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Fatigue analysis and GAG cycles

Fatigue analysis and GAG cycles

Fatigue analysis and GAG cycles

I'm frequently asked to evaluate the impact of a significant change to the flight spectrum that an aircraft was designed to. An example I'm currently working on is a large corporate jet type aircraft that normally has a GAG cycle of TO, climb to between FL300 to FL450, cruise for several hours, descend and land. This aircraft is being repurposed and will now take off, fly at some intermediate altitude for positioning, then descend to 1500' and fly there for several hours, then climb for the return flight, descend and land. In this case, the pressure cycles most likely won't be the issue, but fatigue to other aircraft structures (wings, empennage . . . ) will be impacted due to increased gust loads at low altitude.

The other issue is bird impact issues with driving around for significant times down low and the increased risk of bird impact to the windshield and liberating glass into the cockpit.

I've got a copy of "Fatigue evaluation of wing and associated structures on small airplanes" dated 1973 from FAA where they talk about usage spectrum and it's impact on the structure, and the "standard" spectrum used for different types of aircraft during certification. I was wondering if there might be a similar document that identifies the different spectra to be used for transport aircraft.

RE: Fatigue analysis and GAG cycles

There is quite a bit of useful data for transport category aircraft. Try searching for "UDRI statistical loads data". That should get you started on the right path. But keep in mind that statistical data is not for your flight profile.

Also, "Repair Assessment Procedure and Integrated Design" Version 2.1 (search for that too) Appendices C and D might be helpful.

Sounds to me like the main difference is that your aircraft has an additional cruise-like phase of flight. It might be possible to calculate two different GAG damage ratios (GAGDR) - one for a statistically known flight, and one for your flight. You would have to find how your flight mission changes the exceedences due to maneuver and gusts. You basically have a (slightly?) longer overall cruise, with part of it spent at lower altitude.

You could make a ratio of the the GAGDRs which would basically quantify how much more damaging your flight is than a typical transport flight. A problem you will run into is that the effect will be worse for a "large corporate jet" than something even like a CRJ, because they are lighter and a gust will create a higher exceedence etc. So you may need to compute two spectra with the weight / distribution / aerodynamic data for your aircraft. You might consider using this ratio to factor up your stresses and proceed with your DTA?

Keep em' Flying
//Fight Corrosion!

RE: Fatigue analysis and GAG cycles

Sounds like the two positioning up-downs before and after the middle of the mission may be a bigger effect than you realize.

Bird strike on the windshield (25.775, if you have to comply) is a one-time discrete event, so while the mission may statistically be "in the muck" for more time, the original certification may cover it unless the mission profile is at a faster speed. Also don't forget bird strike on the engines and leading edges, and nose too if there are any critical systems potentially affected.

The references below do have some varying degree of breakdown by flight phase, and the document you reference (as well as its modern counterpart AC23-13A) could probably be used to parse out taxi/climb/crop dusting phases of flight. Number 13 below is rather recent and could be of particular interest.

1. Cavage, W.M., DeFiore, T., and Barnes, T., “An Analysis of Ground-Flight Loads Measured on the Instrumented B-727 N40,” FAA report DOT/FAA/AR-95/82, October 1995.
2. Skinn, D., Miedlar, P., and Kelly, L., “Flight Loads Data for a Boeing 737-400 in Commercial Operation,” FAA report DOT/FAA/AR-95/21, April 1996.
3. Rustenburg, J., Skinn, D.A., and Tipps, D.O., “Statistical Loads Data for Boeing 737-400 Aircraft in Commercial Operations,” FAA report DOT/FAA/AR-98/28, August 1998.
4. Rustenburg, J.W., Skinn, D.A., and Tipps, D.O., “Statistical Loads Data for the Airbus A-320 Aircraft in Commercial Operations,” FAA report DOT/FAA/AR-02/35, April 2002.
5. Jones, T., Rustenburg, J.W., Skinn, D.A., Tipps, D.O., and DeFiore, T., “Statistical Data for the Boeing-747-400 Aircraft in Commercial Operations,” FAA report DOT/FAA/AR- 04/44, January 2005.
6. Tipps, D.O., Skinn, D.A., Rustenburg, J.W., and Zeiler, T.A., “Statistical Loads Data for BE-1900D Aircraft in Commuter Operations,” FAA report DOT/FAA/AR-00/11, April 2000.
7. Rustenburg, J.W., Skinn, D.A., and Tipps, D.O., “An Evaluation of Methods to Separate Maneuver and Gust Load Factors From Measured Acceleration Time Histories,” FAA report DOT/FAA/AR-99/14, April 1999.
8. Tipps, D.O., Skinn, D.A., Rustenburg, J.W., Jones, T., and Harris, D.A., “Statistical Loads Data for the Boeing 777-200ER Aircraft in Commercial Operations,” FAA report DOT/FAA/AR-06/11, November 2006.
9. Rustenburg, J.W., Skinn, D.A., and Tipps, D.O., “Statistical Loads Data for Bombardier CRJ100 Aircraft in Commercial Operations,” FAA report DOT/FAA/AR-03/44, June 2003.
10. Jones, T., Rustenburg, J.W., Skinn, D.A., Tipps, D.O., and DeFiore, T., “Statistical Loads Data for the Embraer-145XR Aircraft in Commercial Operations,” FAA report DOT/FAA/AR-07/61, November 2007.
11. Statistical Loads Data for the Airbus A-320 Aircraft in Commercial ...
12. Statistical Loads Data for Cessna 172 Aircraft Using the Aircraft ...
13. DOT/FAA/TC-16/10, Flight Loads Analysis of Business Jets, March 2017.

