Snowy IFR

Fresh snow! The first big snows of the year have hit the north central and northeast states this week, and have brought with them the usual obscuration that accompanies any precipitation. According to Thomas Sullivan (Northeast Weather Blog), a very January-like pattern has set up for the Northeast this week and there will be an enhancement of snow showers followed by a reinforcing shot of cold air and a reorganization of the lake effect activity.

This morning, 4F dispatchers issued a snowy IFR flight from Wilkes-Barre in Pennsylvania to Nantucket Memorial Airport in Massachusetts. For departure, you can lift off VFR because Wilkes-Barre/Scranton is reporting winds 350 at 10 knots, visibility 10 miles. Scattered clouds at 3000, broken 3700. Temperature 27F, dewpoint 12F - the wind chill is 16F.


KACK 191453Z 36022G31KT 2SM -SN BR SCT014 BKN022 OVC045 00/M02 A2991

As you can see above the METAR indicates that sometime enroute you'll want to pick up that IFR clearance. Nantucket is reporting winds 360 at 22 knots, gusting to 31 knots, visibility 2 miles in light snow and mist. Scattered clouds at 1400, broken 2200, overcast 4500. The temperature dewpoint spread is a little tighter too - with temperature 32F, dewpoint 28F, and relative humidity at 86 percent. Snow is likely to continue to fall until this afternoon as the skies clear, but look for gusty winds to continue through early evening.

Decoding Lesson #1

Austin-Bergstrom International Airport (KAUS) is located in the capitol of Texas. Austin is typically a relatively dry place compared to most of the state to the south, yet conditions can be stormy, or as is the case today, foggy! Here's the current surface observation:

KAUS 221253Z 00000KT 1/4SM R17L/1200V1600FT FG OVC001 17/17 A3000

At 1253Z (7:53 AM CDT) winds were calm (00000KT) and visibility was a quarter of a mile (1/4SM). At this hour Austin is also reporting Runway Visual Range (RVR). The code follows the visibility token because RVR is calculated from visibility, ambient light level, and runway light intensity, and is the maximum distance at which the runway, or the runway lights can be seen.

In this case, the RVR is for Runway 17L, the only runway at Austin that has CAT II/III approaches. The 1200V1600FT part of the RVR token in this METAR indicates that the runway is visibile from between 1200 to 1600 feet (V=variable).

If you were wondering what was causing the poor visibility in the Lone Star State's capitol this morning, just refer to the next token: "FG" - it's the symbol used for fog. Also, the ceiling is reported overcast at 100 feet above ground level.

Finally, the temperature-dew point spread definitely indicates fog. Fog is probable when the temperature-dew point spread is 10°C (50°F) or less and decreasing. Fog usually forms when the dew point and the temperature are within a few degrees of each other, and will start to lift when the temperature-dew point spread begins to increase.

Finally, the surface pressure is reported at 30.00 inches of Mercury (A3000).

I'm listening to Austin Departure and they are reporting that the RVR is currently 6000 feet, so it sounds like conditions are improving - likely that temperature-dew point spread is helping out.

Lots of Abbreviations But Only One Format

In 1996, with the beginning of airline deregulation, the U.S. started using a new format to disseminate current weather and forecasts. Prior to that time the U.S. (and Canada and Mexico) used Surface Aviation Observations (SAO) for current conditions and Terminal Forecasts (TF), while the rest of the world used METARS and TAFS. METAR is an abbrevaition of the French phrase "message d’observation météorologique régulière pour l’aviation", literally "aviation weather observation". A typical METAR includes the date and time the report was sent (in UTC), the winds, could cover and height above ground, temperature and dew point, and barometric pressure. For example, the METAR for this hour for Pontiac, Michigan is:

KPTK 202349Z 29008KT 3SM -RA BR OVC008 11/09 A3008

The four-letter identifer is an ICAO-format desginator for Oakland-Pontiac Airport. 202349Z is interpreted as the 20th day of the current month at 2349 Zulu (Zulu is slang for UTC 'Universal Time Coordinated' or GMT 'Greenwich Mean Time'). The wind direction is presented followdd by the wind speed; here's it's reported as coming from 290 degrees at 8 knots. The next token is the visibility - here's it's 3 statute miles. RA means rain, but the minus in front mean light rain (and +RA means heavy rain). BR is a token for "mist". The cloud cover is overcast (OVC) at 800 feet above ground level (AGL). The temerature is 11 degrees Celsius, and the dewpoint is 9 degrees Celsius - not much of a spread and perhaps indicative of potential icing conditions. The final token "A3008" is interprested as "Altimieter 30.08 inches of Mercury - the surface pressure reading.

