Weather Services and the Go/No-Go

Section 91.103 is the rule that requires weather information before a flight. For a flight "not in the vicinity of an airport," the required information includes "weather reports and forecasts" 1. The rule requires both reports and forecasts for any flight away from the airport. A pilot who reads the METAR and skips the TAF has read the reports and not the forecasts.

The two outlets

The two official outlets are the Aviation Weather Center and Flight Service. The Aviation Weather Handbook: "The website https://aviationweather.gov is operated by the NWS AWC in Kansas City, MO. It is a major aviation weather website for obtaining text and graphical preflight weather information and products" 2. Flight Service is the other. The AIM: "Pilots may access weather information through www.1800wxbrief.com. To contact Flight Service in the CONUS, Hawaii, and U.S. territories; call 1-800-WX-BRIEF (1-800-992-7433)" 3.

A self-briefing is compliant

A self-briefing from official sources meets the rule. The AIM: "Pilots can receive a regulatory compliant briefing without contacting Flight Service. Pilots are encouraged to use automated resources and review AC 91-92, Pilot's Guide to a Preflight Briefing, for more information" 4. A telephone call to a briefer is one way to meet 91.103. It is not the only way. The pilot who prefers the call is "encouraged to conduct a self-brief prior to calling" 4.

The source hierarchy

The sources form a hierarchy. The Aviation Weather Center makes the products. Flight Service delivers and logs briefings: its website "allows pilots to review weather information, receive online preflight briefings, file flight plans, and receive automatic notifications and alerts" 2. Electronic flight bags repackage both. Flight Service records a briefing taken through an account, and that record documents that the pilot met the weather requirement of 91.103. The pilot of N21480 had no such record.

EFB weather is aggregated National Weather Service and FAA data. The app is convenient. When its summary and the raw product disagree, the product is the authority. AC 91-92 warns that the NEXRAD picture in the cockpit "can be 15 to 20 minutes old by the time they are assembled and displayed" 5. The app summarizes the product. The forecaster writes the product.

In-flight weather is not the briefing

A pilot can update the weather in flight, from Flight Service on the radio and from FIS-B on the datalink. Nothing available in flight replaces the preflight briefing. The AIM: "FIS is not intended to replace traditional pilot and controller/flight service specialist/aircraft dispatcher preflight briefings or inflight voice communications," and "FIS should never be used in lieu of an individual preflight weather and flight planning briefing" 6. AC 00-63B: "FIS-B METI and AI may not include all the weather products or NOTAMs that a preflight briefing includes," and "FIS-B does not replace a thorough preflight briefing" 7.

Flight Watch and HIWAS are gone

Flight Watch on 122.0 and HIWAS no longer exist. The current AIM's preflight briefing paragraph names Flight Service, the web, and the telephone, and mentions neither 4. A textbook, a video, or an instructor that still teaches "call Flight Watch on 122.0" is out of date. In flight, the Flight Service frequency is the one on the chart, and the datalink is FIS-B.

A standard briefing is the full briefing. The AIM: "You should request a Standard Briefing any time you are planning a flight and you have not received a previous briefing or have not received preliminary information through online resources" 8. The pilot requests it as close to departure time as possible, for any flight. AC 91-92 on timing: "Obtain a standard briefing as close to your departure time as possible" 9.

The abbreviated briefing

An abbreviated briefing updates an earlier briefing or answers specific items. The AIM: "Request an Abbreviated Briefing when you need information to supplement mass disseminated data, update a previous briefing, or when you need only one or two specific items" 10. It supplements a standard briefing. It does not replace one.

The outlook briefing

An outlook briefing applies when departure is six or more hours away. The AIM: "You should request an Outlook Briefing whenever your proposed time of departure is six or more hours from the time of the briefing. The briefer will provide available forecast data applicable to the proposed flight. This type of briefing is provided for planning purposes only" 10. An outlook briefing is not a briefing for departure. The AIM's next sentence: "You should obtain a Standard or Abbreviated Briefing prior to departure" 10.

The cadence

The three briefings run in a cadence. AC 91-92's list:

• "Plan ahead and obtain an outlook briefing."
• "Obtain a standard briefing as close to your departure time as possible."
• "Obtain an abbreviated briefing just before takeoff if your standard briefing is 1 hour or more old or if the weather is questionable" 9.

The outlook briefing comes first. The standard briefing comes close to departure. The abbreviated briefing comes again when the standard briefing is an hour old or the weather is in question.

