When to start descend from High Altitude Jet route?

[Jonai Chui]

I am wondering when jet liners should start to descend from high altitude jet route.
Is US, the high altitude jet route should be flown above FL180.
When pilots need to descend for landing, can you descend to below 18,000ft on jet route or need to transit to low altitude "V" route at 18,000ft before further descending?

[NacelleStrake]

3x your altitude is the general rule. Therefore an A/C cruising at 350 would start it's descent at approximately 105 nautical miles from their destination...

[Jonai Chui]

Thanks for reply, NacelleStrake.

But my question here is if pilot can descend below FL180 on J routes.
As you say, pilots use 3X rule. Likely they reach FL180 at 54 miles from the destination on Jet route, as they have been flying above FL180.
Now can they descend further on Jet route? or need to maintain FL180 until transit to low altitude route?

[Intruder]

The quick answer is "yes".

When descending for an arrival at the airport, ATC does not usually delineate between the J route and any underlying route (or lack thereof). The routing will be as previously cleared, unless changed. Changes will normally be via a STAR or vectors, and only occasionally via a V route.

[flarepilot]

when you are below FL180 you may get a clearance for a victor airway, the radials of an airway, or as mentioned a standard terminal arrival route(STAR) .

Let' say there is a jet airway you are cleared on and you are cleared to descend to 17,000'. while the jet airway no longer exists, you may be cleared VIA THE RADIALS OF JET AIRWAY you were still on. Or an underlying victor airway.

conversely you may be cleared via a victor airway and when at or above FL180 you may be cleared via an overlying jet air way or the radials of the victor airway.

it is a good technical point and if cleared via a J route below Fl180 or a victor airway above FL180, you should clarify it with ATC...they might chuckle and then clarify...but be sure YOU know which way you are going