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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Wednesday, June 22, 2011

20% or Bust!

In the flight training world, there are lots of old wive's tales and half truths. Some of these are harmless ("you're not allowed to carry the technical log on-board the airplane"), while others are potentially dangerous ("carb ice is caused by the venturi effect and the subsequent expansion and cooling of the air").

One of the sources of these half-truths is based in the misunderstanding of certification standards—even to the point of not knowing that a "rule-of-thumb" originates as a certification standard. As a case in point, today I'd like to discuss the 20% stall-speed crosswind rule. This rule, which is almost invariably presented as a rule-of-thumb, tells us that an aircraft can safely takeoff and land with a crosswind component equal to or less than 20% of the aircraft's stall speed.

The statement "an aircraft can safely takeoff and land with a crosswind component equal to or less than 20% of the aircraft's stall speed" is in fact true. But it is not a rule-of-thumb, and some aircraft can takeoff and land with significantly more crosswind. The 20% rule is a certification standard, and it sets for us the minimum allowable crosswind tolerance for a type certified aircraft. In Canadian regultions, ths requirement is embodied in CAR 523.233:

523.233 Directional Stability and Control

(a) A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VSO.

This regulation establishes two requirements: first that the safe crosswind component for taxi, takeoff, and landing not be less than 0.2 Vso; and second that the safe operation be demonstrated, not just calculated. This requirement for a demonstration is why we often see a "maximum demonstrated crosswind component" published in the flight manual. This published number is the rule, not 20% Vso. However, a careful reading of the regulation will reveal that the demonstrated crosswind component does not have to be a formal limit. For example, if the test program for the aircraft included successful takeoffs and landings with an 18 knot crosswind component, this could become the "maximum demonstrated crosswind component". Even though the aircraft may be capable of handling a 20 knot or even 25 knot crosswind, these higher speeds weren't demonstrated.

So, what about 20% Vso? Is it a useful reference? On some older models of aircraft, the flight manual does not include crosswind information. However, the certification standard tells us that the aircraft can handle at least 20% of the Vso. Maybe it can handle more. Maybe not. But the fact that the aircraft has a type certificate indicates that we have a minimum baseline to work from—even in the absence of published data.

With newer aircraft, CAR 523.1585 requires that the flight manual include information on "The maximum demonstrated values of crosswind for takeoff and landing, and procedures and information pertinent to operations in crosswinds". This suggests that the 20% rule is not only misleading, but obsolete—as we no longer reference it to determine an aircraft's capability, since the capability is published.

Happy Flying!

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