What the briefer needs

A briefer, or the self-brief form, needs the details of the flight. AC 91-92's checklist: "Type of flight planned (IFR/VFR). Aircraft identification. Aircraft type. Departure point. Route of flight. Destination (alternate airports if needed). Flight altitude(s). Estimated time of departure (ETD) and estimated time en route (ETE)" 11. Without the route and the time, the briefing is general weather information. With them, it is a briefing for this flight.

Weather plus aeronautical information

A briefing is weather plus aeronautical information. NOTAMs and TFRs are part of it. The standard briefing's first element is adverse conditions, which the AIM defines as "Significant meteorological and/or aeronautical information that might influence the pilot to alter or cancel the proposed flight; for example, hazardous weather conditions, airport closures, air traffic delays, etc." 8. A clear day is a no-go when the destination runway is closed or a TFR covers the destination.

"VFR flight not recommended" is advisory. The AIM: when conditions "in the briefer's judgment, would make flight under VFR doubtful, the briefer will describe the conditions, describe the affected locations, and use the phrase 'VFR flight not recommended.' This recommendation is advisory in nature. The final decision as to whether the flight can be conducted safely rests solely with the pilot" 12. The phrase is a strong warning and not an order. The AIM bases the decision on weighing "current and forecast weather conditions against the pilot's experience and ratings" 12. For a private pilot with no instrument rating in a Warrior, the phrase usually means no-go.

The self-brief order

The self-brief runs in a fixed order. North Aero's sequence is:

1. the synopsis, the systems and air masses in motion
2. the hazards, the adverse conditions and advisories
3. the current weather, the METARs along the route
4. the forecasts, the TAFs and the Graphical Forecasts for Aviation
5. the winds aloft
6. decide, and document the decision.

AC 91-92's elements cover the same steps:

• "Synopsis. Review the type, location, and movement of weather systems and/or air masses"
• "Current Conditions. Review current observations, including Aviation Routine Weather Reports (METAR), PIREPs, and satellite and radar imagery"
• "En Route Forecast"
• "Destination Forecast" 13.

The order is fixed so that the pilot skips nothing on a day when the sky looks fine.

A METAR is a routine surface observation. The Aviation Weather Handbook: "A METAR includes the airport identifier, time of observation, wind, visibility, Runway Visual Range (RVR), present weather phenomena, sky conditions, temperature, dewpoint, and altimeter setting," and "METARs are sometimes referred to as 'hourly' reports since they are routinely produced near the top of the hour" 14. A METAR is an observation of past conditions. It reports what the station observed, not a forecast.

The SPECI

A SPECI is an unscheduled special observation. The handbook: "A SPECI is an unscheduled report taken when any of the criteria given in Table 24-2 are observed during the period between hourly reports" 14. The criteria include:

• a wind shift
• visibility crossing a threshold
• a thunderstorm beginning
• a ceiling crossing a threshold.

A SPECI alone means that a condition changed between hourly reports.

Wind

The wind group reads direction, speed, and KT. The handbook:

• the direction "is coded in tens of degrees relative to true north using three figures,"
• "The wind group is always coded with a KT to indicate wind speeds are reported in knots,"
• a gust is coded "immediately following the wind speed" as in "27020G35KT,"
• a light variable wind is "VRB,"
• "A calm wind is coded as 00000KT" 15.

The coded direction is true. The runway number is magnetic. The pilot converts one to the other before computing the crosswind.

Visibility

Visibility is in statute miles. The handbook: "The visibility group is coded as the surface visibility in statute miles. A space is coded between whole numbers and fractions of reportable visibility values. The visibility group ends with SM to indicate that the visibility is in statute miles. For example, a visibility of 1½ SM is coded 1 1/2SM" 16.

The weather codes

The weather codes carry an intensity prefix. The handbook: "The intensity qualifiers are light, moderate, and heavy" 17. A minus sign is light, no sign is moderate, a plus sign is heavy. From the handbook's lists, the precipitation includes "drizzle (DZ), rain (RA), snow (SN)," and the obscurations include "mist (BR), fog (FG), smoke (FU)," and "haze (HZ)" 17. TS is the thunderstorm descriptor, and the intensity sign belongs to the precipitation. A METAR that reads +TSRA BR is a thunderstorm with heavy rain, and mist.

Sky condition and the ceiling

Sky condition is reported in eighths, with heights in hundreds of feet above the ground. The handbook's table:

• FEW is 1/8 to 2/8
• SCT is 3/8 to 4/8
• BKN is 5/8 to 7/8
• OVC is 8/8.

