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Other questions currently being answered cover situations regarding planes near, at or even above their MTOW. It raises a more basic question: how is the exact "ramp weight" of an airliner measured or estimated?

On most airlines, luggage is weighed and counted, so the pilot would know within 100lbs or so how much mass is in his cargo hold. He'd also know the "operating empty weight" of the plane itself (the weight of the airframe, all fluids besides usable fuel and the weight of the crew), and the mass of the fuel he's taking on.

The biggest variable, then, is the weight of the passengers and their carryons/personal effects. Last time I flew, I wasn't knowingly weighed and neither was anyone else getting on with me. So, while you know the number of passengers, the total "cabin weight" is a big unknown.

So, do planes have load sensors in their gear which could be used to exactly measure the plane's ramp weight? Or is a median passenger and carryon weight used, with a more conservative total passenger rating used in case a few planeloads err on the heavy side of the bell curve?

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    $\begingroup$ You know how you have to pay a lot of money if your bag exceeds 25 kg, and how you have to buy two tickets if you're too big to fit comfortably in one seat? There's probably more than one reason for that... $\endgroup$ – Landak Jun 25 '15 at 21:15
  • $\begingroup$ Related: Do airlines weigh passengers and baggage before loading the aircraft? $\endgroup$ – fooot Jun 25 '15 at 21:40
  • $\begingroup$ @Landak - I figured that these things factored into the weight calculation. However, passengers aren't weighed directly while luggage is. On my last flight, my toddler daughter required a ticket for her own seat. She weighs 30 lbs soaking wet. I weigh... more, and I was carrying a second child on my lap. So obviously there's still a massive range of possible passenger weight per seat. It falls in a bell curve, like almost anything, but if you're ferrying a pro football team, that's a lot of lean muscle aboard and thus a relatively higher per-seat weight, so how might that be accounted for? $\endgroup$ – KeithS Jun 25 '15 at 21:51
  • $\begingroup$ Related: Do some airplanes have weighing scales on the landing gear? $\endgroup$ – DeltaLima Jun 25 '15 at 21:58
  • $\begingroup$ See en.wikipedia.org/wiki/Air_Midwest_Flight_5481, where total passenger/luggage weight and CofG errors contributed to the crash... $\endgroup$ – DJohnM Jun 26 '15 at 1:00
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Short answer

Determination of mass and balance (or weight and balance) is a critical task for the crew, an error in this operation may have tragic consequences.

  1. Weighing passengers and carry-on items is mandatory per regulation.

    Either standard predetermined weights, or actual weights can be used in this operation. Airlines usually use predetermined tables. Tables values take into account different types of travel (e.g. regular and charter).

  2. While the weight must be checked, the center of gravity location is of equal importance. The distribution of the passengers in the aircraft is also taken into consideration.

  3. Aircraft are not weighed prior to each flight, but the basic empty weight must be known and checked periodically per regulation.

    Aircraft are weighed simply using scales under the carriages :-)

    enter image description here
    (source)

    The other method is to use jacks. See further below for an image.

So this was the short answer. To understand why and how to balance the aircraft, there are quite a lot elements to introduce. For those interested, just continue!


Weight...

The different elements composing the takeoff weight:

enter image description here

All of these elements must be determined prior to taxi and takeoff.

The mass of these elements are necessary to check that the different maximum weights are not exceeded. However there is another aspect of the masses which needs to be secured: Where are the masses located?

  • Taking KeithS' example, if there are many heavy people grouped in the forward section, we may have a problem at Vr, even if the MTOW is not exceeded.

... and balance. Location of the center of gravity vs. the MAC

The masses must be distributed so that the center of gravity of the aircraft is located within limits determined relatively to the aerodynamic chord, and the center of lift.

This is required to ensure the stability and the maneuverability, to minimize the efforts on the horizontal and vertical stabilizers and to increase the maximum lift and reduce the drag. On airliners, the center of lift must be located between the CG and the center of lift of the horizontal stabilizer, so that the latter pushes downward to balance the weight.

enter image description here

On airliners the width of the wing is greatest where it meets the fuselage at the wing root and progressively decreases toward the tip. As a consequence, the chord also changes along the span of the wing. The average length of the chord is known as the mean aerodynamic chord (MAC).

enter image description here
The MAC is a portion of the wing, delimited by LeMAC (leading, 0%) and TeMAC (trailing, 100%). The CG must be located in the MAC.

