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Aircraft weight

Old 23rd Mar 2011, 21:23
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Aircraft weight

I was under the impression that calculation of take off weight etc was based on a standard passenger weight. I've never really thought about cargo weight, how is that assessed?

Why can't the undercarriage be fitted with sensors to "weigh" the aircraft?
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Old 23rd Mar 2011, 21:42
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Why can't the undercarriage be fitted with sensors to "weigh" the aircraft?
Not really needed........

You have the basic empty weight of the aircraft ( from weight schedule)

Add the fuel, Crew, Pax, Pax baggage , Freight .Catering and potable water gives you the actual operating weight.

There is a system which uses a pressure sensor on the nose leg to verify the stab trim setting......ie the "green Band"
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Old 23rd Mar 2011, 21:52
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I've never really thought about cargo weight, how is that assessed?
Each piece is, in principle, weighed either by the airline or by someone the airline believes it can trust.

Systems for self-weighing of aircraft exist, but apparently have their problems and are no widely used. One complication is that a strong wind can provide enough lift for a stationary aircraft to appear significantly lighter to than is really is.
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Old 25th Mar 2011, 03:15
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Interesting statement that the bags are all weighed. I once volunteered to be bumped because our plane was weight limited, not seat limited. Three others also volunteered. They only needed to bump 3 - I was selected to go on the flight because I only had a carry-on bag and the others each had checked a bag. I commented, "Why me? I weigh over 200 pounds and none of the 3 young ladies who were bumped were over 120. It was the old standard weight deal - a passenger is 170 pounds (at least back then) no matter what, and a checked bag is xx pounds (can't remember the figure). Of course there are safety margins so I knew there was no problem with me being on the plane. I have only been on one flight where they actually weighed every person and every bag. That was a very small airline in Australia that had lost a plane 2 months earlier. Surely landing gear sensors would be relatively easy on an airliner. Even on the heavy equipment on which I work we have bucket load sensors so an operator can load a 400 tonne truck to the max and not seriously overload it.
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Old 25th Mar 2011, 04:36
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Some aircraft use weight sensors. Most do not.

Determining the weight of the aircraft is the function of the pilot in command, or those designated to do it for the pilot in command. The PIC will be the person to finally approve the completed weight and balance calculations.

While various sensors describing the weight and balance of the aircraft have been used, some have been deactivated because of maintenance and problems they cause. The empty weight of the aircraft is determined on a regular basis. Whatever is put on the aircraft must be calculated into a final weight and balance document; the aircraft must not only be within weight limits, but a fairly narrow range of balance, to ensure control on takeoff and in flight.

Aircraft, even large airplanes, are not comparable to heavy mining equipment. Aircraft are relatively fragile, and a mistake made on weight or balance can have disasterous consequences. Overload a dump truck and it operates a little slower. Overload an aircraft and it may never get off the ground, instead resulting in a fireball off the end of the runway. Put a little more load aft in the back of the dump truck, it may ride a little nose high. Put the center of gravity slightly aft and one may not have adequate control authority to fly. The implications of even minor improper loading on an aircraft are far more significant than large scale errors on heavy equipment.

When we calculate our weight and balance, we calculate not only for cargo, people, and other load items, but we calculate for the weight of the fuel, and the shift that will occur in flight as fuel is burned. We have intermediate weights we must attend, to include a zero fuel weight; the weight of the airplane before fuel is added. We have balance issues between fuel tanks, and between cargo compartments or sections of the airplane.

The manner in which the airplane is loaded, not just simply it's total weight, affects the way the wings bend in flight. The strength of the wings and their ability to withstand inflight loads such as gusts, is derived in large part from the balance applied to them and the wing bending moment. These are critical issues, and not ones that are generally left to a meter or sensor on the landing gear.

Calculations are done with all individual factors combined and talied, and carefully placed throughout the aircraft to achieve both a weight that is appropriate to the operating conditions and limitations, and a balance that is safe for operation. We can not simply load the airplane and see what it ends up to be. While sensors on the gear might tell us if the airplane agrees with the final calculations, the loading must be determined at the outset. We don't simply herd everyone on board, pack on as much cargo as we can, and then say "Ah, we need to take a little off."

There may be occasions when a calculation finds an error after loading is complete and adjustments must be made, and there are times when ambient conditions such as the temperature changes between the time calcualtions were made and departure time. Recalculations may find that given an increase in temperature, for example, weight must be reduced for a safe takeoff and climb performance.

Weight must be tailored not only to the structural limitations of the airplane,but also to the field conditions at the departure, destination,and alternate. We may be limited by how much cargo, personnel, or fuel we can carry due to temperatures, runway length, or even enroute conditions that might limit our cruising altitude, cause more fuel burn, or less fuel burn (such as strong tailwinds). These affect the loading, and must also be taken into account. These calculations cannot simply be made after everyone is aboard and the door closed; it's too late to throw everyone on, see if onboard sensors give us an acceptable weight,and go. Generally operations are a little more advanced and technical than that.

