RF Vs MS

In the captivating world of aerospace engineering, two giants reign supreme - Airbus and Boeing, which are playfully referred to within the close-knit fraternity of aerospace engineers as the A-Company and B-Company! Behind the scenes of their awe-inspiring aircraft lie intricate stress analysis methods that ensure the safety and reliability of each aircraft while trying to meet the stringent weight and cost targets in their overall design.

If someone has been fortunate enough to delve into the realms of both A & B Companies, he/she would be tempted to become a little adventurous and take a panoramic perspective of those methods. With utmost respect for the proprietary boundaries of each company, this is exactly what I intend to attempt here in a series of articles to explore the similarities and differences that lie beneath the surface of their stress analysis methods.

The Basics of Aerospace Stress Analysis

Before we take a deeper dive into the topic, let us rewind to the basics. The foundation of both Airbus and Boeing stress analysis methods rests upon sound engineering principles and rigorous testing. The primary objective is to balance the contradictory requirements of strength and weight of the aircraft's critical components as they encounter a plethora of loads during their operational lifetime. This involves exhaustive hand calculations, most certainly covered in classical text-books on the subject of strength of materials and structures like the ones from Bruhn, Niu, Peterson, etc; These are complemented almost every time with numerical simulations, using state-of-the-art tools such as Finite Element Analysis (FEA) to evaluate structural integrity and fatigue life.

But then, considering their diverse fleet of aircraft, specific mission profiles, and engineering philosophies and perhaps, driven by the market-competition, both Boeing and Airbus had to develop their own tailored methods and address their respective unique requirements.

RF and MS - A Tale of Similar Goals, Different Approaches

As we delve into the intricacies of stress analysis of either company, we see that the ultimate result of any lengthy and complex calculation is quantified in a simple safety metric which even a lay-man can immediately reckon with. In the Airbus parlance they name it the “Reserve Factor (RF)” and in the Boeing world, it is the “Margin of Safety (MS).” This metric is calculated for every conceivable part, right down to the smallest fastener in the complex load bearing structural systems of the aircraft.

Let us take a look at the formula used to calculate the RF and MS values separately and then compare them. Both of these essential metrics give an idea of the aircraft's resilience in the face of operational stresses which are colloquially referred to as just “loads” in typical calculations.

The RF formula is:? RF = allowable load / applied load

Reserve Factor (RF) is a measure of how much "extra" strength a structure has over and above what is needed to carry the applied load. A higher RF means that the structure is more likely to be able to withstand unexpected loads or damage.

The MS formula is: MS = (allowable load / applied load) - 1

Margin of Safety (MS) is a measure of how much "buffer" a structure has between the applied load and the point at which it will fail. A higher MS means that the structure is more likely to be able to withstand unexpected loads or damage.

In simple terms, RF is a measure of how much stronger a structure is than it needs to be, while MS is a measure of how much "wiggle room" a structure has before it fails.

As it turns out, the minimum RF or MS value that is considered "safe" is also different.

As a thumb rule, RF >1.0 for A-company, and MS>0.0 in the B-Company is acceptable for any of their structural parts. In general, a higher RF or MS is better, as it means that the structure is more likely to be able to withstand unexpected loads or damage.

Units in Diversity

A notable distinction also lies in the system of units used behind the stress calculations that ultimately lead to the safety Metrics of RF or MS. For instance, while Airbus engineers perform their analyses using millimeters and kilograms to arrive at an RF, Boeing engineers use the imperial system, working with inches and pounds.

Coming Up Next:

In the next article, hopefully soon, I shall delve deeper into the intricacies of actual stress analysis, by looking at the two distinct classes of problems addressed by both Airbus and Boeing - Static and F&DT (fatigue & damage tolerance). Using their own system of units, whether it is a static or F&DT stress check, Airbus and Boeing arrive at their RF and MS respectively. These critical safety calculations are derived from their historically developed in-house methods, processes and tools which are validated via rigorous testing.

Despite these differences, we have to understand the overarching goal remains unchanged for both companies – a lightweight but safe design safeguarding the lives of those aboard their aircraft.