As you can see, reading METARS requires a good understanding of the conventions used to report surface conditions using the METAR format. This format, used all over the world now with only minor changes, as spelled out by exceptions for only a handful of airports, makes it easier for pilots to read the current weather at almost any station in the world.

European METARS

This week I'm exploring European METARS. Generally they are very similar to North American weather reports, but have metric celings, sometimes metric winds, and some different weather codes. Stay tuned.

Hannah

I just read the Hannah post by dave over at Flight Level 390 and I have to say it was a lot of fun. FF's LIFR and IFR flight assignments are tailored for the bad weather enthusiast, and I'll share mine and others experiences in the wet and bumpy. In the meantime check it out: "The co-pilot tears off the latest weather from the mini-printer and reads it: Winds 130 degrees at 25 gusting 35 kts. Heavy rain. Ceiling 800 overcast. Visibility 1 mile. Altimeter 29.65 and falling."

Hannah

Auto Refresh Implemented

Assignment pages now have a built-in auto-refresh capability. This way if you are interested in say, only IFR flights, then you can load the page one time and let it idle in a separate tab. As the flight assignments change the page will refresh periodically with the updated information. Mobile pages do not have this capability (who keeps their iPhone plugged in all the time?)

Hurricane Ike

Currently, Hurricane Ike is the big story along the Northern Gulf of Mexico, so this weekend FourFlights will focus on flight assignments in Texas, Louisiana, and Mississippi.

The latest data for the Houston terminal area is winds from 050 at 19 gutsing to 26 knots, visibility 10 miles, overcast at 4000 - I bet Hobby Airport is making good use of Runway 4!

Rules of Dispatch

One of the first rules that FF Dispatchers learn is to select departure airports that are within what we call dynamic weather range (DWR) of the destination airport. The DWR is typically a distance that equates to an estimated time enroute (ETE) of about one hour. The idea behind a dynamic weather range is to dispatch a flight that will terminate with an approach into the selected weather conditions. For example, if you select a VFR flight then you'd like conditions to be VFR as you enter the terminal area.

The rule of thumb used to calculate estimated time enroute is to multiply the straight-line distance (in nautical miles) times 0.15 and add 30 minutes. For example, a flight from Orlando (KMCO) to Miami (KMIA) is 167 NM. The ETE is 167 * 0.15 + 30 minutes which is about 55 minutes. The nice thing about this formula is that it accounts for ground operations. This works out pretty well for commercial jets and probably turboprops on shorter flights (but probably not as well for piston aircraft like a Cessna 152).

Flight assignments will almost always originate with weather conditions as good or better than at the destination. What this means is that a VFR flight will almost always start VFR, but an IFR flight could start out with VFR, MVFR, or IFR weather conditions, but never LIFR, at the time of dispatch. Additionally, IFR and LIFR flights with arrival conditions that include rain, snow, and/or thunderstorms have higher priority - this makes for a more challenging flight, don't you think?

At this hour there are 28 stations reporting IFR in the universe of Part 139 airports in the continental United States, but only 8 are reporting rain. The one that was selected was Newport News with winds from 040 at 5 knots, and visibility 1.5 miles in light rain and mist. If you're looking for rain you came to the right place!

fourflights.com is up!

fourflights.com is a website that delivers flight assignments based on current weather conditions. That is what this blog is about. Every hour, dispatchers update the website with flights to airports with four different flight rules: LIFR, IFR, MVFR, and VFR. If you're a pilot you probably know what this means:

LIFR - Low Instrument Flight Rules
These are really poor conditions for even experienced instrument-rated pilots to land aircraft. A lot of times flights to airports with LIFR require a CAT II or CAT III aircrew certification (more on that in a later post). LIFR is a subset of IFR.

IFR - Instrument Flight Rules
This is no cakewalk and requires special training. Conditions that require instruments (avionics) to complete a flight are usually because of poor visibility and/or low clouds.

MVFR - Marginal Visual Flight Rules
A subset of VFR, conditions do not legally require IFR, yet are probably too challenging for visual-only flying.

VFR - Visual Flight Rules
This is the "see-and-avoid" kind of flying at its most basic. Clouds are no lower than 3000 feet above ground level, and visibility is more than 5 miles (and usually more!)

Here's a chart for comparison.

Category	Ceiling (feet)		Visibility (miles)
LIFR	less than 500	and/or	less than 1
IFR	500 to 1000	and/or	1 to 3
MVFR	1000 thru 3000	and/or	3 thru 5
VFR	more than 3000	and	more than 5

Stay tuned to this blog for updates and more information about fourflights.com