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DocLightning
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What is the exact definition of a "cycle?"

Fri Feb 15, 2019 6:09 am

If an a/c performs a missed approach and raises the gear, does that count as an extra cycle against the gear? Is an aborted takeoff counted as a cycle? Is a touch-and-go counted as a cycle?

I'm just curious.
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Yikes!
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Re: What is the exact definition of a "cycle?"

Fri Feb 15, 2019 6:19 am

A go-around without a landing is not a cycle. A circuit with at touchdown (landing) is considered a cycle.
 
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fr8mech
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Re: What is the exact definition of a "cycle?"

Fri Feb 15, 2019 6:34 am

Doc, normally, we count pressurization cycles, though I do believe, as Yikes! mentioned, each touch and go is considered a cycle.

Neither landing gear cycles nor engine cycles are counted separately from airframe cycles.
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DocLightning
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Re: What is the exact definition of a "cycle?"

Fri Feb 15, 2019 7:23 am

fr8mech wrote:
Doc, normally, we count pressurization cycles, though I do believe, as Yikes! mentioned, each touch and go is considered a cycle.

Neither landing gear cycles nor engine cycles are counted separately from airframe cycles.


So I know that most landing gear components age by cycle, rather than by FH. So if there's a go-around, the airline basically gets a "free" extension/retraction? (Which makes sense. Go-arounds are relatively rare with a given airframe performing one probably no more than 1-3x/yr and they don't put much stress on the gear.)
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747Whale
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Re: What is the exact definition of a "cycle?"

Fri Feb 15, 2019 10:11 am

There are different definitions of cycle. Pressurization cycles are specific to the airframe. We speak of landing gear cycle, in terms of "cycling the landing gear," meaning to raise or lower, but that's not a cycle for maintenance purposes. Aircraft are sometimes generally referred to by "cycles," which typically means a takeoff and landing," and engines have cycles, which are thermal cycles and spelled out in terms of what was done with the engine, and the temperature reached.

The actual terms of what constitute a cycle and what doesn't depends on the manufacturer. Most operators count one flight as one cycle, and typically log them by landings when calculating cycles for inspection or life component purposes. A go-around doesn't alter the calculation of the cycle.

Some manufacturers count partial cycle by fraction or percent; an engine start and shut down may be a thermal cycle for one manufacturer, or a tenth of a cycle for another. A cycle in general terms is a start, a power increase to a takeoff power setting and a shut down. In other words, a cycle from resting to operating temperature and a cooldown. It's the thermal cycle that puts the wear and life limit on the engine; it can be taken to operating temperature and kept there for 20 years (and some have been, continuously)...but the cycles of temperature changes are what wear out and kill engines.

Some equipment calculates pressure cycles apart from engine cycles (which maintain their own tracking, as engines don't stay with one airframe for their lifetime, necessarily), but most operators simply look at one cycle as a flight, takeoff to landing.

I recently did a rejected takeoff; the RTO is an engine cycle, but not an aircraft cycle and there was no landing or pressurization. The engine reached takeoff temperatures and the problem occurred on a max power takeoff, so was a full engine cycle, though not an airframe cycle.

To further complicate the matter, various accessories and components also have cycle life limits and inspection intervals; most of them are calculated together with the aircraft on the basis of cycles for the airframe; one takeoff to landing. Landing gear actuators, for example often have overhaul or inspection criteria based on cycles, and gear trunions, boxes, support structure, and the gear itself typically has a cycle inspection interval or overhaul interval.
 
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fr8mech
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Re: What is the exact definition of a "cycle?"

Fri Feb 15, 2019 11:41 am

747Whale wrote:
I recently did a rejected takeoff; the RTO is an engine cycle, but not an aircraft cycle and there was no landing or pressurization. The engine reached takeoff temperatures and the problem occurred on a max power takeoff, so was a full engine cycle, though not an airframe cycle.


Not us. Our engine cycles equal airframe cycles. Of course, as engines are replaced, the cycles follow the engine, but so far as our tracking goes one takeoff/landing = 1 airframe cycle = 1 engine cycle = 1 landing gear cycle.

I can do max power runs on an engine all day long, within the confines of the engine limitations and not click over 1 cycle under our program.

Like many things when it comes to maintenance programs, it depends on the operator and what he sold to the administrator.
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trijetsonly
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Re: What is the exact definition of a "cycle?"

Fri Feb 15, 2019 1:59 pm

We count incremental engine cycles based on thrust setting.
A derated takeoff with -15% for example counts as .68 or something engine cycles. It's quite some work to keep track but in times of big data it's the way to do it.
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Dalmd88
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Re: What is the exact definition of a "cycle?"

Fri Feb 15, 2019 2:52 pm

fr8mech wrote:
747Whale wrote:
I recently did a rejected takeoff; the RTO is an engine cycle, but not an aircraft cycle and there was no landing or pressurization. The engine reached takeoff temperatures and the problem occurred on a max power takeoff, so was a full engine cycle, though not an airframe cycle.


