Sometimes in the airliners I'm flying I might have a tight turn shortly after TO and I'm unable to stay on the magenta line w/o over-banking so I'll overshoot the line some.  I'm keeping airspeed down but maybe not enough.  We're using very short range for example 2.5mi, whereas at 10 or 20mi I might not notice the deviation off the line.  What's considered too much off the magenta line?  And of course referring to hand-flying up to 10K or what have you.

Edited October 30, 20241 yr by Noel

Depends on where you are and what your doing of course, at cruise deviations aren’t as noticeable as on SIDS or even more crucially approaches.

More modern jets will have  analog NPS navigation performance scales on the PFD that show the accuracy of the aircraft’s navigation, along with a digital display of RNP and ANP - required and actual navigation performance. 
 

These are the things we keep an eye on more nowadays than the magenta line as such, bearing in mind that your pink string is just an FMC generated line, it’s not guaranteed to be accurate to what you’re meant to be doing. If the aircraft is being GPS spoofed for example you may be right on the pink string on your displays but the aircraft isn’t actually where it’s meant to be or thinks it is, it might suddenly realise its mistake and suddenly put your pink string a mile to your left or right, the so called map shift.

To your actual question it’s hard to put a figure to how far from the pink string you can go, as I mentioned it really depends on your phase of flight.

My current operator’s SOP (Boeing)is to set the navigation range to 20nm for departures, so you won’t notice small deviations.

One real world story for you which may be of interest to you regarding your question, I was flying a heavy 747 out of Atlanta one night on the PLMMR RNAV departure. I got a polite call from departure radar who obviously monitor the RNAV departures on a high resolution radar, to clarify I was tracking the departure ok as I appeared to be going slightly wide on the turns, which are close in 90 degree turns on this particular departure, and have also a requirement to accelerate to and maintain 250kts ,following SOP the autopilot had been engaged for the RNAV departure.

Now, according to our pink string on the NDs we were right on track, exactly where we should have been but the controller could see a slight deviation creeping in.

Hardcore sim pilots of the PMDG 747 or PSX , might possibly have an idea of what was happening here ?……..well, the clue was with us being heavy that night, probably up around 330T plus.

The 747 when above 309,000 KG will automatically limit its bank angle to 15 degrees until reaching a speed of flaps up bug plus 20kts. This happens automatically with the autopilot in or if hand flying it’s reflected in the flight director commands.

So, although ATC weren’t necessarily saying we were non compliant, we had obviously drifted off from the track depicted on their high resolution radar for the departure and they were just checking we were ok, even though the aircraft had us right on the pink string.

Because as I mentioned that’s just a FMC generated pink track, and as such it had the turning radius of 15 degrees of bank at 250kts correctly pre calculated and displayed on the ND, 250kts being below flaps up speed, and well below the flaps up +20 required to get full bank.

The answer to wide turns is obviously to keep the speed back, but this particular RNAV departure called for an acceleration straight to 250kt so couldn’t be helped. The SID had obviously been designed for smaller jets and the 747 had been overlooked.

Awesome overview of this and related issues thanks a bunch!  The two take homes for me:

• Slow down more on these tight quick turns in SIDS pretty much is where I see it as I'm hand flying
• Set range to 20nm...had I done this I might not have made the post!

Cheers and thanks again!

No problem glad it was of interest.

I suppose if you were to fly a legacy SID, (one that is not RNAV), in an older jet using a VOR or NDB where the tracking tolerance is +/- 5 degrees and then superimposed that tracking onto a modern jet with an ND, you’d probably see you were way off the pink string in places.

Another example would be the wobun departure out of EGLL, (now called umlat)The plate calls for a northerly track  straight off of the BUR Burnham NDB, but all the heavy jets overcook that turn and have to re establish on the northerly bearing by coming back in from the west.The pink string has a little wiggle in it, so the thing is your pink string might not always follow the ground track depicted on the chart exactly anyway, so being a little off the pink string is nothing to get massively worried about, if hand flying the FDs will always try to get you back onto the pink string should you deviate off of it.

Raw navigation beacon data should always be the overriding factor, however having said that my 787 doesn’t even have an NDB receiver so I just follow the pink string anyway 😉

RNAV SIDs are slightly different, on those the pink string is king.

Edited October 30, 20241 yr by jon b

PBN operation requires less than x0.5 RNP of Flight Technical Error, or occasionally at turn, less than 1xRNP.

Normally for RNAV/RNP SID/STAR, it requires RNAV1 or RNP1, so 0.5nm of XTE, and since you always have like 0.03~0.05 NSE even if GPS is working fine, you'll need to keep XTE under 0.95nm in turning.