FAR PART 25.365 e) Formula 1

[60NUAGE]

Hi.

IS THERE ANYONE WHO CAN ANSWER MY INQUIRY ABOUT THE FAR PART 25.365 e)

The size Ho must be computed by the following formula:
Ho = PAs
Where -
Ho = Maximum opening in square feet, need not exceed 20 square feet.
P =As/6,240 + .024
As = Maximum cross-sectional area of the pressurized shell normal to the
longitudinal axis, in square feet

Inquiry 1 : why the As(Maximum cross-sectional area) is devided by value 6,240?
Inquiry 2 : why P needs + 0.024 ?

Your any comment will be appreciated.

 

[rb1957]

1 and 2 … 'cause FAA say so ! …
0.024ft2 = 3.5in2 … no clear logic for this to be the minimum size opening
6240ft2 is the cross section of a fuselage 44.6ft (!) … no clear logic why this would be a datum

possibly the curve was derived from some companies attempts to justify an opening area

another day in paradise, or is paradise one day closer ?

 

[60NUAGE]

Thank you for your comment, rb1957.

I wonder how the diameter of 44.6ft long became the denominator for the formula. B747 is just 20ft. I guess some model plane was presumed for the formula at the time of rule making.

 

[verymadmac]

If you follow the Final Rule. Docket No. 14779, 14324; Issued on 8/27/80 that approved25.365 (e)(2), it appears to have something to do with AD 75-15-05.

A commenter suggests an alternative to Sec. 25.365(e)(3) which would establish a relationship between the design maximum opening and the cross-sectional area of the pressurized shell. The FAA agrees that the proposed relationship provides an acceptable method for determining hole size. The FAA has determined that the maximum hole size required should be 20 square feet, a value contained in Airworthiness Directive 75-15-05 (August 11, 1975) pertaining to openings in wide-body transports. Section 25.365(e)(3) is revised to allow the maximum opening to vary as a function of the cross-sectional area of the pressurized shell to account for the differences in size between narrow and wide-body transports and is redesignated and adopted as Sec. 25.365(e)(2)

Sooo anybody know what accident drove the requirement for cabin venting to prevent floor rupture during a lower deck depressurization.

 

[SWComposites]

Likely one of the DC-10 cargo door failure incidents.

 

[rb1957]

sure, DC10 … lower baggage door vented the lower fuselage, caused the floor to buckle and jam the control cables and the plane "augered in".

But the OP is right in that this expression is quite "odd" in its parameters, that's why I think it's a curve fit to some testing.

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

It may be a curve fit between 20 sq ft for a DC-10 and some sq ft number selected for say a DC-9.