3.3 Aviation Weather Reports (METAR/TAF) & Flight Planning

3.3 Aviation Weather Reports (METAR/TAF) & Flight Planning

Under UAS.OPEN.060 of Regulation (EU) 2019/947, the remote pilot must make sure the UAS can safely complete the intended flight and must operate within the manufacturer's limitations, which include meteorological conditions. The A2 self-training competencies (AMC2 UAS.OPEN.030(2)(b)) also include checking the current weather and the forecast for the planned time. A general weather app that shows "partly sunny" without gusts or cloud-base figures does not give enough information for that decision.

---

Authoritative Aviation Weather Products vs. Consumer Weather Apps

Before analyzing specific report formats, remote pilots must understand where reliable aeronautical data originates.

Limitations of Consumer Weather Applications

1. Ground-Level Temporal Smoothing: Consumer apps generally report wind averaged over wide geographic regions and temporal windows (e.g. 3-hour blocks). They routinely omit peak gust factors, which represent the primary trigger for multirotor control saturation.
2. Absence of Cloud Base Figures Above Ground Level (AGL): Consumer apps report "cloudy" or "partly cloudy" without distinguishing between high Cirrus at $8,000\text{ m}$ (completely safe) and low Stratus at $80\text{ m}$ (ruling out a 120 m flight).
3. Omission of Atmospheric Moisture Spread: Consumer apps do not display the temperature-dew point spread, leaving pilots unaware of imminent radiation fog development or propeller icing conditions.

Official Aeronautical Meteorological Services

Remote pilots should obtain weather briefings from authorized National Meteorological Services (NMS) and civil aviation authorities complying with ICAO Annex 3 standards. In Europe, authoritative sources include:

• Germany: Deutscher Wetterdienst (DWD) / pc_met
• France: Météo-France / AEROWEB
• Switzerland: MeteoSwiss
• Netherlands: Koninklijk Nederlands Meteorologisch Instituut (KNMI)
• Spain: Agencia Estatal de Meteorología (AEMET)
• Austria: Austro Control

Core Aviation Weather Products: METAR, SPECI, TAF, and SIGMET

• METAR (MÉTéorologique Aviation Régulière / Aviation Routine Weather Report): A concise, standardized statement of observed meteorological conditions at an aerodrome. Issued routinely every 30 minutes (at major commercial airports) or 60 minutes (at smaller airfields).
• SPECI (Aviation Selected Special Weather Report): An unscheduled special report issued immediately when significant deterioration or improvement occurs between regular METAR cycles (such as sudden wind shear, passing squalls, or cloud ceilings dropping below operational thresholds).
• TAF (Terminal Aerodrome Forecast): A standardized aerodrome forecast predicting surface wind, visibility, weather phenomena, and cloud layers over a fixed validity period (typically 24 or 30 hours for large hubs, and 9 or 12 hours for regional airfields). Issued every 3 or 6 hours.
• SIGMET (Significant Meteorological Information): In-flight advisories issued by meteorological watch offices warning all aviators of severe en-route hazards: severe turbulence (SEV TURB), severe icing (SEV ICE), active thunderstorms (EMBD TS or SQL TS), mountain waves, and volcanic ash clouds.
• GAFOR (General Aviation Forecast): Visual route forecasts categorizing flight corridors as Open, Difficult, or Closed based on visibility and cloud ceilings.

---

Anatomy of a METAR Report: Step-by-Step Decoding

A METAR report is written in a standardized, condensed alphanumeric code designed to transmit vital weather data across international networks without language barriers.