Standard weights are used by some operators, and actual weights by others. Every few years the standard weights are revised. The general populace tends to gain weight as a whole, and the standard weights tend to get revised up a bit each time. In unusual cases, each operator always reserves the right to weigh individual persons, if necessary. This is generally done more frequently in small aircraft than large aircraft, as larger airplanes tend to be a little more forgiving, with wider envelopes that can handle changes in center of gravity. Individuals make up smaller percentages of the total payload on large airplanes too; individual excursions from the standard model are more easily tolerated.

Baggage is generally weighed, although standard weights may apply up until a given limit, when a higher standard weight (heavy bag) applies.

The location of personnel and baggage makes a big difference in how the flight may be impacted. On the 747, for example I've seen the movement of a single person from the cockpit to the back of the upper deck make a palpable trim change, and I've seen balance calculations changed under certain loading conditions by moving a single person. Conversely, I've flown some airplanes in which I drop more than half the loaded weight of the aircraft in flight; cargo, jumpers, chemicals, etc, without an appreciable threat to the control of the airplane.

It's far too simplistic to simply think about throwing an electronic scale or meter on the gear and calling it good. Every detail of the load must be accounted, whether it be a passenger, or a case of oil in a lower cargo area.
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Old 25th Mar 2011, 14:08
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I do not disagree that weight and balance is extremely important. That is why I was suggesting that extremely reliable technology, aircraft quality, is available as a final check. Part of my job has been accident investigation, and a lot of my training has been from the aviation industry because crashes get the most intense investigation. You must be aware that many incidents have occurred because planes were improperly loaded.

Still for the main part passengers on large jets are loaded by standard passenger weight. Yes, the standard passenger varies by time of the year, and has grown over the ages, but they aren't weighed. Back in the 80's when I was commuting quite regularly on Metroliners, I would pay more attention to the passengers emplaning with me. Were they all 200+ pounders like me, or a mixed lot? Much more significant when there were only 16 passengers (my commute was always weight limited so we never flew with a full 19 passenger load on my commute.

As for my reference on heavy equipment, it is much more important than you suggest. Tires are the critical factor. The tires are supposed to be inflated with nitrogen but almost all customers inflate with air. Air contains - oxygen. Now think of the size of a tire filled with compressed air - lots of oxygen. Tires are made with flammable materials that tend to deteriorate inside. So we have oxygen, a highly combustable material, all we need is heat for an explosion. Overload, then transport. It is not as simple as you say - sluggish performance. The minimum safety range we say to stay away from an operating truck is 100 meters, but I've located the pieces a lot further from a tire explosion than that. So aircraft are not the only vehicles for which weight and balance are important. Its must that failure to pay attention make for bigger headlines because of the number of people killed per mistake.
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Old 25th Mar 2011, 17:24
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Just to be pedantic you are actually talking about MASS not weight. Kgs and pounds are units of mass, weight is measured in Newtons.

However, it is a very common mistake.
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Old 26th Mar 2011, 00:05
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NWA SLF,

The tires are supposed to be inflated with nitrogen but almost all customers inflate with air.
FAA has mandated (via AD) the use of "nitrogen" in tires since 1987! So any "customer" using air is subject to a big $ fine.
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Old 26th Mar 2011, 00:19
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So we have oxygen, a highly combustible material,
O2 does not burn..it supports combustion of other materials except asbestos, for that to burn you need flourine as the oxidizer

glhcarl---I think he meant heavy lift vehicles...even cars should get N2 imho
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Old 26th Mar 2011, 00:23
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As for my reference on heavy equipment, it is much more important than you suggest. Tires are the critical factor. The tires are supposed to be inflated with nitrogen but almost all customers inflate with air. Air contains - oxygen. Now think of the size of a tire filled with compressed air - lots of oxygen. Tires are made with flammable materials that tend to deteriorate inside. So we have oxygen, a highly combustable material, all we need is heat for an explosion. Overload, then transport. It is not as simple as you say - sluggish performance. The minimum safety range we say to stay away from an operating truck is 100 meters, but I've located the pieces a lot further from a tire explosion than that. So aircraft are not the only vehicles for which weight and balance are important. Its must that failure to pay attention make for bigger headlines because of the number of people killed per mistake
Which is entirely irrelevant, as glhcarl noted, because aircraft tires are inflated with nitrogen.

I'm an airline transport pilot and certificated mechanic, with more than a few years of doing both (as well as inspector, instructor, engineer, and a few other hats here and there). We don't inflate aircraft tires with shop air. We inflate them with nitrogen, for numerous reasons. Combustability of the inflation mixture is one of them.

