Is there a legal definition of a "cycle" on a jet engine?

We must log the cycles, and some maintenance is determined by cycles. From my understanding, this is partially because of the thermal dynamics of an engine cooling and then reheating, and partially because full takeoff power is used.

The "usual" time that you log a cycle is when an engine is started and the aircraft then takes off (using full rated takeoff power), but what about unusual situations like:

  • Engine shutdown and restarted in flight
  • Engine started, aircraft takes off, and then returns for a low pass or a touch and go: Would this be two cycles (does it depend on the amount of power used during the touch and go?)?
  • Engine started and then shut down without a flight
  • 1
    Not "legal definition", but this might help – SSumner Mar 4 '14 at 19:15
  • Im tempted to say the engine start/shutdown without flight isnt a cycle since we routinely did this when delayed but never logged it anywhere. Unless ACARS was taking care of that Im not sure MX would be able to track that. – casey Mar 4 '14 at 20:59
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    @SSumner: No, that's altogether different meaning of a cycle. – Jan Hudec Mar 4 '14 at 21:36
  • @JanHudec - hmmm, didn't know that – SSumner Mar 4 '14 at 23:00
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    Different engine manufacturers will probably have different definitions of a cycle. If you are asking for the meaning in the context of maintenance tracking, then you will definitely want to use the OEM's definition of a cycle. – ra9r Jun 9 at 18:07
up vote 3 down vote accepted

According to the FAA in AC33.70-1:

(b) The applicant should validate and maintain the accuracy of the engine flight cycle over the life of the design. The extent of the validation depends on the approach taken in the development of the engine flight cycle. For example, a conservative flight cycle where all the variables are placed at the most life damaging value would require minimum validation. A flight cycle that attempts to accurately represent the actual flight profile, but is inherently less conservative, would require more extensive validation. Applicants may apply further refinements to the engine flight cycle when significant field operational data is obtained.

A cycle is a start to a shutdown. Lets say there is a flight that is loaded with pax, bags, and fuel. They push back and start both engines since the weather is good and their at an outstation. As they taxi to the departure runway, BAM, ground calls up saying there is a groundstop for the hub and its going to be about 30 minutes. The flight pulls into some empty ramps space, and shuts down both engines. Groundstop lifts, both engines started again, and the flight departs. After landing, pulling into the gate, and shutting down the engines, we can say that for this flight, each engine went through two cycles. ALL of this info is logged and maintenance can access it. With some of the newer engines and higher service packages, OEM

  • 2
    Hi, thanks for the answer. Do you have a reference for this definition? Like I asked, I am looking for the regulation (or other official source) which covers this. I was always told that you had to takeoff for it to be a cycle too. – Lnafziger Mar 7 '14 at 2:29
  • I don't have anything official, unfortunately, as its from my experiences. However, I believe parts 33 and 43 might yield some answers. I'll dig into it when I get home. – h54 Mar 7 '14 at 2:47
  • For what it's worth, the operators I've flown for logged cycle exactly as h54 describes. I have nothing official either. Maybe try calling or emailing your local FSDO (if in US). – acpilot Aug 14 '16 at 14:04

I am an Aviation Maintenance student. The way my instructor explained it to me was: any time that you exceed 70% RPM, you have just run an engine cycle. It is more about acknowledging that the engines reached a certain temperature and was exposed to stresses that maintenance should know about. Different engine manufacturers will probably have different definitions of a cycle.

  • Based on which RPM? Should I count another cycle every time I bring the engine up above 70% in a flight? Or only after a shutdown-restart? – J Walters Nov 5 '16 at 15:44

Engine flight cycle. The flight profile or combination of profiles on which the approved life is based.


To establish a safe life, the applicant needs to establish an appropriate flight profile (or combination of profiles) and consider the expected range of ambient conditions and operational variations to determine the service environment. The engine flight cycles should include the various flight segments that describe a complete flight (or flights). For example, for fixed wing aircraft applications this may include segments such as start, idle, taxi, takeoff, climb, cruise, approach, landing, thrust reverse and shutdown.

AC No: AC 33.70-1

The cycle is one takeoff and landing , meaning engine start,climb,cruise,landing and shutdown. As far as the engine is started in ground and shutdown it is not considered as a cycle since no Aerodynamic forces/Loads are acting e.g. when engine is tested in Test cell it is not considered as a cycle. In the case of IFSD it will be considered as a cycle as the engine went through most of the phases of a cycle.

Refer to the engine manufacture they have received approval from the FAA for their engines. On a TFE731 one engine cycle is one take off and landing which can be found in the LMM.

protected by ymb1 Aug 27 '17 at 21:47

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