1.5 Factor Of Safety - FAR 25.303
 

Hi all,

Does the Factor of Safety of 1.5 under FAR Part 25.303 apply to speeds or just G-loads?

eCFR ? Code of Federal Regulations

In other words, is there a factor of safety of 1.5 above the speed Vne or Vd as well?

In further terms, looking at the following VG diagram, there is an "Overspeed margin" between Vne and the end of the flight envelope.

Does the "Overspeed margin" represent a Factor of 1.5 as well as the G-loadsl?

http://i60.tinypic.com/wufkes.png

Thanks in advance.

No, it does not apply to speeds.

Just think, if max certified is .98 Mach, then if speed were applicable, it would be good for M1.4+........

Thank you for your reply bigduke...

So, if I were to state -

"a 1.5 factor of safety (yellow) is included for ALL loads that planes must withstand, not just the g-loads shown on a Vg diagram" -- with regard to the VG diagram posted above, would that be true or false?

FAR 25 defines a set of loads (not just g loads) that (nominally) may be expected once in the life of an airplane. These are limit loads. The structure must be demonstrated by test to be able to withstand 1.5 times these loads.

These limit loads are defined against structural design speed requirements Va,Vb,Vc and Vd - there are no such speeds as Vno and Vne in FAR 25.

Va,b,c,d speeds are defined in 25.335

The requirements associated with Vd/Md have to be demonstrated by flight test if Md is greater than 0.8M. FAR 25.629 defines an enlarged flight envelope beyond Vd/Md within which certain aeroelastic criteria must be met.

Obviously any margins required over the flight demonstrated limits have to be "proved" by calculation. The aircraft must shown to by calculation be free of flutter, control reversal etc. up to 1.15 Vd/Md (unless this speed would exceed Mach 1.0)

I see this 1.15 factor (was 1.2) as similar in concept to the 1.5 load factor of safety but of course it is a 'paper' margin where the 1.5 factor has to be physically demonstrated.

Hi Owain,

Thank you for your reply. Yes, I agree FAR 25 defines many different types of loads... but would speed be considered a "load" regulated by FAR 25.303?

I always understood that 1.5 factor of safety does not apply to the speeds, as stated by bigduke above.

Yes, I understand the Flutter Free margin (1.15 or 1.2VD) is only based on theoretical calculations and never demonstrated, and it is for equivalent airspeed at both constant mach and altitude (meaning no changes in static or dynamic pressure which might induce flutter at a lower speed).

But the VG envelope is not increased by such a margin due to the fact the VG includes G loads, and therefore would induce flutter above Vd. Correct?

Would I also be correct to say that the "Overspeed Margin" in the above VG diagram is the 10% margin between Vne and Vd as defined under FAR 23.1505?

Or is there some other margin above Vne in a VG diagram under FAR Part 23?

I think your mistake is assuming that all the values are calculated.
in practise they are not.

what far 23 is all about is setting strength standards that allow for foreseeable gust loads in typical operating environments.
this saves a designer from having to do laborious calculations of gust likelihood and effect.
design to the far strengths for each category and the loads will work out ok in practise.

Vne is 90% of whatever was demonstrated during test flying as Vd. Vd is actually reached in flight during the test. if flutter is not experienced at Vd then that sets the value of Vne.

in your diagram the outer edge of the white and green represents the "limit" load.
the outer edge of the yellow is the "ultimate" load.
as a designer you can expect all loads caused by the pilot to be within the limit load.
between the limit strength and the ultimate strength the pilot can expect damage to occur but not damage that would prove fatal.
the aircraft is typically designed to be just stronger than the calculated "ultimate" strength.

Yes, I understand. And in fact these are not my mistakes, rather they are the mistakes made by someone with which I have been having a discussion.

Basically, they are asserting that FAR 25.301, 303, and 305 regulate speeds as loads. And that the 1.5 safety factor defined under 25.303 pertains to speeds.

I have kept trying to tell them that the loads defined in FAR 25.301, 303 and 305 are not speeds, and are good up to Vd, which is defined in FAR 25.305.

For example, there isn't any margin of safety covering a Transport Category aircraft which is accelerating through Vd+50 while pulling 2-3.8 G's (limit load + 1.5 factor of safety).

The person I am having this discussion with seems to think such a maneuver is covered by FAR 25.301, 303, and 305.

Speed as such is not a 'load'. 2.5g at 250kts and a given weight/CG is the same load on the structure as 2.5g at 150 kts and that weight/CG (those speeds chosen to eliminate compressibility effects). The loadings are defined at specific speeds so speed cannot really be also subject to a 1.5 factor.

Not sure what you have in mind here. Equivalent airspeed defines dynamic pressure so Vd at any altitude gives the maximum dynamic pressure there. Flutter gets more likely as dynamic pressure increases so lower speeds are less critical.

