Thunderstorms — Page 238, Lesson 199

I want to walk you through the hazards associated with thunderstorms — specifically lightning, static, pressure variations, and microbursts. These are all phenomena you need to understand thoroughly as a professional pilot, because each one can affect your aircraft, your instruments, or your control of the flight. Let's start with lightning. Lightning is most likely to occur within about 5000 feet of the freezing level, where the temperature is between +10°C and -10°C. That's the band where ice and water coexist in the cloud, which generates the electrical charge separation that produces lightning. The main effects of a lightning strike on the aircraft are several. First, temporary blindness of the pilots — the flash is intense and can disorient you for a moment. Second, minor airframe damage — the strike can leave small burn marks or pits, but modern aircraft are designed to conduct the current safely. Third, and this is important: magnetic compasses may be seriously affected. Errors of tens of degrees have been recorded after a strike, so you must use compass information with caution until you can check it against other sources. Fourth, there can be disruption to electrical equipment — systems may trip or behave erratically. Now, static. Static electricity builds up in the electrically charged environment of a thunderstorm. It causes interference on radio equipment across all the main communication bands: LF, MF, HF, and VHF frequencies. That means your ability to communicate with ATC or other aircraft can be degraded. A visible manifestation of static is St. Elmo's fire. This appears as purple rings of light around the nose, wing tips, and propellers. It looks dramatic, but here's the key point: this is not a hazard in itself. However, it is a very important indicator — it tells you that the air is electrically charged and that lightning is probable. So if you see St. Elmo's fire, you should treat it as a warning that a lightning strike may be imminent. Next, pressure variations. In or close to a thunderstorm, you can get local pressure variations that cover only a very small region. These can cause your QFE or QNH settings to be in error, which means your altimeter readings can be inaccurate by as much as ±1000 feet at all heights. That's a significant error. Combined with gust effects, these pressure variations can cause height errors at low level which can be dangerous — imagine trying to clear terrain or maintain separation when your altimeter is off by a thousand feet. The VSI — Vertical Speed Indicator — will also be subject to errors under these conditions. Because of this, the correct technique is: the aircraft should be flown for attitude rather than altitude. That means you focus on the attitude indicator to maintain the correct pitch and bank, rather than chasing the altimeter or VSI readings that may be unreliable. However, there's a further complication: some attitude indicators may not be able to cope with the severe turbulence likely to be encountered, so you need to be aware of that limitation as well. Finally, microbursts. These are down currents in the cloud that also move outwards by reaction from the ground. Think of a column of descending air that hits the ground and then spreads out horizontally in all directions. Their speeds are considerable: downwards, they can exceed 1000 feet per minute, and can reach up to 6000 feet per minute. Horizontally, they can produce winds of up to 50 knots. The critical hazard here is windshear — specifically a transition from headwind to tailwind as you fly through the microburst, which can cause a sudden loss of airspeed and lift, especially dangerous during takeoff or landing. That covers the main thunderstorm hazards from this section: lightning, static and St. Elmo's fire, pressure variations affecting your altimeters and VSIs, and microbursts with their extreme downdrafts and windshear.