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If an airliner (enroute via IFR rules) discovers a thunderstorm ahead, how much lateral deviation is allowed before a clearance from ATC needs to be obtained ? Is the width of an RNAV corridor only for navigation errors or also for intended deviations ?

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    $\begingroup$ I assume this question springs from the Delta incident over China. RNAV routes have various widths, China has 20 km if I was informed correctly. That doesn't allow a pilot to fly 10km of the centreline, there is uncertainty in the navigation to be taken into account as well. $\endgroup$ – DeltaLima Jul 1 '15 at 15:28
  • $\begingroup$ Are you asking about a specific country or set of regulations, e.g. FAA, EASA, other? $\endgroup$ – Pondlife Jul 25 '18 at 19:48
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4 nm from centerline or 8nm total width of airway. You are not required to instantly turn when transitioning between airways. Required navigational precision for most enroute segments is 2 nm. Being conservative and always assuming an operational error of 2nd means any observed deviation of more than 2 nm is a red flag.

Intentionally deviating within the course width is not prescribed within the regulations, but then there's no regulation prohibiting or allowing fly by (fly over is sometimes mandated).

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If the aircraft is under ATC control then no deviation is allowed except in an emergency. A pilot who wants a course change would need to request it from ATC and obtain a clearance. If the flight is not under ATC control and wants to divert then it's pilot's discretion, he/she would notify ATC of the change if receiving some sort of service.

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    $\begingroup$ IFR automatically means ATC control, does it not? $\endgroup$ – FreeMan Jul 1 '15 at 12:41
  • $\begingroup$ More or less. The aircraft could be flying VFR-on-top in which case altitude changes within the predefined range do not require ATC approval (the plane already has it) but do require VFR-style look-and-avoid. $\endgroup$ – KeithS Jul 1 '15 at 15:52
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    $\begingroup$ @FreeMan Not in the middle of the Pacific! $\endgroup$ – Simon Jul 1 '15 at 18:17

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