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Author Topic: LNAV smart turn limitation  (Read 196 times)

Blake Hauswirth

  • Join date: May 2011
  • Location: YSSY and YPPH
  • Posts: 545
    • http://www.747flightsensation.com.au
LNAV smart turn limitation
« on: Mon, 20 Jan 2020 00:50 »
Hi Guys for the eggheads here.

I was in LNAV over China the other other day.

Airspace is RNAV2
35000 feet or so. So maybe 480 kts
Jet stream 150 kts
90 degree change of course at the waypoint

I noticed the aircraft started to turn well early, intuitively I slowed down 50kts, so I wouldn’t clip the RNAV 2 limiting corner on the inside.

There must be a formula for this scenario. I think the bank is limited to 25 degrees. So if your GS is super fast you might not stay within the airway.

Any idea how to work this out, maybe someone can share a formula. Might be a good exercise in Aerowinx.

Cheers,

Blake


emerydc8

  • Join date: Jun 2015
  • Location: Tucson, AZ USA
  • Posts: 1877
Re: LNAV smart turn limitation
« Reply #1 on: Mon, 20 Jan 2020 03:30 »
Hi Blake. There is a formula based on G/S and DME that we used in the 747 classic to lead the turn when making the left turn toward Antalya, Turkey while over the Med because it was about a 120-degree turn and it exceeded the capability of the Carousel IV. IIRC we had to do it in turn-knob because HDG SEL just wasn't enough bank. I've long forgotten the formula, but I'm sure you can find it if you dig hard enough. With the Boeing LNAV, I would think it would be safer just to leave it coupled and let LNAV do its thing. Probably most everyone else flying that route before you did the same, so even if it's outside the limit, there's "safety in numbers" so to speak and ATC probably expects you to let LNAV do it anyway.
Jon

jcomm

  • Join date: Dec 2011
  • Location: Portugal
  • Posts: 1327
Re: LNAV smart turn limitation
« Reply #2 on: Mon, 20 Jan 2020 07:59 »
But, do those restrictions apply also for cruise ?
Thought it was all about "lower level maneuvering" ( approaches, departures and the like ... ) ?