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?

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"

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.

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.

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.

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.

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.:ok:

FAA has mandated (via AD) the use of "nitrogen" in tires since 1987! So any "customer" using air is subject to a big $ fine.

O2 does not burn..it supports combustion of other materials except asbestos, for that to burn you need flourine as the oxidizer:8

glhcarl---I think he meant heavy lift vehicles...even cars should get N2 imho:)

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.

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.

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.

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?

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

- 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.

BOAC:
 

But you might order a Newton of figs, would you not? :O

It must be a British thing.

- read post #14?

The concept might be a bit advanced for the 'Dubyas':)

I did.

I will not take the exam, as I will not want the certification, thanks.

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.

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. :E

I have used air on light stuff, but they they did have a rubber bag in them :}

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.

They are measuring and recording weight, because the transducers used are usually either load-cells or springs. You can only measure mass with a static balance* comparing the weight of your item to that of a standard, and these aren't used. The units used are actually the weight units "Lbf" or "Kgf", but they don't bother writing down the "f" bit. The calibration assumes a standard G value of 9.81m/sec^2 and isn't changed for use at different altitudes or lattitudes (which it would have to be if they were actually measiuring mass).

If you go into a german market and buy a half-kilo of peaches, measured in a spring balance, then he is selling by weight regardless of whether the unit used is kilos, newtons or punts (500g).

PDR

*OK, you can do a dynamic parametric measurement using velocity when subjected to a certain force, but the only people who do anything close to that are the baggage hadlers at chicago when they throw your baggage down to the tarmac on unloading

But it would only be an objective confirmation in still air. I live in the UK - if we need still air we have to import it...

:E

PDR

In order for an inaccurate weight sensor to be of any use, the error component would have to be consistent in both magnitude and sense. I don't think you can reasonably assume that to be the case. If if isn't, then all you will get is a string of false "overloaded" alarms, which will do nothing for despatch reilability

I recall that quite a lot of work was done, once upon a time, investigating the possibility of weighing aircraft on the taxiway with equipment similar - in principle - to that used for vehicle weight checks.

The need for still air was one problem, maybe insuperable.

The idea was to have a check for gross error that could be followed up if necessary. ATC could advise the crew of their indicated weight. It might even be possible to measure the distribution of weight between the MLG and NLG, as another gross check against CoG limitations.

I have always thought it would be a good idea, used for that purpose. After all, it's usually the errors measured in tonnes that kill, not the smaller discrepancies.

EDIT: Out of interest, I just did a search and found this. It seems I'm well behind the curve, as usual.

Interesting. That brochure, featuring the installation at Bogota, dates from 2004. I think it would be safe to assume that since then the product hasn't exactly been a runaway success.

self-weighing
 

As to one potential error in self-weighing, frequently mentioned here, surely the surface wind is a non-problem. Presumably any aircraft that requires a significant rotation (almost any big airliner I would have thought – perhaps any tricycle aircraft) to lift off is generating little lift on the taxi-way. Indeed, this is the posture of the aircraft when it rolls on the runway, a posture designed to give minimum drag and therefore little or no lift.

Anyway, what lift an aircraft might get from any particular wind can be calculated by the aircraft manufacturer to subtract from the observed weight readings.

Actually an offset wind will generate different elements of lift as between each mainwheel set, giving the crew a clue to the value of the total readings. Perhaps in that case a good reading might be adduced by applying the reading of the out-of-wind mainwheel set to both sides.

In the Bogota prospectus the configuration includes a laser printer but no anemometer. Enough experience of that installation must have accrued to show whether surface wind has been any problem at all.

A quick look at Wikipedia tells us that most of the 747 hull losses over the last twenty years are freighters. At least two of the freighters were disastrously badly loaded. I suggest that the industry knows there is a risk of misloading, but that resulting losses have proved bearable so far. The cure we are discussing is expensive. I assume that the calculation for the airline companies is that freighter losses cost them little – perhaps no more than increased insurance premiums. Misloading of passenger aircraft is relatively difficult to get wrong. So while the litigation and reputational cost of a passenger aircraft loss is very great, airlines have little reason to fear misloading as a cause of such a loss.

So, how much do we care that our freight-dog friends are not being looked after properly by their employers ? (Don’t anybody mention Lithium-ion batteries – OK?)

Worked a 744F with a MAC% reading, was usually pretty correct, but I always trusted my loadsheet, as did the crew.

We had sensors on the gear legs of the 747-200s in the sand box.
It was an option from Boeing, guaranteed to within +-2% if memory serves right.
We probably used it for secondary or back-up to the Load Sheet.

I'm unsure what two 747 freighter crashes you are referring to (I'm not finding any on a quick search), but if one of them was the National 747F at Bagram, that was directly caused by improperly restrained cargo that tried to exit the aircraft when they rotated - not 'badly loaded' (they actually found pieces of aircraft and cargo left on the runway as a result of the cargo punching a hole in the fuselage after it broke loose).
As TowerDog notes, there are onboard load sensors available for the 747F (I know some 747-8Fs are so equipped, although I'm unsure if it's basic or an extra cost option).

