AOPA Hover Power

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.