Consider this representative European METAR from Amsterdam Schiphol Airport:

METAR EHAM 151150Z 24016G28KT 200V280 8000 -RA BKN015 14/11 Q1013 TEMPO 4000 +SHRA BKN008=

[EHAM]  [151150Z]  [24016G28KT]  [200V280]  [8000]  [-RA]  [BKN015]  [14/11]  [Q1013]  [TEMPO...]
  |         |            |           |         |       |       |        |        |         |
Station   Time       Wind Group   Variability Vis   Weather  Clouds   Temp/DP   QNH      Trend

Station Identifier and Timestamp

1. METAR: Identifies the report as a routine hourly/half-hourly observation. (If automated with no human observer, the word AUTO appears after the station identifier; if an error was corrected, COR appears).
2. EHAM: The 4-letter ICAO location indicator. The first letter identifies the region (E = northern Europe, L = southern Europe/Mediterranean), the second usually the country (EH = Netherlands, LF = France, ED = Germany, LE = Spain), and the last two the aerodrome (EHAM = Amsterdam Schiphol, LFPG = Paris Charles de Gaulle, EDDF = Frankfurt, LEMD = Madrid Barajas).
3. 151150Z: The observation date and time in Coordinated Universal Time (UTC / Zulu). The first two digits represent the day of the current month (15 = 15th day), while the next four digits represent the time (1150 = 11:50 UTC). Remote pilots must always convert UTC to local standard or daylight saving time.

Surface Wind Group (Direction, Sustained Speed, and Gusts)

1. 24016G28KT:
   • Wind Direction (240): The first three digits indicate the direction from which the wind is blowing, expressed in degrees relative to True North (rounded to the nearest 10 degrees). In this example, the wind is blowing from $240^\circ$ (west-southwest).
   • Sustained Speed (16): The next two digits indicate the mean sustained wind speed over the 10-minute observation period: 16 knots.
   • Peak Gust (G28): The letter G denotes gust. The digits after G give the maximum wind speed (the highest 3-second average) in the previous 10 minutes: 28 knots. The G group appears only when that maximum exceeds the mean by 10 knots or more; otherwise it is omitted (e.g. 24016KT), even though smaller gusts may still occur.
   • Units (KT): Knots (nautical miles per hour). In some countries (such as Russia and parts of Eastern Europe), MPS (metres per second) is used.
   • Aviation Unit Conversions: $1\text{ knot} \approx 0.514\text{ m/s} \approx 1.852\text{ km/h}$ $\text{Sustained: } 16\text{ kt} \times 0.514 \approx 8.2\text{ m/s} \quad | \quad \text{Peak Gust: } 28\text{ kt} \times 0.514 \approx 14.4\text{ m/s}$
2. 200V280 (Directional Variability): If wind direction varies by $60^\circ$ or more but less than $180^\circ$ and the mean wind speed is 3 knots or more, the two directional extremes are reported separated by V. Here, the wind direction fluctuates between $200^\circ$ and $280^\circ$.
3. VRB02KT (Variable Wind): If the direction varies by 60° or more with a mean speed below 3 knots, varies by 180° or more, or cannot be determined (for example in a thunderstorm), VRB replaces the three-digit direction.

Horizontal Visibility Group and CAVOK

1. 8000: Horizontal surface prevailing visibility expressed in metres as four digits. 8000 indicates a prevailing visibility of 8,000 metres (8 km). Other examples:
   • 0800 = 800 metres (indicates Fog).
   • 4500 = 4,500 metres (indicates Mist or precipitation).
   • 9999 = Prevailing visibility is 10 kilometers or greater.
2. CAVOK (Ceiling And Visibility OK): A critical aeronautical term pronounced KAV-oh-KAY. When CAVOK appears, it replaces the visibility group, present weather group, and cloud group entirely. It certifies that three simultaneous conditions are met:
   • Horizontal visibility is 10 km or greater (9999).
   • No cloud below $1,500\text{ m}$ ($5,000\text{ ft}$) AGL or below the highest minimum sector altitude (whichever is greater), and no Cumulonimbus (CB) or Towering Cumulus (TCU) at any level.
   • No significant weather phenomena (no precipitation, fog, mist, or thunderstorms).

Present Weather Group (Qualifiers, Descriptors, and Phenomena)

1. -RA: Indicates light rain. Weather groups are constructed systematically using standardized two-letter abbreviations, qualifiers, and intensity symbols:

Examples: +TSRA = Heavy thunderstorm with rain; FZFG = Freezing fog; -DZ BR = Light drizzle and mist; VCSH = Showers in the vicinity.

