Thunderstorm Avoidance in Aviation: Strategies for Safety and Efficiency

Thunderstorms pose a significant threat to aviation because of their potential to cause turbulence, lightning, hail, and even tornadoes. It is important for pilots to understand effective means of avoiding thunderstorms and their associated hazards to ensure the safety of flight and to maintain operational efficiency. The following article delves into thunderstorms themselves as well as the strategies and technologies used to avoid them.

What are Thunderstorms?

A thunderstorm is a local storm produced by a Cumulonimbus (CB) cloud. Thunderstorms are always accompanied by lightning & thunder, and typically produce several phenomena that are hazardous to aircraft, including, but not limited to: strong wind gusts, heavy rain, and hail.

There can be up to 40,000 thunderstorms on a daily basis across the globe, and a significant portion of them occur in the United States.

They can produce dangerous conditions such as:

• Low ceilings and visibility: Visibility is near zero within a thunderstorm cloud, and visibility can be further inhibited by precipitation and blowing dust.
• Turbulence: All thunderstorms create turbulence, and it can sometimes be violent and severe. A severe thunderstorm can destroy an aircraft and wind gusts can increase an aircraft’s load factor, which can be severe enough to stall an aircraft or cause structural damage. Outside the thunderstorm cloud, turbulence can be encountered several thousand feet above and up to 20 Nautical Miles (NM) laterally.
• Lightning: By definition, every thunderstorm produces lightning and thunder. Lightning can damage or disable an aircraft in the following ways:
  • It can puncture the skin of an aircraft
  • It can damage communications
  • It can damage electronic navigational equipment

In addition, lightning in the vicinity can blind a pilot, which renders the pilot momentarily unable to navigate either by instrument or visual reference and unable to carry out his/her duties.

• Hail: Per the FAA, hail is precipitation in the form of balls or other irregular lumps of ice produced by thunderstorms; an individual unit of hail is called a “hailstone”. Hailstones can range in size from a pea-sized which is about ¼ inch in diameter to hailstones larger than a softball, which is about 4 ½ inches in diameter. Hailstones that are ¾ inch in diameter and larger can cause significant damage to aircraft and make it difficult to control.
• Microbursts: These sudden, powerful downdrafts can cause rapid and dangerous changes in wind speed and direction, impacting takeoff and landing operations. Specific characteristics of microbursts include:
  • Horizontal diameter of less than 2.5 Statute Miles (SM)
  • A depth of 1,000’
  • A lifespan of ~5 – 15 minutes
  • Downdrafts of up to 6,000’ feet per minute (fpm), which is mindboggling considering pilots flying single-engine training aircraft are climbing out at 500 fpm on a good day
  • Headwind losses of 30 knots to 90 knots (this significantly degrades performance)
  • Strong turbulence and windshear

 

Strategies for Thunderstorm Avoidance

1. Pre-Flight Planning and Weather Briefings: Effective thunderstorm avoidance begins with thorough pre-flight planning. Pilots should review weather forecasts, satellite imagery, and weather radar to identify potential thunderstorm activity along their planned route. Tools like Aviation Routine Weather Reports (METARs) and Terminal Aerodrome Forecasts (TAFs) provide essential information on weather conditions and storm predictions. Pilots should also review AIRMETs, SIGMETs, and Convective SIGMETs as a part of their preflight planning & weather reviews.

Convective SIGMETs are severe weather advisories concerning convective weather (i.e., thunderstorms). Weather conditions detailed in Convective SIGMETs affect the safety of all aircraft, not just General Aviation (GA) aircraft. Specific items pilots will find in Convective SIGMETs are as follows:

• Surface winds greater than 50 knots
• Hail at the surface greater than or equal to ¾ inch in diameter
• Embedded thunderstorms
• Lines of thunderstorms
• Thunderstorms with heavy or greater precipitation that affect 40% or more of a 3,000 square mile or greater region
• Tornadoes

 

2. In-Flight Weather Radar: Modern aircraft are equipped with onboard weather radar systems that help pilots detect and avoid thunderstorms during flight. The radar provides real-time images of precipitation and storm intensity, allowing pilots to make informed decisions about altering their route to avoid severe weather.

