Weight and Balance (A Sensual Approach)
Weight and balance tends to be a math intensive subject and you should understand the principles: Weight and Balance Principles. Because some airplanes are more sensitive than others and even those that are center-of-gravity-sensitive tend to be okay most of the time, we tend to let our guards down on this subject. That is too bad, because getting weight and balance wrong can be deadly. (See CL-600 N370V.)
Even if you aren't a math wizard, having a "feel" for weight and balance can come in handy. If you can visualize where on your airplane the center of gravity resides, the forward and aft limits of your center of gravity, and how the seats and fuel tanks all relate to those points, you will have the skills you need to approach your weight and balance sensually. Sensually? Yes: weight and balance by feel.
Photo: The center of gravity on Eddie's aircraft.
Do you know where your CG resides?
- Balance — Center of gravity management is all about keeping the airplane in balance. If you think of balancing the airplane on a pyramid, you will know what you need to know about what comes next.
- Limits — The limits of your longitudinal center of gravity are determined by where exactly the airplane's center of lift resides and how much elevator authority you really have. The manufacturer gives these numbers to you in terms of a percentage of the mean aerodynamic chord, but what does that really mean? There is an easier way.
- Sensitivity — Not all airplanes are created equal and when it comes to weight and balance, you have to worry more about some than others. Knowing your airplane's "center-of-gravity-sensitivity" will help you focus on the most important issues.
- Impacts — Your center of gravity will be impacted by where on the aircraft you have stored fuel, cargo (baggage), and people. When these things move around, so does the center of gravity. Understanding where the CG moves is key to having a feel for your airplane's weight and balance.
- Legal Issues — You need to know what the Fed has to say about this, it could affect your license.
If you could place the airplane on a point so that it remains level, you will have found the center of gravity for that airplane as currently loaded.
With any kind of relative wind over the wing you will have an aerodynamic force which can be broken into two components. The component that operates perpendicular to the relative wind is known as lift. Just as you can visualize the airplane's center of gravity going through one point along the fuselage, you can also visualize all of the lift generated in an upward direction as the "center of lift." An airplane with a conventional layout — wing in the middle, tail in the back — the center of lift will be behind the center of gravity.
Of course having the center of lift behind the center of gravity induces a pitch down moment. We counteract that moment with a downward force from the tail. You might think the ideal arrangement would be to have the center of lift precisely over the center of gravity and have no tail at all. This would work in theory until any kind of movement within the aircraft moves the center of gravity. The tail is necessary to compensate for these movements and to give the pilot some pitch control over the airplane.
If we replace our balancing pyramid with a graphical representation of our center of gravity, we will have the classic illustration of the weight and balance problem. Our task as pilots is to always keep the center of gravity forward of the center of lift, but not so far forward that we overwhelm the amount of downward force available from the tail.
In the example center of gravity chart, we see our G450 has an aft limit of 45% MAC and a forward limit of 36% MAC. In the case of a takeoff problem, we know that too far aft will mean the nose will rotate on its own and we won't be able to keep the airplane from pitching up regardless of our inputs, and too far forward we won't be able to rotate at all. But where exactly are these points?
The limits of your longitudinal center of gravity are determined by where exactly the airplane's center of lift resides and how much elevator authority you really have. The manufacturer gives these numbers to you in terms of a percentage of the mean aerodynamic chord, but what does that really mean?
The MAC is the "Mean Aerodynamic Chord" of the wing. The chord is simply a line drawn from the leading edge of the wing to the trailing edge. Of course most aircraft have wings that change shape from the fuselage outward to the wing tip. The mean aerodynamic chord is an average of all the chord lines from root to tip.
For more about this, see Lift.
We can better visualize the limits of our center of gravity by looking at a bird's eye view of the aircraft.
Gulfstream chose a reference point 4" forward of the nose and all manufacturing measurements take place from this imaginary point. They then drew a horizontal reference datum 72" behind that. In the case of our G450:
- The Leading Edge of the Mean Aerodynamic Chord (LEMAC) is 387.7" aft of the horizontal datum.
- The Trailing Edge of the Mean Aerodynamic Chord (TEMAC) is 166.22" aft of the LEMAC.
- The forward limit of our CG is 36% of the MAC, or 59.84" behind LEMAC.
- The aft limit of our CG is 45% of the MAC, or 74.80" behind LEMAC.
- The distance between the forward and aft limit is 74.80 - 59.84 = 14.96".
- Our CG must reside within those meager 15".
