A Weighty Issue

May, 2003

A weighty issue. And a balancing act too...

The civilian-contracted transport rolls down the runway laden with servicemen destined for overseas duty. Shortly after rotation and with less than a hundred feet of altitude in the climb the aircraft stalls, rolls to the right and crashes with total loss of life.

The commuter airliner has all chairs occupied as the Captain rotates. After a brief normal flight profile the nose of the airplane pitches up to the extreme. After a breathtaking stall the airplane plunges back onto the airport surface killing all on board.

What happened here?

In the first instance investigators believe that the airplane had accumulated some ice on the airfoils during the previous approach. After loading the soldiers, the aircraft departed without being de-iced. Because standard passenger and baggage weights were used for weight calculations there was an excellent chance that the aircraft was overweight. Combined with the airfoil ice the flight was doomed.

In the second case there are three possibilities. Because the flight was full, and standard passenger and baggage weights were used, there is a possibility that the aircraft was severely overweight and out of CG. The second possibility is that the baggage in the aft cargo bay shifted after rotation creating an extreme aft CG. Lastly, there is a potential mechanical problem in the elevator mechanism itself.

Purpose of this tutorial.

To point out to the reader how passenger weight and baggage weights are determined in Transport and General Aviation Aircraft, .and how these calculations can be flawed. A discussion of how weight and balance affect aircraft handling and performance, and what you as a pilot can do about it.

Weight and balance

Weight is what you shove in the airplane, that's pretty obvious. If you try to takeoff with more weight than the aircraft is certified for it may, or may not get off of the ground. Balance is how this load is distributed. The two are so closely entwined that you seldom hear one without the other- the Yin and the Yang of aircraft performance.

Definitions

Gross weight: The actual weight of the aircraft at any given time.

Maximum gross weight: The maximum amount that the aircraft can weigh. (Sometimes called "Maximum Certificated Gross Weight.)

Empty weight: The airplane itself, including all permanently attached options. Plus any ballast that is a part of the design and any undrainable oil and fuel.

Useful load (General Aviation only): The combined weight of the pilot(s), passengers, baggage, usable fuel, and drainable oil. If you subtract the empty weight from the maximum gross weight you get the useful load.

Basic operating weight: Add the crew to the empty weight and you get this number. Note that there is no fuel included.

Payload: The total weight of the stuff that makes money- passengers and cargo.

Zero fuel (maximum) weight: The maximum weight, outside of fuel, that you can put in the aircraft. Any weight added after reaching the zero fuel weight must be in the form of fuel. Why is this so? Weight distribution. All of the weight that isn't fuel is concentrated in the fuselage of the aircraft. For structural weight distribution purposes, any added weight from this point must be fuel out in the wings.

Ramp (or taxi) weight: This is the maximum take-off weight plus the fuel that will be burned off from the gate to the departure end of the runway.

Maximum take-off weight (mtow): This may not be the same as the maximum gross weight. Runway length, weather and other variables may make this number lower.

Maximum landing weight: This may be a reduced number due to runway conditions (rough or short for example), or atmospheric conditions (high altitude runway). Most aircraft have a lower maximum landing weight than their maximum take-off weight. This is primarily due to the increased stress on the airframe during the touchdown moment.

Wow. The weight guesser from the County Fair could have a field day at an airport.

What do we know that are "absolute givens" when it comes to the weight department?

Well, the manufacturer gives us the weight of the airplane. As the pilot, you know, or will be informed, of the cargo weight. You know how much fuel is onboard. This just leaves out the weight of your passengers and their bags. Come to think of it, have you ever been weighed when you checked in for a flight? How about your baggage? Ever seen that weighed? What's going on here? Ah Grasshopper, enter the world of the FAA "weight standards" and the standard passenger and baggage weights.

The FAA has established that the things that fit into airplanes have standard weights. Some of these are exact, some are speculative.

Exact weights

Gasoline: 6 pounds per U.S. gallon

Jet fuel: 6.7 pounds per U.S. gallon

Oil: 7.5 pounds per U.S. gallon

Water: 8.35 pounds per U.S. gallon

"Standard passenger and baggage weights"

(As an aside, recall that this phase popped up in both of the accidents that were presented at the beginning of this tutorial. We may be on to something here.)

Passenger, summer: 160 pounds

Passenger, winter: 165 pounds

(This actually is a "derived number" based on a 60% male, 40% female passenger ratio.)

Children, two to twelve years old: 80 pounds

Cabin attendant, male: 150 pounds

Cabin attendant, female: 140 pounds

All other crew members: 170 pounds (Cabin attendants must be leaner from eating those in-flight meals.)