I've never heard of anyone checking flutter at speeds above Vd/Md combined with pulling 'g'. The aircraft is of course cleared for flutter at Vd/Md and 1.5g (at least) as that is part of the Vdf/Mdf demonstration. The additional calculated margins are intended to make sure that there is no 'cliff edge' just beyond Vd/Md

Sorry, I thought from your OP that you were asking about Part 25 aircraft. I'm not sufficiently familiar with Part 23 to give you a reliable answer

Agreed.

FAR 25.629 says, "....enlarged at all points by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude."

Basically that tells me it applies to an aircraft which is not experiencing changes in altitude and airspeed which would induce flutter at a speed lower than 1.15 Vd.

The following I posted above your last post, but you may miss it since I need to wait for mod approval....

In short, I am having a discussion with someone who feels the Loads defined in FAR 25.301, the factor of Safety in FAR 25.303, and the Strength and Deformation definitions in FAR 25.305 will provide a margin of safety for an aircraft which hypothetically would be accelerating through Vd+50 and pulling upward of 2-3.8g.

I contend no such margin exists nor is defined by Part 25.301, 303 and 305.

Thanks again for all your replies.

They just don't build them like the used to
 

Read this NTSB report from the mid 70's regarding TWA flight 841. A 727 that fell from flight level 390. The old style flight recorder showed 6g and 470Kts at one point.

I wouldn't want to test that in an Airbus.

http://libraryonline.erau.edu/online...s/AAR81-08.pdf

Factor of Safety of 1.5
 

Bill,
This factor of safety is also used on the components and systems of the aircraft build-up.
Hydraulic pipes and cables are proof tested 1.5 times the normal operating value, wings are over bent to demine wing loading and fuselage pressure cycled such that when you fly your aircraft the whole 'component' has been well and truly proof tested to cover your normal flying plus.

Don't ask about the computers please......

Additionally, the margins required by regulations - whether for loads or for anything else - are not there to be used by someone who feels a bit "adventurous" today. They are there to cater for many circumstances, which include inadvertent excursions in 'g' or speed, but also include errors or uncertainties (especially the latter) in the underlying engineering. They also allow for simplification of the design cases, to a few cases with decent factors of safety, rather than many thousands of cases considering all kinds of combinations of manoeuvres and inputs. It would be a foolhardy man who took the margins in the regs and decided they were all available for use "on demand".

I know of a christen eagle that is aerobatted to ultimate strengths because limit strengths are too tame.

the aeroplane has required substantial rebuilds and has almost structurally failed in flight a number of times.

I know, its a FAR23 aircraft not FAR25 but there really are pilots out there who don't really understand all their knowledge.

the aeroplane has required substantial rebuilds and has almost structurally failed in flight a number of times.

.. why are we not surprised ?

The fellow involved ought not to buy lottery tickets .. his luck has been used up several times over in his flying experiences.

I am glad you brought this up, because 470 knots is 80 knots over Vmo for the aircraft involved, and it suffered severe structural damage.Well, that depends I suppose.Scroll down in the above source to see the video of flight testing certification out to the A380 Vd/Md.

FAR 25.301, 303 and 305 do not pertain a 1.5 Factor of Safety in relation to speeds. In other words, if Vd were 400 knots, the 1.5 Factor of Safety defined under FAR 25.303 would not offer a 'margin of safety' in speed of (hypothetically) 420, or, 450, or 500... or 600 knots (1.5 x Vd).

Is there anyone here who disagrees?

When considering 25.303, 25.301 above might be relevant as well.

I would not neither, but 6g alone does not mean anything for the load on the airframe, you need to know the weight of the airplane at that point in time. The certified g load is defined for maximum takeoff weight, at lower weight more g is possible, however 6g is quite a number.

Another extremely important aspect is, that the load is multiplied by a factor of 1.5, and not the material strength reduced by a factor of 1.5. What sounds equivalent at first, becomes relevant when we talk about stability (e.g. buckling) or internal loads of a deflected structure. An item loaded in bending and compression (e.g. members of an upper wing skin) sees more than 1.5 times the limit stress, when you load it to 1.5 times of limit load. The load stress relation becomes nonlinear if you take into account deformation, which occurs quite a bit on aircraft, as all of us have probably already seen.

at lower weight more g is possible

.. for those without Volume's background ... the bits in the aircraft and bolted to the tin are designed for specific restraint factors and don't have the weight variation consideration .. ie the wing might not fall off but the pilot might have bits and pieces floating about his/her ears.

Loss of Safety factor
 

The 1.5 safety factor is also generally applied to military aircraft.
One popular light attack jet had wing tip lights fastened by 6 screws, 3 on top of the wing, and 3 on the bottom of the wing.

Aircrews and maintenance workers did not generally understand that flying with one screw missing then provided NO safety factor for that particular piece of equipment. Aircraft would often lose these lights on flights where the maximum allowed g was being approached.

You bring up a good point.

At a lower weight, Va/Vra actually decreases. So, are you saying more G is possible at a decreased Vra due to a lower weight?

Or can one increase G loading at the same Va/Vra for a lower weight?