3. Next Generation Weather Radar System (NEXRAD): NEXRAD is a useful tool that pilots can use to avoid thunderstorms; it detects weather, specifically precipitation and winds, by emitting a burst of energy that reflects or bounces off an object (e.g., raindrop, snowflake, hailstone, birds, bugs, etc.) in all directions. A small fraction of that reflected energy is received by the radar itself, which helps paint a picture of the weather conditions on a screen in a mosaic format.

NEXRAD is best used to understand big picture weather conditions, and when coupled with the in-flight weather radar for real-time information, both can be used as effective tools for thunderstorm avoidance.

NEXRAD, however, has its limitations. For example, the radar image is not in real-time and can be up to 5 minutes old. In some extreme latency cases, the actual age of the oldest NEXRAD data in the mosaic can exceed the age indication in the cockpit by 15 to 20 minutes. Therefore, it’s important to use NEXRAD for big picture information and it should not be used as the primary means of navigating safely around a thunderstorm.

4. Flight Path Adjustments: Pilots should be prepared to adjust their flight path to circumvent areas of intense thunderstorm activity. This may involve deviating laterally around storm cells or altering altitude to avoid the most severe turbulence. Air Traffic Control (ATC) can provide guidance and clearance for such deviations. “Salt Lake Center, Cessa 123AB, requesting left deviations for weather.” – “Cessna 123AB, Salt Lake Center, left deviations approved. Proceed direct ZAVAD when able and advise.”

5. Altitude Management: Thunderstorms can have significant vertical development, with storm tops reaching altitudes at and above where aircraft operate. Pilots should use weather radar to determine the altitude of storm tops and avoid flying through or near them. Climbing above or descending below the storm, if possible, can help minimize exposure to turbulence.

6. Avoiding the Storm’s Periphery: Thunderstorms can produce dangerous weather conditions beyond their immediate vicinity. Pilots should be cautious when flying near the periphery of a thunderstorm, as associated hazards like turbulence can still pose risks. The FAA recommends that pilots avoid thunderstorms laterally by at least 20 NM (i.e., 23 SM).

7. Training and Simulation: Regular training and simulation exercises help pilots prepare for and respond to thunderstorm-related challenges. Training programs often include scenarios that simulate thunderstorm conditions, enhancing pilots’ ability to make quick and effective decisions in real-world situations.

Specific Guidance from the FAA:

Both the FAA’s Aviation Weather Handbook (FAA-H-8083-28) and the Aeronautical Information Manual (AIM) provide pilots with a significant amount of information on thunderstorms and strategies to avoid them. The AIM, 7-1-27, “Thunderstorm Flying” (pg. 860) lists specific ways pilots can avoid thunderstorms. The methods are as follows:

• Don’t land or takeoff in the face of an approaching thunderstorm. A sudden gust front of low level turbulence could cause loss of control.

 

• Don’t attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and wind shear under the storm could be hazardous.

 

• Don’t attempt to fly under the anvil of a thunderstorm. There is a potential for severe and extreme clear air turbulence.

 

• Don’t fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded usually can be visually circumnavigated.

 

• Don’t trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm.

 

• Don’t assume that ATC will offer radar navigation guidance or deviations around thunderstorms.

 

• Don’t use data-linked weather next generation weather radar (NEXRAD) mosaic imagery as the sole means for negotiating a path through a thunderstorm area (tactical maneuvering).

 

• Do remember that the data-linked NEXRAD mosaic imagery shows where the weather was, not where the weather is. The weather conditions depicted may be 15 to 20 minutes older than indicated on the display.

 

• Do listen to chatter on the ATC frequency for Pilot Weather Reports (PIREP) and other aircraft requesting to deviate or divert.

 

• Do ask ATC for radar navigation guidance or to approve deviations around thunderstorms, if needed.

 

• Do use data-linked weather NEXRAD mosaic imagery (for example, Flight Information Service-Broadcast (FIS-B)) for route selection to avoid thunderstorms entirely (strategic maneuvering).