Baggage

25 pounds for domestic flights

30 pounds for international flights

"Heavy" bags: Any bag considered by the "operator" to weigh more than 70 pounds must be weighed exactlly and will be considered as part of the cargo.

Cargo

Cargo is actually weighed.

The problems of "guesstimating."

For transport category aircraft on average flights this method of accounting usually does not present a problem. However, consider a full plane with every chair occupied- the flights that you and I always seem to be scheduled on. Given the trend for people to weigh more today, especially in America, do you really think that the average passenger weighs 160 pounds? Do you really think that their luggage is only 25 pounds? There is a possibility that a U.S. flight that is full is also exceeding the certified maximum take-off weight for the aircraft. (As an aside, many modern transport aircraft have strain gauges mounted on the gear that display very accurate weight information.)

Look back on the first incident at the top of the page. A plane-load of soldiers, and their gear. Not your average manifest by a long shot. Throw in a little ice on the flight surfaces, and ......

Weight distribution

Here it can be a little tricky. Most of the transport aircraft have baggage holds along the belly area of the plane. Handlers can distribute the load along the longitudinal axis. Now take a look at corporate and commuter aircraft. Almost all of them have their baggage areas in the rear of the aircraft, a setup that makes weight distribution almost impossible.

In fact, there is one very popular commuter airplane that, even though loaded under the maximum take-off weight, may not steer properly on the ground while taxiing if there is too much weight in the aft baggage compartment. There just isn't enough weight on the nosegear steering rack for it to function properly. More than one crew of this type has turned the plane around and returned to the gate to have some of the baggage in the rear removed just so the nose steering works properly.

Comforting, huh?

The balancing part of the equation

Weight is only one of the two parts- balance is also critical. Actually, between the two, proper balance is more critical than the weight. Weight is relatively easy to understand, balance in an aircraft can be a little more difficult. There are many fine papers published in books and on the internet and I certainly bow to their author's superior knowledge on the subject, My intention in this tutorial is to make the reader aware of how improper weight and balance can have an adverse effect on the safe operation of an sircraft.

Definitions

Center of gravity

Usually just written and spoken as "CG." If you could suspend an airplane from a cable in the sky and find the just right spot to hook that cable up to where the plane was perfectly level, fore and aft and left to right, you would have the center of gravity.

The aircraft manufacturer, under strict FAA guidelines, establishes how far forward and aft they can load the airplane before the the airplane will no longer become controllable.

Envelope

If you take those "dimensions" of how forward and how far aft the CG can be in the airplane you have what is called the envelope. Sometimes it is referred to as the CG range.

Chuck Yeager (the first pilot to break to sound barrier in the Bell X-1) could operate out of the envelope when he was a test pilot- the performance envelope. As good as the General was, he wouldn't operate out of the CG envelope.

Datum

The datum is an imaginary point somewhere toward the front of the airplane, often times forward of the nose out in space. It is not uncommon for an aircraft to be only loaded in an "aft CG status." In other words there are no chairs or cargo compartments located forward far enough to counter aft CG.

Stations

Stations are points along the CG envelope and are measured from the Datum (point). Every chair, every cargo compartment, every fuel tank, etc. has an established station that is determined by the aircraft manufacturer.

Arm

The arm is the length (distance) from the datum. Each station has an arm. A 100 pound weight placed on a "long arm" (away from the datum) will have more of an adverse effect of aft CG than the same 100 pound weight placed on a "short arm" (toward the datum).

Moment

OK, our last definition. The weight of our 100 pound object, multiplied by the length of the arm (or station) gives us the moment.

Deriving the weight and the CG

Each aircraft has a chart that shows where various objects, including oil and fuel, can be loaded- often referred to as stations. Each station has an arm that is measured from the datum. (Please don't doze off on me here, I'll get off of this tech. stuff in just another couple of sentences.) You add up the total weights and the total moments. Divide the total of the moments by the total of the weights and you end up with a number that is the CG. You then take this number and see if it falls within the CG envelope. If it does, you're good to fly- if not, you'll have to make some adjustments.

For example, say that you're under your maximum weight limitaion but your CG is too far aft. By shifting a passenger, or some cargo forward (a shorter arm), then you may be able to shift the CG forward enough to be in the envelope.

Dead weight

Some smaller aircraft, the Beechcraft Bonanza comes to mind, actually have weight permanently attached to the airframe, in the engine compartment or as far forward as possible, at manufacture- just to give the airplane a more forward CG. Some transport aircraft need to have bags of sand loaded into a "forward CG" compartment just to keep the CG from being out of range. In other words, they end up weighing more because of this, BUT they are in the CG envelope.

Outside of the envelope (the CG one, that is.)

