Boeing Serial Numbers and it's System

[Syckes]

Hi everyone.

I was looking at some aircraft and at their history at the airlines to which they were initially delivered and found something that is odd to me. I know Boeing makes use of a two number serial number (one for Boeing and the other for the line eg 737 or 747) for instance:

19700/673 Boeing 727-223 delivered to American Airlines (first Flight - 14 Dec '68)
19701/677 Boeing 727-223 del to American Airlines (FF - 6 Jan '69)
19702/680 Boeing 727-223 del to American Airlines (FF - 8 Jan '69)
19703/684 Boeing 727-223 del to American Airlines (FF - 21 Jan '69)
19704/689 Boeing 727-223 del to American Airlines (FF - 31 Jan '69)
19705/675 Boeing 707-344C del to South African Airways (FF - 5 Feb '69)
19706/691 Boeing 707-344C del to South African Airways (FF - 18 Mar '69)

and

19700/673 Boeing 727-223 Del to American Airlines (FF - 14 Dec '68
19975/674 Boeing 727-251 del to Northwest Airlines (FF - 18 Dec '69)
20201/675 Boeing 727-284 del to Olympic Airllines (FF - 15 Oct '69)
19914/676 Boeing 727-222 del to United Airlines (FF - 3 Jan '69)


This is just a short extract to show how the numbers usually follow. Now my question is why in some instances the numbers don't follow this form. Sometimes only the line number is out of place/sequence in the production order and other times the Boeing number is out. On other occasions the number appears to be missing. I also noticed some of the orders for an airline would have sequential numbers (Boeing number) and on other orders they would be split into two or three different sequences. Is this due to the date the aircraft have their first flight? Are the Boeing numbers assigned in batches to individual lines? Are the numbers assigned by order and then the split in the sequence is due to the Options being made into firm orders? Are the Boeing numbers assigned to lines by year or am I missing something? Does anyone have some more info on this.

Don't know if someone else noticed something similar and found an answer or did I just notice something querious.

Thank you very much.

[airsmiles]

I could be corrected on this but my understanding is:

The first number (e.g. 19700) equates to an order or sales booking. These can be changed or cancelled quite a bit as options are not converted to orders, orders for one variant are swapped to other variants etc.

Later, when orders are confirmed and delivery dates are agreed with the customer, the line numbers are allocated. (e.g. 675 in your example). The line numbers represent the expected sequence aircraft will be manufactured. There will still be some late changes due to cancellations, short-term customer or technical issues etc.

In your example 20201/675 for Olympic has a much later sales number. I expect Olympic placed an order and took up an existing slot on the production line, thereby gaining a relatively quick delivery. Possibly line number 675 was for a previous order for an airline that cancelled.

BTW; 20201 was actually line number 765 and not 675. The principle above remains the same though.

Just to add another twist, Boeing did start something different in the late 1990's I think because they seemed to miss out quite a few order/sale booking numbers. In particular, there aren't many aircraft with numbers in the 31000 to 31999 range for some reason. Just a handful of B737's I think.

No doubt someone more knowledgeable can explain the system further.

airsmiles