Thunderstorms — Page 266, Lesson 238

I want to walk you through the effects of thunderstorms on aircraft systems, starting with a very serious one — the magnetic compass. When lightning strikes an aircraft, the effect on magnetically slaved compasses can be severe. I'm talking about deviations of many tens of degrees having been recorded. That's not a small wobble — that's a complete loss of reliable heading information. So the rule is clear: magnetic compasses should not be relied upon after an aircraft has been struck, and they should be checked as soon as possible after the event. Now let's move to static electricity. This phenomenon will generally first be noticed as noise on the High and Medium frequency radio bands — that's HF and MF — and also, to a lesser extent, on VHF receivers. As the static electricity increases in severity, the noise will increase. In extreme cases, a visible discharge known as St Elmo's fire will be seen on some parts of the aircraft, particularly around the edges of windscreens. Let me define St Elmo's fire for you: it's a visible electrical discharge that occurs when the static electric field around the aircraft becomes intense enough to cause the air to glow. You'll see it as a bluish or violet glow on the windscreen edges, wing tips, or propeller blades. Now, static electricity is not associated only with thunderstorms, but such conditions are particularly favourable to its creation. Although it is not normally dangerous, there have been rare incidents when a static discharge has occurred across a windscreen or plastic panel causing it to break. So while it's not a primary safety threat, it can cause structural damage in extreme cases. An understanding of the effect of static electricity on radio equipment is important. It is detrimental to the performance of MF equipment — that's Medium Frequency, which includes the ADF or Automatic Direction Finder — and HF equipment, High Frequency. But it has little or no effect upon VHF and UHF — Very High Frequency and Ultra High Frequency. On HF, static may cause the signal-to-noise ratio to be such that communications are impossible. The signal-to-noise ratio is simply the strength of the desired radio signal compared to the background noise — when static overwhelms the signal, you can't hear anything useful. In these conditions, navigational aids such as ADF must be used with extreme caution due to the fluctuating or erroneous indications that may occur. Finally, let's look at instrumental errors and limitations. Starting with altimeters and vertical speed indicators. Local pressure variations can occur in or very close to a thunderstorm at all heights. This, together with local gusts, may give rise to errors in the indications of altimeters and vertical speed indicators. There is some doubt as to the magnitude of altitude errors — the excerpt leaves that as an open question, but the key point is that you cannot fully trust your altitude and vertical speed readings when you're in or near a thunderstorm cell. Let me show you the three stages of thunderstorm development so you can visualise where these effects occur. So to summarise what we've covered: lightning strike can cause compass deviations of tens of degrees — don't rely on the compass after a strike. Static electricity causes radio noise on MF and HF, can produce St Elmo's fire on windscreen edges, and can make ADF unreliable. And pressure variations near thunderstorms cause errors in altimeters and vertical speed indicators.