In a "classic" aircraft design, one with a wing and normal tail surfaces (I'm trying to eliminate cunards, delta wings and flying wings here) the usual limiting factor is the horizontal part of the empanage- the horizontal stabilizer and the elevator. From here on we'll be referring to airplanes that are built that way. It is these surfaces that control the "up/down" of aircraft pitch, and it is pitch authority that is a problem with an airplane that is "out of CG."

Generally speaking, if you have a CG "problem" it will an aft one- not because the CG is too far forward.

Out of CG?

The manufacturer tests to see how far forward a plane can be loaded before the nose pitches down and can no longer be "brought back up" by the horizontal control surface(s). The same applies to aft loading. Now, why did I write that word "surface(s)" like that? Because the horizontal control area of the tail is comprised of the fixed susrface- the horizontal stabilizer- and the moveable surface- the elevator.

Let's say that you are enroute and the elephant cargo that you're carrying decides that the hay in the back of the airplane looks a whole lot tastier than the stuff where he has been loaded and secured- right within CG. As Dumbo moves rearward, you can feel the plane pitching up (as the tail goes down). You feed in more and more elevator until our pachyderm is satisfied with his selection and quits moving toward the tail.

Two things can happen here.

1. Dumbo stays put in his new location. You may, or may not, have enough elevator authority to prevent the tail from dropping down as you reduce airspeed for landing. It's entirely possible that just a little reduction is airspeed (reducing the airflow over the horizontal surfaces and making them less effective) will cause you to run out of elevator. The aircraft will pitch up, stall, and a nasty descent will follow. Or, you make your descent OK, but at some point during the approach as you continue to slow down, or during the flair for landing, the nose will pitch up, stall, and a nasty....

2. Dumbo goes all the way aft where the best hay is. After using all of the elevator authority available to you the aircraft pitches up, stalls, and a nasty...

Neither "1." or "2." is very acceptable. What if you stored the best hay toward the front of the airplane and Dumbo makes a move forward for it? Well, at least you won't stall. But on either the approach or enroute you'll likely run out of "up elevator" and enter a nasty descent that will result in an elephant packing you firmly into the ground.

What can you do?

In Dumbo's case, tie him down securely so that there is no chance for movement- and keep the good hay nearby.

For non-elephant flights, assuming that you are not overweight, move items from one station to another to satisfy the CG envelope. Move cargo out of an aft compartment (long arm) to a forward one (short arm). Move a heavy passenger to a forward seat.

You do not want to take-off if the airplane is out of the CG envelope.

What about fuel? What about stuff out there in the wings?

Some aircraft have fuselage tanks in addition to wing tanks. As these tank burn dry they will have an effect of the CG- it varies from aircraft to aircraft. Generally speaking, the airplane manufacturer recommends that the fuselage fuel be burned first, then the wing fuel- it makes for better weight distribution. Wing fuel usually does not adversely affect the CG as it is distributed along the same datum point- the aircraft becomes lighter as the fuel is burned, but the CG usually does not appreciably shift.

So, weight AND balance do matter

Big time. Look, you can over-load an aircraft by a fairly high percentage and get away with it. It's a little known secret that for years operators in Alaska could take almost any plane operating under the Federal Aviation Regulations part 91 (old 91.38, new 91.323) and legally overload the airplane. Perfectly legal. Why? As a fact of life they were doing it anyway, the Feds. figured that they might as well establish some guidelines. If you've ever seen a deHavilland Beaver take-off for the interior with a full cabin and a canoe or two strapped on the outside of the airplane to the wing struts you know what I mean.

Then it's OK to be overweight?

No. It's not legal. Well, except for that Alaska thingy.

Will an airplane fly when it is overweight?

Sure, usually no problem. Depending on the type aircraft you can sometimes grossly exceed the maximum certified weight.

So, what's the problem?

You're a test pilot. All of the performance charts in the book, the airspeed limitations on the airspeed indicator, the effectiveness of the controls, the strength of the landing gear, the entire structure- all are in uncharted waters when you exceed the maximum certified gross weight for an airplane. If you do lift off smoothly, fly the route through to your destination in totally calm air, and grease the landing when you get there, well then you probably won't have any problems.

But: You may not be able to get enough speed going before running out of runway. Or, no matter what your speed, you may not have enough elevator authority to bring the nose up off of the runway. You're going to be burning more fuel. If you move the controls abruptly, encounter any turbulence, plant the old girl a little too firmly on landing, any of a myriad of possibilities... you might just encounter structural failure. With an engine loss you might not be able to stay airborne.

If anything goes wrong the safe conclusion to your journey will be in doubt.

The bottom line

It's not legal, and it can be dangerous to be overweight.

It's not legal, and it most likely will be hazardous if you are out of CG.

Weight and balance DO matter.

Period.

Fly safely.

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May, 2003