We know. Nobody is ready to start talking about winter but it's fast approaching. This means it's time to start considering icing. As helicopter pilots the vast majority of us fly under Visual Flight Rules (VFR) and consequently never fly through clouds. This reduces the amount of time we are exposed to icing conditions but it doesn't mean you won't see them.
The National Oceanic and Atmospheric Administration (NOAA) has completed several studies into aircraft icing. They found that in visible moisture most icing is likely to occur between 0° Centigrade and -20° Centigrade, with over 50% of that occurring between minus 8 to minus 12° Centigrade. Temperatures we're already seeing overnight and before long during the daytime too.
So as helicopter pilots when is icing a concern? Any time you are flying in below zero temperatures with visible moisture, that is, supercooled water droplets. The obvious scenarios are freezing rain or drizzle which, if you remember your ground school, occurs when precipitation falls from a warmer layer aloft through a layer with a sub-zero temperature. Remember, if you have ice pellets on the ground you could be climbing into freezing rain. Keep an eye on those GFA's for winter warm fronts and check the upper winds too. They give you a temperature and may indicate warmer air aloft.
Once again we're going back to our ground school. What was the difference between mist, fog and cloud? If it's reducing your visibility and you can see one half statue mile or less it's fog, more it's mist and if it's not touching the ground it's a cloud. If you can get icing in a cloud you can get icing in mist or fog. Even with the best weather planning it's possible to unexpectedly find yourself in these conditions and having to make a decision.
So where are you likely to see the first indication of icing? First off don't believe what you're looking through. Thin objects deflect less of the air flow and therefore have a higher catchment rate. If your flying in icing conditions your thin rotor blades will start picking up ice long before you can see any on your bubble.
Another factor of the catchment rate is speed. The faster an object moves the more supercooled water droplets it will hit. On a Robinson the tail rotor spins around 6 times faster than the main rotor. If you're picking up icing this is usually where it will start. Not so easy to see when you are flying.
So how can you tell if you're picking up ice? Take note of your power requirements. Ice on your blades is going to change the lift, drag, weight and thrust characteristics. If you have to pull more power to maintain the same altitude and airspeed as you fly something isn't right. There's a very long list of reasons that could be causing this power increase but if you're in conditions conducive to icing it might be wise to set down and have a look.
Is this increasing power demand your biggest concern? Not yet. The helicopter isn't about to drop out of the sky just because there's ice on the blades. It will demand more and more power until the helicopter decides it's landing whether you want it to or not. Make sure you've picked a spot and are on the ground while you can still choose.
As it continues to build asymmetrical shedding of the ice can cause vibrations so severe you can no longer see the instrument panel. There are documented cases of flight test pilots aborting icing tests for this very reason. A very serious concern however is how ice degrades the helicopters ability to autorotate.
During an autorotation only the driving region in the diagram below is providing effective lift. Back in 1990 the US military did some tests using a Bell UH-1H Huey. They discovered that when then ice had built up to just one half of an inch on the driving section of the blade they could no longer maintain the minimum rotor RPM for an autorotation and that's with a high inertia rotor system. Also remember ice build up around the engine air intake could cause an engine failure by starving the engine of air or by ice ingestion. You are certainly at more risk of an engine failure in icing conditions than flying in beautiful blue sky. (Diagram from Principles of Helicopter Flight by W J Wagtendonk).
So there you have it. A little recap on icing, what could happen if you encounter it and what to look for if you suspect you are in icing conditions. See our Facebook page for a link to the Flight Safety Foundations article on Helicopter icing.
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