I got this unable message tonight while decending to the KOOLY 4 arrival to KPHX. ITEMM Is the first waypoint in the STAR, and has a restriction of 280kts and FL240. It didn't make sense to me why the FMC was unable, and sure enough it blew the airspeed restriction, flying the waypoint at 261kts. Note that 280 was well below the barber pole at that altitude. After that is another 280kt waypoint and it unabled that one too. At this point is used spd int to override, and the autothrottle came up to MCP SPEED, and we hit the next waypoint on altitude and speed.So the question is, why the unable?Thanks!George

Edited February 24, 201214 yr by gmohr

Winds, ISA Dev, Correct PERF page set-up, could be a bunch of things..sorry I cannot help much further.90% of the time when I have VNAV problem with the NGX it is because I screwed up something, somewhere. Maybe on intial descent the wind start "backing" and threw you off VNAV. VNAV really needs to be "babysat", but as long as the FMC has accurate infromation in it and the Aircraft is capable of performing the duty called up by the FMC then you should have no problem.

Are you sure it's an exact 280kt restriction? I really doubt it is. Or is it a MAXIMUM of 280kt to cross ITEMM? I fly many STARs which say you should cross a fix AT OR BELOW a certain speed. For example, I have to cross MENDS at 250kt or below, but the FMS shows it like I had to cross the fix AT 250kt. If it's an "at or below" restriction, just type the speed, in your case 280, with a B and a forward slash after. You'll type 280B/ and insert that at ITEMM. The plane should fly this one much better.

Edited February 24, 201214 yr by barfra1995

^ Yes, I'd also say it was exactly this. It's just like the REDFA1A approach in Tutorial #1.

Hi,It's because it's conflicting with the global decend speed. Had that been fx. 300/.78 then you wouldn't have got that warning.regards,

So the question is, why the unable?

Your planned descent speed was less than the restriction. The speed your C/I wanted was below the 280. Next time put a 280/.78 or faster restriction in the descent and you won't have that problem. Or use a higher C/I.

You're violating the FMC's monotonic path concept - your calculated ECON descent speed is lower than 280 knots so the FMC will never command the airplane to speed up or climb once the descent has started. It always proceeds downward in both speed and altitude. That's what monotonic means - one direction. This applies in climb too, the plane is not going to descend or slow down - it only remains constant or goes up.

You're violating the FMC's monotonic path concept - your calculated ECON descent speed is lower than 280 knots so the FMC will never command the airplane to speed up or climb once the descent has started. It always proceeds downward in both speed and altitude. That's what monotonic means - one direction. This applies in climb too, the plane is not going to descend or slow down - it only remains constant or goes up.

Ah, that's a very interesting piece of information! I usually never have problems at all getting things done with the 737NGX but sometimes I don't quite understand why the FCM is 'saying' something which can be achieved anyway (using other modes). This explains a lot: didn't know that!

Yes, excellent info. Thanks Ryan et al. So the answer is to enter 280 in the decent speed field, and ride the higher speed all the way down. Because the speed of decent is never increased, the FMC will have no trouble hitting the next set of restrictions (all lower).barfra1995, the waypoint is indeed a hard restriction of 280, check out the chart for yourself (KOOLY4 @ KPHX).Ryan, One follow up question...At T/D, FL380, 280 kts is above the barber pole (if memory serves). So starting the decent at that speed isn't workable. What's the RW procedure to handle this decent? Is this profile possible without using SPD INT?Thanks!!George

Edited February 24, 201214 yr by gmohr

Yes, excellent info. Thanks Ryan et al. So the answer is to enter 280 in the decent speed field, and ride the higher speed all the way down. Because the speed of decent is never increased, the FMC will have no trouble hitting the next set of restrictions (all lower).barfra1995, the waypoint is indeed a hard restriction of 280, check out the chart for yourself (KOOLY4 @ KPHX).Ryan, One follow up question...At T/D, FL380, 280 kts is above the barber pole (if memory serves). So starting the decent at that speed isn't workable. What's the RW procedure to handle this decent? Is this profile possible without using SPD INT?Thanks!!George

On the decent page the constraints are Mach and Konts. As mentioned above use something like .76/280

Edited February 24, 201214 yr by falcon999

At T/D, FL380, 280 kts is above the barber pole (if memory serves). So starting the decent at that speed isn't workable. What's the RW procedure to handle this decent? Is this profile possible without using SPD INT?

Next time put a 280/.78 or faster restriction in the descent and you won't have that problem. Or use a higher C/I.

That's why you have both KIAS and Mach, around FL260, they'll cross over and you'll go from being on Mach to KIAS.

George,The limiting factor here is probably the Cost Index you're entering. Use something like 40 and you'll easily get 280 in the descent.

Setting a descent speed of 280kt could also do it.

yeah I've been running 20 CI. Thanks! -G