Equilibrium is one of those areas that many new student pilots have trouble understanding. It's actually a pretty simple concept, but we tend to over-complicate these things sometimes. If we want to be picky about our terminology, we really should use the term "mechanical equilibrium". But since we don't normally talk about other forms of equilibrium in aviation (e.g. – hydrostatic, thermal, chemical, etc.), we can stick to the shortened and more generic "equilibrium".
So, equilibrium (the mechanical kind) refers to the absence of acceleration. If we think in terms of Newton's Laws of motion, this suggests that there are no net forces acting on the aircraft—so all of the forces balance (cancel out) and the aircraft will travel in a straight line at a constant speed.
That seems pretty simple (and it is!), but the confusion comes from our intuitive understanding. Equilibrium is often considered (intuitively) to refer to a stationary object—which isn't a requirement at all. We generally don't have trouble extending this intuitive concept to steady straight-and-level flight. But we often run into trouble trying to apply it to climbs and descents.
The "logic" (for lack of a better term) applied seems to be that if the altitude is changing, there must be acceleration, and we therefore must not be in equilibrium. This is wrong. In a steady climb or steady descent, there are no net forces acting on the aircraft, and there are therefore no accelerations.
So straight steady climbs and descents are included in the list of maneuvers that qualifiy as equilibrium—along with straight & level at constant airspeed and sitting stationary on the ramp (and hovering if you fly helicopters). However, the transition from straight & level into a climb/descent is not equilibrium, nor is the transition back into straight & level.
Why is this important? Well, equilibrium (or the lack thereof) is related to the forces acting on the aircraft. We need to be conscious of these forces in order to anticipate aircraft behavior. This is especially important at low speeds, where an increase in lift brings us closer to the stall. If we have more lift than weight, then we are not in equilibrium, and the stall speed is increased. This will happen when we make adjustments to the flight path using pitch.
About the Author
- Steve Pomroy
- 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.