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Richard McDonald Woods

Unable RTA

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The limits set for RTA seem to be very tight.

 

I set the RTA for a waypoint which is 1900+ nms away as 18:09, but when it is calculated that I will cross at 18:09:50 I receive the Unable RTA message.

 

Is this realistic? If so, what are the limits to this?

 

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Is this realistic? If so, what are the limits to this?

 

Was it calculated that you'll cross that waypoint at 18:09:50 prior to you setting the RTA, or after?

 

Remember that, while at altitude, your max and min speed aren't too far apart. That being said, if you're in the higher altitudes at which the aircraft can cruise, you can't speed up or slow down too much. Of course, with distance, a small change in airspeed can have a larger impact, but there's only so much that you can do.

 

For the sake of argument here, I'm going to use GS instead of airspeed. Since they're linked, it's not an unfair comparison. You min speed is 470 and your max is 500. You're cruising at 485. Your original Time Over Fix (1000nm away) is 0203.7Z (123.7 minutes), which means the current time is 0000Z.

 

If I tried to set an RTA of 0207Z (127 minutes), this would be okay. The resultant speed to cross that point in 127 minutes is about 472 knots, which is above the minimum. Setting it one minute more (to 0208Z), however, would result in 468 knots, resulting in an UNABLE RTA (and I believe the FMC would give you a closest-to-RTA value of 0207.7Z). The same thing would happen on the max side. So, depending on the situation, the "margin" might appear rather slim. A single minute is all that stands between an acceptable and unacceptable time value.

 

 

 

The thing to remember, here, is that RTAs aren't pilot-assigned. RTAs come from ATC, and the values are calculated out by the trajectory systems. It isn't some best guess metric that a lot of people are using in the sim here. In other words, how did you come up with your RTA value?

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Hi Kyle,

 

Many thanks for your usual comprehensive reply. Much appreciated!

 

No, to your first question. My flight plan, KJFK-CYVR, said that I would cross at 04:02 after takeoff, so I just did a little bit of advanced maths and got the expected crossing time for MLP of 18:09z.

 

I understand that RTA is used by ATC for aircraft sequencing, particularly on oceanic crossings. I was playing with RTA just to see how the commanded IAS would be set.

 

Thanks for your help.

 

Cheers, Richard

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No, to your first question. My flight plan, KJFK-CYVR, said that I would cross at 04:02 after takeoff, so I just did a little bit of advanced maths and got the expected crossing time for MLP of 18:09z.

 

Gotcha. I'll take a look again tonight and see what I can find. I remember trying to break it in the Beta, but haven't used it since.

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Yeah, I tried to break it in beta and succeeded and commented but apparently got buried in all the other things we were focused on. I recall a typical KIAH-PHNL trip and I didn't set the RTA for DENNS, which joins the MAGGI arrival, until I was about two hours ETA at the fix.  I set the RTA for 10 min later than the ETA and the FMS responded accordingly by adjusting cruise. The problem didn't arise until the last airway fix before DENNS, where upon passing the UNABLE message appeared. As reported by Richard, there was a very tight margin for RTA and I cancelled by setting a 0.84M cruise and continued on to the fix crossing it within a minute of my original RTA.

 

I should have ran more tests, and I should have pushed my finding but I moved on to other things.  Bottom line, I could recreate same symptom Richard is reporting in a different scenario.  My workaround is to use RTA until close enough to make the fix 'manually.'

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We used to get crossing restrictions in to YYC (Calgary) all the time due to the configuration of the runways (or lack thereof) and the amount of traffic (dinner rush).

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Gentlemen,

 

I can now report the experiences of setting RTA for crossing MLP (Mullan Pass) from my completed flight into CYVR from KJFK. I accept that RTAs are normally only used by ATC to improve traffic flow.

 

At around 16:30z, I set my RTA as 18:09z, and the RTA page of PROGRESS showed to achieve the restriction the A/P commanded RTA SPD as .842M, with a maximum speed of .850M.

 

At 17:40z, the commanded airspeed had increased to .85M to cross MLP at a calculated 18:08.2Z, presumably only for wind variations.

 

I crossed MLP at 18:08.6z!

 

Really rather good, don't you think?

 

Cheers, Richard

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