Turbine Pilot

Tipping the scales in your favor 

Few things are more basic to the notion of heavier-than-air flight than weight itself, and bigger airplanes mean a bigger variety of weight-related issues to deal with. Moving up to your first jet or turboprop aircraft presents a good opportunity to review what need not be a heavy subject, aircraft weight limitations.

In the realm of popular entry-level jet aircraft, the Cessna Citation I and I/SP (a variant certified for single-pilot operation) remain perennial favorites. From 1977 through 1985, 345 were delivered. Many of these early Citation models continue to soldier on in the used-aircraft market. According to Jay Mesinger, president of Boulder, Colorado-based J. Mesinger Corporate Jet Sales Inc., an average-condition Citation I series with 5,500 airframe hours will fetch around $1.5 million in today's preowned market. If you can say "$1.5 million" without wincing, you're ready for your first jet. So by way of example, let's look at the ins and outs of weight in a Citation I.

A logical starting point when speaking of turbine aircraft weights is empty operating weight, or EOW. Much like the familiar empty weight listed in the flight manual of a light GA aircraft, it includes everything that has a fixed location and is installed in the aircraft. Hydraulic fluids, engine oil, and any ballast that may be required are all part of EOW. This number will vary depending upon an owner's choice of interior furnishings and the like, but for the Citation I it will be in the neighborhood of 7,000 pounds.

Next in line is basic operating weight (BOW). To determine this figure, begin with EOW and then add everything of a nonpermanent nature, other than cargo, fuel, and passengers, that is needed for a particular flight. This includes miscellaneous items such as catering supplies and emergency equipment, as well as the crew and their baggage. BOW is the usual starting point for weight and balance planning, especially in the case of an owner operator who normally flies with the same kinds of items aboard. A typical Citation I number here is about 7,400 pounds.

The standard Citation I has a useful load of 4,600 pounds, a figure that can include as much as 3,807 pounds of fuel. (Note that the weight of fuel alone is considerably higher than the weight of many single-engine GA aircraft.) Adding this useful load to BOW reveals the certified maximum ramp weight (MRW) for the Citation I, which is 12,000 pounds. This is a structural limit developed by Cessna during the certification process. Staying at or below MRW ensures that the design limits of the tires, brakes, wheels, and other components won't be exceeded during ground maneuvers. MRW for the Citation I is 150 pounds heavier than the aircraft's maximum takeoff gross weight (MTOGW) of 11,850 pounds. In other words, if the airplane weighs 12,000 pounds leaving the chocks, be prepared to burn off 150 pounds of fuel before accepting a takeoff clearance.

Note that we said 150 pounds and not 22.4 gallons. When dealing with turbine aircraft, fuel is usually discussed in terms of pounds in the United States or kilograms in some other parts of the world. Fuel quantity and fuel flow markings on cockpit gauges are normally marked in pounds or kilograms as well. For the record, 6.7 pounds per gallon is a generally accepted figure used in calculating the weight of jet fuel. However, fuel density can vary, and weights in the range of 6.5 to 6.8 pounds per gallon are possible depending upon the grade of jet fuel being used.

For many light GA aircraft, if you satisfy maximum gross weight requirements for takeoff, you automatically satisfy structural weight requirements for landing as well. Take off at the 2,300-pound maximum gross weight limit in a Cessna 172 and you are legal to land immediately. Not so with the Citation I. It has a certified maximum landing weight (MLW) of 11,350 pounds, or 500 pounds less than MTOGW. Unless you must land for safety-of-flight reasons (you forgot the golf clubs doesn't cut it), you are required to burn off sufficient fuel so as to arrive over the numbers at or below MLW should you need to land soon after takeoff.

Generally speaking, an overweight landing in a jet aircraft is nothing to worry about so far as damage to the aircraft is concerned. A normal or even firm landing should result in no harm done. An especially hard touchdown might result in structural damage that would not occur at normal landing weights. However, any overweight landing calls for an inspection by an A&P mechanic, just to be sure that all is well and to satisfy any curiosity on the part of friendly FAA inspectors who might later look into the incident. And of course, landing over MLW means taking extra care to ensure that the chosen runway is long enough to do the job.

Some jet aircraft, predominantly airliner types, incorporate fuel dump systems because of the very large spreads possible between MTOGW and MLW. But these systems are generally not found on light business jets like the Citation I. In an emergency such as an immediate return to the takeoff runway following an engine failure, the certification process ensures that the Citation I can either land or perform a single-engine go-around safely when operating within weight limits.

The next operationally significant weight to consider is maximum zero-fuel weight (MZFW). This is simply the maximum weight to which the aircraft can be loaded without fuel. Any weight increase above this point must come from fuel alone. For the Citation I, this number is 9,500 pounds, or 2,500 pounds less than MTOGW.

The reason why weight above MZFW must come only from fuel has to do with the load-bearing capability of the wings. The Citation I carries all fuel within its wing tanks. Consider that the lifting force generated by the wing places an upward load on the wing structure. The weight of fuel carried internally in wing tanks helps counteract somewhat the upward wing flexing caused by this lifting force, in effect making the wing act "stronger" than if it carried no fuel at all.

Flights in most aircraft involve compromises in payload, fuel, and range planning, and the Citation I is no different than other aircraft in this respect. Taking off with full fuel tanks means that only about 800 pounds of passengers and bags can be hauled, depending upon the flight's actual BOW starting point. But even when some seats go unused and the tanks could otherwise be topped off, it might be operationally advantageous to launch with less-than-full tanks (as long as all fuel required for the flight is on board, of course). The Citation I can't climb straight to its maximum certificated altitude of Flight Level 410 at heavier takeoff weights, and a lighter takeoff weight allows a broader choice of initial cruise flight levels. Same-direction cruise altitudes at and above FL290 require 4,000 feet separation, so having to settle for FL370 rather than FL410 might mean going around an extensive weather system rather than over it, or being stuck with less favorable winds or a turbulent ride.

Another advantage gained by not hauling excessive fuel around is that the total fuel used for the flight is reduced. Carrying un.-necessary weight means that some of the additional fuel is burned just to move what's leftover from one airport to another. While this relationship holds true for any aircraft, it really only becomes a significant factor in larger types. One rule of thumb suggests that for jet aircraft, around 10 percent of any excess fuel carried is burned for this reason. Thus for a typical Citation I flight, carrying 1,000 pounds of unneeded fuel could result in a total fuel burn about 100 pounds greater than would otherwise be the case.

If the weight of indecision over buying your first jet has you straddling the fence, one helpful mental technique is to convert big numbers into small ones. That $1.5 million price tag works out to just $125 per pound. Compared with a brand-spanking-new Beechcraft Baron 58 at around $192 per pound, it's a bargain.

Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 737 captain for a major U.S. airline.