Cloud Coverage Group and Base Heights

1. BKN015:
   • Coverage Code (BKN): Broken cloud coverage (5 to 7 oktas), representing an aeronautical cloud ceiling.
   • Base Height (015): The height of the cloud base above ground level (AGL) expressed in hundreds of feet. Multiply this number by 100 to obtain feet AGL: $\text{Height in Feet} = 015 \times 100 = 1,500\text{ ft AGL}$ $\text{Height in Metres} = 1,500 \times 0.3048 \approx 457\text{ metres AGL}$
   • Significant convective cloud types are appended to the height: BKN015CB (Broken cumulonimbus at 1,500 ft) or SCT020TCU (Scattered towering cumulus at 2,000 ft).
   • If vertical visibility into an obscured layer (such as dense surface fog) is reported, VV is used: VV002 = Vertical visibility of 200 feet ($60\text{ m}$). Treat this as a ceiling at 200 ft.

Temperature, Dew Point, and Spread

1. 14/11:
   • Air Temperature (14): Ambient surface temperature in whole degrees Celsius ($+14^\circ\text{C}$).
   • Dew Point (11): Surface dew point temperature in whole degrees Celsius ($+11^\circ\text{C}$).
   • Sub-zero temperatures are prefixed by the letter M (Minus): M02/M06 indicates an air temperature of $-2^\circ\text{C}$ and a dew point of $-6^\circ\text{C}$.
   • Dew Point Spread: Subtract dew point from temperature: $14 - 11 = 3^\circ\text{C}$. A small spread ($0^\circ\text{C}\text{ to }2^\circ\text{C}$) indicates near-saturated air with high probability of fog or low stratus.

Altimeter Setting (QNH)

1. Q1013: The atmospheric pressure reduced to mean sea level according to standard atmospheric lapse rates, expressed in whole hectopascals (hPa). In North America, altimeter settings are prefixed by A in inches of mercury: A2992 = $29.92\text{ inHg}$.

Trend Forecasts (NOSIG, BECMG, TEMPO)

1. TEMPO 4000 +SHRA BKN008: European METARs include a 2-hour trend forecast appended at the end:
   • NOSIG: No Significant Change expected in the next 2 hours.
   • BECMG (Becoming): A permanent change reaching specified values over the 2-hour window.
   • TEMPO (Temporary): Temporary fluctuations expected to last less than 60 minutes in each instance and covering less than half of the forecast period. Here, visibility will temporarily drop to 4,000 metres in heavy rain showers with a broken cloud ceiling descending to $800\text{ ft}$ ($244\text{ m}$) AGL.

---

Anatomy of a Terminal Aerodrome Forecast (TAF)

While a METAR tells the pilot what is happening, a TAF tells the pilot what will happen. TAF reports use the same meteorological codes as METARs but include explicit validity timeframes.

Consider this European TAF from Paris Charles de Gaulle Airport:

TAF LFPG 151100Z 1512/1618 22015KT 9999 BKN020
    BECMG 1515/1517 24020G35KT 4000 TSRA BKN012CB
    TEMPO 1518/1522 1500 +TSRA BKN005CB
    FM160200 28012KT 8000 NSW BKN015=