 

• Do advise ATC, when switched to another controller, that you are deviating for thunderstorms before accepting to rejoin the original route.

 

• Do ensure that after an authorized weather deviation, before accepting to rejoin the original route, that the route of flight is clear of thunderstorms.

 

• Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo. This is especially true under the anvil of a large cumulonimbus.

 

• Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage.

 

• Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm.

 

• Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is visually sighted or determined by radar.

 

• Do give a PIREP for the flight conditions.

 

• Do divert and wait out the thunderstorms on the ground if unable to navigate around an area of thunderstorms.

 

• Do contact Flight Service for assistance in avoiding thunderstorms. Flight Service specialists have NEXRAD mosaic radar imagery and NEXRAD single site radar with unique features such as base and composite reflectivity, echo tops, and VAD wind profiles.

 

If you cannot avoid penetrating a thunderstorm, following are some Do’s before entering the storm:

• Tighten your safety belt, put on your shoulder harness (if installed), if and secure all loose objects.

 

• Plan and hold the course to take the aircraft through the storm in a minimum time.

 

• To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of -15ºC.

 

• Verify that pitot heat is on and turn on carburetor heat or jet engine anti-ice. Icing can be rapid at any altitude and cause almost instantaneous power failure and/or loss of airspeed indication.

 

• Establish power settings for turbulence penetration airspeed recommended in the aircraft manual.

 

• Turn up cockpit lights to highest intensity to lessen temporary blindness from lightning.

 

• If using automatic pilot, disengage Altitude Hold Mode and Speed Hold Mode. The automatic altitude and speed controls will increase maneuvers of the aircraft thus increasing structural stress.

 

• If using airborne radar, tilt the antenna up and down occasionally. This will permit the detection of other thunderstorm activity at altitudes other than the one being flown.

Following are some Do’s and Don’ts during the thunderstorm penetration:

• Do keep your eyes on your instruments. Looking outside the cockpit can increase danger of temporary blindness from lightning.

 

• Don’t change power settings; maintain settings for the recommended turbulence penetration airspeed.

 

• Do maintain constant attitude. Allow the altitude and airspeed to fluctuate.

 

• Don’t turn back once you are in the thunderstorm. A straight course through the storm most likely will get the aircraft out of the hazards most quickly. In addition, turning maneuvers increase stress on the aircraft.

 

Challenges and Considerations

1. Unpredictable Nature of Thunderstorms: Thunderstorms can develop rapidly and change course unpredictably. Pilots must rely on real-time weather data, including PIREPs, and be prepared for sudden changes in weather conditions.

2. Communication with Air Traffic Control: Effective communication with ATC is vital for thunderstorm avoidance. Pilots should provide ATC with accurate information about their position and intentions, and ATC can assist with route adjustments and provide updated weather information. While ATC may provide you with radar vectors to fly you safely around thunderstorms (i.e., if you’re flying under IFR or receiving flight following), it is important for the Pilot-in-Command (PIC) and the crew to proactively identify and request radar vectors to safely avoid thunderstorms and their associated hazards.

3. Passenger Comfort and Safety: Severe turbulence and unexpected weather conditions can impact passenger comfort and safety. Pilots should communicate with all crewmembers to ensure passengers are informed and securely fastened during turbulent conditions.

4. Operational Constraints: In some cases, operational constraints such as airspace restrictions or traffic congestion may limit the options available for avoiding thunderstorms. Pilots must balance safety with operational considerations and work closely with ATC to find the best solution.

Thunderstorm avoidance is a critical aspect of aviation safety. It requires a combination of pre-flight planning, in-flight decision-making, and effective use of technology. By leveraging weather radar, adjusting flight paths, managing altitude, and maintaining open communication with ATC, pilots can navigate around or through thunderstorms with greater safety and efficiency. Ongoing training and preparedness further enhance pilots’ ability to handle the challenges posed by these formidable weather phenomena, ensuring the safety and comfort of passengers and the successful operation of flights.