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Is there really any difference between Vortex Ring State(VRS) and Settling with Power?
Examiners tend to make a big deal out of VRS on check rides.You know what it is and how you get into it- you have to explain it on every check ride – so there is no need to explain it here. However, seriously, are we confusing pilots and especially our students, and putting them at a disadvantage when we mislabel this aerodynamic phenomenon? The real destroyer of helicopters and killer of helicopter pilots is Settling with Insufficient Power, a structural or environmental power issue induced by the pilot.
Settling with Insufficient Power and exceeding the operational limitations of a helicopter accounts for approximately 16% of all helicopter accidents each year according to research data collected by the United States Helicopter Safety Team (USHST). In ten years of accident analysis compiled from one thousand helicopter accidents, I could only find one accident that was truly attributed to VRS, which the USHST grouped into the loss of control category. Lack of performance management, or settling with insufficient power, makes up the bulk of the accidents in this category.
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The confusing term of Settling with Power, in my Opinion, should be eliminated from every ACS, helicopter manual, and textbook forever. Transport CANADA is the first to remove the reference in their Helicopter Flight Training Hand Book. In it, they explain the difference between VRS and Settling with Power, yet the FAA is stuck in the Past, calling ”VRS Settling with Power” in the world standard, the Helicopter Flying Handbook.
Demanding more power than the engine(s) can deliver, then continuing to increase collective input can be disastrous. When your engine(s) reach their limit, this, in turn, introduces rotor droop. As you pull up on the collective, the rotor drooping induced then drags RPM down with it. As rotor RPM decreases, the knock effect is a reduction in the effective disc area due to the increased coning angle.
In an OGE hover or slow flight above the helicopter’s service ceiling, at a particular engine and rotor RPM, the machine will simply stop flying and descent out of control. At the bottom end of an approach, if you pull a large amount of collective to arrest a descent rate that it is too fast, the output of the engine(s) cannot match the power required to maintain rotor RPM. Some manuals refer to this as ”overpitching”.
Most pilots after experiencing a settling with insufficient power situation, are confused by what happened. The first thing most pilots say (providing they survive) is: “I had an engine failure.” Which can usually be easily disproven in a post-crash investigation. In many cases the NTSB sends the engine(s) for analysis, only to discover each engine was operating normally right up until the point of the impact. In the accidents I reviewed when researching this topic, in my opinion, could have been completely avoided with a little bit of knowledge and efficient flight planning.
Logically, to prevent Settling with Insufficient Power accidents due to performance planning shortfalls, we need to start training like we will be, or could be, tasked to fly. Most flight training occurs in a controlled environment, either at or near sea level altitudes, at a low gross weight, and with minimum fuel on board. This is a disservice to new pilots because they are not receiving the training they need to properly operate at the upper limits of their helicopter’s performance envelope.
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Flight manual performance chart calculations are an integral part of this planning. However, charts vary widely across manufacturer models. Military flight manuals are chock full of performance charts, making it easy o find a certain pressure altitude, temperature and weight calculation for a certain mission. Some other calculation issues involve manuals that are not detailed as others, which could require a degree of interpretation to be made by the pilot using available charts that match a planned flight environment.
When working off Airport, one of the things a pilot should prepare for is imperfect predictions and to plan for variances in as many ways as possible while also thinking through their “plan B” response should conditions not match what is predicted. Ambient conditions at a destination are rarely the same as the ones you based your planning on. Flight manual performance graphs denote the operation of a brand new or “spec” engine that was tested with clear rotor blades and new equipment. With that in mind – remember that values should always be verified with an actual power check under the ambient conditions that exist at the operating site.
Pilots, after loading or reloading passengers and cargo, would be served well to re-verify those numbers with a power check each time before attempting another takeoff as your and your passenger’s lives could depend on it.
Special thanks to Heliweb for this important post written from Mark Colborn, a Senior Corporal and Instructor Pilot for the Dallas Police Department Helicopter Unit. Credits also to the United States Helicopter Safety Team for their valuable data and work.
The enjoyflying team.