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Author Topic: RTA: INVALID ENTRY  (Read 827 times)

emerydc8

  • Join date: Jun 2015
  • Location: Tucson, AZ USA
  • Posts: 2008
RTA: INVALID ENTRY
« on: Sat, 25 Jul 2020 00:26 »
I did a trip between SEA and YVR last week that was really unusual. As soon as we got to our cruise altitude of 16,000/290 knots they handed us off and we were told to begin slowing to the slowest possible speed. ATC said we had an ETA for EGRET of 1648 or later. They issued the clearance at 1628 -- about 50 miles from EGRET. I looked down and saw the EGRET ETA -- 1638! We slowed to 210 knots and put 210 in the cruise page. The new ETA was 1642. Then we tried entering 1648 into the PROGS RTA: INVALID ENTRY! No help there.

It was time to do some basic math and forget about the automation. We ended up extending flaps to 15 and slowing to 150 knots. That was about 2.5 miles per minute and allowed us to cross EGRET at 1647. Missed it by a minute but ATC was happy. I guess a cruise speed that low was just too much for the FMC. We thought about entering 150 knots in VNAV once we slowed but at that point we knew we were in the ballpark and didn't want to risk short-circuiting the old computer for the rest of the arrival!

I couldn't find any info in my books about the parameters the FMC looks at in determining when a requested RTA will get kicked out as INVALID, as it did here. I'm presuming that anything below the zero flap maneuver speed will not be considered. If anyone has info on this please post it.

It turned out that a single-engine Cessna pilot got lost and was heading west, right through Vancouver's airspace at 3000'. We listened as ATC lectured the guy about having to move us and a China Airlines Super and then he gave him a phone number to call when he landed. You could hear the shame in the poor guy's voice over the radio. I suspect a 609 ride (or the Canadian equivalent) will be in his future.

Jon




Jeroen Hoppenbrouwers

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Re: RTA: INVALID ENTRY
« Reply #1 on: Sat, 25 Jul 2020 00:56 »
Do you know why ATC did not take the "easier" action (for you) to vector you out over the ocean in a U-turn? Was it because they were too busy with that Cessna to take the additional workload of having to vector a few heavies? Or do they not even own that airspace?


Hoppie

emerydc8

  • Join date: Jun 2015
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Re: RTA: INVALID ENTRY
« Reply #2 on: Sat, 25 Jul 2020 02:31 »
I didn't ask him, but after listening to it for a while, I think ATC was taken by surprise that this aircraft just wondered into their airspace. We were coming up from the south at 290 knots and I think Seattle just wanted to keep us away from the guy. We weren't talking to YVR approach until we got closer to EGRET. I suspect China Southern was coming in from over the water. I think he just wanted to delay our getting close to the entire mess until he could get the guy heading east. I don't know exactly what he did with China Southern but I don't think it would have gone over too well to ask them slow to 150 knots 60+ miles from the airport.

I suspect if someone dug around the ATC tapes on 7/14 at that time they would hear what went on.

EDIT: Start the tape at 07:30 in if you're interested. Listen for TQF. https://archive-server.liveatc.net/cyvr/CYVR1-App-Dep-Jul-14-2020-1630Z.mp3
« Last edit: Sat, 25 Jul 2020 03:32 by emerydc8 »

Hardy Heinlin

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Re: RTA: INVALID ENTRY
« Reply #3 on: Sat, 25 Jul 2020 07:48 »
I couldn't find any info in my books about the parameters the FMC looks at in determining when a requested RTA will get kicked out as INVALID, as it did here. I'm presuming that anything below the zero flap maneuver speed will not be considered. If anyone has info on this please post it.

Quote from Boeing's RTA White Paper:

"The minimum calculated RTA speed will also not be less than the Best Hold Speed."

Are you sure you got "INVALID ENTRY" and not "UNABLE RTA"? When the calculated RTA speed drops below the Best Hold Speed (which refers to the flap-up configuration obviously), the RTA function exceeds its limits.


