Aircraft Fatigue Life

RAAF fatique life

Bager1968 since you asked……

I read a very interesting paper out of Oz about this, 6 months ago. I had a look on the net and have found it again. It looks at the various methods used to calculate fatigue.
I also located an article from the 50’s to show that a few things have changed over time.

http://www.flightglobal.com/pdfarchive/view/1955/1955%20-%200347.html

http://www.dsto.defence.gov.au/publications/2796/DSTO-159.pdf

The US actually uses a hybrid hours/FI method… certain events (g-forces above a certain level, flight through turbulence above a certain level, etc) add extra “hours” to accumulated “flight hours”.

A very “eventful” flight of 2 hours duration can actually add many more hours to the airframe’s official total.

The DH/BAe Trident had a life of 22,000 cycles/landings which seems fairly low. I seem to remember it was a contractual figure used during the initial purchase contracts, expected to rise if the world fleet size had been large enough, then BAe would have seen value in looking at life extension programmes, but that never happened.
Some small extension was allowed though if certain ‘critical’ components were replaced, one of these being the flap track beams, which were ‘I’ section and made of titanium alloy.
BA had wanted to keep a small number of Trident 2 in service as Shuttle backups so placed an order on BAe for a batch of beams, with a lead time in many months, unfortunately after a long wait a mistake was made in the machining and most were scrap. This brought foreward the earlier than planned retirement of Trident 2 fleet.
The Trident 3B had already suffered wing cracking and had been fitted with reinforcing plates and re rigged ailerons so had no life extension potential.

Richard

To follow on from the previous post, FI is most commonly calculated from fatigue testing of the airframe assembly which is tested to destruction. That’s where the number crunching comes in and the designer will apply a Factor of Safety (FOS)to the component life. If a structure fails at 18,000 repetitions of applied stress a FOS (commonly 3) would be applied to give a ‘safe’ life of 6000 repetitions.

For Military aircraft the means of operation can accrue more FI depending on the operating environment. A good example of this is the Valiant which was designed to operate at high altitudes with a more benign flight regime and suffered spar fatigue problems when switched to low level operations, where more violent manouevres were needed.

well firstly the term “fatigue” relates to accumulated cyclic damage sustained during the course of normal operation which will evetually cause an item to fail, Aluminium structures are prone to fatigue whereas steel structures are less so, and below certain thresholds do not accumulate fatigue damage at all.

Aircraft fatigue can be calculated in a number of ways, the two most commonly used are Fatigue Index (FI) in the case of aircraft equipped with apparatus to measure the forces going through its structure, and Flight Hours, in the case of aircraft not subjected to high stress manouevering loads such as airliners although some military types are covered by FH.

FI for an airframe is calculated during destructive static testing of one or more airframes to gain a Fatigue Value for the airframe type, the critical componenet of the airframe which exhibits the lowest fatigue tolerance will generally be used as the yardstick for that type’s total FI, which is set at 100FI, the fatigue measuring apparatus is then calibrated for the airframe and installed, it will monitor G-loadings, shock loads, ect and display a cumulative total of FI units consumed, the aircraft will then be determined as life expired or “lifex” when this value reaches 100 unless the aircraft has been modified to allow a higher total of FI.

Aircraft lifed by flight hours will generally have airframe stress measuring apparatus but will be flown to a pre-determined houred lifespan, the testing process to acquire this hour number is much the same as for aircraft being FI lifed, with the hourage at first failure of a critical airframe componenet being accepted as the maximum houred lifespan, as an example the A320 Airbus series is lifed to 85,000 flight hours, once an A320 aircraft reaches that total it is considered lifex and can no longer be safely flown, it seems generally that US military aircraft manufacturers tend to use houred lives instead of FI lives, the F-15E Eagle having an houred life of 15,000 flight hours and a B-52 37,500

A number of factors.
Cycles (take off and landing), touch and go’s, weight, pressurisations, ‘G” loading, flying hours and any number of things I can’t think of off the top of my head.