Sunday reading. Fatigue and damage tolerance are covered in a handbook free on Kindle, with three other titles, until Tue 10 Nov 23:59 PT. Time remaining: ends Tue 10 Nov, 11:59 pm PT.
This damage tolerance explainer was published during a free Kindle week for four engineering books in November 2026.
It may sound alarming, but airliners aren't designed on the assumption that they stay free of cracks. They are designed on the assumption that cracks will start, and that they will be found before they matter. This idea is called damage tolerance, and it was learned the hard way.
Lessons from accidents
In 1954, two de Havilland Comet jets broke up in flight. Investigators found that repeated pressurisation had caused metal fatigue in the fuselage. Each flight's cycle of inflating and relaxing the cabin was doing small, cumulative damage.
The industry's first answer was a "safe life": prove by test that a part lasts a set number of cycles, then retire it. But test results scatter, and a hidden flaw can upset the calculation. So thinking moved on to structure that can survive damage.
In 1988, an Aloha Airlines Boeing 737 lost a large section of its upper fuselage in flight, and the crew landed it. The aircraft had many cycles behind it, and small cracks along a row of fasteners had linked up. The accident drew attention to widespread fatigue damage in ageing fleets.
How damage tolerance works
Today, 14 CFR 25.571 requires an evaluation of strength, detail design and fabrication so that catastrophic failure from fatigue, corrosion or accidental damage is avoided throughout the aircraft's operational life.
In practice, engineers estimate how fast a crack grows, from the smallest size inspectors can reliably detect to the size at which the part would fail. The time between gives the basis for inspection intervals. Check often enough, and a crack gets at least one good chance to be found.
Structure is also arranged so load has another path if one element cracks. Crack-stopping features limit how far damage can run.
Why this is good engineering
Damage tolerance accepts that nothing is perfect, then builds a system of analysis, design and inspection around that fact. Maintenance is part of the safety case, not an afterthought.
For the wider picture of how static strength is shown first, see our explainer on proving a part.
The four books
A humble request from the author
If you download one of these books while it is free, I would be truly grateful if you could leave an honest rating and review on Amazon once you have read it. Whether you loved it or found gaps, your honest view helps other engineers decide if the book is right for them, and it tells me what to improve in the next edition. Thank you for reading. — Sandeep Bandyopadhyay
Practical Airframe Stress Analysis, Volume 1: Metallic Structure
GD&T Professional Exam Guide
Practical Airframe Stress Analysis, Volume 2: Composite Structure
Python for Aerospace Stress Engineers (Volume 3)
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Related gear
- Practical Airframe Stress Analysis, Volume 1: Metallic Structure (Sandeep Bandyopadhyay) on Amazon — Related gear
- Aero Structures and Composite Materials with Artificial Intelligence (book) — Sandeep's book
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Sources
- Cornell LII: 14 CFR 25.571, Damage tolerance and fatigue evaluation
- Wikipedia: de Havilland Comet
- Wikipedia: Aloha Airlines Flight 243
