Happy New Year! I’ll start the year off with a subject everybody understands: weight and balance.
OK, maybe not everybody…
In experience gained providing innumerable aircraft checkouts, I have concluded the subject of W&B is actually a mystery to a majority of pilots. Most can do the necessary computations in only one model of aircraft, and almost none understands the underlying theory.
We need a definition of terms and consequences for both WEIGHT and BALANCE.
Weight is by far the simpler half, the result of a mysterious attraction, gravity. It has a primary effect on two separate areas of aircraft operations: takeoff/climb performance and structural integrity in turbulence.
All aircraft have very specific restrictions about the maximum weight authorized for takeoff (MTOW). There are also potential maximum “ramp” and “landing” weights, but these go beyond the purposes of this discussion.
On the performance side, pilots of aircraft boasting lots of horsepower sometimes become cavalier about MTOW after discovering that it is possible to operate as much as 5-10% over the limit with no serious penalty. These discoveries are often inadvertent and innocent, the result of realizing after the fact that a passenger/baggage/fuel combination had been too much.
If it worked once, though, it’s probably OK all the time, so a habit is formed.
This is understandable, the kind of flaw often called “human,” but quite dangerous. To start with, there are two obvious performance drawbacks. The first centers around the possibility of mechanical failure: if you suffer power loss, it is best not to have excess weight as an added complication.
The second dangerous area involving performance and over-loading is specific to high density altitude operations. Every year’s accident statistics offer up a predictable number of events involving pilots trying to fly in too little air with too much aircraft weight. The odds are good that many of these pilots had become accustomed to a careless approach to weight restrictions at lower altitude airports, only to find that the combination of a thin atmosphere and an overloaded airplane can have disastrous results.
If performance arguments are not sufficient to change a pilot’s approach to over-weight operations, the structural consequences might be more convincing.
The design process of every certificated aircraft incorporates development of a “Vg diagram” that clearly defines the relationship between desired cruise airspeed, G loading and structural integrity (the “g” stands for “gust”). The chart is used to determine design maneuvering speed (Va), maximum structural cruising speed (Vno), and never-exceed speed (Vne); limits are predicated on vertical gusts of 15 feet per second.
The first two of these speeds are developed to provide pilots with absolute limits for use in avoiding structural damage in turbulence. Is there any turbulence at all? Slow down below Vno. Are conditions rough enough to require abrupt or full control extensions? Slow down to Va or lower.
As you might expect, these airspeed values are developed in conjunction with specific gross weight expectations. If you exceed that weight, you lose guarantees of structural integrity in turbulence.
Now, if you are an inveterate over-loader you might be willing to put up with decreased climb performance and extended takeoff rolls, but how comfortable are you with the idea of bent and missing airframe pieces? Although there are lots of safety margins built into the design of your aircraft, I can assure you there are few extra pieces. You need them all, and if you knock a couple off as a consequence of overloading, you will end up badly.
Next month, I’ll start to tackle the second and more complicated half of W&B: balance, the teeter-totter part.