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.

Friday, August 3, 2012

Clearing Obstacles

It's been quite some time since I've posted here. I guess it's time to get back at it! SkyWriters Publishing is progressing well, and book sales are slowly taking off. The PPL Exam Prep book is especially popular.

Recently, I was contacted by a student pilot who blogs about her experiences learning to fly. She has reviewed and posted her thoughts about the Exam Prep book HERE.

So check out Vera's Blog, and then come back here and read a little about climbing for obstacle clearance.

Clearing Obstacles:

We're all taught in groundschool that in order to clear obstacles, we should climb at Vx, or the best angle of climb speed. With a little bit if theory, we also learn that Vx corresponds to the speed at which we get the most excess thrust (not to be confused with excess power, which provides us with climb rate).

So far so good, all of this seems pretty straightforward. But in some aircraft types, the speed provided in the flight manual for an obstacle-clearance takeoff is at odds with the separately published Vx. For example, the flight manual for some models of the Cessna 172 call for a speed through 50 feet of 68 mph for obstacle clearance takeoffs, and list Vx as 75 mph. What gives?

The key to understanding this discrepancy is in the distinction between equilibrium and non-equilibrium maneuvers (discussed here: http://www.flightwriter.com/2010/11/equilibrium.html). Vx is determined under the condition of equilibrium. During takeoff, we are not in equilibrium, we are accelerating—so some (or all) of the excess thrust being created by the engine is being used to change our airspeed instead of our height. Because of this, we need to make a choice: start climbing early (i.e. – before reaching Vx) and accept a shallower initial climb gradient, or accelerate just above the runway to Vx and climb at our best available angle. Which option is better?

The prefered technique depends on the height of the obstacle. Consider the geometry of the two climb options. By climbing early, we will initially gain more height than climbing at Vx. But because of the reduced climb gradient, another aircraft climbing at Vx will eventually catch up and gain more height. Early in the climb, the aircraft climbing early and slow will be higher. Later in the climb, the aircraft climbing at Vx will be higher. At some height, which will be different for different aircraft (and may even be different for the same aircraft under different operating condition, such as weight and density altitude), the altitude of the two aircraft will be the same. For obstacles below this height, climbing early is preferable. For obstacles above this height, accelerating level and then climbing at Vx is preferable.

How does this information affect operational decisions? Understanding the (generally) poorly explained flight manual data enables us to get more performance out of our aircraft with better safety margins. A perfect example would be taking off toward rising terrain with an elevation change of several hundred feet. In the above C-172 example, do we climb at 68 MPH or 75 MPH? An uncritical review of the performance tables would suggest 68 MPH. But we'll get better terrain clearance at Vx = 75 MPH because such a long climb gives us plenty of opportunity to establish equilibrium.

Happy Flying!