RE: Fatigue analysis and GAG cycles

Thanks for the replies. I agree that the aircraft already meets the regulatory requirements, but flying low level will place the airplane at greater risk for bird strike, and the 25.775 requirement is only for a 4 lb bird at Vc, and only withstand that without penetration. Studies have found much larger birds, and the requirement doesn't talk about liberating glass shards into the cockpit. The risk of injury to the crew will be greater while flying low altitude due to the greater number of birds. That's one of the things I'm trying to quantify.

As it relates to fatigue, with the small airplane document I referenced, an aircraft performing aerial survey would be accumulating fatigue at almost twice the rate as the same airplane flying a normal profile.

RE: Fatigue analysis and GAG cycles

dgapilot... war story, this topic.

A GA company proposed a small twin corporate jet for student pilot's transitioning to transport acft for use starting in the late 1980s[?] as a contracted-service. Their standing experience with jet-trainer/light-attack made for a VERY unique/deep experience-base in all 'real-world' aspects of jet-trainers RE design/durability, performance, support, logistics, etc.

The mission included 'typical' student training [TO/LD, high-altitude all-weather-IFR, etc]... and also had to include 'high-speed low-level maneuver' training. When analyzing their proposed all-metal corporate jet, the 'high-speed low-level maneuver' training was the long-pole in the tent. Fatigue durability [high speed, abrupt maneuvers, gusts, etc], bird-strike and weather threats [hail, freezing rain, extreme dust/sand/debris, etc] drove their analysis. Conclusion: the design was basically sound... but needed significant fatigue and impact/erosion durability improvements.

Primary aluminum structural components were slightly 'beefed-up', shot-peening and fretting-resistant finishes were applied, fatigue-critical holes were cold worked, all-other fastener holes carefully sized for interference... and some joints were also adhesive bonded for added fail-safety... etc. The leading edges [radome/windshield, wing/stab leading edges and the engine inlet-lips] were 'improved' for impact strike/wear resistance. The twin engines [considered redundant, already] also had minor compressor improvements for impact/erosion.

The company was awarded the contract and produce the fleet to meet these tough criteria. These aircraft went into service and worked well for several-years. The in-production improvements to these jets made for a 'unique' fleet of 'mission specific' acft that instructors, students and the maintainers really liked... especially after surviving major bird-strikes and massive hail-damage several times. The only known criticism was a 'very rough ride' in extreme gusts due to the 'long-straight wings'.

When the initial training contact ended it was abruptly 'not-renewed'... but was [re]awarded to another company [Brand-X, a favorite of politicians, a war-story in-and-on-itself].

The company put-in a lot of experience/intellect to 'get-it-right the first time'... and from what I've heard [later-on], Brand-X had to re-learn many of these lessons/elements 'the hard-way'. OH WELL.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Fatigue analysis and GAG cycles

I'd've thought that bird strike requirements would be unchanged from the original type. Ok, if the mission now has a low level phase there is more opportunity for a bird strike which I see as an operational issue (rather than a certification one) ... ie if the plane had a bird strike mid-mission, would it be able to return to base ?

but the new mission will have a much higher mission severity than the original OEM mission. As mentioned above 2 pressure cycles per mission, 2 GAG if you will, plus extra gust damage from low level search. You can estimate this analytically (ESDU 69023) or you can get loads monitoring to estimate the effect. Note different parts of the plane are affected differently (fuselage mostly pressure, wing and empennage mostly gusts, flaps and LG not at all).

In addition to DER810's references I have dot-faa ar05-35 (fire-fighting loads) and ar11-7 (tanker manoeuvre loads).

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

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