The height "is recorded in feet AGL" 18. BKN025 is a broken layer at 2,500 feet above the ground.

A ceiling is the lowest broken or overcast layer. The handbook: "The ceiling is the lowest layer aloft reported as broken or overcast. If the sky is totally obscured with ground-based clouds, the vertical visibility is the ceiling" 18. Scattered is not a ceiling. A METAR reading SCT008 BKN030 has a ceiling of 3,000 feet, with clouds at 800 feet that cover up to half the sky.

VV

VV means the sky is obscured. The handbook: "Vertical visibility is coded as VV, followed by the vertical visibility into the indefinite ceiling. An 'indefinite ceiling' is a ceiling classification applied when the reported ceiling value represents the vertical visibility upward into surface-based obscuration" 18. VV003 is an indefinite ceiling of 300 feet with no layer base under it. There is no gap under an indefinite ceiling, because the obscuration reaches the ground.

Temperature and dewpoint

Temperature and dewpoint are reported in whole degrees Celsius. The handbook: "Temperature and dewpoint are coded as two digits rounded to the nearest whole degree Celsius," and "Sub-zero temperatures and dewpoints are prefixed with an M. For example, a temperature of 4°C with a dewpoint of -2°C would be coded as 04/M02" 19.

A closing temperature and dewpoint spread across successive METARs is the fog warning. A spread of 3 degrees on a clear, calm evening means "expect fog by morning." AC 91-92's caution: "Be especially cautious when the temperature and dewpoint spread is 3 °C or less: fog may form" 9. Three successive METARs with the spread at 6, 4, and 2 degrees show the trend toward fog. The weather theory Module explains the mechanism.

The altimeter group

The pilot sets the altimeter group before flight. The handbook: "The altimeter group always starts with an A and is followed by the four-digit group representing the pressure in tens, units, tenths, and hundredths of inches of mercury. The decimal point is not coded. For example, an altimeter setting of 29.92 inHg would be coded as A2992" 19. With A2992 set, the altimeter on the ramp reads the field elevation. A reading far from the field elevation is a discrepancy to write up.

AO2

AO2 in the remarks means an automated station with a precipitation discriminator. The handbook: "AO1 or AO2 is coded in all METARs/SPECIs from automated stations. Automated stations without a precipitation discriminator are identified as AO1; automated stations with a precipitation discriminator are identified as AO2" 20. An automated station reports what its sensors detect overhead and nearby. It can miss what a human observer would report, such as a thunderstorm ten miles away. The remarks identify which kind of station produced the report.

The whole route

The briefing covers stations along the whole route, not the endpoints alone. AC 91-92: "Review information at the departure, along the route, and at the destination" 13. Two airports 100 miles apart can both report clear while a stratus deck lies between them. The en route METARs show the conditions between the endpoints. The pilot reads three or four of them before the destination's.

A TAF is an aerodrome forecast. The handbook: "A TAF is a concise statement of the expected meteorological conditions significant to aviation for a specified time period within 5 SM of the center of the airport's runway complex (terminal)" 21. It does not cover the en route segment. The departure TAF and the destination TAF do not cover the 100 miles between them. The Graphical Forecasts for Aviation and the en route METARs cover that segment.

Issuance and validity

TAFs issue four times a day. The handbook: "Scheduled 24- and 30-hour TAFs are issued four times per day, at 0000, 0600, 1200, and 1800Z," and "The majority of TAFs provide a 24-hour forecast for the airport, while TAFs for some major airports provide a 30-hour forecast" 21. The header carries the issuance time and the validity window in day and hour Zulu: "The first two digits (Y1Y1) are the day of the month for the start of the TAF. The next two digits (G1G1) are the starting hour (UTC)" 21. A validity of 1512/1612 runs from the 15th at 1200Z to the 16th at 1200Z.

FM

FM marks a rapid change to a new prevailing condition. The handbook: "The change group FMYYGGgg (voiced as 'from') is used to indicate when prevailing conditions are expected to change significantly over a period of less than one hour" 22. Everything after the FM is a complete new forecast, from that time until the next FM or the end of the period.

BECMG

BECMG marks a gradual change across a stated window. The National Weather Service's TAF format uses FM, TEMPO, and PROB. The handbook: "The forecast change indicators FM, TEMPO, and PROB are used when a change in any or all of the forecast elements is expected" 22. BECMG appears in the military and international TAFs the handbook shows for joint-use airports, where a change occurs during the group's window. A pilot who reads a BECMG has a military or foreign TAF. The change arrives at some time inside the window, not at its start.

