If you consider the picture with an aircraft on scales .. the CG position projected to the horizontal axis will be dependent on the attitude of the aircraft .. if nose down, the apparent CG is further forward, if tail down, further aft.
To make sense of the numbers, CG data needs to be repeatable and this requires the numbers to be referred to some reference attitude. This level position is defined by the OEM and is significant only as a rigging reference.
All aircraft are required by the Design Standards to have a means for checking the level reference for weighing.
Typical means are
... put a spirit level
(a) on two screws on the side of the aircraft
(b) on the top of the tail cone
(c) on the seat track
.. etc
or ... drop a plumb line
(a) to pass two screws mounted vertically on the side of the aircraft
(b) from a jig point to an inscribed plate
.. etc.
There may be other variants but the techniques listed above cover the great majority. Sometimes for tailwheel aircraft the OEM may schedule a taildown weighing method but this is far less satisfactory as it presumes a vertical location for the CG for the trigonometry necessary to convert the measured CG to a presumed level CG.
There will also be a means declared to achieve a lateral level .. especially important on rotary machines.
The weighing will be done either with
(a) weigh bridge style platforms (not common)
(b) glorified bathroom scales (but much more accurate, repeatable, and expensive) for smaller aircraft. Typically these are either strain gauged beam construction or mechanical balances.
(c) compression disc load cells which sit on top of the jacks and beneath the aircraft jackpad fittings. For smaller aircraft the aircraft is then jacked clear of the floor .. best if the aircraft started off level otherwise one can see some dreadful side load scale errors. For jumbo size birds, some hangars are set up to have the jacks remain stationary while the floor area under the wheel trucks is lowered clear of the wheels. There have been some hydraulic disc cells but of limited success in the marketplace.
And every now and then some clown rebuilds an aircraft ... losing the level jigs in the process .. I have only had this once .. turned up to weigh a C402 (as I recall) and found no jig references in the airframe .. which had had a major rebuild. The maintainer concerned hadn't bothered to find out what the bits he discarded were intended for ... As I recall we wasted quite some time with another example and the manuals working out an equivalent jig for the weigh.
Last edited by john_tullamarine; 19th February 2006 at 21:52.