Ref. TLI (Total Life Index) 1

[Julio757]

Hi, I´m not familiar with the Total Life Index (TLI) concept. I know that if TLI reaches 2.0, the aircraft is permanently grounded; to adopt 1.75 as TLI limit is based upon acceptance of risk and probably of failure. I wonder if someone could tell me how to calculate the TLI for one aircraft ?
julio

 

[rb1957]

sounds like a variation of miner's rule, except of course the limit is 1.

 

[Julio757]

Thank you for your prompt response.

But in this case, would you know if is it possible to calculate the limit life of one aircraft with miner´s rule?

If I have an airplane that has 25.000 hs flight limit and I see that it is flying harder, is it possible for me to estimate in advance a new limit life?

 

[rb1957]

sure, if an a/c has a life limit of 25,000 hrs there should be a typical flight profile associated with this (may or may not be public knowledge). so i guess you can say that the plane has acculumated X hrs of typical usage. now you're anticipating a more severe usage, say 2x typical; then the number of hours available is (25000-X)/2.

 

[GregLocock]

What's the tradeoff of number of cycles vs stress?

Is n*s^5 =constant a reasonable first sight approximation for an aircraft?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Julio757]

Thank you for your prompt response.
and what du you think would be the best structural inspection to attain this reduce service life goals for the a/c? I put outer wing surface upper planks and center wing surface upper planks. Horizontal stabilizer. other ?
thanks

 

[rb1957]

that all depends on the plane. but the uuper wing planks are generally in compression .. the lower wing skins are more sensitive to fatigue.

is the fusealge pressurised ?

is the wing struted ?

 

[Julio757]

Thanks, the fuselage is pressurised and the wing is struted, and has four engines.

 

[rb1957]

so then the fuselage and the strut are fatigue critical structures, like the outer wing lower panels and the H. Stab. and they all have different loads, sprectra, fatigue details.

i'm trying to think of a pressurised, 4-engine plane with a strut wing ... nope, not getting any ...

 

[Julio757]

Many thanks again.
But not, forgive me, it is not an airplanes with external braces. I speak about a P3 a/c, which by his nature is very demanded in his maneuver of flight. So, I try to be attentive to the first indications of appearance of fissures. Because of it I wanted to see what might be that of initial damage and incorporate him into this zone an inspection.

 

[Julio757]

Also. the fuselage is pressurized from the forward bulkhead of the flight station to the aft bulkhead in the cabin.

 

[rb1957]

ok, a P3 has known fatigue limits. if you're trying to extend the life of a P3 there's Way more to be done ...
how does your mission compare with the assumed flight profile ?
talk to Lockheed; the operator should have a support agreement with them.

or are you trying to add something onto the airplane ??

 

[Julio757]

Thanks for your answer. The plane dropped its limit flight hours. For now, I try to inspect key points where fatigue can be initiated and to limit its operation. I contacted the company. What i know, the L-188 was flying with cracks controlled and an inspections plans. This should be similar. Points below the wing. If something started to happen should start to see loose rivets, oder ? The same for back, oder ?

 

[Julio757]

I don´t try to add something onto the airplane. The plane is light.

 

[rb1957]

ok, i think that you're trying to do a life extension program for a P3. you know something of fatigue, but i sense not much formal training or experience; forgive me if i'm wrong in this.

this is something that is Way to big to solve in a forum discussion.

if the palne has consistently been flying at a lower AUWt than Lockheed's assumed mission, then there is something to be gained for Most of the structure. the landing gear probably won't change. the fuselage probably won't gain much either, dominated by pressurisation.

Some military planes fly with known cracks, but only after Alot of analysis. They need to prove that the crack will still hold ultimate strength untill the next inspection.

Get some formal training in ASIP, DTA, fatigue, etc. Get some experienced engineers to work through the problem with you. talk to Lockheed. this is a big project, several manyears of effort.

 

[Julio757]

Thanks again for your response. Yes, you are right, i know but i´m not an specialist. Last week, I entered in touch with specialists. Please, what is AUWt ?

 

[rb1957]

All Up Weight ... take-off weight, maximum weight the a/c operates at (implied, somewhat less than the type's maximum).

word of caution ... if the "missing" weight is fuselage stores, then this is all good; the plane's structure is less stressed. if the "missing" weight is wing fuel then there's a "plus-minus" game being played ... most of the plane isn't less stressed, because wing fuel's weight is reacted by wing lift it comes along for free (as far as the structure is concerned), and if the fuselage is fully loaded then clearly there's no change here.

 

[Julio757]

Forgive me again, but how does it play the pressurization of cabin in all that? Surely there must be a level limit of flight for which the differential pressure affects less the effort for fatigue. It should be low. Is there a practical rule for this ? Thanks

 

[rb1957]

a fuselage is pressurised to maintain a comfortable environment in the cabin (about 8,000') whilst the plane is criusing at 36,000'. the fuselage is a pressure vessel (< 10psi), pressure causes tension stresses (hoop stress = pR/t, typically 12ksi); this is the dominant fatigue stress on the fuselage.

if the plane cruises at about 10,000' pressure stresses would be quite small (insignificant??) but there'd be a whole bunch of other issues (fuel burn, area surveyed, ...). this really doesn't matter, 'cause you're working on a plane that has been operating for several years (no?) and is approaching it's fatigue life, and you're looking for ways to extend it (no?).

 

[Julio757]

Yes, and these planes are very good but have many hours of flight. What do you try to say me with: " this really doesn't matter, ' cause you're working on to plane that are been operating for several years (not?) "Is there some advantage that the plane has operated for several years? Thanks again.