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Old 30th Sep 2010, 20:39   #1 (permalink)
 
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Calculating an aircraft's weight

I've always wondered this, but never been able to find the answer. Sorry if this has been asked before.

Do the pilots need to know:

(a) how many passengers are on-board;
(b) how many bags are in the hold

in order to calculate the take-off performance data? What weight difference would be required to affect V1, Vr? I'm thinking B737 / A320.

A friend recently argued that the reason checked-in bags are weighed is so that the pilots know the weight in the hold. I disagreed, saying that this information does not go to the flightdeck; rather it is used to check compliance with the maximum weight allowance (and make the airline extra revenue). Plus, you can never know what the hand-luggage weighs, so that can't be relevant? Then I thought I should check this...

Sorry if there's a really simple answer to this, but I'd be interested to know.

Nick
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Old 30th Sep 2010, 21:15   #2 (permalink)
 
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Generally speaking, the dispatcher and flight crew will use standard weights for male passenegers, female passengers, children, and crew.. Standard weight figures are also used for baggage...

After filling out a loadsheet - either the dispatcher or flight crew will do this - the centre of gravity can be calculated and various types of weights ie take off weight. This is calculated by factoring in the 'weight and position of each hold, position of passangers sometimes if the a/c is fairly empty, the fuel on board etc...

Once the various weights are worked out, and by using environmental factors suchs as air temp, pressure, surface type and condition, wind component etc... various conclusions can be made - take off distrance required, accelerate stop distance if aborting at V1 etc...
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Old 30th Sep 2010, 21:30   #3 (permalink)
 
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Thanks for the reply One9iner.

I'm still not sure though whether you know how many bags have been loaded? Do check-in pass this information to you?

I understand that by law you must know how many passengers are on board and that there is a passenger manifest for this purpose. However, what happens if passengers check-in but then don't board the flight? You know, the ones who go to the pub in departures and then miss the flight. Equally, what about the people who check-in online but then never take the flight?

These passengers are, presumably, factored into your calculations. So how can you be that accurate? Or are you able to calculate everything once the CC have done a manual head-count? That surely doesn't allow you enough time.
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Old 30th Sep 2010, 22:09   #4 (permalink)
 
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Nicholas,

Talking of me experiences in my younger days, and charter rather than scheduled airline, we (Ops) would receive passenger figures the evening before.

With B737/A320 and larger aircraft computerised flight plans need to be generated by Ops a few hours ahead of the flight, not more than a few hours before the flight because en-route winds can change affecting the flight time and fuel burn.

Airlines work on standard weights for males, females, children, infants and per bag, bags are weighed and often recorded but humans are not, the passenger figures passed the day before may only itemise adults (including children) plus infants, there may be no breakdown of males, females, children thus Ops may need to, on the flight planning stage, need to guesstimate this breakdown, some airlines may take worst case scenario and presume that all are male adults ..... the heaviest of the species

When the crew would subsequently report for duty they would be presented with the computerised flight plan, i.e. this is your routing and your payload is X amount of kilos, per kilo the aircraft needs to fuel accordingly, the heavier the payload the more fuel the aircraft will burn.

Prior to departure the handling staff present the crew with a loadsheet breaking down males, females etc, Ops may have presumed all male adults, the crew receive a lesser payload which, in theory, they've got more fuel than they require but, in one sense, one can never have too much fuel unless it affects the maximun landing weight of the aircraft at destination.

But excess fuel represents payload, it costs more fuel to carry that excess fuel/payload, I, personally, would monitor Ops estimates compared with actual payloads and, for the next flight to that same destination, perhaps reduce the weight per passenger by a kilo or so until more actual figures were being achieved for fuel planning purposes.

So, to answer your question, yes, the crew do know how many bags, they know how many males, females etc, if you watch these programmes on TV you might hear the term 'LMC' used, LMC is a 'last minute change' to the loadsheet whereas they may need to add one male and a bag, or whatever, and recalculate payload.

