1.5 Factor Of Safety - FAR 25.303

[john_tullamarine]

Or can one increase G loading

no ... the bits bolted on are only designed to handle the certification G loads .. quite apart from the max G loads being certification limitations.

[Bill Serger]

no ... the bits bolted on are only designed to handle the certification G loads .. quite apart from the max G loads being certification limitations.

Agreed.



Thanks JT!

[Owain Glyndwr]

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?

No. The applicable FAR is:

(c) Design maneuvering speed VA. For VA, the following apply:
(1) VA may not be less than VS1 √n where—
(i) n is the limit positive maneuvering load factor at VC; and
(ii) VS1 is the stalling speed with flaps retracted.
(2) VA and VS must be evaluated at the design weight and altitude under consideration.
(3) VA need not be more than VC or the speed at which the positive CN max curve intersects the positive maneuver load factor line, whichever is less.

So unless the manufacturer chooses (3) Va is, by definition, the minimum speed at which you can pull 2.5g without stalling. If you define Va using (c) (1) and (2) then you cannot pull more G at a lower weight.

At the dive speed, excessive aircraft vibrations develop which put the aircraft structural integrity at stake. Source - VD/MD | The Flying Engineer

I'm afraid the Flying Engineer is writing for dramatic effect! All FAR25 aircraft have to demonstrate freedom from dangerous vibrations (flutter) in flight at VDf/Mdf and, as I wrote earlier, by calculation to well beyond that speed.

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".

Absolutely 100% agree !!!

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.

A quick check on the numbers says that the aircraft at the time of the incident referred to was at 130400 lb. MTOW for a B727 31 was, I think, 190,500 lb so the structure, even if it just met the FARs, would be good for 5.5g

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.

There are NO requirements relating to strength and deformation above Vd/Md so formally arguing about margins is meaningless. In practice, we agreed earlier that excluding compressibility effects the loads at a given weight/g are independent of airspeed in which case the margins above VD/Md would be the same as at Vd. But, if compressibility kicks in then the centre of pressure moves aft and at any given g one needs more download on the tail, and to get a given total aircraft g the loads on the wing will be increased, reducing the structural margins. OTOH, the increased tail loads from trim changes and from airspeed increases will increase the control hinge moments significantly, so depending on the size of the hydraulic jacks you may not have enough control power to pull 2.5g anyway.

[Bill Serger]

No.

Agreed... but I was actually not asking you. I was waiting for a reply from "Volume". But yours will suffice.

In short, a 'VG' moves to the 'left' as weight decreases.

I'm afraid the Flying Engineer is writing for dramatic effect!.

Have you seen the A380 Flight Test in the link provided?

There are NO requirements relating to strength and deformation above Vd/Md so formally arguing about margins is meaningless.

Agreed.... and this was the basis of my OP. You and others have already answered my questions in spades. I thank you.

[Genghis the Engineer]

There is no significant difference between the wording and requirements of parts 23 and 25, the only real difference is the level of rigour required by part 25 in the assessment, which is considerably greater. Also however, because weight is usually more customer-critical on part 25 aircraft they are usually designed as close as possible to the margins, whilst they may be larger on part 23 aircraft.


The 1.5 safety factor in paragraph 303 is the backstop requirement for the margin between limit loads (that which may be seen in service) and ultimate loads (what it's supposed to take for 3 seconds without failing for 3 second, but a failure after 4 seconds is acceptable in the standards). Other margins may be added in - for example for bolted joints, composite materials, loaded hinges - but the basic principle is always there.


And the principle is only partly related to either speed or normal acceleration. The safety factor is applied to the actual loads in the structure - which are inevitably complex and dependent upon multiple factors.

So within a wing, for example - there are loads in the mainspar due to weight and Nz, but these will be modified also by speed (because of drag and torsion loads acting aerodynamically), and also will be affected by the alleviating weight distribution of the fuel within the wing tanks - increased fuel in the tanks, particularly outboard, will decrease the stresses on the wing structure.

At high speed, the most significant loads on the wing are actually torsional, and yes the 1.5 factor is applied to the predicted torsional loads. But they are not linear with speed - they're roughly proportional to the square of speed. So, the simplistic factor is SQRT(1.5)=1.22. In practice there are more players than that, which include potential for instability, downwash, balancing and torsion loads on the tailplane (which will also be dependent upon CG position).


The only thing that you can really rely upon in either a part 25 or part 23 aeroplane, is that when the aeroplane was flight tested, it was taken to Vdf and Mdf - flight test diving speeds; above that for whatever criteria were used by the flight test and airworthiness teams, it was deemed unsafe to fly.

Vne is set not above 0.9Vdf, and Mne is set not above 0.9Mdf.

1/0.9=1.11 or 111%.


So basically your actual speed margin (in theory at-least) is another 11% on speed.

But that was a flight test team, in a new and instrumented aeroplane - you don't have either of those things to your favour.


So, there is a region between maximum speed and 11% above that which can be labelled "here live dragons", but until 1.11Vne (or 1.11Mne) you don't have a guarantee that the dragons will eat you. Above that, you can probably rely upon that happening.

[Bill Serger]

So, the simplistic factor is SQRT(1.5)=1.22.

So, are you saying that one could reasonably expect a margin of safety of 1.22*Vd, at ultimate G loading of 1.5 x limit load?

In other words, if one were flying a Utility Category aircraft with a Vne of 180 knots (.9 of Vd), a Vd of 200 knots, the pilot could expect to be safe at 244 knots (1.22*Vd) pulling 6.6 G's (1.5 * limit load)?

[dubbleyew eight]

bill a little conservatism here may save lives.
if you have a look at the thread about the ATR grounded at albury.
ATR expected the aircraft to be flown at 180 knots in turbulent weather descents.
it was actually flown at Vmo 230 knots.
probably all that saved the passengers was the absence of further turbulence after the catastrophic damage to the T tail had occurred.
the ATR is a FAR25 aircraft and maybe there was some surplus strength helping the gods. there was a lot of luck in that incident.

I wish you designers lots of luck like that.

an inflight breakup of one of your designs must permanently ruin your outlook on life.

(btw a pilot should expect Vd only. the rest is a lottery)

[Genghis the Engineer]

Originally Posted by Bill Serger

So, are you saying that one could reasonably expect a margin of safety of 1.22*Vd, at ultimate G loading of 1.5 x limit load?

In other words, if one were flying a Utility Category aircraft with a Vne of 180 knots (.9 of Vd), a Vd of 200 knots, the pilot could expect to be safe at 244 knots (1.22*Vd) pulling 6.6 G's (1.5 * limit load)?

In that ballpark, but there's a lot not amenable to analysis which has only been tested to Vdf, and also for example a bit of turbulence at that corner of the envelope can easily push the aircraft over the edge.

I wouldn't trust my life to it, but it would be interesting to test with a model.

[Gysbreght]

the pilot could expect to be safe at 244 knots (1.22*Vd) pulling 6.6 G's (1.5 * limit load)?

If he exceeds limit load but not ultimate load, he may expect to bend the airframe but not break it. A bent airframe must be repaired before further flight. If he exceeds ultimate load he may expect to break it.

[Bill Serger]

I think Genghis summed it up best here.....

So basically your actual speed margin (in theory at-least) is another 11% on speed.

... when referring to actual margins of safety with regard to speed... Vne - Vd.

Thanks all.