Welcome to the CPL Performance question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
The present definition was introduced (I'd have to dig about in the archives to locate just when but) around 35-40 years ago. I had some involvement in the change and, to some extent, I think DCA probably got on with it to shut me up at the time. The previous definition specified undrainable oil and this reflected the fact that, in the earlier days of aviation, we had plenty of large radial-engined aircraft for which oil was more of a consumable than it is for the modern HO engines. In that situation, it made more sense for empty weight to start with minimal oil quantity.
As the old radials progressively disappeared (not many on the register these days), it became a nuisance for us WCOs to schedule both an empty weight and some other weight to reflect full oil, which was more appropriate for the smaller GA engine aircraft. A few of us made a nuisance of the issue with DCA (CASA's antecedent) to have the definition changed to reflect full engine oil and this, eventually, came to pass.
Now, why do you still get bedeviled by two definitions ? The exams (and, hence, the training courses) have evolved over the years and parts of the work still date back prior to the ANO (CAO) definition change, so you get stuck with old documents raised with the earlier definition and newer documents raised with the present definition.
When you go out to your aircraft for a fly, though, the AFM/POH should specify a quantity in the weight and balance information called "empty weight" which includes full engine oil. It may well then go on to define one or more other weights (which can be called anything the operator/WCO agree on) to provide starting data of more use to the particular aircraft's operation. The only other weight definition for us is "operating weight", also in CAO 100.7, and which can be pretty well anything the operator/WCO choose it to be. A different matter overseas and various authorities and professional bodies have various lists of definitions which, generally, only serve to muddy the waters.
The main thing is, anytime you read a weight term, there should be a definition associated with the term in the document so that you can get an idea of what the term refers to. A bit of a nuisance, I guess, but that's the way things are.
Loading System variations
The requirements for loading systems are in CAO 20.16 and 100.7. In general, your aircraft should fit into one of the following categories -
For the simpler aircraft it may be that the aircraft can't be loaded outside the envelope while observing weight and compartment limits so, there may be no specific loading system beyond a statement saying that such a document is not required, usually with a note to remind the pilot to observe weight and compartment loading limitations.
If the aircraft can be loaded outside the envelope, then there must be a loading system approved and used for the aircraft's operations. In the simplest style, this might comprise a simple set of load rules along the line of "with so many seats occupied, maximum baggage is x kg" or similar to cover the situation while providing an easy set of rules for the pilot to use in loading his/her aircraft. While you may see this style, typically, in 2-4 seaters, some larger aircraft are sufficiently benign in loading that simple rules are the general story. For example, the HS125 family, near invariably, will have simple loading rule style loading systems, unless the operator, for whatever reason, wants to have something a little more complex.
The next step up the ladder is the typical tabular do-it-yourself calculation system and check the answers against a graph or table of the CG envelope, either presented as weight by arm or weight by moment. For the latter, moment may be replaced by index units but, to all intents and purposes, the two are the same thing. Loading system Charlie is after this sort of style.
Then we look to simplifying this a bit by giving the pilot some simple graphs (or tables) to look up the moments (ie save the effort of doing the sums). Overall, just a minor variation on the do-it-yourself exercise. Loading systems Bravo and Echo are in this style.
This all gets a bit tedious for larger aircraft for which we generally move to trimsheet loading calculations. The advantage of trimsheets are two fold -
(a) if well designed and executed, the functional accuracy is as good as a longhand tabular calculation
(b) they make things easier and quicker if there are numerous loading stations to incorporate into the loading calculations.
Loading system Alpha is one such trimsheet. Be aware that there are two slightly different versions of this sheet floating around the theory training side of the Industry. Some discussion is in this thread
. Subsequent to my posts in that thread, I confirmed elsewhere that the Alpha sheet originally was lifted by the then theory examiner from a Piper and was one of Norm's sheets. Later on, another examiner asked Bruce Clissold (another very experienced WCO) to update the sheet a little and that resulted in the two similar documents remaining in existence.
Then, as you go further up the totem pole, it is the province of computers. These sorts of systems fall into two categories -
(a) a simple computerization of any of the above manual systems. Often these are done poorly and there are numerous traps for young players along the way.
(b) for the larger, typically airline aircraft operations, the loading systems are incorporated into any of the very sophisticated departure control systems. For those of you who end up in airlines, you will get to know the ins and outs of these when you get onto the larger aircraft.
So why do we have a multiplicity of system styles ? In essence, it is a case of endeavouring to tailor reasonably simple, but suitable, systems to increasingly complex aircraft requirements. Does it matter much which you use for a given aircraft ? Not really, providing that
(a) the system is accurate and well engineered and
(b) is called up the load data sheet as the approved system.
Please do ask any other questions you have and I'll do my best to answer them here.
Engineering specialist in aircraft performance and weight control.