I don't think any of the 15 or so 747 accident hull losses in the last 20 years that can be attributed to being badly loaded, but it's certainly true that two-thirds of them involved freighters.

The 747s with load sensors were pax planes. The airline had ticked of all the boxes on the option sheet, we also had 3 autopilots and did Cat 3 approaches, pretty good for old analog aero planes.

Mis-loaded B747 freighters
 

tdracer #29 and DaveReidUK #30.

I agree I have overstated the problem, after reading through the ASN Aviation Safety Database for the 747.

As it happens the two hull-losses that I had in mind are not straight-forward arguments for self-weighing: MK 1602 at Halifax 14 Oct 04 and Cargo B 3101 at Brussels 27 Oct 08. In both cases the physical loading was not in question; the data were mis-loaded by the flight crew. In those situations probably neither flight crew was enough on the ball to make prudent use of any cautionary information from the undercarriage.

Indeed, the Cargo B crew belatedly corrected their load figures, so an undercarriage cross check would only have comforted them; their disastrous mistake was not to recalculate the take-off speeds to fit the 100 tonnes extra weight they had stumbled upon.

As to the Bagram loss, I would insist that “improperly restrained cargo” is “badly loaded”, but that has nothing to do with self-weighing. Still, of the three cases, this was one where the captain could have felt “if they get that wrong, what else are they going to get wrong ?” To my mind the critical failure – and unbearably sad to tell – was that the aircraft loadmaster, on the ground at Bagram, seems to have seen that the restraints fitted at Camp Bastion were failing but could not bring himself to get them fixed before his aircraft again took off.

If I was to persist with my case, I could look into all the crashes of old airliners of other makes or models for evidence that they were misloaded freighters. I do not think I will bother.

Really? For any wind aspect from 0 through 359 degrees? And when the aircraft is at the gate with the wind swirling around all those buildings? so that some surfaces might see the full force of the wind while others might be blanked? With jetways and freight-loading kit further messing up the flow field around the aeroplane?

Oh yes, and don't forget to factor-in the tailplane trim position, because the wind will be producing loads on the tailplane as well as the wings...

PDR

Well yes really.

We are talking about a check figure not primary planning information. A crew would be ill advised to take a reading in the hurly-burley you have in mind or, having taken such a reading, would be free to discard it.

We have been told of the ground installation at Bogota. I expect to be told that it is/was positioned somewhere along a long open taxiway.

A built-in system can be read at whatever point the crew finds the least turbulence.

I would be amazed if it was difficult or expensive for an aircraft maker to model the ground effect of surface wind on a free standing airframe.

I wait for Ex Cargo Clown #27 or Tower Dog #28 to tell us whether surface wind was even considered in using the installations they know of.

Go ye unto the archives: https://www.pprune.org/archive/index.php/t-395476.html

:ok:

Nah, can’t remember any wind correction when using the built-in weight/balance system.

loadsheets weight and balance

we mostly used notional weights 75kg males 65 kgs females
can't remember child weight 35 kgs?? and 10 kgs for an infant
bags were 15 kgs

so an example loadsheet would look like 48M 42F 15C 2I plus 108 bags
plus 128 kgs of freight
= your actual load sheet payload - plus add the crew and catering
all in kilos (sometime in lbs which was a pain in the arse)
TOB 105 + 2 INF

occasionally the baggage was actual weights used (weighed on scales at check in)
and mail and bulk cargo was all weighed and manifested

sometimes if the size of the baggage and type of flight was abit abnormal - then maybe we would calculate everyone as a male at 75 kgs + 20 kgs bag weights for all pax - depending on what skipper wanted
oil rig workers and ski flights always drew attention

All cargo freighter a/c - all palleted/ULD or bulk cargo was actual weights plus add the weight of a ULD or pallet as well (which were standard weights)

all quite simple maths really

one of our 707's had a CofG MAC meter in the cockpit which was pretty accurate - i think it worked off the nose wheel but cannot remember

BTW what are today's notional pax weights? - I assume they have increased somewhat than what I have quoted above

It was on a static aircraft and pretty sheltered, so was usually +/- 0.1%MAC off the loadsheet. As the pallets were loaded you could pretty much second guess if there was a gross error, as you'd know just how much it should change by.

One has to watch for some fiddling of reported weights from some shippers. Some packed their containers at their facility but of course, our Air Freight people weighed each one as it was received, or at least so they assured us they did so. When a loader-lifter refuses to raise two LD-3's, its hydraulics screaming in protest, one gets suspicious. Each had a weight of 3200-lbs chalked on it as testimony that our people had weighed it.
Fortunately the Chief Pilot at JFK was walking the ramp with me and he insisted they take the two back to air-freight for "reweighing". Each one of those two LD-3 exceeded 11,000-lbs, somewhat in excess of the 3500-lb limit. They were packed with flat sheets of aluminum stamping going to a factory in Puerto Rico for bending and assembly into TV sets. Our local Air Freight department suffered a change in local management as a result.
We had some B747-100's that had strain gauges on the gear feeding a built-in W&B system. They never worked very well and ended being deactivated.