Avoiding turbulence in a Robinson helicopter
One of the key skills of a helicopter pilot is knowing when to fly, when not to, and when to turn back.
The generally excellent weather forecasting available to pilots is obviously a big help in the decision whether to lift off or not. But to get the benefit, you have to use it, understand it, and make the right judgements from it. One of the many good reasons for maintaining your relationship with your flight school, is that you can ask for help and advice in interpreting the suitability of the weather for the safe enjoyment of your planned mission.
As time goes by, and the excellence of your theoretical knowledge on the day of your final handling test fades a little, it’s easy for helicopter pilots to slip into the mentality that suggests that if you like what you see out of the window, it’s ok to fly. This can be risky, especially if you are going on a long trip, where the weather may be different on arrival from that on departure. The universal accessibility of aviation met services can help with this, as products like SkyDemon can show the weather en route on an iPad, and the Met Office aviation briefing service is legible on something as handy as a smartphone.
In the UK, when we’re using the Met Office services, just looking at the TAFs, though quick and convenient, does not give the complete picture. The information about turbulence, for example, one of the real hazards in a light two bladed helicopter, is shown not in the TAF but on the F215 chart. It’s in the right hand column. The F215 takes a little more effort than the TAFs to decode, but the extra information is definitely worth the effort. There’s a guide to decoding them at the end of this article. It’s well worth becoming quite an expert at it. While you’re there, the F214 will give you the wind strength aloft. If the winds seem unusually strong to you, try checking some of the TAFs of the en route airfields, looking for winds over, say, 15kts, high gust spreads and big swings in wind direction. It can all add up to an unsuitable day. In Switzerland, equivalent information is available in www.skybriefing.com.
The Robinson Helicopter Company’s own Safety Notice No SN-32 strongly urges avoidance of flight in high winds or turbulence. This is because in the severe down-draughts experienced in turbulence, the loading of the main rotor disc can become negative. In other words, instead of the weight of the fuselage and its contents pulling down on the centre of the main rotor disc, resulting in good disc stability, the fuselage can become weightless or even effectively push up on the main rotor disc, both resulting quickly in the main rotor disc becoming unstable. Co-inciding with this instability, the tail rotor thrust, which is now the biggest or only influence on the fuselage’s orientation, pushes the fuselage so it rolls to the right. It does this because it is above the centre of gravity of the fuselage. Think of it as being the same effect as if the tail rotor was placed at the top of the main rotor mast, still pushing the same way as normal. It would try to push the fuselage over to the right. This fuselage roll is independent of the main rotor disc, which is busy doing its own thing without reference to the pilot’s inputs or to the position of the fuselage. So as the fuselage rolls right, pushed over by the TR thrust, the main rotor disc, which has no reason to follow it immediately, is effectively now tilted to the left, relative to the fuselage. What do we helicopter pilots usually do to correct a roll to the right? Input left stick! This input, in these circumstances totally inappropriate, can be the straw that breaks the camel’s back. It tilts the main rotor disc even further to the left, often within reach of the front left skid, the front left door, or the tail boom.
Appropriate control inputs in turbulence recommended by Robinson can be summarised as slow down, reduce power, don’t over control, allow the helicopter to go with the turbulence, and restore straight and level flight with gentle inputs. It’s worth reading the Robinson Safety notice on turbulence, linked at the end of this article.
This is pretty extreme stuff, which happens in pretty extreme conditions, and can be avoided with the right pre-flight preparation.
Pre flight, check your route on your chart for turbulence inducing terrain, ask for local advice if possible, and in flight keep your eyes out of the window. Nearby ridges, hills, or even tall buildings upwind of you can all cause severe down draughts even on a cloudless sunny day. Distant mountains can cause the same effect. A mountain wave is a powerful air mass downstream of a mountain range across the path of the wind, rotating about a horizontal axis. Think of a stream encountering a fallen tree across its path. The water rises up to cross the tree, rolls turbulently down the far side, continues to roll over and over for a bit, and eventually resolves into a pattern of waves up and down, until finally it smoothes out. Similarly, with wind downwind of mountains, there can be a succession of such waves, starting immediately downwind of the mountain where the turbulence can be at its most violent, and continuing for some distance in a succession of gradually decreasing waves. You may see lenticular clouds forming where the wave takes the airflow into colder air above then down again. The key safety point here is that all of this can be present even on the clearest of blue sky days, so you need to consult the F215, and study the enroute TAFs. This is pretty extreme weather, so it will jump out at you from the F215, but you have to look.
Other possible turbulence risks: Heavy aircraft landing, taking off or even passing nearby can produce enough disturbance of the air in their wake to pose a threat to lighter aircraft nearby. Both fixed-wing aircraft and helicopters create vortices at the wingtips or rotor tips as a direct result of producing lift. The most powerful vortices are generated by heavy aircraft flying slowly, and these pose the greatest risk to nearby light aircraft. These vortices can be strong enough to result in complete loss of control, for up to several minutes after the heavier aircraft has passed.
Summary: to stay safe, go the extra mile and read the F215 for the turbulence information, check the enroute TAFs and the F214 for unusually high wind speeds and gust spreads, don’t assume that just because it looks like a lovely day out there that it’s safe to fly, ask experienced pilots for their local knowledge if possible, stay away from the lee side of mountains and other turbulence-inducing terrain in high winds, and be prepared without hesitation to make a diversion as Martin Rutty does in the video above.
Know the safe recovery control inputs from weightlessness and from tail rotor induced roll.
Here are the links: