Aircraft Weight and Balance, it is serious
Many people have died as a result of miscalcutated weight and balance, or weight that shifted during flight, or even as the result of pilots who just didn't take it serious to begin with. Weight and balance is a tough subject for some, so I am going to try and make it a little clearer and easier here.
With regard to helicopters, there are longitudinal CG limits as well as lateral CG limits. With regard to Airplanes, usually only longitudinal limits apply. Why? Helicopters have very limited control available in all directions whereas airplanes have a lot of control available in all directions, but especially laterally with consideration to the comparatively narrow fuselages. Lateral weight does affect fixed wing aircraft but normally only when it is out on the long lever of the wings in the form of fuel tanks. That is one reason that fuel management in fixed wing aircraft that have multiple fuel tanks is so important.
Can you calculate Weight and Balance Manually?
If you have the knowledge, yes you can, and you can do it for any aircraft. Despite the fact that a manually calculated weight and balance is clearly the most accurate when it is done correctly, most flight manuals these days have nothing more than an "envelope" as the method of figuring out if you are safe or "within the envelope". All flight manuals have the datum stated and from this information you can calculate the weight and balance manually for any given aircraft. Some flight manuals have some of the more important "arms" stated as well which can make the job much easier.
What is the Datum?
The Datum, also known as "Station 0", is the location which the manufacturer has declared as the point of reference for all weight and balance calculations. It is a declaration and it could be anywhere; even a point located a measured distance out in front of the aircraft. The datum could be an actual location on the aircraft such as the forward most point of the aircraft, the mast centerline, or a point x-number of inches forward of a given object such as the pitot tube mount. If the datum is located somewhere aft of the forward most point on the aircraft, anything forward of the datum is a negative and anything aft of the datum is a positive. It is important that you never forget the positives and the negatives when calculating your weight and balance.
What is the Arm?
The Arm is the location of the CG (Center of Gravity) of any given object or a predetermined location such as the pilot station. Stated arms are often the center of a given location such as the baggage compartment where the arm could be the middle of a baggage compartment that is 20-inches or more wide. Lets say that you have placed a package that weighs 50-lbs and is 10-inches by 10-inches in the forward half of the baggage compartment that has a center CG station of 174. Is the CG of that package 174? No, it is actually 169. This is the center of the 10-inch package; 5-inches forward of the stated arm of the center of the baggage compartment. The arm is the balance point, or the "fulcrum" of any given item. This example also demonstrates how weight shifting can occur in flight. All weight is important and it is equally important that weight is prevented from shifting during flight.
Can you as the pilot determine an arm yourself?
Yes of course you can, afterall it is the pilot who is charged with ensuring that the aircraft is in and airworthy condition and if it is not within the CG limits, it is not airworthy until adjustments are made to bring it within those limits. To find the arm of a given location or item, all you have to do is measure from the datum to the center of the location in question.
What is the Moment?
The Moment is the result of a given object when it's arm has been multiplied by it's weight. For example, a package that weighs 7.5 lbs which is located at station 75 has a moment of 562.5. If it were located at a station forward of the datum such as station -12, then the moment would be -90.
What is the CG?
CG is an abreviation for "Center of Gravity". This is also the balance point of any given item. When it comes to finding the CG of a given aircraft in it's planned flying state, the CG can be found by dividing the total Moment by the Total Weight of the given aircraft after all weights and arms have been calculated and added.
What about Lateral CG?
Lateral weight and balance calculations are an important part of Helicopter Weight and Balance, but not so important with regard to fixed wing aircraft. The lateral Datum is the centerline of the aircraft. Anything to the left of this datum is a negative, and anything to the right is a positive. If you are calculating the weight and balance for a helicopter and you haven't done the lateral, then you haven't completed the process.
With regard to fixed wing aircraft, lateral does make a difference and is noticable to the pilots, but due to the amount of control available the lateral CG isn't near as critical as it is with regard to helicopters which have very limited control available.
Negligable Weight and Balance change
If you review aircraft logs, STC's and 337's, you will see this statement frequently, "Weight change negligable, no adjustment to weight and balance necessary". The problem is that if you put several items in your aircraft that have a negligable change, sooner or later you have a gross change that isn't so "negligable". This brings us around to the subject of reweighing your aircraft or any given aircraft from time-to-time. How frequent is enough is something that the operator should decide based on changes that have occurred. I would think that every 5-years or so would be a good idea. Getting an aircraft weighed is not an expensive operation, and it doesn't take long either but it does a lot when it comes to knowing gross change.
Failure to take Weight and Balance serious
Not long ago, I had a student who was doing his helicopter add-on to his private airplane certificate. I had just landed with another student and asked him to have the attendant add a specified number of gallons to the main tank and then do his pre-flight. When I came down to the helicopter I found that he had the attendant top off the tanks contrary to my orders. I said the aircraft is over-gross and we can't fly it that way. He stated in part that, "we don't worry about weight and balance where I normally fly". I was shocked, but we didn’t fly until he had drained off the fuel. On another occasion, I watched as two rather large individuals got out of an R22 and ordered a fuel top off of both tanks as they landed enroute ferrying the aircraft to south Florida. Having significant experience in the R22 I knew this helicopter was severely over-grossed. Over-gross is an issue which in all helicopters can cause premature damage leading to premature failure of many components.