Regards,

|-|ardy
« Last edit: Sat, 25 Jul 2020 08:15 by Hardy Heinlin »

emerydc8

  • Join date: Jun 2015
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Re: RTA: INVALID ENTRY
« Reply #4 on: Sat, 25 Jul 2020 09:59 »
Thanks, Hardy. I'll have to ask my FO. I thought he said invalid entry. I was busy slowing down and trying to figure out how slow we needed to go.

emerydc8

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Re: RTA: INVALID ENTRY
« Reply #5 on: Sat, 25 Jul 2020 19:41 »
The FO said he got an INVALID ENTRY when he tried to enter a 1648 RTA at EGRET.

Will

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Re: RTA: INVALID ENTRY
« Reply #6 on: Sat, 25 Jul 2020 20:16 »
Maybe both are possible? Maybe UNABLE RTA if the required speed is below the best hold speed, but INVALID ENTRY if the required speed is below some hard limit like the flap up maneuvering speed? That's just a guess, I have no idea how it really works.
« Last edit: Sat, 25 Jul 2020 21:22 by Will »
Will /Chicago /USA

emerydc8

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Re: RTA: INVALID ENTRY
« Reply #7 on: Sun, 26 Jul 2020 04:40 »
You may be right. I'll have to play with it the next time I go out. The RTA feature is a good idea, but wouldn't have helped us in our situation anyway.

Hardy Heinlin

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Re: RTA: INVALID ENTRY
« Reply #8 on: Sun, 26 Jul 2020 10:09 »
Or you were in ACT DES. When you have left the CRZ phase, an RTA progress (that started during the CRZ) remains displayed until the RTA fix is passed, but maybe you're not supposed to change the RTA anymore as there is no RTA speed mode in ACT DES (just ECON and SEL SPD).

When you are before the DES phase and you have entered a valid RTA, that RTA may become unreachable when the wind data changes. In that case the UNABLE RTA message occurs. Perhaps the FMC can calculate a worst case scenario with, say, a wind change from 250 kt tailwind to 250 kt headwind, and then promptly check if the entered RTA is still reachable within the max/min RTA speed limits; if not, promptly trigger the INVALID ENTRY message. But it would have to be an extremely tolerant check -- with a 500 kt wind speed change. I think it would be absurde to do this check, and also absurde if it did this check with wind speed changes smaller than 500 kt. Either way, an INVALID ENTRY check of this kind would make no sense, in my opinion. And the UNABLE RTA message would inform about the problem anway.


|-|ardy
« Last edit: Sun, 26 Jul 2020 10:32 by Hardy Heinlin »

emerydc8

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Re: RTA: INVALID ENTRY
« Reply #9 on: Sun, 26 Jul 2020 22:08 »
I'm sure we weren't in DES mode but there was a lot going on in a short period of time and my FO might have misread what the CDU displayed. Let me check it out when I get back to work next month.

emerydc8

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Re: RTA: INVALID ENTRY
« Reply #10 on: Mon, 27 Jul 2020 09:44 »
I talked to a buddy at JetBlue and he said that he will sometimes use the RTA for maximum forward speed on transcons when he doesn't want to mess with a new CI or cruise speed. They pick a waypoint on the arrival and then subtract 10 minutes from the ETA to that waypoint, then they put that into the RTA. Apparently the Airbus FMC will try to make it by going as fast as possible, even if there is no way it will. He wasn't sure what it would do if you were trying to go the other way and go as slow as possible.

Hardy Heinlin

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Re: RTA: INVALID ENTRY
« Reply #11 on: Mon, 27 Jul 2020 09:53 »
It makes sense to stay in RTA mode when the RTA cannot be reached anymore. Why should the RTA mode disable itself and reactivate the ECON mode? The wind may change unexpectedly in a few minutes and make the RTA reachable again. When that happens, the UNABLE RTA message clears itself.


|-|ardy

cavaricooper

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Re: RTA: INVALID ENTRY
« Reply #12 on: Tue, 28 Jul 2020 01:18 »
I talked to a buddy at JetBlue and he said that he will sometimes use the RTA for maximum forward speed on transcons when he doesn't want to mess with a new CI or cruise speed. They pick a waypoint on the arrival and then subtract 10 minutes from the ETA to that waypoint, then they put that into the RTA. Apparently the Airbus FMC will try to make it by going as fast as possible, even if there is no way it will. He wasn't sure what it would do if you were trying to go the other way and go as slow as possible.