This may not seem like much, but this Gulfstream's fuel tanks are all in the wings and the airplane is certified to allow C.G. computations without regard to fuel. Even without the first drop of fuel on board the airplane weighs nearly 43,000 lbs. So getting outside this 15" when you are dealing with 14 people who weigh on average only 200 lbs will take some doing.
Not all airplanes are created equal and when it comes to weight and balance, you have to worry more about some than others.
In the case of our G450, a typical airplane with a 42% basic %MAC can load 900 lbs of baggage in the furthest aft location and still be okay. From that point passengers can be loaded aft-to-front or front-to-aft and the airplane will still be within tolerances. (The furthest aft seat is still forward of the center of gravity, so any passenger at all brings this CG forward.
Note: A G450 with a forward galley may run into forward CG issues.
This particular center of gravity chart is used for takeoff computations and does not include fuel. Gulfstream found that the wing-only fuel arrangement will not put the airplane out of CG no matter the quantity of fuel, and was able to certify it this way. While fuel loading does impact takeoff trim, it can never place the airplane in an unacceptable CG range.
This particular G450 can be said to be "CG-insensitive," it can tolerate wide movements of passengers, baggage, and fuel and still be within CG limits. It is helpful to know how sensitive your airplane is.
A Challenger 605 can be said to be "CG-sensitive." While it can tolerate movements of passengers, baggage, and fuel and still be within CG limits, it can also find itself out of limits without too much effort. With fuel distributed from nose to tail those 605's with forward galleys tend to be nose heavy. Fuel mismanagement can also jeopardize a safe balance:
- In a typical 600-series Challenger, about half the seats are forward of the CG, half are aft. Where you seat passengers matters.
- Fueling the wings only will start to move the CG forward but at about the halfway point moves it aft.
- After the wings are full, fuel should be computer controlled to distribute between the auxiliary and aft-auxiliary tanks in a 2.65 to 1 ratio. That generally moves the CG forward.
- After the auxiliary tank is full, the rest of the fuel goes to the aft-auxiliary tank and then to the saddle and tail cone tanks. All of that tends to move the CG aft.
This is a lot to consider when you are making changes. Most of these airplanes have forward galleys and tend to be nose heavy until they become very heavy. This airplane is CG-sensitive.
Your center of gravity will be impacted by where on the aircraft you have stored fuel, cargo (baggage), and people. When these things move around, so does the center of gravity. Understanding where the CG moves is key to having a feel for your airplane's weight and balance.
CG-insensitive Example (G450)
Photo: The CG Envelope on a G450 (Those 36% and 45% markers are both aft of the aft-most passenger seat), from Eddie's aircraft.
In the case of our example CG-insensitive G450:
- The LEMAC is 387.7" behind the horizontal datum.
- The forward-most center of gravity allowed is 36% of MAC (166.22") which comes to 59.8". That means the forward-most center of gravity allowed is 387.7 + 59.8 = 447.5" aft of the horizontal datum.
- The aft-most center of gravity allowed is 45% of MAC (166.22") which comes to 74.8". That means the aft-most center of gravity allowed is 387.7 + 74.8 = 462.5" aft of the horizontal datum.
- The aft-most seat on a typical G450 is 426" behind the horizontal datum.
- The cabin door in front of the aft baggage compartment is 555" behind the horizontal datum.
So what does this mean to us as pilots? Simply, for this example G450, any aft baggage moves the C.G. aft and any passenger moves the C.G. forward. You might be alarmed that the distance from one limit to the other is only 15" but don't be. The airplane weighs 43,000 pounds before you add any fuel and you would need to move a lot of passengers and cargo to get the C.G. out of that range.
CG-sensitive Example (Challenger 605)
Unlike the G450, visualizing the center of gravity on a Challenger 605 does not yield easily discernible results. The range is still pretty narrow but the range moves with weight. You can still profit from knowing exactly where on the airplane that range resides, so you can better understand the impacts of moving people, bags, and fuel.
If everything is working by the book, the airplane's center of gravity should not be a problem. But there has been at least one Challenger 600 series crash so it bears looking into. (See CL-600 N370V.)
Let's say you are flying without passengers on a leg requiring 15,000 lbs of fuel. Your aircraft has a fully stocked galley and is generally nose heavy as Challengers go. Your Zero Fuel Weight (ZFW) comes to 30,000 lbs and 28% MAC. This is a pretty normal condition for you without passengers.
A 15,000 fuel load should automatically distribute itself thusly:
- The first 9,720 lbs will fill the left and right wing tanks.
