One of the most common questions that we receive as a result of pilot reports in AOPA Pilot is: "Why do new airplanes have so little payload?" Similarly, readers often complain that on few aircraft can you fill the seats and the fuel tanks and not be over the maximum gross weight.
It's true that aircraft designs gain weight over time as new equipment, snazzy interiors, and accessories are added. But why not just increase the maximum gross weight to gain new payload? It's a good question.
However, even if the max gross weight could be increased easily, it's still not necessarily a good thing to be able to fill all of the seats and the tanks at the same time. In fact, in most cases, an aircraft with such capabilities would be less efficient than one in which you had a choice between a lot of fuel or a lot of passengers.
In general terms (there are always exceptions), purchasing an aircraft that can carry full fuel and full seats is a bit like buying a 12-passenger van to haul your aunt Mildred and all of your cousins around town during their two annual visits — it's not very economical financially or, to carry the analogy over to airplanes, aerodynamically.
The typical load on most four-seat general aviation airplanes is fewer than two people. Mooney Aircraft's surveys of its customers show that their average load is 1.7 people. With two people aboard, you can fill the tanks in any of the Mooney models, stow a week's worth of luggage, and travel 800 nautical miles or more without stopping. If Mooney's goal was to produce an aircraft that can carry four 170-pound adults (the FAA standard person) and full fuel, it would have to severely limit the amount of fuel available, which would reduce the range significantly for every flight, whether there were four people on board or one. Why pay that penalty on every flight when you can simply fly with less fuel on those couple of trips a year when you need to fill the seats?
The Cirrus SR20 is an example of an airplane design that offers a good compromise between fuel and passengers. With its maximum 60 gallons, it can carry four adults and bags a distance of 800 nm, which equates to an endurance of about five hours with reserves. It is an airplane that was designed from the beginning to offer buyers a simple, efficient airplane. There are faster airplanes and there are airplanes that go farther, but the Cirrus offers a fair give-and-take among speed, performance, and comfort. You can top the tanks after every flight and know that, no matter what the next trip brings, you can go a reasonable distance even with the seats full.
Of course, the SR20 is a brand-new design. Undoubtedly, in a few years, customers will want air conditioning, anti-ice or deice systems, datalink, a satellite telephone, more insulation for an even quieter ride — and who knows what else. Each of those will wick away its payload. It will then join the ranks of older designs that have seen their payloads erode.
At Mooney, Tom Bowen, vice president of engineering, says that "weight is a battle we fight on an ounce-by-ounce-basis." Customers demand a "Lexus interior," he says, which eats up valuable payload. People forget, he continues, that older interiors were made of plastic and had little or no sound or thermal insulation. Today's Mooneys often come with leather interiors and sidewalls stuffed with lots of insulation to quiet the ride and keep the heat in on cold days and at high altitudes.
Pilots' seemingly insatiable desire for new avionics also takes its toll. According to Bowen, an Ovation with a one-tube electronic flight instrumentation system and the usual avionics stack contains more than 75 pounds of wire. Bowen believes, however, that we have reached the pinnacle as far as avionics wiring weight goes. "The wiring morass is winding down," he says. New integrated avionics systems, such as the Garmin GNC 430 — which combines a color moving map with an IFR GPS, VHF navcom transceiver, and glideslope receiver — will reduce weight. In addition, the combination box requires only one installation tray and rack and it contains its own annunciators. Likewise, audio panels combined with intercoms and music interfaces, which are the trend, also reduce weight.
Without a doubt, though, new accessories that buyers will demand will be developed. So, as payload is whittled away, why not just increase the maximum gross weight? It sounds like a simple solution, but increasing the maximum weight is like opening a Pandora's box. As with attempts to increase cruise speed, increasing payload causes more and more compromises in the airplane. "The compromises ripple through the entire aircraft. It means a lot of changes and compromise for very little gain," explains Bowen.
One way to increase payload without having to increase max gross weight is to decrease empty weight. Most manufacturers have long since chiseled away all unnecessary weight, but Mooney recently found a way to shed pounds on the Encore. Switching from a metal cowl to one made of carbon fiber reduced the Encore's weight by 18 pounds. As a result of that decrease ahead of the center of gravity, Mooney was able to reduce the ballast in the tail by 12 pounds, leaving a net payload increase of about 30 pounds, according to Bowen. "I figure it cost us about $3,200 a pound to get it, though," he reports. Testing, tooling, materials, development, and equipment for the weight-reduction project cost nearly $100,000. "That was a cherry pick; there aren't many more opportunities like that left on the airplane," he says. Mooney has subsequently stopped production of the Encore. A similar change for the long-body Mooneys is in development, however, and a retrofit for the Mooney 252, the Encore's predecessor, may be offered later.
Bowen and Pat Waddick, vice president of engineering at Cirrus, agree that the biggest impediment to a higher maximum gross weight is the federal aviation regulation that requires single-engine aircraft to have a stall speed of less than 61 knots in the landing configuration. A higher gross weight would push the stall speed above that limit for most aircraft. Another factor for lesser-powered aircraft is the climb gradient set forth in the regs. Some aircraft, particularly on hot days, have difficulty meeting the balked-landing climb requirement at their current weights; greater weight would only exacerbate the situation. The third greatest challenge in increasing weight, according to Bowen, is the airframe and its components. The landing gear, spars, and load transfer paths would all need to be reexamined and possibly changed to accommodate more weight.
Single-engine aircraft can be granted a waiver to the 61-knot rule, but only if it can be shown that they offer an equivalent level of safety to the occupants. That can be accomplished by installing stronger seats and cabin structures that crumple in ways that absorb loads during an accident. However, those changes themselves increase weight. In addition, testing for the waiver is difficult and expensive.
Certification testing for aircraft with a stall speed greater than 45 knots but less than 61 knots places great emphasis on seat strength. The latest regulatory amendments demand that the seats withstand 19 Gs in the "vertical lumbar load test," according to Waddick. In other words, at the point of greatest deceleration during the testing — where the seat on a sled is dropped to the floor — the seat must withstand 19 Gs. During the horizontal test, in which the seat is crashed into a wall, the maximum restraint loads must stand up to 26 Gs.
In order for an aircraft to receive a waiver to the 61-knot rule, its airframe, along with the seats, must go through an expensive battery of tests. Usual methods of improving safety to meet the waiver requirements are crushable floor areas and firewalls that help to absorb loads during the crash.
Cirrus tested a number of preproduction composite fuselages in a NASA-sponsored program. In the testing, the Cirrus fuselages showed significant crushing at the firewall and floor-structure juncture and at the spar carrythrough structure. Such crushing disperses loads during crashes, easing loads on the occupants. Aluminum fuselages do an admirable job of absorbing such loads — often to a fault. They crush so much that the cabin area is compromised and the occupant dies not from the loads, but from the panel's or engine's squashing him. The trick, according to Waddick, is to allow the structure — whether it's aluminum or composite — to crush without compromising the cabin volume to the point that the occupants can't survive.
While we'd all like to carry more — whether it's fuel or people — most of today's aircraft, new designs and old, do an adequate job of allowing us to carry a reasonable number of people in comfortable cabins over a fair distance. Adequate. Reasonable. Fair. Sound like a compromise? It is, as is everything in aerodynamics.