Walkthrough of TAF Change Groups

1. Header & Validity (TAF LFPG 151100Z 1512/1618): Forecast issued by Paris CDG on the 15th at 11:00 UTC. The validity group 1512/1618 spans 30 hours from 12:00 UTC on the 15th to 18:00 UTC on the 16th.
2. Base Forecast (22015KT 9999 BKN020): Wind from $220^\circ$ at 15 knots, visibility 10 km or more, broken ceiling at $2,000\text{ ft}$ ($610\text{ m}$) AGL.
3. Gradual Change (BECMG 1515/1517 24020G35KT 4000 TSRA BKN012CB): Between 15:00 UTC and 17:00 UTC on the 15th, conditions will permanently transition to: wind $240^\circ$ at 20 knots gusting to 35 knots, visibility dropping to 4,000 metres in moderate thunderstorm with rain, and a broken cumulonimbus ceiling descending to $1,200\text{ ft}$ ($365\text{ m}$) AGL.
4. Temporary Deterioration (TEMPO 1518/1522 1500 +TSRA BKN005CB): Between 18:00 UTC and 22:00 UTC, temporary periods of heavy thunderstorms with rain will drop visibility to 1,500 metres with a cloud ceiling descending to $500\text{ ft}$ ($152\text{ m}$) AGL.
5. Rapid Permanent Transition (FM160200 28012KT 8000 NSW BKN015): The code FM (From) indicates a rapid, complete change starting at exactly 02:00 UTC on the 16th (160200). Winds ease to $280^\circ$ at 12 knots, visibility improves to 8,000 metres, NSW (No Significant Weather) indicates rain has stopped, and ceiling lifts to $1,500\text{ ft}$ AGL.
6. Probability Groups (PROB30 / PROB40): A probability group indicates a 30% or 40% chance of a hazardous phenomenon occurring (e.g. PROB30 TEMPO 1514/1516 2000 TSGR = 30% chance of temporary hail storm). In aviation risk planning, a 30% or 40% chance of thunderstorms or gale gusts is treated as an operational No-Go.

---

Systematic Go / No-Go Decision Framework for A2 Remote Pilots

To ensure consistent, defensible flight operations, the remote pilot must establish quantitative personal and aircraft minimums before arriving at the flight site.

METAR / TAF REVIEW
        |
        v
[ STEP 1: WIND & GUSTS ] ===> Exceeds 80% of C2 Limit (e.g. > 9.6 m/s)? ===> NO-GO (ABORT)
        | OK (Within limits)
        v
[ STEP 2: CLOUD CEILING ] ==> Ceiling < 120 m AGL? ====> RESTRICT ALTITUDE OR NO-GO
        | OK (Ceiling > 120 m)
        v
[ STEP 3: VISIBILITY ] =====> Visibility < 1,000 m (FG)? ============> NO-GO (VLOS)
        | OK (Vis >= 5,000 m)
        v
[ STEP 4: ICING / MOISTURE ] => Temp +3°C or below with Drizzle/Mist/Fog? => NO-GO (ICING)
        | OK (No icing risk)
        v
   MISSION GO!

Step 1: Wind and Gust Margin Assessment

• Consult the Manufacturer's Instructions: Identify the stated maximum wind resistance (e.g. $12.0\text{ m/s} \approx 23.3\text{ kt}$). This is a manufacturer limitation that UAS.OPEN.060(2)(e) requires you to respect.
• Apply 20% Safety Buffer: Environmental wind increases with altitude due to the ground friction gradient. A wind measuring 8 m/s at 2 m AGL may reach 11 m/s at 100 m AGL. A prudent personal minimum is a 20% margin below the manufacturer's limit: $v_{\text{operational limit}} = v_{\text{mfr limit}} \times 0.80 = 12.0 \times 0.80 = 9.6\text{ m/s} \quad (\approx 18.6\text{ knots})$
• Check Peak Gusts: The go/no-go evaluation must be based on the peak gust (G), not the mean sustained wind! If a METAR reports 22012G24KT, the peak gust is 24 knots ($12.3\text{ m/s}$). Because 24 knots exceeds the 23.3 knot manufacturer limit and exceeds the 18.6 knot operational threshold, the flight is an immediate NO-GO.

Step 2: Cloud Ceiling vs. 120 m Legal Altitude Buffer

• Identify the lowest broken (BKN) or overcast (OVC) layer in the METAR/TAF.
• Convert hundreds of feet to metres: multiply by 30.48.
• If the cloud ceiling is at or below $400\text{ ft}$ ($120\text{ m}$) AGL (e.g. BKN003 = 91 m AGL), the flight altitude must be constrained to at least 30 metres below the cloud base ($60\text{ m}$ max flight altitude), or canceled if mission objectives require higher altitude.