Hope this helps.
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Old 30th Sep 2010, 22:33   #5 (permalink)

 
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Nicholas,

Two methods of calculating the weight and balance are used. One involves standard weights for male and female adults and children, and for baggage. The other method involves using actual weights.

Due to the immense difficulties of weighing each passenger and each bag and calculating precisely where each item goes, standard weights are preferred. When standard weights are used, bags are considered either "standard" or "heavy." A bag which weights in excess of what a standard bag should weigh is considered "heavy," and a higher weight penalty is applied to the airplane. Even if the bag is a pound over the standard weight, it becomes a "heavy bag," and may count for as much as two standard bags.

From a revenue perspective, the aircraft can only hold so much weight. Even though a bag may not weigh twice as much, once it becomes a "heavy bag," the airline must consider the bag as having weighed the full standard "heavy" amount.

Getting standard weights for bags in a lower cargo space makes sorting the bags easier. Can you imagine weighing each individual bag, then sorting them one by one to establish balance? The bags are packed in tightly, in no particular order, when weighed and determined to be "standard." This saves time and still allows the flight to be made safely.

Weight and balance are critical considerations for aircraft control, and for performance. Weight affects speeds, but balance is also very important. We consider the weight of the fuel, and even the way in which that weight continues to be distributed as it burns off in flight. Likewise, we carefully consider the placement of passengers and cargo as we determine the best way to safely load the airplane.
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Old 30th Sep 2010, 22:33   #6 (permalink)
 
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flying out of a performance challenging airport we use real bag weights. so yes when i sign the loadsheet i can tell you how many and what they weigh in each hold. We use standard weights for passengers.

In terms of hand luggage that`s why there is a weight limit! we use an average fiigure so it should even out over the flight. If weight is a problem then on smaller aircraft you can always even ask for every passenger and hand luggage to be weighed! and use real weights. Average weights work better with more pax. Average weights used are approved figures that are correct for the type of flight you are doing and are checked to ensure that they are correct.
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Old 1st Oct 2010, 09:08   #7 (permalink)
 
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Thank you, thank you for your answers.

So, just to confirm, when I weigh my bag at check-in, its weight is recorded, it is added to the weight of all other checked-in bags for my flight, and the total weight is sent to Dispatch and then on to the Flight Crew who use this data to amend the payload they were given when they reported for duty, if necessary?

On airlines where you pay to check-in bags, do the Operations team just use a standard weight calculated based on the number of people who paid for checked-in luggage when they bought their ticket for that particular flight? Or do they simply use an average for that route/time of day etc.?

In the example I gave above, let's say 150 passengers have checked-in online for their flight. Now let's assume that 50 (I exaggerate for effect) passengers do not show at the airport or miss the flight, meaning that only 100 passengers go to the gate and board the flight. The aircraft will be much lighter. Am I correct in thinking that it is only when boarding cards are checked at the gate and cabin crew do a manual head count that the Flight Crew know for certain how many passengers have boarded? Consequently, these Last-Minute Changes to take-off performance can only be made once passengers are boarding/have boarded the flight?
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Old 1st Oct 2010, 09:58   #8 (permalink)
 
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Nicholas,

The airline may use actual baggage weights or they may use figures that each bag weighs the allowable baggage limit, 10kg, 15kg, 20kg etc.

Check-in closes X amount of time before departure, that's when the payload is calculated and these figures passed to the crew, because Ops, at the flight planning stage, would have exaggerated figures, but not by too much, the aircraft should have been fuelled for a slightly heavier payload than the actual payload. In the event of no-showing passengers then the aircraft has more than enough fuel on, better to have too much than too little fuel!

I've never worked for a lo-co but I guess they use nominal figures whereas X% of passengers will check-in bags weighing Xkg each and these figures, at the flight planning stage, should be exaggerated also but on a 25 minute turnround, and check-in closing 15 minutes before the aircraft is actually on the ground, the crew can receive the actual payload figures before fuelling the aircraft.

