Small Unmanned Aircraft Ceilings and Visibility
Jan 24, 2018
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Small Unmanned Aircraft Ceilings and Visibility
Mountain flying and a few other things to finish up this module. Can you believe we’re finally here? We’re going to trek right on through here, a lot of information in this module.
But, again, it’s what the FAA says you need to know, so, let’s just keep right on trucking.
Mountain Flying: When planning a flight over mountainous terrain, gather as much preflight information as possible on cloud reports, wind direction, wind speed, and stability of air. Satellites often help locate mountain waves.
Adequate information may not always be available, so remain alert for sign posts in the sky. Wind at mountain top level in excess of 25 knots suggests some turbulence. Wind in excess of 40 knots across a mountain barrier dictates caution. Stratified clouds mean stable air. Standing lenticular or rotor clouds suggest a mountain wave.
Expect turbulence many miles to the lee of the mountains and relative smooth flight on the windward side. Convective clouds on the windward side of the mountain means unstable air. Expect turbulence in close proximity to, and on either side of, the mountain. Structural Icing: Two conditions are necessary for structural icing in flight.
Number one is the aircraft must be flying through visible water, such as rain or cloud droplets.
Small Unmanned Aircraft Ceilings and Visibility
Number two is the temperature at the point where the moisture strikes the aircraft must be 0° Celsius or colder. Aerodynamic cooling can lower temperature of an airfoil to 0° Celsius, even though the ambient temperature is a few degrees warmer.
Thunderstorms: As pilots, we’re asked this on every single test we ever take, over and over and over. It doesn’t matter what type of pilot you are, you have to know the three stages of a thunderstorm. They are: cumulous, mature, dissipating. I can tell you this is going to be on your test.
You have to know this now. You’re going to need it when you retest. Memorize these: cumulus, mature, and dissipating. It is virtually impossible to visually detect the transition from one stage to another.
The transition is subtle and by no means abrupt. Furthermore, a thunderstorm may be a cluster of cells in different stages of a lifecycle. Cumulous stage: Although most cumulous clouds do not grow into thunderstorms, every thunderstorm begins as a cumulous. The key feature of the cumulous stage is an updraft.
The updraft varies in strength and extends from very near the surface to the cloud top. Growth rate of a cloud may exceed 3,000 feet per minute. It’s inadvisable to operate a small unmanned aircraft in an area of rapidly building cumulous clouds.
Early during the cumulous stage, water droplets are quite small but grow to raindrop size as the cloud grows. The upwelling air carries the liquid water above the freezing level, creating an icing hazard.
As the rain drops grow still heavier, they fall. The cold rain drags air with it creating a cold downdraft. Coexisting with the updraft, the cell has reached the mature stage at that point.
Mature Stage: Precipitation beginning to fall from the cloud base is your signal that a downdraft has developed and a cell has entered the mature stage. Cold rain in the downdraft retards compressional heating and the downdraft remains cooler than the surrounding air.
Therefore, its downward speed is accelerated and may exceed 2,500 feet per minute. The surface wind surge is called a “plow wind” and its leading edge is the “first gust”. Meanwhile, updrafts reach a maximum with speeds possibly exceeding 6,000 feet per minute.
Small Unmanned Aircraft Ceilings and Visibility
That’s pretty crazy, right? Updrafts and downdrafts in close proximity create strong vertical shear and a very turbulent environment. All thunderstorm hazards reach their greatest intensity during the mature stage.
We move to the dissipating stage. Downdrafts characterize the dissipating stage of the thunderstorm cell and the storm dies rapidly. When rain has ended and downdrafts have abated, the dissipating stage is complete.
When all the cells of the thunderstorm have completed this stage, only harmless cloud remnants remain. Ceiling: For aviation purposes, a ceiling is the lowest layer of clouds reported as broken or overcast, or the vertical visibility into an obstruction like fog or haze.
Take note of this. This will be on your test, I guarantee you. Clouds are reported as broken when eighths to seven-eighths of the sky is covered with clouds. Overcast means the sky is entirely covered with clouds.
Current ceiling information is reported by the Aviation Routine Weather Report (METAR) and automated weather stations of various types. You learned about METARs, or if you haven’t yet, you will. If you passed it somehow, go back to it. METARs and TAFs are covered in detail.
Visibility: Closely related to cloud cover and reported ceilings is visibility information. Visibility refers to the greatest horizontal distance at which prominent objects can be viewed with the naked eye.
Current visibility is also reported in METAR and other aviation weather reports as well as by automated weather systems. Visibility information, as predicted by meteorologists, is available for a pilot during a preflight weather briefing. Now, let’s apply all this to the small unmanned aircraft.
The manufacturer may provide operational and performance information that contains the operational performance data for the aircraft, such as data pertaining to takeoff, climb, range, endurance, descent, and landing.
To be able to make practical use of the aircraft's capabilities and limitations, it’s essential to understand the significance of the operational data. The use of this data in flying operations is essential for safe and efficient operation.
It should be emphasized that the manufacturer’s information regarding performance data is not standardized. If manufacturer-published performance data is unavailable, it’s vital to seek out performance data that may have already been determined and published by other users of the same small unmanned aircraft manufacturer model and use that data as a starting point.
Small Unmanned Aircraft Ceilings and Visibility
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Gary Cleveland, Chief Pilot
FAA Safety Representatives
574-767-1797
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