Not us. Our engine cycles equal airframe cycles. Of course, as engines are replaced, the cycles follow the engine, but so far as our tracking goes one takeoff/landing = 1 airframe cycle = 1 engine cycle = 1 landing gear cycle.

I can do max power runs on an engine all day long, within the confines of the engine limitations and not click over 1 cycle under our program.

Like many things when it comes to maintenance programs, it depends on the operator and what he sold to the administrator.

I've worked at three different airlines, including one major, plus a stint as a contractor of the ANG. All cycles on everything were airframe cycles. Engine runs to full power did not count for anything,
 
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DocLightning
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Re: What is the exact definition of a "cycle?"

Sat Feb 16, 2019 12:01 am

747Whale wrote:
There are different definitions of cycle.


Thank you very much for your comprehensive answer. I had wondered if the industry hadn't just agreed on one simplified definition and accepted that variances to specific components will occur, or whether it was a complex and nuanced question. It appears to be the latter.
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stratclub
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Re: What is the exact definition of a "cycle?"

Sat Feb 16, 2019 1:34 am

Yup. Cycle of _______________ appears to be the correct answer.
 
stephanwintner
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Re: What is the exact definition of a "cycle?"

Mon Feb 18, 2019 8:27 pm

Dalmd88 wrote:
I've worked at three different airlines, including one major, plus a stint as a contractor of the ANG. All cycles on everything were airframe cycles. Engine runs to full power did not count for anything,


Speaking an a gas turbine engineer, currently working on ground power turbines, they ought too ! ;) As noted by 747Whale, it's the thermal cycle on the engine that matters. It might not have been logged, under your logging system, but it really should be. Derated takeoffs can count for less than a full cycle, short flights can count for less than long ones, and it gets very complex when we speak of military jets - a long, gentle cruise flight with no augmentation can count for much less damage than even a brief one with augmentation. For some components, dwell time (e.g. at cruise) is the driving factor (creep), rather than fatigue (cycles). But, LCF and creep interact, so you can get a nice complex mix.

One of the reasons the USAF eventually got GE to build engines for the F15 was that they were racking up some very tough cycles on the engines, and Pratt had not been told to design for the kind of cycles the air force was actually flying. Pratt and the USAF couldn't agree on fixing that oversight.
 
Yflyer
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Re: What is the exact definition of a "cycle?"

Mon Feb 18, 2019 9:41 pm

747Whale wrote:
Most operators count one flight as one cycle, and typically log them by landings when calculating cycles for inspection or life component purposes. A go-around doesn't alter the calculation of the cycle.


One thing I've always wondered is if a flight stays below 10,000 feet and is never pressurized, does that count as an airframe cycle? Like say a very short ferry flight from OAK to SFO or something like that. From this it sounds like for most operators one flight = one cycle, regardless of altitude.
 
747Whale
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Re: What is the exact definition of a "cycle?"

Tue Feb 19, 2019 12:20 am

Again, cycle has a number of definitions, applicable to different components. If the flight stays below 10,000' and doesn't pressurize, (though most would pressurize regardless), it's still going to have a landing, which is also a type of cycle. Generally engines have landings, and airframes have cycles. An airframe that doesn't pressurize lacks some of the rigidity that comes from pressurizing, and consequently the strength, and may be subject to twisting and flex in flight that it might not otherwise experience; it may experience greater stresses, depending on the nature of the flight.

We use large airplanes doing firefighting, that don't pressurize on most missions; they still log landings and cycles.
 
stephanwintner
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Re: What is the exact definition of a "cycle?"

Tue Feb 19, 2019 4:59 pm

747Whale wrote:
Again, cycle has a number of definitions, applicable to different components. If the flight stays below 10,000' and doesn't pressurize, (though most would pressurize regardless), it's still going to have a landing, which is also a type of cycle. Generally engines have landings, and airframes have cycles. An airframe that doesn't pressurize lacks some of the rigidity that comes from pressurizing, and consequently the strength, and may be subject to twisting and flex in flight that it might not otherwise experience; it may experience greater stresses, depending on the nature of the flight.

We use large airplanes doing firefighting, that don't pressurize on most missions; they still log landings and cycles.


Oh, good point. I didn't mention that a lot of the stress on an engine comes from the actual takeoff rotation and landing - maneuvers, along with hot restarts and perhaps turbulence, is what rubs out the seals. While that type of "cycle" won't likely limit the life of the engine, it will cause performance losses, and raise temperatures throughout the engine. Maneuver loads in a high-G fighter have the same effect, but are of course pretty challenging.
 
747Whale
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Re: What is the exact definition of a "cycle?"

Tue Feb 19, 2019 5:21 pm

My previous reply should contain a correction but I can't adjust it, so will here:

Generally engines have cycles, and airframes have landings.

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