About the Author

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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!

Tuesday, June 7, 2011

Harbour Grace

Continuing on from my last post (Flying on Grass), a little discussion of one particular grass field:

This picture was taken in Harbour Grace, Newfoundland—by far, my favourite place to take students for some initial exposure to real challenging conditions (as distinct from the simulated conditions we often set up on long paved runways for training purposes).

Aside from being a great little airstrip, Harbour Grace has some history. It was the launch site for some of the original successful international and trans-atlantic flights (read up on some of the history HERE), although Alcock and Brown launched out of St. John's, just a few minute flight time to the East.

I took the picture while I stood on top of the 65 foot rock (read "obstacle") that must be cleared prior to landing. The aircraft was being flown by two of my instructor students. At this point, they had cleared the obstacle, and were completing their approach to the grass strip, which also has a 4% downgrade. In light winds, this is one of the few one-way runways outside the Rockies. You can land uphill with no obstacle on the approach, and takeoff downhill away from the obstacle. Unfortunately, this is Newfoundland, and light-wind days are few and far between. So, with prevailing winds out of the West, landing over the obstacle onto the downslope is the norm.

This is a good example of a place where you might push the limits of not only the aircraft, but the pilot. Precision control of glidepath, aim point, and airspeed are vital, or the approach will be unsuccessful. I have yet to see a pilot (private, commercial, or instructor candidate) make a successful, unassisted landing on their first attempt. On the other hand, I have yet to see a pilot who couldn't land here safely after a bit of practice. Perhaps the most important skill one needs before attempting to land in a place like this is the overshoot/go-around. The overshoot is your "out" if things don't go exactly according to plan. Don't be surprised if you need to use it here.

During takeoff, there is a clear preference to takeoff downhill and away from the obstacle. Luckily, the prevailing winds usually support this preference. However, I have had one occasion of taking off on the upslope and over the obstacle. We had landed uphill with a light headwind, with the plan to takeoff downhill with a light tailwind (less than 10 knots and within the limits of the aircraft we were flying). But we stopped for a break and wouldn't you know it, but the winds picked up. leaving us with three options:
  1. Takeoff downhill with no obstacle, but with a tailwind outside my comfort zone. This was a non-starter.
  2. Sit tight and wait for the winds to change. This showed some promise, although it might have been a long wait.
  3. Takeoff into the headwind uphill over the obstacle. But only after confirming it could be done and establishing an "out".

Option 2 woud clearly be the safest. But the question we faced was, could we make option 3 work safely? The answer came down to two actions and a possible third.

First, we spent some time crunching numbers—to see if it worked on paper. With a margin of error. Accounting for the gradient, the grass, and the non-standard (i.e. - not 50 ') obstacle required the application of some rules-of thumb. Not being manufacturer-approved, it was important to make sure the rules we used were conservative—that they erred on the side of caution. As it turned out, the winds really helped. Too strong to allow for a tailwind takeoff, they contributed greatly to shortening our takeoff and climb distance when applied as a headwind.

Second, we identified and marked a reject point. As it happend, the "mark" was pre-existing, but getting out and placing a deliberate stack of rocks (or some other marker) would have been an option if there weren't a natural mark to use. The ability to reject the takeoff was our "out" if the calculations were wrong, or if the engine wasn't actually performing as advertised. Or if any one of a host of other things didn't go as planned. The ability to reject a takeoff is just as important as the abilty to go-around from a bad approach or landing.

Our possible third option was leaving our passenger (a second student observing from the back seat) behind. If the takeoff was too marginal, the weight reduction could make enough of a difference to get us out. That would have meant returning without the front seat passenger (the student), and would have been a bit of a hassle. But it might have made the difference if we couldn't all get out together. As luck would have it, this wasn't necessary. But it was discussed as an option.

After all of this planning and thought, the takeoff itself was almost anti-climactic—a non-event. We accelerated (slowly, as anticipated) to our reject speed, after which we passed our reject point and made the "continue" decision. Then we lifted off and climbed over the obstacle—with a shallow left turn to take advantage of the slope of the obstacle (it's lower on the left than on the right), clearing it with plenty of margin. All in all a good day, a good flight, and a valuable unplanned learning experience.

Landing on (and taking off from) grass surfaces is something we train for. But almost always under "simulated" conditions—where we get to practice the technique, but are forced to skimp on much of the decision making. The opportunity to try things out on real grass is invaluable for students, and very rewarding for experienced pilots.

Happy Flying!