FAR 27.562(c)

Sourches:

FAR Part 27.562(c) It appears in NASA/TM-2002-211733 and NASA/TM-20030000682. 

Description: This is a safety rule that says how much pressure a pilot’s back can take before the seat is considered unsafe. 

How it is calculated: It is a rule for people who build helicopters. It says that when a helicopter lands very hard, the force pushing on the pilot's back must not be more than 1,500 pounds. This ensures the "pillar" of the back doesn't get crushed.

FAR 27.562(c) (Spinal Load - Force Absorption Ratio)

The FAR (Force Absorption Ratio) is a biomechanical indicator used to evaluate how effectively a protector reduces the force transmitted to the human body during an impact.

FAR does not describe how violent the impact is in absolute terms.
Instead, it describes how much of that violence is absorbed by the protector versus how much reaches the body.

In simple terms:

FAR tells you how good a protector is at “taking the hit” instead of passing it to your back.

Simple explanation for the user

When an impact happens, two things matter:

• how strong the impact is,

• how much of that force is stopped by the protector.
  
  

Two protectors can experience the same impact energy, but:

• one spreads the load smoothly,

• the other transmits a sharp force spike to the body.
  
  

FAR quantifies this difference.

• A low FAR value means the protector absorbs most of the force.

• A high FAR value means a large portion of the force still reaches the body.
  
  

So:

Lower FAR = better protection

Difference between FAR and peak force / peak G

Peak acceleration or peak force only tells you:

• the single worst moment during the impact.
  
  

They do not tell you:

• how long the force lasted,

• how much total loading the body experienced,

• how effectively the protector worked over time.
  
  

FAR, instead:

• looks at the entire impact event,

• considers how force evolves during the impact,

• evaluates force transmission efficiency, not just peaks.
  
  

In short:

Peak G measures how bad the worst instant was.
FAR measures how much of the impact was actually absorbed.

What FAR measures conceptually

FAR measures the ratio between force that reaches the body and force generated by the impact.

Conceptually, it answers:

“Out of everything that hit the protector, how much passed through?”

It is therefore a dimensionless ratio, not a force, not an energy, and not an acceleration.

How FAR is calculated (textual description)

The FAR calculation follows these logical steps:

1. The acceleration signal measured during the impact is filtered using the same standardized filtering applied to all other OPS / LEAI metrics, ensuring consistency.

2. The impact window is identified, corresponding to the first contact event.

3. From the filtered acceleration signal, the instantaneous force acting on the system is computed by multiplying acceleration by the total test mass.

4. The force signal over time represents the total force generated by the impact.

5. In parallel, the effective absorbed energy of the protector is evaluated from the force–motion interaction during the impact.

6. Using this information, the algorithm determines how much of the force-time history effectively contributes to energy absorption, versus how much is transmitted as residual load.

7. FAR is then computed as the ratio between transmitted force contribution and total force contribution over the impact window.
   
   

The result is a single scalar value representing force transmission efficiency.

Interpretation of FAR values

FAR is interpreted qualitatively as follows:

• Low FAR: highly effective force absorption, smooth load transfer

• Medium FAR: partial absorption, noticeable load transmission

• High FAR: poor absorption, sharp force transfer to the body
  
  

FAR is best used to compare different protectors under the same test conditions.

Relationship between FAR and other metrics

FAR complements other impact metrics:

• Peak G → worst instant

• Jerk → how fast force is applied

• DRI_biom → overall biomechanical severity

• REF → residual load on the body

• FAR → absorption efficiency
  
  

Together, they describe how bad the impact is and how well the protector handles it.

Important limitations

• FAR is not an injury criterion

• FAR does not predict fractures or pain

• FAR is relative, not absolute

• FAR depends on correct impact window detection and filtering
  
  

It should always be interpreted together with severity metrics.

One-line summary

FAR (Force Absorption Ratio) quantifies how effectively a protector absorbs impact forces by comparing the force transmitted to the body with the total force generated during the impact. Lower FAR values indicate better protective performance.