By Bernie Willis
No, I’m not trying to shame anyone into losing weight. But I am frustrated at a number of “absolutes” that may be good for primary flight students but keep aviators from understanding the broad impact of their decisions. “Have you ever flown with an aft CG?” “Have you ever taken off downwind?” “Do you always know your Gross Weight?” I believe pilots will make good decisions if they know the consequences of their choices. What are the consequences of aircraft weight?
Many years ago as a copilot on the B737-200 I observed one of our most experienced captains make a perfect approach to Dead Horse on the North Slope. He was right on speed and crossed the threshold perfectly about to touch down on the marks when all of sudden the stick shaker banged the control column, the plane shuddered and went splat on the runway, There wasn’t even enough energy left for a bounce. The captain knew what had happened and I soon learned. There was a large pump in cargo position one and it had been either misweighed or mislabeled. It turned out to be twice as heavy as listed, besides exceeding the position weight limit. Our reference speed above stall should have been a bit higher. The aerodynamic effects of a forward of mid CG increases down pressure from the tail which increases the necessary lift provided by the wing which in turn increases the necessary angle of attack which increases the induced drag and slows the airplane down or requires more fuel to maintain the normal speed which shortens range. In other words the further forward the CG is the more fuel will be used to maintain speed. Some large cargo Jets move fuel around between wing, fuselage and tail tanks to maintain an optimum balance for range or fuel economy. Lighter weights provide many benefits. I’m very proud of Alaska Airlines service to Dutch Harbor. With a runway only about 4000 feet long we were able to operate safely for many years by using reduced weight for takeoff and landings. We could factor in additional performance with head winds. That was the only time I really liked a wind on the nose.
It’s not uncommon in the Cessna 100 series to have a very forward empty CG. Rumor has it that the 182 has a weak nose gear. I’m not so sure of that. But what I do know for sure is that in some early models the forward CG limit is exceeded with two normal adults in the front seats and no baggage or passengers behind. In this configuration it’s very easy to land flat or nose wheel first. Another situation we should all be aware of in this day of increased power through engine conversions is that the additional weight of the larger engine and constant speed propeller while increasing performance also pushes forward the empty CG only to be aggravated by only one or two crew in the front seats. Beware! Do stall characteristics change with a too far forward CG? Yes. they do! Your aircraft may not stall at all but instead will mush at a decent rate where upon impact your back will be broken. Full up elevator will not stop the decent. Only power will help and you may not have enough. The escape is, wings level, power up and nose down elevator to return to flying speed. How close to the ground do want to do this?
If a forward CG isn’t desirable then what about a rearward CG? Since about 1925 in the US stall characteristics for certified aircraft require that if the wing stalls the aircraft must pitch nose down to initiate recovery on its own. How would we ever recover from a stall if the aircraft pitched up after the wing quite flying? Should a spin occur after a stall and the nose not go down far enough for the aircraft to gain speed or decrease angle of attack to begin flying again by definition it would be in a flat spin and probably not recoverable. As a result the aft CG limit is usually a couple inches ahead of this very dangerous position in light aircraft. Never the less, pitch sensitivity increases as the CG approaches the limit.
It’s established then, that CG position is critical to safe flight but what about total weight? Most pilot operating handbooks (POH) list performance at various weights. Older CAR 3 certified aircraft may only show performance figures for the Gross Weight. Some handbooks have been revised over the years by the manufacturer. I know of one that originally listed a glide ration of 15:1 now revised to 10:1. Stall speeds are a system dependent number. If your pitot static system is tight, which they seldom are, the book values may be accurate but if not, some flying near the margins will give you valuable information. Total weight or gross weight is the total of the empty airplane and everything in it. The max gross provided by the manufacturer is the basis for maneuvering speed, VA, an interesting topic for another day, as well as the ability of the aircraft to sustain loads imposed by flight maneuvers and turbulence. Weight on a graph is linier compared to load factor. In round numbers if you have a max gross weight listed at 2000 pounds and a capability of sustaining 4 g’s then if you could fly at 1000 pounds the aircraft could sustain 8 g’s. In reality aircraft get loaded over the listed max gross so the capability is decreased. In our example the 2000 pound airplane if loaded to 4000 pounds could only sustain 2 g’s. Yes, airplanes are flown over the max listed gross weight legally but only in certain conditions. Ferry flights on ocean crossings require extra fuel and cabin tanks are often installed. Max Conrad delivered a 250 Comanche from New York to Italy. Besides the normal 60 gal. tanks he arranged for aux tanks to carry an additional 265 gal. of fuel. That fuel load put his takeoff weight about 1000 lbs over normal gross weight. Our local Bill Compton was over normal GW to fly his Bonanza to Hawaii. Check out FAR 91.323. In some circumstances it allows for a 15% increase of max gross weight in Alaska. Don’t use this as authorization because your aircraft must have been certified by the Department of Commerce by Jan 1, 1931. It should be obvious that weight decreases performance. Take off distance increases, climb rate decreases, fuel burn increases, engine cooling may become marginal, but no one knows the numbers for these over- weight flights. To get approval for overweight flights check out 14CFR 21.197.
The pictures from the FAA form 337 in the logs of N4567D when it was owned by Dr. Kraft. He flew it from here to Narobi, Africa a number of times. The plane took off over gross by about 400 lbs and had restrictions on flight conditions and routing.
Everyone enjoys a good STOL competition. They’re good for general aviation, manufacturers, spectators and pilots. But it’s the ability to carry a payload that pays the bills. It’s the load carrying ability that puts us up against gross weight. Hunting season will be here soon and knowing your performance at gross will make the difference between safety and another trip. The STOL competition I’d really like to see is one with each aircraft loaded to legal Gross Weight. Then it would be more than a game, useful information would be shared for all of us.