TEMPO

TEMPO marks temporary fluctuations. The handbook: TEMPO is used for conditions "expected to: Have a high percentage (greater than 50 percent) probability of occurrence; Last for one hour or less in each instance; and In the aggregate, cover less than half of the period" 22. A TEMPO 1 1/2SM SHRA BKN010 in a window of two hours is likely, short, and worse than the prevailing conditions. The pilot plans for the TEMPO conditions, not the prevailing conditions.

PROB30

PROB30 marks a 30 percent chance. The handbook: "The probability group (PROB30 YYGG/YeYeGeGe) is only used by NWS forecasters to forecast a low-probability occurrence (30 percent chance) of a thunderstorm or precipitation event" 22. A 30 percent chance is low and still large enough to plan for. A PROB30 thunderstorm at the destination at the ETA means an alternate with fuel to reach it.

TAF AMD

An amended TAF cancels the old one at once. The handbook: "Whenever an amended TAF (TAF AMD) is issued, it supersedes and cancels the previous TAF. That is, users should not wait until the start of the valid period indicated within the TAF AMD to begin using it" 21. The forecaster issues the amendment because the old forecast is no longer correct. Planning does not continue on the old TAF.

CB

CB is the only cloud type a TAF names. The handbook: "The only cloud type included in the TAF is CB. CB follows cloud or obscuration height (hshshs) without a space whenever thunderstorms are included in the significant weather group" 23. OVC020CB in a TAF means the forecast includes thunderstorms for that period.

The flight categories are a summary of ceiling and visibility. The AIM: "LIFR (Low IFR). Ceiling less than 500 feet and/or visibility less than 1 mile. IFR. Ceiling 500 to less than 1,000 feet and/or visibility 1 to less than 3 miles. MVFR (Marginal VFR). Ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 miles inclusive. VFR. Ceiling greater than 3,000 feet and visibility greater than 5 miles; includes sky clear" 24. The colored dots on the map show these four categories.

MVFR is a decision zone

MVFR is not an automatic go. A ceiling of 2,000 feet and 4 miles is legal VFR in most airspace and below the personal minimums of a new private pilot. Legal VFR can be below the written personal number. AC 91-92's first "do": "Establish personal minimums that reflect your level of proficiency," and its first "don't": "Plan flights that exceed your personal minimums or level of proficiency" 9. The category shows the pilot where to examine the products more closely. It does not make the decision.

The flight of N21480

On March 25, 2023, a Piper Warrior flew into a cloud layer near Johns Island, South Carolina, and dove into a marsh. The pilot, a private pilot with no instrument rating and about 740 hours, died. He was on the second day of a flight from Machias, Maine, to Sun 'n Fun in Florida 25.

The first day was three legs and nearly ten hours, with a night in a pilot's lounge in North Carolina. The next morning he flew to Conway, South Carolina, and stopped for fuel. "According to an employee at HYW, the pilot said that he was headed to Sun 'n Fun and had been fighting a strong headwind the entire trip. The pilot appeared tired, and the employee could see 'fatigue in his eyes.' The pilot checked the AWOS and mentioned that he might stay the night and wait out the approaching weather. However, the employee saw the pilot walk out to his airplane, check the fuel, and depart" 25.

He took off at 11:56 and leveled at 1,600 feet, on flight following. At 12:43 a controller asked whether he wanted to climb. "The pilot said that he wanted to stay low to maintain VFR 'due to any clouds up ahead.'" At 12:51 the controller saw the airplane turn right and descend, and asked whether he was maneuvering to stay below the clouds. "The pilot responded, 'Mayday Mayday…in the clouds…I'm going down.' This was the last communication from the pilot" 25.

The forecast included the clouds. "A Graphical Forecast for Aviation cloud forecast, issued at 1100 and valid for the accident time, forecasted cloud bases around 2,100 ft msl in the area of the accident site." The observation two miles away "at 1235, reported broken cloud layers at 1,300 ft and 1,700 ft above ground level (agl) with 10 statute miles visibility. The 1255 observation reported an overcast ceiling at 1,300 ft agl" 25. The NTSB on the briefing: "the pilot did not request a weather briefing through Leidos Flight Service. According to information provided by ForeFlight, the pilot accessed his ForeFlight account and updated the route about 1130 on the day of the accident; however, he did not file a flight plan or obtain a weather briefing through the service" 25. The probable cause: "The pilot's decision to continue flight under visual flight rules into instrument meteorological conditions, which resulted in the pilot's spatial disorientation and loss of control. Contributing to the accident was the pilot's degraded judgement and performance due to fatigue" 25.

