2010-07-26
settling with power
昨日（25日），埼玉県秩父市大滝の山中で，山岳遭難の救助活動にあたっていた防災ヘリコプター「あらかわ１」が墜落し，機長や副機長，県の防災航空隊員ら５人が命を落とすという事故がおきました。
学校からの帰り道，田邉先生の車中で聞こえたラジオ放送によると，事故の原因はsettling with powerという現象かもしれないということです。辞書で調べてみましたが，当然ながら学習辞書には載っていません。そこでインターネットを使ったところ，ヘリコプターの専門サイトを発見。settling with powerについての解説を引用します。（院生 小山本）
Settling with power can be a dangerous condition that any pilot may face, and if he or she is not on their toes, it may cause a serious uncontrollable situation. Settling with power is basically when the helicopter settles into the rotor wash produced by its own main rotor system. It requires 3 key elements to occur, and these conditions should be avoided in combination with one another. These are: A near zero airspeed, up to 100% power applied, and a better than 300 foot per minute rate of descent. Once you have all of these situations in occurrence, the aircraft will settle in its own down wash from the rotor system. The only way to recover is to gain forward airspeed and allow the rotor system to fly into "Clean air". Once the rotor system is clear of the rotor-wash, it will become efficient again, and the settling with power conditions will cease to exist. This can become a real problem at an out of ground effect hover (Above 10 feet from the ground), and during landings. I have personally experienced it during night operations while taking off from a pinnacle. We started to descend once we left the ground effect of the hill top we were departing from. Because it was at night and we were not under night vision goggles, it was hard to recognize the situation. Corrective action was applied, and we flew out of the situation without incident. A good lesson was learned that night without a major accident taking place.
If you go to the section of this web site that discusses the V-22, you will read a lot about "Vortex Ring State", "Settling With Power" (Army), or "Power Settling" (Navy). Unfortunatly, the different branches of service have different meanings for the latter two terms. The Navy uses the term "Settling With Power" as a means to explain how high temperatures and high humidity will reduce the available power a helicopter can generate and use. The Army uses the term "Power Settling" to describe the same phenomena.
In addition, the Army uses the term "Settling With Power" to describe the "Settling in your own downwash" phenomena where the Navy calls the same exact thing "Power Settling". The term "Vortex Ring State" is used to describe the actual swirling of the air within the rotor system itself that causes "Settling With Power" (Army) or "Power Settling" (Navy). Not only that, but the Army manuals say that Vortex Ring State can begin to occur when you have 300 Feet per minute (FPM) as a rate of descent. The Navy says 800 FPM is a more accurate figure. Of course, I would always side with the lower number as being the more safe. As you can see, this gets very confusing when the branches interact with one another or manuals are made by a company that normally deals with one specific branch of the military. Are you confused yet?
How ever you wish to describe it, and which ever terms you wish to use, it is a dangerous situation that any rotary wing machine can experience. Pilots need to be aware of the situation and avoid it at all cost. For more information, see the V-22 section. There is a copy of the Congressional hearings which discuss the phenomena covered here.