Just to be pedantic you are actually talking about MASS not weight.
Actually, we're not.

While weight varies with height and separation from the gravitational body (earth, hopefully); we don't account for those changes, and we calculate weight and balance forms, not mass and balance forms. We worry about mass when we can't get stopped, but weight when we can't get off the ground.

Given our gravitational constants, weight works just fine. Unless we're planning a voyage into space or to another planet, we largely stick with weight calculations when figuring aircraft performance, as one of our four chief forces in defining flight is, of course, gravity. While gravitational acceleration works upon mass, the definition is weight, and that's what we use.

A sure way to get a black eye would be to tell my wife "My my, dear, have you lost a little mass lately?"

Pedantics, semantics. I just want to live.
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Old 26th Mar 2011, 11:23
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As a Licensed Engineer with multiple types on my Licence, in over 30 years in Aviation i've never seen aircraft tyres inflated with anything other than Nitrogen.
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Old 27th Mar 2011, 12:22
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and we calculate weight and balance forms, not mass and balance forms.
They are only called Weight and Balance forms by common usage, but that is still wrong. As I said before, if you are entering values in pounds or kilos then you are entering values as masses, the unit of weight is the Netwton. You don't enter Newtons on your forms , do you?
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Old 27th Mar 2011, 14:17
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Well said groundloop we had to watch out when doing manuals a few years ago when weight had to be replaced with mass. So we now have Mass and Balance sheets and Mass and Balance manuals
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Old 27th Mar 2011, 14:56
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They are only called Weight and Balance forms by common usage
- certainly Euroland (gawd blessit) changed sometime around 2000 and although it is more correct I only talk now of 'DOW' and 'MTOW' because I get vacant stares when I refer to 'MTOM' etc. You will not get an Eu-OPS licence if you talk of weight in the exam, I fear.

"Grocer, old chap - a Newton of bananas please" just does not sound right...................

Of course, Newtons for 'weight' are only relevant to those wot weigh ('mass'????) things in kg.

NB In avoirdupois, a pound is now considered to be a two-part measurement pounds-weight and pounds-mass.

This is a weighty topic and could amass a few posts I feel.
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Old 28th Mar 2011, 21:35
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BOAC:

But you might order a Newton of figs, would you not?
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Old 1st Apr 2011, 11:46
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So we now have Mass and Balance sheets and Mass and Balance manuals
It must be a British thing.
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Old 1st Apr 2011, 12:39
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It must be a British thing.
- read post #14?

The concept might be a bit advanced for the 'Dubyas'
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Old 1st Apr 2011, 12:51
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- read post #14?
I did.

You will not get an Eu-OPS licence if you talk of weight in the exam, I fear.
I will not take the exam, as I will not want the certification, thanks.

The concept might be a bit advanced for the 'Dubyas
I haven't flown with the ex-president, so I wouldn't know. I didn't vote for him, and generally think he was an idiot, so you may be right. Completing a lucid sentence was a challenge for him, let alone having to worry about reality. Frankly, he's probably in a study somewhere, cowering from a pair of shoes.

Insofar as the terminology might apply to the USA, of course, it's correctly weight and balance. We obtain the empty weight of the airplane by weighing it, and we add thereto the weights of fuel, cargo, passengers, and other, until we arrive at the loaded weight for the airplane in it's myriad forms and descriptions, and determine the balance thereof for suitability for flight.

Mass is something one attends on Sundays out of guilt, allegiance, or boredom.

Newtons are, of course, for regularity.
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Old 1st Apr 2011, 14:21
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As a Licensed Engineer with multiple types on my Licence, in over 30 years in Aviation i've never seen aircraft tyres inflated with anything other than Nitrogen.
Seem to remember a Phantom in RAFG having it's tyres inflated with breathing Oxygen, didn't roll far on landing, well at least when it had tyres on it.

I have used air on light stuff, but they they did have a rubber bag in them
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Old 31st Jan 2018, 11:05
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Aircraft Weight

On the current thread “Rumours & News – DC87 Unsafe Departure” we get several references (implicitly concerning freight aircraft operating out of airfields a long way from Heathrow) to misleading manifests.

On this thread the discussion has concentrated at length on the proper weight (mass)-and-balance calculations as if that was being challenged. They are not.

If undercarriage sensors are feasible, I would have thought they would make a valuable check on the proper procedures in action. However inaccurate, their indications can be plotted against the manifest calculations from flight to flight, until an anomaly in the comparison warns the captain of a hazard.

I can visualise the tiny crew of a freight 747 in the middle of the night expecting to depart Limpopo International Airport or wherever. They may not know the ground handling team and will not know the pressures on that team.

The captain would then be grateful for an objective confirmation that his aircraft is safely loaded, provided by the suggested undercarriage sensors.
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