- The remaining fuel should distribute itself in a 2.65 to 1 ratio between the auxiliary and aft-auxiliary tanks: 3,384 lbs auxiliary and 1,446 lbs aft-auxiliary.
- This airplane can have fuel system problems if the fuel pressure is interrupted which is what happens just as the aft-auxiliary gets to 1,000 lbs. At that point it stops accepting fuel and the remaining 446 lbs all ends up in the auxiliary tank.
So you've had a very minor glitch but your margin of safety was wide enough so the airplane is still within acceptable center of gravity limits for your planned takeoff. It is highly unlikely you would notice the fuel glitch. The information of fuel distribution is available, but few pilots would notice 446 lbs in the wrong place. If, on the other hand, you computed your resulting weight and balance and center of gravity, you would be aware that you are very close to your forward limit.
Now let's say you are fully loaded and about ready to depart when your phone rings. Your charter company found six passengers with 200 lbs of baggage wanting to fly the exact city pair so it is an easy revenue pick up. You gladly accept the six male passengers who climb on to the airplane and take the first six seats, all forward.
If you hadn't bothered with the weight and balance routine you wouldn't have realized just how far forward your center of gravity is. It was okay before, but now?
Now you are well beyond your forward CG limit. Takeoff rotation is doubtful!
If, on the other hand, you were aware of your starting weight and balance issue you would have been forewarned. Simply placing your passengers in the aft-most seats keeps you within your authorized limits, though just barely. (You might want to consider burning off some fuel before takeoff.)
You need to know what the Fed has to say about this, it could affect your license. Besides, if you have a center-of-gravity-sensitive airplane, it will help you understand . . .
Transport category aircraft must be built with published center of gravity limits established.
[14 CFR 25, §25.23(a)] Ranges of weights and centers of gravity within which the airplane may be safely operated must be established. If a weight and center of gravity combination is allowable only within certain load distribution limits (such as spanwise) that could be inadvertently exceeded, these limits and the corresponding weight and center of gravity combinations must be established.
[14 CFR 25, §25.27] The extreme forward and the extreme aft center of gravity limitations must be established for each practicably separable operating condition. No such limit may lie beyond — (a) The extremes selected by the applicant; (b) The extremes within which the structure is proven; or (c) The extremes within which compliance with each applicable flight requirement is shown.
There isn't anything regulatory that says a general aviation pilot must compute weight and balance data, however . . .
[14 CFR 91, §91.7 (b)] The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur.
[14 CFR 91, §91.9 (a)] no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane...Flight Manual, markings, and placards.
These are the citations you will see on the accident report if you fail to compute a weight and balance for every takeoff. Besides . . .
[FAA-H-8083-1A, pg. 1-1] The pilot in command of the aircraft has the responsibility on every flight to know the maximum allowable weight of the aircraft and its CG limits. This allows the pilot to determine on the preflight inspection that the aircraft is loaded in such a way that the CG is within the allowable limits.
If you are flying commercially, or if you want to employ best practices . . .
[14 CFR 135, §135.63]
For multiengine aircraft, each certificate holder is responsible for the preparation and accuracy of a load manifest in duplicate containing information concerning the loading of the aircraft. The manifest must be prepared before each takeoff and must include:
1) The number of passengers;
2) The total weight of the loaded aircraft;
3) The maximum allowable takeoff weight for that flight;
4) The center of gravity limits;
5) The center of gravity of the loaded aircraft, ...
(d) The pilot in command of an aircraft for which a load manifest must be prepared shall carry a copy of the completed load manifest in the aircraft to its destination. The certificate holder shall keep copies of completed load manifests for at least 30 days...
Weight and balance control program
[AC 120-27E, ¶5.a] This document provides guidance to both passenger and cargo operators that are either required to have an approved weight and balance control program under parts 121 and 125, or choose to use actual or average aircraft, passenger, or baggage weights when operating under part 91, subpart K of part 91, or part 135. The guidance in this AC is useful for anyone involved in developing or implementing a weight and balance control program.
Standard Average Weights
[AC 120-27E, ¶5.b.] As shown in Table 1, the FAA has divided aircraft into three categories for this AC to provide guidance appropriate to the size of the aircraft.
|For this AC, an aircraft originally type-certificated with—||Is considered—|
|71 or more passenger seats||A large-cabin aircraft.|
|30 to 70 passenger seats||A medium-cabin aircraft.|
|5 to 29 passenger seats||A small-cabin aircraft.|
NOTE: Aircraft with fewer than five passenger seats must use actual passenger and baggage weights.
