ATPL · Meteorology · Meteorology Atpl Ground Training Series 2014 lesson — Page 266, Lesson 239

I want to walk you through the effects thunderstorms have on your flight instruments — specifically the altimeter, vertical speed indicator, airspeed indicator, and attitude indicator. This is critical knowledge for operating near or inside thunderstorm areas. Let's start with the altimeter and vertical speed indicator. In and around thunderstorms, pressure variations can cause errors in the indications of both the altimeter and the vertical speed indicator. Now, there is some doubt in the meteorological community about the exact magnitude of these altitude errors, but there is evidence that they can be as much as plus or minus 1,000 feet. That's a significant potential error — plus or minus 1,000 feet means your altimeter could read 1,000 feet higher or 1,000 feet lower than your true altitude. For ground clearance purposes, it is essential that you make due allowance for such errors when flying in or near thunderstorm areas. Near the surface, periods of heavy rain are an indication of the likelihood of pressure variations and gusts — so if you see heavy rain at low level, expect those pressure fluctuations that cause instrument errors. Now let's move to the airspeed indicator. Despite the precautions taken in the design of pitot heads — the forward-facing probes that measure ram air pressure for airspeed — there is still a possibility that very heavy rain may cause an airspeed indicator to give a false indication, even when the pitot head heaters are used. The heaters are meant to prevent ice from blocking the pitot, but heavy rain can still disrupt the pressure measurement. Here's the key operational procedure: if you have selected the power setting that gives the safest speed for penetration before entering a storm, you should take no action to correct for violent or short-period airspeed indicator oscillations, provided you maintain a reasonably level attitude. In other words, once you're in the storm, trust your pre-selected power and attitude — do not chase the airspeed needle if it jumps around. Finally, let's look at attitude indicators. Attitude is indicated by instruments presenting pitch and roll information alone, or by other more complex flight directors that contain attitude indication amongst other elements. The simple artificial horizons fitted to most aircraft — either as the main attitude indicator or as a standby instrument when remote-reading indicators are installed — provide indications of pitch angle up to 85 degrees nose up and nose down, and may have complete freedom in the rolling plane. That means they can show you any bank angle without limitation. Except in rare circumstances, these instruments give an adequate range of indication. However, they may lack referencing — meaning they don't give you enough visual cues to assess attitude accurately at large angles of pitch, or to give you maximum assistance in recovery from any unusual attitudes. So while the range is there, the precision and recovery guidance at extreme attitudes may not be.