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A flight leg is basically flight from one point to another point, (I'm not sure if flight leg number increase with touch and go around). I would like to know more about flight leg under what all conditions usually it increases and when it is reset.

Also, most important why is it so important for an aircraft/airlines because usually it is 1 or 2 flight leg per travel. From what I know, flight leg is most important parameter in case of recording or storing data in NVM but I believe flight phase should be sufficient enough.

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    $\begingroup$ What do you mean by "NVM"? This TLA does not seem to have any ASM. $\endgroup$ – Jan Hudec Jun 2 '14 at 8:00
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    $\begingroup$ @JanHudec NVM = Non-Volatile Memory; it's a computer/avionics term referring to memory where configuration, settings, maintenance and other data can be stored persistently. $\endgroup$ – TypeIA Jun 2 '14 at 13:20
  • $\begingroup$ If I understand it right, a flight leg is just one part of a flight cycle - the time between take off and landing. Correct? Therefore the number of flight legs shall be equal to the number of flight cycles during the lifetime of an A/C. $\endgroup$ – Hristo Oct 29 '14 at 9:38
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With respect to avionics and fault logs, flight legs is just one more data point. It's not any more important than any other piece of data, but it does have value. Avionics are not structure and don't care as much about pressurization cycles, but it can be a factor - the pressure change between sea level and 8000 cabin altitude does have some effect. It's just that avionics can easily count flight legs and it makes for an easy way to organize the fault logs along with flight phase.

It is useful data when trying to find the root cause of an intermittent fault in a piece of avionics. Fault logs typically log at least 20 legs before they "roll over" and overwrite old data. With some aircraft flying 6 or more legs a day, that ensures the plane can get to a base with maintenance before the fault gets overwritten.

The value is the ability to help detect patterns. I've seen faults that only occur on the first leg of the day after the plane was parked overnight at a very humid airport. Re-creating the conditions allowed us to duplicate the fault. The problem was a flaw in the conformal coating (sealant) on a circuit card that allowed moisture to cause a short. Once the unit had operated for an hour or so, the heat had dried it out and it worked fine.

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I don't think flight leg is important for either aircraft no airline.

The reasons why most flights have one or two legs is purely economic. Aircraft have to be operated with rather high load factor to be profitable. If you have a line with less passengers, you really want to use smaller plane or run it less often. Because different links have different demand, it's rather rare to have many consecutive links with similar demand so it would make sense to fly them with one plane as multi-leg flight. And few people would want to fly the whole trip as the stop-overs take rather long.

For aircraft what matters is not legs, but cycles, meaning pressurization cycles. For pressurized aircraft the material fatigue is most closely related to the cycles and since each cycle also involves one period of running the engines at high thrust, the engine wear is also somewhat related. It does not matter whether the aircraft accumulates 4 cycles in 4-leg journey or 4 separate journeys.

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    $\begingroup$ Yes from Material Fatigue point of view, aircraft maintenance are more concerned about pressurization cycles. But from other point of view like investigation, analysis etc would like to know flight leg importance. For example primary flight control computer (part of Flight Control System) while storing faults in memory it stores date, time, flight no., flight phase, flight leg etc. And also special importance is to flight leg, whenever flight leg increase base address of memory is increased for fault storage so that faults can be grouped per flight leg. $\endgroup$ – ToUsIf Jun 26 '14 at 9:34
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what is flight leg?

A flight leg is a trip of an aircraft, from take-off to landing.

Use of the term implies a multi-leg flight.

From the point of view of an aircraft's operator, it is a smaller part of an overall journey which involves landing at an intermediate airport. Each leg starts and ends at a different airport.

From the point of view of a passenger it is a part of a journey that may start or end with a stopover, a change of plane or a change of airline.

why is it important for an aircraft

It isn't really. See Jan's answer regarding aircraft cycles.

It is obviously important administratively for scheduling and financially for optimizing return on investment etc.

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