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Getting the Maximum from VFR Minimums

Posted 14 years 8 days ago ago by Admin

By Kerry J. Sullivan - The article by Susan Parson in the March issue titled “Personal Minimums: A Development Guide” provides a systematic way for pilots to determine realistic safety margins for weather. The EMS operator I fly for requires its pilots to develop their own personal weather minimums which are to be more restrictive than those contained in the Operations Specifications. I have found more restrictive minimums necessary because I do not believe the generally used weather minimums are adequate to keep me out of Inadvertent Instrument Meteorological Conditions (IIMC). Despite strict weather minimums, detailed weather products and annual training in weather and pilot decision making we still have all too-frequent incidents of IIMC. As we are all painfully aware, some of these IIMC occurrences result in fatal accidents.


Rather than just raise the ceiling and visibility figures already provided by my employer I have tried to look at theems helicopter reasoning behind the minimums and compare that reasoning to my own experience. For example, let’s look at the concept of a “ceiling” as a VFR minimum. At some point in aviation’s history a decision had to be made as to when an airport had to be limited to IFR traffic only. At some specific weather condition VFR and IFR traffic no longer can operate safely together. Mixing IFR and VFR traffic at an airport requires the ability to both “see and avoid” and to be able to climb and descend visually. The term handed down to us today is “ceiling.” It was decided long ago that if more than half the sky is covered by clouds it is called a ceiling. A ceiling below 1,000 feet means VFR traffic may not operate at an airport without a clearance. All pilots know the definition of a ceiling and its use is very common. Using ceilings in helicopter EMS minimums is therefore understandable, if perhaps, arbitrary.


EMS helicopters seldom operate to or from airports but must use reported weather from airports to extrapolate for the weather conditions where they do operate. Experienced sky watchers know how much sky conditions can change over time and distance. The sophisticated ASOS weather stations are a tremendous asset but they do not provide instantaneous weather information. Sky condition is measured by a single narrow beam reflected back to a sensor. An algorithm is used over time to determine the sky condition. The difference between a broken or scattered layer is one octal, or one-eighth sky coverage. There is little practical difference from my perspective between half the sky covered and more than half the sky covered in clouds but one is a ceiling and one is not. It would be prudent to consider a report of scattered clouds as a ceiling because it is just as likely as not to be one when extrapolated beyond five miles from the ASOS weather station. For example, if I have an 800-3 company weather minimum for cross-country flight and the nearest weather station reports 700 scattered, I would consider that below my personal minimums. The lower scattered clouds are the more I avoid them. I would never fly if the weather was reported or forecast to be 300 scattered, even though scattered clouds are within the minimums since they do not constitute a ceiling. Any clouds close to the deck should be a red flag for potentially unsafe conditions.


There are two critical factors that I apply to all weather decisions. These factors are directly related to my personal minimums. First, what is the trend? Should I expect stable, deteriorating or improving conditions? I have flown under a low overcast all day without concern when I was confident it would not lower any more and visibility was unrestricted but I have balked at flying with a higher ceiling when conditions were likely to deteriorate. The various forecast products can give you an idea of the weather trend. If conditions are forecast to deteriorate at all, I give all reported weather during the period a wider margin. For example, a 1200-foot ceiling may be two hundred feet above my absolute minimum of 1000 feet but with a narrowing dew point, winds off the water or an approaching front it’s likely to drop below that 1,000-foot minimum quickly. The cooler temperatures after sundown may drop both cloud layers and visibility even in otherwise stable conditions. I never recommend flight in rapidly deteriorating weather, but how much margin to apply to weather that deteriorates over a longer period of time is problematic. Points to consider are the forecasted time weather is expected to worsen, the length of a flight and how low conditions are forecast to go. I suggest that if a cloud layer exists below 3,000 feet or moisture limits visibility to less than five miles (MFVR conditions) and the forecast trend indicates deteriorating conditions long flights are a bad idea.


The second critical factor is nighttime. EMS unaided night flights are dangerous for many reasons; weather is just one reason they need to be conducted with an abundance of caution. The VFR weather minimums detailed in Operations Specifications typically apply to Class G airspace only, which is generally uncontrolled airspace 700 to 1200 feet above the surface. The basic VFR weather minimums of §91.155 in Class G airspace below 1200 feet at night require three miles of visibility, and 500 feet below, 1,000 feet above and 2,000 feet horizontal distance from clouds. As all helicopter pilots know we have an exemption that allow us to operate clear of clouds, but not all helicopter pilots know the condition that must be met to apply that exemption. To quote the applicable section…”A helicopter may be operated clear of clouds if operated at a speed that allows the pilot adequate opportunity to see any air traffic or obstruction in time to avoid a collision.” How much of a speed reduction is required? Logically, a helicopter would need to fly slower than an airplane to justify the excemption to the cloud clearances. Many airplanes fly as slow as ninety helicopter lightningknots or less. The very slow speeds necessary to meet the apparent intent of the regulation are contrary to the purpose of EMS flights and are not typically observed by helicopter pilots.


Clouds are not self-illuminated. Without sufficient ambient light to be reflected from the surface of a cloud it will not be visible. It is difficult to operate clear of clouds you can’t see. It is even more difficult to avoid air traffic or previously obscured obstacles that suddenly emerge from hidden clouds. If you are not willing to slow below ninety knots you should remain 500 feet below any clouds to ensure compliance with the intent of the regulation. As helicopter pilots know all to well, the lower you fly the more obstacles you may encounter. Most antennas are below 500 feet in height. That’s one reason most operators have a five-hundred foot minimum cruise altitude for night flights. Therefore, I recommend you consider not flying at night when ANY clouds are reported or forecast below 1,000 feet.


These recommended personal minimums may seem overly restrictive. However, the real value of using a helicopter for EMS is to reduce the time it takes to get a patient to a facility that offers a higher level of care. It is estimated that one patient flight out of ten will make enough difference to save a life that would otherwise be lost. However, it also follows that delays in transporting a patient because of weather aborts or diverts en route could have fatal consequences. I believe the standard to apply is that if you as the pilot do not have complete confidence that you can complete the flight to the higher-level of care without delay, you should turn the flight down. The patient will get to the appropriate hospital without a helicopter. A helicopter is the right choice only when it will expedite that transport. You should never accept a flight in which there is a risk of weather delaying patient transport. That demands restrictive weather minimums.