In large aircraft, center of gravity limitations and the actual center of gravity are often expressed in terms of %MAC (on the roll axis, similar limits exist on the two other axis).

enter image description here (image and text around: Aircraft Design, Mohammad H. Sadraey)

In general, in large transport aircraft, the most forward CG is located as forward as 5% MAC, while the most aft location as aft as 40% MAC. The average CG limits in large transport aircraft are between 20 and 30% MAC.

As commented by Calchas, using wrong weights for the passengers (adults instead of children) may move the CG to a wrong location, not balanced by the trim, and create strong difficulties during the takeoff and the initial climb.


PAX weight applicable procedures

For an airliner, the PAX portion is relatively small compared to the total mass, so the passengers mass is usually estimated based on standard passenger mass tables and the number of PAX per cabin section. Each airline may provide its own standard mass tables, but standard tables are proposed with the applicable regulation:

For the EU, a new survey was conducted by the European Aviation Safety Agency in 2009 and a proposal for table updates issued:

At this point you may want to download the documentation I'm mostly using for this post: Mass and Balance in Aircraft. We are at page 54.


Special cases

There are cases where it is obvious the actual mass and balance will be erroneous if the standard tables are used (this is especially true for a small cabin).

In these cases the operator must improve the evaluation by actual weighing of either all or a sample of passengers and their baggage. The procedure already mentioned for establishing standard tables is used (JAR - OPS 1.620).


Putting all elements together

All masses, arms (distance to the reference center) and moments are finally reported on the aircraft manifest to calculate the correction that must be used for the takeoff (trim amount).

enter image description here


Basic empty weight determination

The aircraft is not weighed for each takeoff, still it has been weighed to determine its basic empty weight and its center of gravity. This operation must be done every 3 or 4 years (9 or 18 years in a large fleet, this is a specific process) according to JAR Ops 1.605, Appendix 1.

Platform scales under the carriages may be used:

enter image description here
(source)

Another method is to use jacks equipped with a transducer at their top:

enter image description here
(source)


Related FAA regulations and JAR-OPS

  • FAA AC 120-27E and JAR-OPS 1.620 to determine the mass of the baggage, passengers and carry-on items.
  • FAA AC 120-27E, JAR-OPS 1.607, JAR-OPS 1.615 for the determination of operational items, which include the crew and the crew baggage (there is also a table, that I won't include to prevent comments on the differences with the passenger table -- All tables are based on survey from airlines). The operational items also include catering, emergency equipment, spare parts..

May be interesting too:

  • There is a confirmed trend of weight increase among the population (including myself). This has an impact on the operational costs. The way airlines manage this impact can be read in a two-part article: The true costs of heavier passengers, part 1, part 2.

Related:

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    $\begingroup$ Aren't those weights changed in recent years? $\endgroup$ – DeltaLima Jun 25 '15 at 22:00
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    $\begingroup$ So what happens when there is an expected deviation from the mean/median weight per passenger? An American football team, plus coaching, training and support staff, would represent the bulk of the average airliner's passenger capacity, and would have a substantially higher per-passenger mass. How might the pilot account for the difference? The team would weigh their players regularly and so could furnish relatively recent specs on the players at least... $\endgroup$ – KeithS Jun 25 '15 at 22:37
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    $\begingroup$ @KeithS. If the operator suspects the standard mass will be exceeded, then they must sample the passengers using rules detailed in my (updated) post. $\endgroup$ – mins Jun 25 '15 at 22:46
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    $\begingroup$ @KeithS Recently about 90 school kids on a QF 737 were incorrectly loaded as adults, and worse, sat at the back. The centre of gravity was therefore much further forward than the pilot expected. The pilot only noticed when he tried to rotate off the runway and found the nose very heavy; having to pull back so hard made him very nervous about a tail strike during the take off. The incident was investigated by the Australian Safety Board: atsb.gov.au/media/5092920/ao-2014-088_final.pdf $\endgroup$ – Calchas Jun 25 '15 at 23:24
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    $\begingroup$ The initial version of this answer was good; the updates with contingencies and total trip planning is excellent. Tick. $\endgroup$ – KeithS Jun 26 '15 at 14:40

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