Friday, January 14, 2011

Six for safety: Cold Weather Flying

Starting at the January general meeting, PFC is reviving an old tradition: "Six minutes for safety" (or six-for-safety). Each month we will have a quiz and discussion on an aviation safety issue. Try your knowledge of aviation safety. This month: winter weather flying.

The 2011 winter season might be one for the record books, especially after our latest Nor'easter. Maybe you're like me and prefer to sit the winter out waiting until the Spring thaw. It doesn't have to be that way. But if you go, you should know what you are up against. Try completing this short quiz based on a couple of winter weather scenarios. Answers and discussion can be found at the end of the quiz. 

1) Given the previous nights snowfall, you call the FBO the next morning and have the plane towed to the hangar. This, your thinking goes, should help melt away accumulated snow on the wings, horizontal stabilizer and control surfaces. And given very cold temperatures and wind, you will have a nice warm place to preflight the plane. You arrive at the FBO 20 minutes later, brush off some remaining snow, and give the plane a thorough preflight and verify that controls are free and clear. You then have the plane moved to the ramp, preheat the engine, start-up, taxi and then line up and wait. What potential danger might develop during the time the plane is on the ramp and eventual takeoff?
a) The three hour rule may require you to taxi back so your passenger can disembark.
b) The engine might experience a vapor lock given the sub-freezing temperatures.
c) Your feet will get very cold due to a lack of air movement and gangrene will quickly set in.
d) The control surfaces could freeze and bind due to runoff from the melted snow refreezing.
2) You have planned a flight from KCDW to KBVY (Beverly, MA just north of Boston). Current conditions at CDW are light and variable winds and 34 degrees Fahrenheit. The Boston Area Forecast for MA, CT and RI is OVC035 and OTLK... MVFR CIG. TAF, at estimated time of arrival, reports breezy winds from the east, temperature 32 degrees Fahrenheit and dew point 30. You decide to fly the route VFR direct. Given the preceding information plus the surface analysis pictured below, what kind of weather might you encounter during your flight.
Source: NOAA Surface Analysis, March 23, 2010
a) Coastal fog.
b) Scattered clouds below 3,500 feet MSL.
c) Freezing rain.
d) All of the above.

3) You decide the smart thing to do is cancel your flight to KBVY. By the following morning the Low is still influencing the New England coastal areas with weather. So instead, you decide to fly VFR to KCHO (Charlottesville, VA) which is 67 miles SW of KIAD (Dulles airport). Your route of flight will take you west of the DC SFRA. The area forecast reports broken to scattered clouds at 5,500 feet. TAF, at time of arrival, reports VFR conditions with winds 12 kts gusting to 18 out of the west, temperature 35 degrees Fahrenheit. Given the preceding information plus the satellite image below, what kind of flight conditions might you encounter.
Derived Source: CIMSS Satellite Blog
a) Low visibility due to ceilings.
b) CAVU.
c) Could get bumpy; better check Airmet Tango.
d) All of the above.

4) How long can the average person, given fair weather conditions, survive in the water after an unplanned ditching with a surface temperature of 34.5 degrees Fahrenheit?
a) 30 minutes to 1.5 hours
b) 1.5 to 2 hours 
c) 2 to 3 hours
d) It depends on many factors but probably not long.

Answers and discussion:
1) The correct answer is d). The control surfaces could freeze and bind due to runoff from the melted snow refreezing. A big thank you goes to Stanley for bringing this to our attention. And AOPA even makes reference to it as part of their winter checklist. As for the remaining choices: a) the three hour rule does not apply to Part 91 operations; b) vapor lock is not likely to occur in cold temperatures for a fuel injected engine; c) gangrene is highly unlikely though a long wait in an open cockpit might lead to some very cold feet.

2) The correct answer is d). All of the above. Robert Buck stated it best in his book Weather Flying, "the big action is up there ahead of the warm front" (Weather Flying, Buck, McGraw-Hill, 1998). Given an incomplete briefing combined with some favorable reporting elements, we might be lulled into giving this flight a go. But the real danger lies along the warm front which is moving counter clockwise in relation to the Low's center and happens to parallel our line of flight. At the same time, the proposed altitude takes the aircraft through a sub-freezing air mass underneath a sloping warm front. This provides the perfect setup and potential for freezing rain and aircraft icing. Given the graphic, falling precipitation in the form of rain or melted snow in the upper layers of the warm front could easily turn into freezing rain (supercooled water droplets) upon entering the lower and significantly colder air mass. In general, once the supercooled droplets hit any surface having a temperature at or below the freezing point, the droplets will freeze and cause structural icing. And that's not a good thing, of course, for a lot of GA aircraft. The remaining choices reflect other types of weather phenomena that might occur for this scenario. In looking at the isobars, we see a general air flow from east to west. Given the low temperature to dew point spread, advection fog is a possibility and even more so if there is an existing snow pack. And don't forget that a forecasted or reported ceiling is the lowest broken or overcast layer of clouds. The report may or may not forecast lower scattered layers. But given the FA's degrading outlook, that's a real possibility under these conditions.

3) The correct answer is c). Could get bumpy; better check Airmet Tango. Without even looking at a forecast report, this satellite photo pretty much tells the whole story -- strong winds at lower altitudes. The phenomenon seen in the photo are wave clouds. In ground school, we're generally taught that they occur on the downwind side of mountains and in particular, in and around the Rockies or Sierras. They produce extreme turbulence. However, as can be seen in the photo, this phenomena occurs along our geologically older and significantly lower East Coast mountains. For a nice overview of wave clouds, visit CIMSS Satellite Blog.

4) The BEST answer is d). In creating this question, I was interested in knowing the coldest temperature of the Hudson River. That turned out to be about 34.5 degrees and occurs during February. Given that bit of information, scenario estimates range between 30 minutes up to 2 hours of survival time. In reviewing a couple of sources, many factors come into play including ability to remain conscious, body weight, clothing, fitness, age, etc. Best advice -- don't ditch in cold water! For additional information, visit any boating safety site and check out this interesting Coast Guard presentation.

Safe flying!

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