Should a passenger, that has checked-in, not board the aircraft then there is a security risk if that passenger's bag is in the hold, then all hell breaks loose and they need to find the passenger or offload the bag. When the cabin crew do the head count is it to ensure that the actual figure matches the loadsheet figure to establish that no security risk exists and they can close the doors etc.
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Old 1st Oct 2010, 10:35   #9 (permalink)
 
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Thanks PF.

Quote:
In the event of no-showing passengers then the aircraft has more than enough fuel on, better to have too much than too little fuel!
Yes, I understand that. What I meant was: if the aircraft is significantly lighter than expected (because a substantial number of passengers have not shown up and/or fewer bags have been loaded than expected), would this not mean that the V1 / Vr figures etc. need to be re-calculated? A lighter aircraft takes off at a slower speed, right? How "late in the day" can you do this?

Unless I am mistaken (please correct me if so), it is not possible for an airline to know the definitive number of passengers it will have on a particular flight until they present themselves at the gate, especially where online check-in is possible (rather than going with the figures at close of check-in) and said passengers do not check-in any luggage.

I apologise if I'm missing something fundamental here - it just seems that with the ever-increasing "ease" of flying, it makes knowing actual numbers harder. Perhaps the odd no-show simply has no effect in terms of flight performance calculations, but does present a security risk, as you say.
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Old 1st Oct 2010, 12:44   #10 (permalink)
 
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Nicholas,

This, payload, fuel, aircraft weight etc, all goes on the loadsheet and from the loadsheet the crew know their performance figures pertinent to aircraft performance.

The airline, crew, will not know the definitive number of passengers until they have arrived at the airport and subsequently boarded the aircraft, even if we check-in online there is no guarantee we won't have a mishap on the way to the airport.

We see it on TV, even once checked in, a passenger may become drunk, develop a fear of flying or whatever, that's minus one male, one female, X number of bags or whatever to be deducted as last minute changes on the aircraft loadsheet.
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Old 1st Oct 2010, 13:26   #11 (permalink)
 
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Quote:
How "late in the day" can you do this?
Generally the V1, Vr speeds etc are calculated just before the doors are closed.

So just to recap, in general the process is:

1. (-24 hours) Online checkin opens.
2. (-2-8 hours) Ops make a 'best guess' at passenger/baggage/catering load based on booked figures. A flight plan is produced, with these estimates, and given to the crew.
3. (-2 hours) Airport checkin opens.
4. (-30mins) Checkin closure. The final(ish) ZFW is known, and the fuel figures are finalised by the crew and passed to the dispatcher. A loadsheet can be produced now if needs be.
5. (-10mins) All pax boarded, minus any no shows. If a loadsheet has been produced at checkin closure this can be LMCed to reflect any changes (i.e. the no show pax). However it is much easier to produce a loadsheet now, as the actual final load is known.
6. (-10mins) The crew input the final figures into the FMC, this gives the appropriate V1, Vr speeds etc, alternatively they can be looked up on charts. These are used for the take off.
7. (-5mins) Close doors
8. (dep time) Wait 10mins for a tug to show up
9. (+10mins) A/c pushes.

This obviously varies between airlines and airports, but in general its how it happens!

So the V1, Vr speeds are calculated at the last minute. There would have to be a fairly major change in weight to affect the fuel requirement, so this doesn't really change (certainly on the shorter domestic/European routes I deal with).

Hope that's some help!
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Old 1st Oct 2010, 21:15   #12 (permalink)
 
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And it can become somewhat more complicated if the aircraft is 'tankering' fuel

I'm going back quite a few years but, as an example, I think it was Kalamati, Greece where the fuel would be circa 50% more expensive than buying fuel at a London airport thus the operator would want to carry, out of London, as much fuel as possible based on aircraft, or airfield, performance, whichever is the most limiting factor.

To carry the excess fuel that distance might cost circa 15% of the excess in fuel burn but that's still a saving of circa 35% on the cost of the excess fuel carried, if Ops, when producing the computer fight plan, have over exaggerated the payload that would cost the airline significantly, the crew would take that payload figure before fuelling up, last minute fuel top ups might delay the flight but perhaps they could perform such a fuel top up whilst waiting for the tug
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Old 2nd Oct 2010, 19:45   #13 (permalink)

 
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The weight of individual bags and individual passengers becomes more and more important as the airplane becomes smaller, as individual weights have a proportionately greater impact on balance, as well as performance.