[AC 120-27E, ¶6.] Who can use standard average or segmented weights?
- Standard Average Weights. Use of standard average weights is limited to operators of multiengine turbine-powered aircraft originally type-certificated for five or more passenger seats who hold a letter of authorization (LOA), OpSpecs, or MSpecs, as applicable, and were certificated under 14 CFR part 25, 29, or part 23 commuter category or the operator and manufacturer is able to prove that the aircraft can meet the performance requirements prescribed by part 23 commuter category aircraft. Single-engine and multiengine turbine Emergency Medical Service Helicopter (EMS/H) operators may use standard average weights for EMS operations, provided they have received an LOA.
- Segmented Weights. Segmented weights are provided for, but not limited to those aircraft that are multiengine turbine-powered aircraft originally type-certificated for five or more passenger seats and that do not meet the performance requirements of part 23 commuter category aircraft or part 29.
- The FAA’s recommendations and advice on the safe use of standard average weights and segmented weights are contained in this document. In the FAA’s view, it would be unsafe for an aircraft operator to use standard average weights or segmented weights in any of the following aircraft:
- All single-engine piston-powered aircraft.
- All multiengine piston-powered aircraft.
- All turbine-powered single-engine aircraft.
A segmented weight program is simply a standard average weight program with a statistical pad added to each weight.
NOTE: All multiengine turbine-powered aircraft certificated under part 23, except for commuter category aircraft, may only use an actual weight or segmented weight program. Operators that elect to use a segmented weight program must meet the requirements in paragraph 6b and curtail the CG envelope as specified in Appendix 3, 4, and 5. Commuter category aircraft may use standard average weights and should refer to paragraph 200f for further guidance.
Most of us in the transport category aircraft world use "standard average weights" when computing our weight and balance. You might say, for example, that a typical adult male passenger weighs 175 lbs, a typical female adult passenger weighs 150 lbs, and so on. That usually works in the airline world because they are dealing with large cabins and have the necessary regulatory requirements met. But it more than likely is the wrong approach in the 14 CFR 135 business jet world and could get your operator into some trouble if caught. Moreover, it is the wrong thing to do in a 14 CFR 91 operation too, just because it could be dangerous.
So if you don't use standard average weights what is the alternative? If you have fewer than five passengers seats you don't have much choice, you have to use the actual weights of your passengers and baggage. (That means you either weigh them or ask them, "how much do you weigh?")
[AC 120-27E, Chapter 2, §5]
An operator may determine the actual weight of passengers by—
- Weighing each passenger on a scale before boarding the aircraft (types of weight scales and scale tolerances will be defined in the operator’s approved weight and balance control program); or
- Asking each passenger his or her weight. An operator should add to this asked (volunteered) weight at least 10 pounds to account for clothing. An operator may increase this allowance for clothing on certain routes or during certain seasons, if appropriate.
NOTE: If an operator believes that the weight volunteered by a passenger is understated, the operator should make a reasonable estimate of the passenger’s actual weight and add 10 pounds.
To determine the actual weight of a personal item, carry-on bag, checked bag, plane-side loaded bag, or a heavy bag, an operator should weigh the item on a scale.
If you have more than five passenger seats you have another option: you can curtail your center of gravity envelope and then use standard average weights. Setting up a curtailment program can take a little brain power, and if you are operating under 14 CFR 135 it will also require Operation Specification A097 (or equivalent). But once that is done there is very little to worry about other than watching out for unusual passenger loads. If, for example, you are flying the front line of the New England Patriots, you might want to increase your standard weights.
More about this . . . Weight and Balance: Curtailment.
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
14 CFR 135, Title 14: Aeronautics and Space, Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft, Federal Aviation Administration, Department of Transportation
Advisory Circular 120-27E, Aircraft Weight and Balance Control, 6/10/05, U.S. Department of Transportation
Advisory Circular 121-29B, Carry-on Baggage, 7/24/00, U.S. Department of Transportation
Air Force Manual (AFM) 51-9, Aircraft Performance, 7 September 1990
Bombardier Challenger 605 Weight and Balance Manual, A/C 5701 and subs, Publication No. CH 605 WBM, Feb 01/2010.
FAA-H-8083-1A, Aircraft Weight and Balance Handbook, U.S. Department of Transportation, Flight Standards Service, 2007
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013
Gulfstream G450 Weight and Balance Manual, Revision 3, March 2008