By Bjorn Fehrm, Henry Tam, and Andrew Telesca.
November 26, 2021, ©. Leeham News: Last week, we started our analysis of the serial production phase. If development is filled with revelations and problems to solve, production has it as well.
We start this week by looking at the beginning of serial production, where several issues are overhangs from development. There are areas of the aircraft, though we have achieved our Type Certificate, that are not quite to the maturity level we want for long-term serial production.
Figure 1. A typical aircraft Final Assembly Line (FAL) site. Source: ATR.
Serial production is typically started before the end of development and before the design Type Certificate has been earned. This means we have parts in the aircraft that are not fully mature and we fight with:
Logistics & Coordination Issues:
- Parts and systems that are not to final specification, or the wrong revision for the aircraft
- Late arrival of parts that are difficult to produce for one reason or another
- The introduction of customer delivery reviews may introduce the need for changes and reworks that we as the OEM accept. We could also run into contractual disputes (especially if the program was delayed) that can slow deliveries and increase production costs as the aircraft for this order might be taken off the line and parked until we know how the dispute is resolved.
Design Variation and Optimization:
- Parts and systems that are overweight create variant problems. The first aircraft in a new program are typically 5% overweight compared to what our preliminary design put down. Aircraft development and production is a constant fight against weight creep, a fight that never stops. As we go into production many small changes will be introduced to try and correct this. Gradually the product reaches a mature state regarding revisions on parts and systems and their final weight.
- We will need to produce the variants sold to different customers. Each customer has a slightly different specification of the aircraft, especially in the cabin. Such first aircraft to a slightly different specification are called “heads of versions”, and these require extra work to get the revised parts to fit and our assembly team to get comfortable with the new parts. A changed specification means options management and certification, the introduction of new interior bits, and validation of the design with original or new foreign authorities who may require design changes or where delays for inspections could cause hiccups in the delivery process.
Slow Defect Resolution:
- Parts and systems that need extra work consume more work hours to complete, and therefore cost more than planned. If we find structural parts that are not to specification, the best line of action might be a repair of the part instead of rejection and production (and therefore wait) for a new part. Such repairs will take longer as our repair manuals may not be completed. It then involves intense dialogs with engineering to understand what can be done and how. A “use as is” dispositions may not be possible as we don’t have the data to support the variance of the part, and we might have to wait and see if the defect is common enough to warrant the additional engineering effort to investigate a corrective repair. As we wait, it means a higher rate of expensive “remove and replace” dispositions.
- Traveled work packages are fitted to the aircraft out of position, something that costs extra. This can be a result of late arrival or not to spec parts. Traveled work means the final state of an area like an air duct or cabin fitting will only be installed once the aircraft has rolled off the FAL. This keeps the production line moving, but if the volume is too large it pushes the bottleneck to our delivery center, where the work is often less efficient (more costly) to perform.
- Early aircraft are more likely to discover issues during their production flight tests after the FAL process. It will increase our bucket of out of sequence work.
- Certain problems might be too complex to find an immediate solution to. We can then have an agreement with the customers that certain functionality will be fitted after delivery and entry into service, via a modification kit. An example is how Boeing agreed that the vertical tail tank for the 747-8 was not usable initially until its full influence on the aircraft was cleared in flight testing. A modification was defined (it can be as simple as a paper change or just a software update for the systems involved) which was installed on the aircraft after it had entered service.
The reasons for the learning curve
The initial high cost of the first aircraft is because of all the above problems. But it also comes from the time it takes for assembly mechanics to confer with designers how it all is intended to come together. Parts that run into tolerance conflicts will not fit and need adjustments. The need for adjustments must be reported back to design and production engineers so that the procedure for deficiency correction can be verified. “Is it OK if we machine this surface a further 0.2mm to make the part fit?”. This can only engineering answer as it has the stress calculations and fit principle for the part in its design data.
Often the designers/product engineers/planners come to production to physically see the problems. Each problem area is thoroughly discussed. A fix is devices and is tried in subsequent aircraft.
When a production chain runs into problems with parts, it tries to continue without the part or fit it provisionally while waiting for a part that fills the specification. This means this area of the aircraft must be opened again (if it was behind something) and the final part put in place.
All this cost time and time is paid work-hours. If one analyses a simple area like an aluminum structure for example the horizontal tail, one can see that the cost of production has almost no relation to the cost of the aluminum alloy used. The all dominant cost item of the horizontal tail is the amount of work and machine hours that the item has consumed in its production.
The extra work hours spent in making different sections of the aircraft is, therefore, later found in the company’s “time used for this aircraft serial number” reports and finally in Cost of Goods Sold, COGS.
The initial part of the production is struggling with a very large consumption of work-hours to handle all the disturbances in the production. Late deliveries mean flying in parts that should have arrived with normal transport, and purchasers and production planners literally carry certain items to the production site.
As production settles down, items fit and the specification of the 500,000 items in our aircraft reach their mature and stable state. It’s when normality arrives and by it, lower costs.