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rondon9897

Unable 250 kts by xxx

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Hi guys,

 

Every time I insert performance data after a routing, I get the message "Unable 250kts by XXXX" in the FMS. I can't find a post on the forum or the FCOM and wondered if someone could tell me the exact repercussions of this message. What am I doing wrong, if anything? Is it suggesting that the cruise level for the flight is too high and that, even with speedbrakes, the aircraft will never reduce to 250kts by 10,000ft? It can't be that because I recently did EKCH-OMDB in the 737 and still got the warning. Perhaps it is suggesting that the altitude constraints in the STAR are too close together to reduce to 250kts? Not quite sure. Is there anything I can do to avoid it and am I making some kind of mistake?

Cheers,

John Price

 

 

Edited to add my name - for some reason my signature pic isn't working :(

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This is an advisory message and I don't think your cruise altitude before descent has much to do with it. The FMS will adjust TOD to compensate for higher/lower cruise and winds. I agree with your suggestion that constraints might play a role but highly unlikely because terminal procedures are designed by very trained specialists and subject to criticism by the users. What about winds? If your tail wind component is very high then the procedure's constraints might become an issue. In my experience, the VNAV path calculation done by the NGX is amazing and I've seen it pull off some difficult tasks.

 

John, add some more info such as arrival route from TOD and arrival constraints and what how you set up your weather.

 

EDIT:  Could this message apply to the depart segment of your trip? You're still in preflight phase, right?

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Not quite sure. Is there anything I can do to avoid it and am I making some kind of mistake?

 

Sometimes it's not a mistake, but a conflict between what the plane wants to do and what the STAR says to do. In the 737, it will automatically use a 240 knot restriction at 10000. This gives you a 10 knot buffer on the speed limit. If a STAR then assigns 250/10000 for a particular fix, the CDU will prompt you with a message of "I can't do 250 at FIXXX" because the speed restriction (240 at 10000 on the VNAV DESC page) takes higher precedence. Since I'd prefer to "fit into the picture" more than I would give myself a 10 knot buffer on the speed, I usually just re-set the restriction to 250.

 

My bet is that you're seeing a conflict more than an error of some sort.

 

Example conflict:

Fly the NGX into DCA using the OJAAY1. It'll give you an error at OJAAY because the STAR calls for 250/10000 at OJAAY, while the speed restriction calls for 240 at 10000.

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Now I'm trying would you believe it I can't recreate it. Next time it happens in the sim I will post it on here but your suggestion is very convincing Kyle if it happens again I will stick 250/10000 into the VNAV section of the FMS and see if the conflict goes.

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Now I'm trying would you believe it I can't recreate it.

 

Welcome to the life of the beta team / anyone-who's-worked-helpdesk.

 

Definitely let us know if you see it again.

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Sometimes it's not a mistake, but a conflict between what the plane wants to do and what the STAR says to do. In the 737, it will automatically use a 240 knot restriction at 10000. This gives you a 10 knot buffer on the speed limit. If a STAR then assigns 250/10000 for a particular fix, the CDU will prompt you with a message of "I can't do 250 at FIXXX" because the speed restriction (240 at 10000 on the VNAV DESC page) takes higher precedence. Since I'd prefer to "fit into the picture" more than I would give myself a 10 knot buffer on the speed, I usually just re-set the restriction to 250.

 

My bet is that you're seeing a conflict more than an error of some sort.

 

Example conflict:

Fly the NGX into DCA using the OJAAY1. It'll give you an error at OJAAY because the STAR calls for 250/10000 at OJAAY, while the speed restriction calls for 240 at 10000.

This.

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Every time I've had that message come up it has been due to the conflict that Kyle has mentioned. I solve it one of two ways. One, change the default 240/10000 in the FMC by changing on the DES page of the CDU. Two, on the legs page change the 250/10000 constraint at what ever waypoint to 250B/10000 so that the FMC is allowed to go below 250 as it passes the waypoint.

 

Dave

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Two, on the legs page change the 250/10000 constraint at what ever waypoint to 250B/10000 so that the FMC is allowed to go below 250 as it passes the waypoint.

 

Not that 10 knots will matter much, but I'd hesitate to use that option just in case the FMC decided to use some other lower value. 240 would be fine, but if you're coming across a fix at 220 when everyone else is doing 250, you're going to jam up the flow and likely get a phone number when the controller figures out why one aircraft is out of place.

 

Granted, I doubt it would use anything other than 240 (the limit in the VNAV DES page), but I wouldn't want to be the one person who finds out the moment that it choses something below it.

 

I wouldn't say it's wrong, but I'd hesitate to do that myself.

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Yes, I was worried about that same thing. So I've tested it a bunch of times at using 250B/10000 at the fix has always resulted in crossing the fix at no slower then 240. So far it's worked out fine each time. But, of course, as you've pointed out many times, everyone should always be ready to, "fly the airplane", just in case automation decides to do something unwanted.

 

Dave

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I've flown with a bunch of guys who will grumble all the way across the US because of this issue. I've tried to explain it to a few of them, but their eyes just glaze over. . .I've finally started to just change it as a preflight item.

 

The grumblers better than the guys who will fly at 249knots on descent.

 

Me: "But, why not 250?"

Them: "Because the limit is 250!"

Me: "But you fly the climb at 250 for the same reason, what's the difference?"

Them: "Uh, well, uh, on the -200 we used to blah blah blah."

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Me: "But, why not 250?"
Them: "Because the limit is 250!"
Me: "But you fly the climb at 250 for the same reason, what's the difference?"
Them: "Uh, well, uh, on the -200 we used to blah blah blah."

 

"...because that's the way it is and I don't want to use logic because it might question or void my current world view!!!"

 

*eyeroll*

 

Okay boss. You're clearing the messages this flight then...haha.

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I've flown with a bunch of guys who will grumble all the way across the US because of this issue. I've tried to explain it to a few of them, but their eyes just glaze over. . .I've finally started to just change it as a preflight item.

 

The grumblers better than the guys who will fly at 249knots on descent.

 

Me: "But, why not 250?"

Them: "Because the limit is 250!"

Me: "But you fly the climb at 250 for the same reason, what's the difference?"

Them: "Uh, well, uh, on the -200 we used to blah blah blah."

 

Those guys usually give up and switch to V/S anyway, rendering the whole argument moot.

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Yes I can cofirm it is a conflict issue. Had it doing ENGM-EDDM today on BAGMI 3A though still not completely sure why. I think the automatically computed descent in the legs page had me going down to FL110 then back up again. Anyway it's the STAR clearance limits that cause it so now I know.

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