Where the pilot could have prevented the accident

The pilot said at the fuel pump that he might wait out the weather, then departed. Each decision below is marked with the PAVE category that applied to it.

The briefing's adverse conditions section is the no-go trigger list. Known or forecast icing, turbulence aloft, or convection, for a trainer with no deicing, means "no" before the pilot decodes any product. The AIM's standard briefing: "Pilots should also be alert for any reported or forecast icing if the aircraft is not certified for operating in icing conditions. Flying into areas of icing or weather below minimums could have disastrous results" 8. AC 91-92's "don't" list: "Plan flights in or near current or forecast convective activity," and "Flight into known icing conditions is prohibited for all aircraft not properly certificated for flight in icing conditions" 9. The Warrior is not certificated for icing. The pilot reads the trigger list first because it can end the briefing early.

Winds aloft

The winds aloft from the briefing feed the nav log. The Aviation Weather Handbook: "Forecast winds aloft will be provided in knots and tens of degrees and referenced to true north. The briefer will interpolate wind directions and speeds between levels and stations as necessary to provide expected conditions at planned altitudes" 26. A large difference between the winds aloft and the surface wind indicates shear and turbulence in the climb and the descent. A calm surface wind and 35 knots at 3,000 feet is a shear layer that the airplane crosses twice.

Personal minimums are written numbers

Personal weather minimums are written numbers: ceiling, visibility, wind, and crosswind. The pilot sets them before flight, above the legal minimums. AC 91-92's first instruction is "Establish personal minimums that reflect your level of proficiency" 9. North Aero's Personal Minimums Sheet carries the numbers for each pilot, with a line for each of the four. A new private pilot's sheet might read 3,000 feet, 5 miles, 15 knots, and 8 knots of crosswind.

The comparison is mechanical

The personal minimums comparison is mechanical. The pilot sets the forecast against the written number. Above the number, go, and below it, no. The number does not change on the ramp, because the ramp is where the pressure to go is strongest and the judgment is weakest. AC 91-92's "don't": "Plan flights that exceed your personal minimums or level of proficiency" 9. The pilot of N21480 had no written number to compare with the AWOS, so a tired man with friends waiting made the comparison.

The continue-or-divert decision point is set before launch. It is a place on the route plus the condition there that triggers the divert. "At the river crossing, if the ceiling at the next station is below 2,500 feet, turn back to Conway." AC 91-92's "do" list includes "Have a contingency plan for alternates if unexpected circumstances arise," and its "don't" list: "Continue VFR into IMC. Instead, wait it out or turn around if you find en route weather lowering below your personal limits" 9. The pilot names the point before takeoff so that the decision in the air is a comparison, not a choice.

Recognizing the circumstance

Weather worse than briefed and trending below personal minimums is the circumstance that makes diversion prudent. The diversion Module covers how to fly the diversion. The recognition has three parts:

• the briefed conditions
• the conditions out the window
• the direction of the difference.

Conditions worse than briefed and getting worse are the trigger. The pilot of N21480 saw all three parts from 1,600 feet.

The fatality rate

About two-thirds of general aviation accidents in reduced-visibility weather are fatal. The NTSB's safety alert: "Historically, about two-thirds of all general aviation (GA) accidents that occur in reduced-visibility weather conditions are fatal. These accidents typically involve pilot spatial disorientation or controlled flight into terrain" 27. The go/no-go call is the safety decision with the largest consequence that a private pilot makes. A landing accident damages the airplane. A reduced-visibility weather accident, two times in three, is fatal.

Each product has an age

A METAR is minutes to an hour old when the pilot reads it. A forecast's reliability decreases through its period. The AIM has the pilot check "the type of weather product (for example, current weather or forecast weather), the currency of the product (that is, product issue and valid times), and the relevance of the product" 28. The pilot reads the issue time with the contents. A TAF issued at 1200Z and read at 2300Z is eleven hours into a forecast that ends at 1200Z. If an amendment was issued, it has replaced that TAF.

The go/no-go is a conclusion

The go/no-go is a conclusion from the framework: brief, compare, decide, document. It is not a feeling about the sky. The framework:

• brief, in the fixed order, from the official sources
• compare, each forecast against the written personal minimum
• decide, and say the decision out loud
• document, the briefing on the account and the decision in the planning notes.

A pilot who can say "no, because the destination TAF has a TEMPO BKN008 at my ETA and my ceiling number is 2,500 feet" has made a decision. A pilot who says "it looks fine" has made a guess.