This is an area where many pilots think that they can beat the FAA and/or the manufacturer often mentioning the built in fudge factor. There is no doubt that in the right conditions, many helicopters will lift more than their maximum rated weight. In most cases however, if you are over gross, there will be an over-torque. Keep in mind that just because it is called manifold pressure on a piston engine does not mean that when you pull past the limit it is not an over-torque; it most certainly is. You are pulling the guts out of that engine over-stressing the blades and other parts, and something won't last as long as it would if you flew right.
Weight and balance is a limitation, and limitations must NEVER be exceeded. There are enough occasions when a limitation is inadvertently exceeded; we don't need to do it intentionally first and then accidentally again later. You could become a statistic just like many others who thought they were smarter than the engineers and the FAA.
What happens when an aircraft is over-grossed? FATIGUE, parts gets tired! The fatigue takes its toll on everything. Rotor blades often suffer internal damage that will cause premature failure. Transmissions are often the power limiting factor and therefore transmission damage can occur. The engine life is reduced. Transmission mounts, the airframe, every part of the aircraft gets fatigued. It could be something that may be found on a pre-flight, and it just might not. It could cost your friend, a fellow pilot; an innocent person, his or her life.
If you are about to fly and the pilot or instructor says it's ok to over-gross the aircraft with a little extra fuel, get another pilot or instructor because that one isn't worth his salt!
Liar, Liar, Pants on Fire
Back in the old days before fat food like McDonalds and Doughnuts were everywhere, the average man weighed 170 pounds, while the average women weighed 140 pounds. Finally new data has come out, now the average weight of an American is 191 pounds, male or female. This means simply that everyone has porked out, but more than anything women have. It is just to easy to say, "I can't help it, it's my thyroid!". Doctors know that they can skin people for some big bucks if they help them make excuses so there won't be any help from the medical profession. Don't take responsibility for your fat, blame it on someone else, it must be the Oreos! Put down the chips.
In aviation, when you ask your passengers weight, most of the time they will just plain lie. This is true with most students as well, they all want to weigh less so they say it. They tell you a weight that they were 5 or 10 years ago when they looked better in the mirror.
As a pilot, you must become a good judge of weight, and don't be shy about it. Tell people that weight is an issue in aviation. In some cases it may even be necessary that you weigh people just like they do at the commercial tour operations. I always weigh students, and if necessary I'll weigh others.
If you're over-weight, go on a diet. No operator wants a fat pilot. Even a flight school should not mislead a student by training an over-weight person who has career intentions without informing them of weight standards in the industry. Any potential helicopter pilot should certainly not weigh more than 220 pounds, and that is really pushing it. It is better if you weigh less than 180 pounds. When you apply for a job your employer will ask your weight, and with aviation this is acceptable and legal as it is relative to what we do. Once you have built experience and a respectable name for yourself the weight issue diminishes to a point. I know several commercial pilots who weigh more than 220 pounds, but they are flying larger aircraft and have gained the weight since they paid their dues which does make a little difference. In one particular advertisement for EMS pilots currently on the web, the weight limit is 215 pounds while in yet another the weight limit is 220 pounds, when I hired in it was 195 lbs.
In some cases, school operators will not hire instructors that weigh more than 180 pounds. Now as I stated above if you have a lot of flight hours and a good name, this changes some, but only some. Keep in mind that the more the instructor weighs, the less the student can weigh and the student is paying so he/she wins. Also keep in mind that commercial operators know that the more the pilot weighs, the less pay load they can carry.
Keep in mind that being proportionately heavy is one thing, but being over-weight is another. This however does not change the fact that weight is an issue and an aircraft can only carry just so much. If you're over-weight and you want to become a helicopter pilot, trim up.
Does the FAA take weight and balance serious?
The FAA takes weight and balance especially serious if an accident or event occurs, not so much until then unless they can use it as an excuse to dig into what you are doing. If you are flying part 135, this is a subject that the FAA can be keen on if it happens to be the subject that they are focused on for the time. I find it curious that the FAA will allow averages for 121 and 135 operators based on AC 120-27E, especially after the crash in Charlotte, NC on Jan 8, 2003, where NTSB investigators found that the aircraft was several hundred pounds overgross having used this system. As a result of that crash, the FAA did revise the weights in the aboved mentioned AC, but I challenge that it is still way off, and about as accurate as other information the FAA publishes based on data sampling. The NTSB has testified that the FAAs methods for determining these numbers is greatly flawed.
Until recently, the FAA has held that the PIC is responsible for an accurate weight and balance. However, under the approval of the FAA, an operator can mandate a weight and balance method which relieves the pilot of the responsibility of it's accuracy. This is a double edge sword so-to-speak; the pilot under this system can't use any other method, though he/she could pad the numbers for a safety factor without a problem. The good part for the pilot though is the relief of responsibility for accuracy. This is fitting for the pilots who didn't care about the weight and balance anyway.
Enjoy your training, and fly safe!! Jump to Top