Aviation Safety Letter 3/2004
To the Letter — Slow for Thunderstorms.
Your letter is always interesting reading prior to the joy of updating my A.I.P. Canada (AIP). One item was evidently missed in your primer for thunderstorms in the "Take Five" feature of ASL 3/2003.
The most important action to take if one is unable to avoid flying into a cumulonimbus (CB) is to slow down. This means to fly below the manoeuvring speed for the airplane at its current loading. This requires a knowledge and understanding of Va [design manoeuvring speed] and its implications. Also, lowering the gear in a retractable will help stabilize the A/C [aircraft], although this action must be weighed against the additional surface for ice build-up if icing conditions exist. Having had a few unplanned encounters myself with CB's, I wonder if I would still be around if I had not applied the knowledge relative to Va? Pilots also need to be reminded that the placarded value for Va is for gross weight and that the speed diminishes for lower indicated airspeeds. Having given well over 6 500 hr of instruction, I can state that Va is still not well understood amongst many pilots.
Thank you D.S. The AIP, AIR section 2.7 covers inadvertent flight through thunderstorms quite nicely and indicates that you should set the power settings for turbulence penetration airspeed recommended in your aircraft manual. Some publications do not use the term Va, as it is considered that understanding of the words "turbulence penetration airspeed" (shown in the aircraft manual) is more important at the early stages of training than learning V speeds. — Ed.
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