Thursday, October 13, 2011

It Looks Like Summer's Over

A friend says that there are two seasons here in the Northeast: "Leaves On" and "Leaves Off". We're now well into the transition to the latter, and the times call for a different approach to flight planning. It's now less about, "Will there be convection issues?" and more about "Will there be ice?"

The plan for tomorrow, as usual on a Friday, is to depart Bridgeport (KBDR) late in the afternoon for the flight down to the DC area (KVKX). I expect that my clearance will take N631S and I west to the Sparta VOR (SAX) then south to Solberg (SBJ), west again to the Allentown, PA area, south across Reading, Lancaster and Baltimore, MD, then on into my destination. The first question is, "What's the big picture look like?"

We've got a surface low just north of Lake Huron, with an occluded front trailing off to the southeast. The map (from the NCEP Hydrometeorological Prediction Center site, reflecting the forecast for 00Z Saturday, i.e., 8 PM EDT) depicts an area of showers covering much of New England and New York along with parts of New Jersey and Pennsylvania. Maryland and Virginia are not included. So, I can may get wet on departure but the arrival at KVKX is likely to be dry.

The latest Terminal Area Forecast (TAF) for KBDR (at 00Z Friday, 8 PM EDT, from the AviationWeather.gov site's TAF page) indicates that the rain will stop about 1 PM tomorrow afternoon and the ceiling will be broken at 2,000 feet – not so bad! Here's the TAF:

KBDR 132342Z 1400/1424 06007KT 2SM BR OVC007 
     TEMPO 1400/1402 1SM -DZ BR OVC006 
     FM140400 10007KT 3SM -RA BR OVC003 
     TEMPO 1409/1412 1SM BR 
     FM141400 14010KT 5SM -RA BR BKN008 OVC012 
     FM141700 15012KT P6SM BKN020 
     FM142200 20011KT P6SM SCT030 BKN100

For another indication of the flight conditions to be expected, I can look at the relative humidity (RH) aloft. The chart at left (collected from the NCEP Model Analysis & Guidance site), also valid for 00Z, shows the RH for the 850 millibar level of the atmosphere (or, about 5,000 feet). Most of my expected route is covered by the lighter green, indicating RH of 70% to 90%. In that range, I can probably expect nice, juicy clouds. (An RH above 90% says that rain is very likely.) I'll be starting out at 8,000 feet and then probably descending to 6,000 somewhere in Pennsylvania, so it's a good bet that I'll get to log a significant bit of actual instrument time.

The really important question is whether icing is a hazard. Given the expected RH data, it's pretty clear that if the Outside Air Temperature (OAT) aloft is below freezing then airframe icing is a real possibility. Let's look at the forecast.

Here's the forecast (from the AviationWeather.gov Winds & Temperatures page) for 00Z tomorrow night, showing isotherms at the 725 Mb level (about 9,000 feet). At an altitude of 8,000 feet (required to stay above the Newark arrivals) N631S and I ought to see OAT's a Celsius degree or two above those depicted. It might be a bit close, but conditions ought to be acceptable going over to Sparta and down to Solberg.

Once in Allentown Approach airspace (i.e., about at the Pennsylvania border), Newark arrivals will no longer be a factor and I'll be able to request a descent to 6,000 feet. (The chart at left shows 00Z temperatures at 800 Mb, about 6,000 feet.) That will be well below the freezing level and should be the end of concerns about icing. The rest of the trip, in warmer air and toward improving conditions, should be uneventful.

It will not, however, be quick. At left, the 00Z forecast winds at 9,000 feet (and the forecast conditions for 6,000 to 8,000 feet are similar). As you see, there will be substantial headwinds to deal with. According to FlightPlan.com, the average headwind component for the flight will be 18 knots and the expected time en route is 2 hours + 24 minutes – a rather tedious trip!

As for arrival conditions, the TAF for nearby Washington National Airport (KDCA) is suggesting that from 20Z (6 PM) there will be showers in the vicinity with generally good visibility and scattered clouds at 1,500 feet under a broken ceiling at 5,000.

KDCA 132335Z 1400/1424 12008KT P6SM VCSH BKN030CB 
     FM140600 18005KT 5SM BR BKN008 OVC015 
     FM141300 22007KT P6SM VCSH BKN015CB 
     FM141800 26010G18KT P6SM SCT015 BKN050 
     FM142000 27012G24KT P6SM VCSH SCT015 BKN050
If one of those "showers in the vicinity" (VCSH) chooses to park right over the airfield, then the RNAV Rwy 6 approach may be necessary but there's nothing to suggest any real difficulty getting in. The forecast wind (for KDCA, 12 knots out of the west, gusting to 24 knots) looks a bit energetic, but the field at KVKX is nestled in a valley and the winds there are rarely as strong as forecast at nearby airports.

So, to summarize:

  • A departure into fairly low, possibly showery conditions;
  • Some near-freezing OATs possible in northwestern New Jersey at 8,000 feet;
  • Steady, fairly strong headwinds throughout;
  • Reasonably good arrival conditions, with a possible shower and perhaps gusty winds.
In short, a fairly typical Fall IFR flight.


Shortfinals said...

Just a thought, looking at the TAF, what is the acceptable crosswind component for N631S?

An historical note - the MAXIMUM acceptable crosswind - at 90 degrees to centreline - for the Royal Air Force's Avro Lancaster of the Battle of Britain Memorial Flight is exactly 20 knots! Although you CAN touch down on the mains and hold the tail off for a bit!

Frank Van Haste said...

Hi, Ross!

The POH says that the Maximum Demonstrated Crosswind was 15 knots for the 182. Of course, this is not a limitation. I've done a 15 knot xwind, and it pretty much uses up all the rudder...you might get away with 17 but that would be about it, save using tricks like cutting an angle across a wide runway.

With 27012G24 at KVKX it would call for RNAV Rwy 6 circle-to-land 24 where the xwind component would be about 6 -12 knots...not a big problem.

That said, years ago KVKX was named Rose Valley Airfield. It is nestled down in a valley which really limits the winds. The down side is, sometimes the fog is very persistent.

Thanks for your comment.

Best regards,