FDT

Question 1

What loads are included in a spectrum?

Answer 1

• Flight loads (gust & maneuver)
• Ground loads (towing, taxi, engine run-up, turning, landing, breaking, & thrust reverse)
• Pressure (repeated application of normal operating differential pressure + flight loads + external aero pressure)

Question 2

What is spectrum clipping?

Answer 2

Removing load levels near the upper end of the exceedance curve due to elasticity concerns (think Comet)

Question 3

What is spectrum truncation?

Answer 3

Removing load levels near the lower end of the exceedance curve – lowest levels of remaining loads create little to no fatigue damage.

Question 4

What is safety-by-inspection?

Answer 4

Damage Tolerance

Question 5

What is safety-by-design?

Answer 5

Fail-safe

Question 6

What is safety-by-retirement?

Answer 6

Safe-life

Question 7

What is the damage tolerant concept?

Answer 7

Damage due to fatigue, corrosion, manufacturing defects or accidental damage: structure remains capable of taking reasonable loads without failure until the damage is detected. (Ended with Aloha-Air)

Question 8

What is the fail-safe concept?

Answer 8

Catastrophic failure is not probable after fatigue failure or obvious partial failure of a single PSE (ended by Pan-Air / F111).

Question 9

What is the safe-life concept?

Answer 9

Structure can withstand repeated loads expected during service of life without detectable cracks (ended with Comet).

Question 10

What are the DT Analysis steps?

Answer 10

• Is it a PSE?
• Determine load spectrum
• Appropriate damage criteria (betas)
• Test & Service History
• Inspection Interval & Methods
• ICA Development
  o Order 8110.54A (Instructions for continued airworthiness), AC 25.571 (FDT Evaluation of Structure), AC 25.1529 (2 stage approval) all provide guidance on ICA’s

Question 11

What is the Limit of Validity (LOV)?

Answer 11

Fatigue quality of the airframe – it’s the period of time that the aircraft has been demonstrated by analysis and test to remain free of widespread fatigue damage (WFD).

Question 12

What AC addresses Limit of Validity?

Answer 12

AC 120-104 (Establishing and Implementing LOV to prevent WFD)

Question 13

What is the widespread fatigue damage?

Answer 13

WFD is the simultaneous presence of cracks in multiple details that are sufficiently large or numerous to reduce the residual strength below the required level.

Question 14

For WFD, what is the difference between Multi-Site Damage (MSD) and Multi-Element Damage (MED)?

Answer 14

Multi-Site Damage (MSD) where cracks are at different locations in the same element

Multi-Element Damage (MED) where cracks are in adjacent structural elements.

Question 15

What are inspection intervals based on?

Answer 15

Inspection intervals are based on crack growth life divided by a factor which depends on if testing was performed and if it’s single load path (SLP) or multi-load path (MLP). Initial interval is based on time to grow from (rogue flaw to failure) / factor while repeat is time to grow from (detectable to critical) / factor.

Question 16

Define Category 1 Structure.

Answer 16

Large damage capable, detected during normal maintenance or operation. Initial flaw is visual. Generally multi-load path (MLP).

Question 17

Define Category 2 Structure

Answer 17

Does not have large damage capability and/or damage cannot be detected during normal maintenance. Directed inspection (typically via NDT) is required. Initial flaw size is Rogue Flaw. Generally single-load path (SLP).