When standard weights are used, these values are applied to the final weight numbers for performance calculation (V speeds, climb performance, etc).

When passengers sit in seats, balance is a fairly simple matter with standard weights; the precise location of each estimated (standard) weight is fixed by the seat position.

When bags are packed in a cargo area, the values become a little more critical, as variances, packed closer together, can add up to larger changes from the norm. That is, each bag, ten pounds over or under, for example, makes a significant change from the expected value over the space of a packed cargo compartment, than it does for passengers in their seats. This becomes a balance issue.

Performance numbers are based on the weight of the aircraft and the ambient temperature; performance numbers are calculated just prior to takeoff, and just prior to landing. Performance is also calculated for the flight, by dispatchers or schedulers, when determining how much fuel to carry, etc. A heavy airplanes will require more thrust and more fuel for a given flight than a light one.

The actual takeoff weight of the airplane can vary somewhat from the planned or estimated weight that the schedulers use. The amount of variance that can be tolerated depends on the airplane in question. A big airplane can handle a bigger weight difference.

If our schedulers estimate the fuel for our trip based on a given weight, for example, they have used flight planning data to set up the route, considering forecast winds, to have us arriving at the destination with a particular fuel reserve. When we get the flight plan, generally two hours before our departure, this has all been precalculated. If our weight is not the same as the weight that the flight planners used, then we could have a problem.

We have a 10,000 lb variance. That is, unless the difference between our takeoff weight and the planned takeoff weight exceeds 10,000 lbs, we don't need to recalculate our flight planned performance. If it does exceed that value, we are required to have a new flight release issued, which recalculates our enroute fuel burn and other factors associated with the flight. This is a company policy applicable to our B747 aircraft. Obviously with smaller airplanes, that much difference would be entirely unacceptable.

Our takeoff fuel for a given trip can vary 30,000 to 40,000 lbs depending on weight, for the same length of leg. That is to say, a lighter airplane may require forty thousand pounds less fuel for a given trip than a heavier one. We normally try to land with a minimum of thirty to forty thousand pounds. Mistaking the planned weight could conceivably result in insufficient fuel for a trip. Obviously underestimating weight may also result in serious takeoff penalties; our takeoff roll is long enough that we always see the red lights at the end of the runway during the takeoff. We've got to have realistic numbers when planning the takeoff.

Ironically, even in a big airplane, the positioning of people can have a noticeable impact. When calculating an empty takeoff, which is just crew, the addition of an extra person or two on the upper deck means we need to add ballast fuel to the center fuel tank to make the airplane balance for takeoff. In smaller airplanes, in some cases one can feel the airplane require pitch trim changes, in some cases, when someone walks through the cabin or changes seats. In the extreme case, in a hang glider...one controls the entire aircraft simply by shifting weight.
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Old 15th Dec 2012, 08:03   #14 (permalink)

 
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Calculating an aircraft's weight

Reference
----------

Determining Aircraft Weight
Weighing scale - Wikipedia, the free encyclopedia

Can we design and construct a electronic instrument panel (Weighing Scale) which will display the Total weight of the aircraft before take off ?

If Overload occurs, the electronic weighing machine will buzz alarms to the Pilot and crew members.

Thanks & Regards,
Prashant S Akerkar
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Old 10th Apr 2013, 06:37   #15 (permalink)

 
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Calculating an aircraft's weight

Thank you.

Is it possible to design and construct a electronic instrument panel which will determine the total weight of the aircraft before flight take off ?

Thanks & Regards,
Prashant S Akerkar
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Old 10th Apr 2013, 11:36   #16 (permalink)
 
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It is already available.
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Old 10th Apr 2013, 11:39   #17 (permalink)

 
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Calculating an aircraft's weight.

Thank you.

Thanks & Regards,
Prashant S Akerkar
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