Step 3: Visibility and Practical VLOS Operational Radius

• If visibility is below 1,000 m (FG), treat it as a no-go: the drone will usually disappear from unaided view within a short distance, and continuous VLOS is a legal requirement.
• If visibility is reported between 1,000 m and 5,000 m (BR or HZ), evaluate the operational radius. For a typical C2 drone (diagonal dimension 35–50 cm), an unaided human eye with 20/20 vision loses orientation cues at approximately 150 to 200 metres on a clear day. In mist or haze, orientation is lost at 80 to 100 metres. The operational volume must be compressed accordingly.

Step 4: Temperature-Dew Point Spread and Structural Icing Risk

• Check ambient temperature ($T$) and dew point ($T_d$):
  • If $T$ is about $+3^\circ\text{C}$ or below AND present weather reports visible moisture (-RA, -DZ, BR, FG), assume a propeller icing risk. NO-GO.
  • If $T - T_d \le 2^\circ\text{C}$ during late afternoon or evening operations with clear skies and winds between 2 and 5 knots, radiation fog will form rapidly. NO-GO for extended missions.

Example Personal Minimums for an Unrated Class C2 UAS (12 m/s Wind Limit)

---

Comprehensive Operational Scenario Walkthrough

To consolidate your skills, evaluate the following real-world operational planning scenario.

Operational Briefing and Mission Parameters

• Client: Structural engineering audit of an industrial warehouse roof complex.
• Location: 6 km northeast of Brussels National Airport (EBBR), Belgium.
• Aircraft: Class C2 multirotor (MTOM 3.4 kg, manufacturer maximum wind limit $12.0\text{ m/s} \approx 23.3\text{ kt}$, unrated IP protection IPX0).
• Planned Flight Profile: Rooftop grid photogrammetry at 50 metres AGL, maximum operational radius 180 metres from pilot ground station.
• Mission Schedule: 09:30 to 11:30 local time (08:30 to 10:30 UTC).

Current METAR and TAF from Brussels Airport (EBBR)

METAR EBBR 220820Z 20014G25KT 4000 -DZ BR BKN004 03/02 Q1008 TEMPO 2500 DZ BKN002=

TAF EBBR 220800Z 2209/2306 21016G28KT 5000 -RA BKN006
    BECMG 2211/2213 24022G36KT 3000 RA BKN003
    TEMPO 2213/2218 1800 +SHRA BKN002CB=

Step-by-Step Risk Evaluation

1. Wind Analysis:
   • Current METAR reports wind from $200^\circ$ at 14 knots with gusts to 25 knots ($12.8\text{ m/s}$).
   • The peak gust of 25 knots exceeds the manufacturer's stated limit of 12.0 m/s (23.3 knots), and is well above the 80% personal limit of 9.6 m/s.
   • The TAF also shows the wind strengthening to 22 knots gusting 36 knots ($18.5\text{ m/s}$) between 11:00 and 13:00 UTC, just after the planned window, so conditions are forecast to get worse, not better.
2. Moisture and Ingress Protection:
   • The METAR reports active light drizzle and mist (-DZ BR).
   • The C2 aircraft possesses an IPX0 rating (zero water ingress protection). Liquid drizzle will infiltrate open motor cooling ports and short-circuit sensitive ESCs.
3. Cloud Ceiling and Clearance:
   • METAR reports a broken cloud ceiling at $400\text{ ft}$ ($122\text{ m}$) AGL (BKN004), with a temporary trend lowering to $200\text{ ft}$ ($61\text{ m}$) AGL (BKN002).
   • While the planned flight altitude is 50 m AGL, the temporary lowering to 61 m leaves an unacceptable 11-metre vertical margin from cloud immersion.
4. Structural Propeller Icing:
   • Ambient temperature is $+3^\circ\text{C}$ and dew point is $+2^\circ\text{C}$ (spread $1^\circ\text{C}$).
   • This sits inside the icing-risk band from Step 4 (visible moisture at $+3^\circ\text{C}$ or below). With drizzle and only $3^\circ\text{C}$ at the surface, there is little margin above freezing, so treat propeller icing as a real risk.

Final Operational Decision