Outside the envelope

Pilots seem to be more conscientious about complying with weight limitations. This is because the ramifications of an excess load are easier to understand (and calculate): performance deteriorates, and structural integrity can be compromised (especially in turbulence).

But pilots do overload their aircraft, often inadvertently. As a designated examiner, I was chagrined to discover how many pilots calculate gross weight using the FAA’s nominal 170-pound passenger. Also, many incorrectly use the sample empty weight shown in the pilot’s operating handbook.

There is no valid use for the FAA’s nominal 170-pound passenger. It should be stricken from training manuals and handbooks because using it to determine gross weight can lead to dangerous miscalculations.

A Hollywood celebrity owned a Beech Bonanza V35B and occasionally loaded it with four men and full tanks. He sensed, incorrectly, that the aircraft was loaded within limits and saw no reason to perform any calculations. The trouble is that his aircraft (and many others) cannot be loaded this way without being overweight (and possibly risking an excessive aft CG). Just because the aircraft has room to carry the load does not mean that it is legal and safe to do so.

This is particularly true of large aircraft. They often can fly with full tanks or full cabins (and cargo compartments) but not both. This explains why passenger loads might be limited on some long-range flights in favor of additional fuel.

Although pilots are aware of the problems associated with overweight takeoffs, they don’t seem to be as aware of the hazards associated with an excessively aft or forward CG, which could be why they are cavalier about it. They assume that the aircraft might be a bit nose- or tail-heavy, but that’s about it. They don’t appreciate how dangerous an out-of-bound CG can be, nor do they appreciate how easy it can be to exceed a CG limitation.

The same pilot with the V-tail Bonanza ultimately loaded four big men in the cabin and stuffed his baggage compartment with heavy TV equipment. Upon returning, he complained of difficult handling characteristics. I suggested that this might be related to an aft CG and that we calculate what it was when he departed. We discovered that his CG was not just outside the envelope, and it was not just outside the graph. The CG was so far aft that it had to be plotted off the page, and burning fuel during flight only worsened the condition.

Perhaps the most unusual flight characteristic he experienced was a significant loss of longitudinal (pitch) stability. This can make the aircraft difficult to control—especially in turbulence or when flown on instruments.

I once flew an aircraft somewhat like that. It was an improperly loaded—yes, it was my fault—Transavia Airtruk, which is as strange in appearance as its name implies. The flight was to Dubbo, a small city in eastern Australia. Every time the airplane would gain or lose airspeed in turbulence it would continue gaining or losing; when I added power, the nose would go down and vice versa. It was weird, unnerving, extremely fatiguing, and, yes, dangerous.

In addition to losing pitch stability, an excessively aft CG also causes stick forces to lighten, which means that fewer pounds of pull are needed to create a given G load. As a result, it is easier to inadvertently overstress an airplane, especially in turbulence.

Other consequences of an excessively aft CG include potentially violent stall characteristics and the tendency for a normal spin to become a flat spin, recovery from which might be impossible. On the flip side of the coin, an aft CG does improve cruise performance slightly and results in lower stall speeds. This explains why cargo in jetliners is often loaded in such a way as to shift the CG aft, but obviously within limits. This fuel-saving technique, however, is seldom worth the effort in light aircraft.

Forward CG limits typically are more difficult to violate. But when operating with an excessively forward CG, a pilot can expect excessive pitch stability, which is more unpleasant than you might expect. The aircraft becomes less responsive in pitch, and the elevator becomes heavy, like trying to maneuver an automobile with its power steering turned off. An excessively forward CG also can make it difficult to flare during landing. Flying an overloaded and out-of-balance aircraft is analogous to flying an airplane collecting ice. Both are conditions where even test pilots fear to tread.

BarrySchiff.com