Jon- wouldn't this work differently in Fifi?  The FMCGS develops a soft cruise profile for ALT and SPD and the window allows this to work without pilot intervention.  Boeing won't allow that, as it uses definite numbers, nicht?  I'm very new to the MCDU so just trying for better understanding.

Hope you are staying safe as you fly!

C

Carl Avari-Cooper, KTPA

emerydc8

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Re: RTA: INVALID ENTRY
« Reply #13 on: Tue, 28 Jul 2020 06:20 »
Hi Carl. I'm sure you know more about the Airbus FMS than I do. Maybe someone familiar with that box can answer your question. My buddy did mention that even the Neo RTA feature is different and he's not that familiar with it since he only flew it a few times. I'm not even that familiar with the Boeing RTA feature since I've used it so little. I plan to film some of the experiments when I go back and I'll post them here. I don't know what's used less on the Boeing -- the RTA feature or the off-path descent feature.

Jon
« Last edit: Wed, 29 Jul 2020 04:21 by emerydc8 »

cavaricooper

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Re: RTA: INVALID ENTRY
« Reply #14 on: Tue, 28 Jul 2020 13:01 »
Jon-

Good to hear from you. I’m just delving into Fifi now, so no real grasp atm.

Best- C
Carl Avari-Cooper, KTPA

emerydc8

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Re: RTA: INVALID ENTRY
« Reply #15 on: Thu, 6 Aug 2020 20:15 »
I had a chance to experiment and see what happens when asking the FMC for an RTA that would require it to slow below the best holding speed. As you can see, it took about 90 seconds for the FMC to figure out it was unable. I didn't execute in this video but when I did, the speed would slow to correspond to whatever RTA speed it was showing. Interesting how it displayed the incremental decrease in speed until it finally gave up.

https://youtu.be/RB3typitiPk

Jon


Hardy Heinlin

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Re: RTA: INVALID ENTRY
« Reply #16 on: Thu, 6 Aug 2020 23:48 »
Fantastic. This confirms my theory of the cost index based RTA calculation process. Thanks. The same happens in PSX. To determine the (lower) RTA speed, the FMC decreases an internal cost index (not the CI shown on the PERF INIT page) a bit and checks if the new ETA matches the RTA. If not, it decreases the CI further, and so on. The RTA speed is an ECON speed, not a SEL SPD, so it has to calculate the RTA speed by several iterations of various CI settings. Only when the best holding speed is reached, the RTA speed is no longer cost index based.

When the RTA is an A/B time frame and the current ECON speed keeps the ETA within that frame, the RTA speed simply uses the current ECON speed and cost index.


|-|ardy

David Palmer

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Re: RTA: INVALID ENTRY
« Reply #17 on: Mon, 17 Aug 2020 00:22 »
Hi Jon,

Does this look familiar...?

Regards,
David.

emerydc8

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Re: RTA: INVALID ENTRY
« Reply #18 on: Mon, 17 Aug 2020 10:56 »
Yep. That's definitely me! Interesting that it shows the speed, even on the ground. Taxiing from the north cargo area to RW34R at SEA is a long downhill roll and I'm pretty sure I exceeded 30 knots at some point. It's easy to do when you're really light. That was one flight I'll never forget.

United744

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Re: RTA: INVALID ENTRY
« Reply #19 on: Fri, 21 Aug 2020 02:42 »
ADS-B is evil. It reports everything. Worse than QADR, etc.

Interesting story! Thanks for sharing!

EDIT: The ATC tape isn't available anymore, and can't be found on the site by searching, either.