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Jan
05
2010
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Posted 14 years 323 days ago ago by Admin
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By Dan Bitton
For those law enforcement agencies that operate in cold weather environments, winter adds dangers we must consider before launching.
Winter brings a combination of high moisture content and cold temperatures that pose a threat of engine, airframe, and blade icing. During day and night operations, snow and ice become significant threats in many ways. Ice on a ramp or ice or snow on a helipad during a rapid spool-up or rapid spool down can cause the helicopter to yaw. Snow can lead to a whiteout during takeoff and landing. During night NVG operations, a gradual increase in snowfall can lead to loss of situation awareness and CFIT, or to instant IIMC conditions that can result in loss of control.
An awareness of these conditions along with proper training, mission planning and crew-coordination can prevent flight crews from getting caught in these traps and thereby prevent accidents. The bad news is that accidents involving loss of situational awareness and the resultant loss of control and CFIT are usually fatal accidents. The good news is that these accidents are typically avoidable.
At the risk of boring you with physics, the basics a pilot and all crewmembers should know about icing begins with the fact that icing can occur in conditions of high humidity when the ambient temperature is at or below 0 degrees C. Note that this does not require you to be in the clouds. Icing can also occur at higher temperatures during times of falling barometric pressure, so for safety sake let’s just say that with temperatures at or below 5 degrees C, high humidity, temperature and dew point being nearly even, we must be aware of the dangers of ice accumulation.
The primary effect of ice on a helicopter’s blades is drag and loss of lift. During temperatures between 0 and -3C, ice will form on the leading edge from the root outwards toward the tip, covering about 70% of the blade, as kinetic energy or the heat from the blade tip inboard typically inhibits ice accumulation in that area. This kinetic energy benefit decreases with temperatures below -3C. Ice accumulating on a blade will cause vibration – from very slight to significant dependant on the type of blade and the rate of accumulation. The increase in the torque required to sustain level flight is an indication of significant accumulation. If in icing conditions, it is helpful to know that the faster the helicopter is traveling, the faster ice accumulates. Any indication of ice on typical law enforcement helicopters, any vibration, should be cause for a Land As Soon As Possible decision.
Icing conditions also pose a danger to turbine engines. While many of our helicopters have engine anti-ice systems, these systems are meant for light icing and ice may accumulate at the throat near the first compressor stage. This in itself will not cause the engine to fail or flameout, it is the breaking of this ice and its being ingested into the engine that can cause damage and posssible flameout. Engine manufactures have conducted testing and found that as little as 350cc of water – at one time—into the engine can cause a flameout. Aircraft with particle separators are less susceptible to these engine induction hazards.
Perhaps a more common danger in winter is snow. Snow has caused many helicopter accidents during takeoff and level flight, and during landings due to whiteout conditions and loss of situational awareness. Prior to departing from a snow-covered ramp or helipad, whiteout should be considered. In fresh snow, pulling just enough pitch to become light on the landing gear can cause the snow to blow away from the aircraft. Depending on the helicopter model, I teach our flight crews to use the chin bubble as a reference point, as it seems ground contact can often be seen through this window during takeoff and landing even when a snow cloud is obscuring the primary windows.
Off-site landings and takeoff’s pose a problem of their own we must consider. In addition to the whiteout described above, landing in snow-covered fields can become a dangerous situation as the snow may be masking uneven terrain or hazardous obstructions.
Similarly, landing on frozen snow can be dangerous, as just after landing the weight of the aircraft can break through the frozen snow and in both cases the PIC may not be ready for a slope situation that may exceed the helicopter’s limitations. After landing in a field, and sitting for some time, when departing, a skid may freeze to the ground and dynamic rollover situation may occur. The best prevention during off-airport landings is to have a ground support officer assist in assessing the landing area, and in all cases, land slowly and treat each snow covered landing and takeoff as you would a slope landing.
The increased use of Night Vision Goggles requires us to be ever vigilant of light snow, as while flying with NVG’s in light snow, the snow may only be seen as a slight increase in the graininess or amount of “visual static” in the goggles, thus allowing us to see through light snow. As we fly in remote country areas with ambient light sources few and far between, during times of low moon illumination from one second to the next, the snow can increase and when viewed unaided we can quickly be in an inadvertent meteorological conditions. Crew coordination becomes critical, and any unaided crewmember should advise the flight crew of diminishing visibility.
Whiteout situations, low visibility, CFIT, inadvertent IMC and resultant loss of control deserve our attention, as these preventable accidents continue to occur. When faced with inadvertent IMC conditions, hopefully a situation we constantly train for, it is important to note that accidents occur when attempting a turn, as a further loss of situational awareness often happens. Having this knowledge, and obstructions permitting, climb straight and advise ATC you have an inadvertent IMC emergency condition. Let the TFO tune any radio’s, as the PIC, just concentrate on straight and level flight. Today many pilots have never heard of the no-gyro and/or radar surveillance approach that controllers used to provide. While a senior controller recently advised me that very few ATC facilities still train for or provide true ASR approaches, they are able to help and advise you of turns, climbs, descents, etc by issuing commands such as “turn – stop turn”, “climb – stop climb”, and therefore vector onto and down a localizer approach during IMC conditions. Check with your local controller to see what services are available in your area. If you have a military base located near you, they most likely can provide you with a true ASR approach. You just listen, fly to their commands.
Again, good judgment, early on, is the best prevention. If you are in diminishing visibility and ceiling, land as soon as possible, any parking lot, field, etc – being ever so mindful of obstructions. During these landings, slow is good and slower is better.
Many of us in law enforcement have been flying with NVG’s for some time so it’s important to remember that the moon phase, azimuth and relative position are all important factors influencing image quality and operational capability. Once the moon is less than 20% above the horizon, the atmosphere begins dispersing more lunar light resulting in less image detail. It is interesting to note that most military recorded accidents while operating on NVG’s occurred when the angle of the moon was less than 30 degrees above the horizon. Lunar levels can be obtained at the NOAA.gov website. This was all good stuff I learned from Scott Baxter when he pioneered the Bell NVG initial class, and for all those able, I highly recommend this initial and recurrent training.
Mission planning, crew coordination and plain old good judgment remain the best manner in which to prevent being in dangerous and deadly situations. Good crew coordination involves flight leadership, two-way communication, and the ability to maintain situational awareness. Often times, as flight crews become more proficient with NVG operations, overconfidence and complacency can compromise safety of flight. Risk assessment and management must be continuous from the initial pre-flight briefing to the post flight de-brief.
Author Dan Bitton is a Commander with the Winthrop Harbor Police Department Law Enforcement Aviation Coalition