robinson Helicopter to issue a service letter to all approved service centers describing the mast-rocking vibration that can occur in the main rotor assembly of R44 helicopters and instructing service centers to report all incidents of mast rocking to the manufacturer. (A-11-84)
• Require Robinson Helicopter to amend the R44 helicopter flight manual to inform pilots of the potential for mast-rocking vibration in the main rotor assembly and how to safely exit the condition. (A-11-85)
• Require that the Robinson Helicopter R44 pilot training program be revised to provide pilot instruction in the recognition and mitigation of in-flight mast-rocking vibrations in the main rotor assembly. (A-11-86)

Most helicopter pilots are aware of mast bumping in semi-rigid (two-bladed) rotor systems, but this issue is new and not as well-known and raising awareness is important to safe operations.

 Wind is defined by Webster’s dictionary as a strong current of air. Although simple in definition, the affects of wind on a helicopter can be profound. To a pilot who clearly understands this, wind can be very helpful. Yet, helicopter pilots sometimes underestimate the risks of flying in gusty wind conditions.

On March 27, 2002, the pilot of a Hughes 269 helicopter lost control while hovering at the Fort Collins Downtown Airport. The flight instructor reported that the wind was about 2 knots at takeoff, but forecasted to be gusty in the afternoon. While hovering at about 3 ft with the student pilot at the controls, the helicopter encountered a very strong gust and began to wobble. The instructor took control of the helicopter and climbed to about 15 ft when another gust hit the helicopter, turning it sideways and then downwind. The instructor stated he was attempting to get it on the ground, but the wind continued to drive the helicopter forward with excessive nose-over tendency. With the tail rotor into the wind, creating a high power demand and limited tail-rotor authority, the helicopter skipped along the dirt two or three times. The helicopter traveled forward 180-200 ft. The right strut failed, and the helicopter rolled over on its right side.

The instructor reported that he thought the wind was gusting to 60 knots at the time of the accident. The reported weather at the Fort Collins-Loveland Municipal Airport, 8 nm south of the accident site, was wind from 260 deg at 13 knots, gusting to 25.

Extra care should be taken when sitting on the ground with less than 100% rotor rpm in windy conditions. Wind can affect the flexing of a rotor blade at low rpm much more than at normal speed. An Enstrom 280FX helicopter was substantially damaged when the main rotor blades struck the tail boom while sitting on the ground. In an interview with the NTSB, the pilot stated he landed in a corn field and got out of the helicopter while the rotors were still under power. Then, he said a gust appeared and the main rotor severed the tail boom.

It should go without saying that leaving the pilot station of a helicopter with power still applied to the rotor system is just a really bad idea under any circumstances.

A certificated flight instructor (CFI) had his student were practicing hover taxiing before concluding the last of three flights in a Bell 47D–a model known for its docile flight characteristics and forgiving nature. The student had trouble that day maintaining rotor RPM during maneuvers, so the CFI looked inside to check as the student started to apply collective. When the CFI looked back outside, the helicopter was nose high and rolling to the right. He tried unsuccessfully to recover. The main rotor blades struck the ground. No one was hurt.

Due to the highly responsive characteristics of helicopters, the briefest bit of inattention by a CFI can result in an accident. This has haunted anyone who has ever worked as a CFI. Yet, in defiance of logic, we rely on the least-experienced pilots to do the vast majority of primary flight instruction. It should be no surprise that flight instruction has the highest accident rate among commercial helicopter operations.

One problem is that many pilots instruct just to build the time needed to get a better job. Competency as a CFI requires more than that. To be effective, it requires an interest in and desire for instructing. A CFI applicant needs only a cursory knowledge of teaching theory to pass the FAA’s fundamentals-of-instructing written test. It is a far more complex matter to understand how the mind processes information and learns, but a thorough understanding of this is what separates a professional teacher from a time-builder.

Flight instruction is demanding. A CFI must allow extremely inexperienced people to manipulate the flight controls, typically in a light, highly responsive, and unforgiving Robinson R22, in which most primary flight instruction is done. Instructors must continually weigh when the time is right to take over the controls. A student can benefit from correcting his own mistakes, but allowing a student to go too far might make the helicopter unrecoverable. Accident reports from the NTSB consistently list delayed remedial action and inadequate supervision as probable causes in training accidents. Such reports offer a wealth of information, and their complete review can be a great learning tool for CFI applicants.

If you have ever closely watched a hovering helicopter, you might have noticed that most times the skids are not level with the ground. In other words, one of the helicopter’s skids is lower than the other. Although wind and loading can cause this, the tail rotor thrust determines the base line for either right skid low or left skid low. Tail rotor thrust tends to make the helicopter drift in the same direction and is called translating tendency.

The tail rotor is designed to produce thrust to oppose the torque that tries to spin the helicopter in the opposite direction of the main rotor. Some of this thrust applies a force to the fuselage that causes the helicopter to drift laterally in the same direction. A tilt in the main rotor causes a small sideward thrust opposite the tail rotor to counteract the drift. The tilt can be accomplished by mounting the transmission at a slight angle or designing the flight control system to tilt the rotor disc when the cyclic control is centered.

The direction the rotor spins makes a difference. In a system the turns clockwise when viewed from above, the tail rotor thrust causes the helicopter to drift to the left. Tilting the main rotor disc to the right to counter this causes the right skid to hang low. A counterclockwise turning system will cause a right drift and a left tilt making the helicopter hover left skid low.