Delete VNAV's 240 Knot Speed Limit Below 10,000

[emerydc8]

While descending via the JFUND 2 arrival into Boston this morning, I noticed that VNAV cycled to 240 knots as it normally does out of 10,000', but the command speed did not go back to 250 knots to cross WINTA. It's unusual to get a crossing speed of 250 knots below 10,000', so it's something I've never noticed in the past. Just prior to WINTA, I inserted 250 knots on the VNAV descent page, but the command speed would still not speed up to 250 knots. It did slow to cross SPYSD at 220 knots. Aside from speed intervention, I'm not sure how else to do this arrival in VNAV. Any thoughts?



Jon

[Hardy Heinlin]

In the 744 FMC (probably all Boeing types) waypoint speed limits are always maximum limits, never minimum limits. On the arrival route, when your speed is already below the limit, there's no reason to accelerate. In ACT DES, the FMC will reduce the SEL SPD to the active limit when it's above that limit. It will never increase the SEL SPD.

In ACT CLB it's different, but then the FMC doesn't use the SEL SPD; it just changes the CLB page title according to the current acceleration phase while the speed mode in 2R remains in ECON.

The chart shows 250 KT at WINTA because there you cross 10000. It's the usual 250/10000 transition where the FMC always sets 240 to get a 10 knot safety margin. This safety margin is not applied in other speed limit situations. So you're at 240 after WINTA, and since it's only 15 nm to the next limit 220 KT, the FMC will soon reset it to 220.


Regards,

|-|ardy

[emerydc8]

waypoint speed limits are always maximum limits, never minimum limits.

Not to ATC. There is an arrival going into ATL that has you cross a fix at 13,000 and 250 knots. The box does fine but still slows to 240 knots when it descends below 10,000. One morning we were going in there and ATC asked us our speed. Being in VNAV we replied 240. He asked us who told us we could slow and then went off on a tirade about our being in front of a Delta "heavy jet" (like we weren't too) and made a big deal about vectoring us out of the way to let Delta cut in front. In fact, he switched us at the last minute from 08L to 09R which added 30 minutes to our taxi. I  thought about getting a phone number but I was just too tired and didn't feel like fighting.

I'd bet money if it was busy in Boston and we did that route segment between WINTA and SPYSD at 240 knots, they'd have said something.

[Hardy Heinlin]

When ATC instructs to maintain a certain speed during the arrival phase, I would use the MCP SPD. There's no other way to remove all FMC speed limits. There's at least one last limit at the FAF, even if it's displayed in small font.

[emerydc8]

True. But if you open the speed window and set 250,  you're in charge of making all the altitude restraints because you'll be in HOLD VNAV SPD.

[Hardy Heinlin]

Why? When the aircraft is within a few hundred feet of the active waypoint altitude constraint, VNAV SPD will change to VNAV PTH and it will level off at that altitude -- even in MCP SPD mode.

[emerydc8]

If you're descending via an arrival (continuous descent) with the bottom altitude in the MCP and you open the speed window, it's up to you to keep it on the glide path using power or speed brakes. You will be in VNAV SPD. It won't go to VNAV PTH even if you're right on the path.

[Hardy Heinlin]

That's clear. I thought you were talking about altitude constraints rather than descent rate management.

[Mariano]

I wonder if creating WINTA/-5 at 250/10000 and editing WINTA to 250/10000 (hard) would work, or just editing WINTA to 250/10000 (hard), without even bothering with WINTA/-5.

Best regards,

Mariano

[emerydc8]

I have a suspicion that once it gets under 10,000 it's going to cycle to 240 knots no matter what, even if you manually set 250 on the VNAV DES page. I go back there in about 9 hours. I'll try your suggestion if I remember.  Thanks Mariano.

[Hardy Heinlin]

You can delete the 240/10000 SPD TRANS in 3L on the DES page (it cannot be changed but it can be deleted). Then set a SPD RESTR of 250/10000 in 4L. Or set a waypoint constraint at WINTA for 250/10000 (the respective deceleration phase before WINTA will be started automatically). But the SPD TRANS in 3L must be deleted, otherwise you'll get 240/10000.


Edit: If WINTA has a 10000/8000 altitude window, then better 250/10000B8000A

[Mariano]

I’m hoping that by you demanding that WINTA be crossed NOT below 10,000 feet, the FMC will hold 250 (if you hard-select it to 250/10000). It should then hold 10,000 feet as it decelerates to 220 knots after crossing WINTA, before pitching down again.

Glad that you can test it so soon.

Best regards,

Mariano

[andrej]

Hi John,
thanks for sharing your real world challenges. It is always interesting to read, how operators deal with these issues. I also wonder, if designers / creators of terminal procedures somehow take into account performance of various planes that could potentially fly in / out of their area.

For example, few B77W pilots told me that it is very slippery plane. Considering that B744 has sometimes difficulty to meet some constrains, I can only imagine how challenging it is for them.

Yesterday, I tried a short hop from KALB to KBOS, and I did exactly the same as Hardy suggested (I wanted to ask this earlier, but felt a little silly as I am not a pilot). By deleting the 240 / 10000 SPD TRANS and manually selecting 250 kts on CDU, I was able to obey the speed limitation. VNAV crossed all waypoints on the upper limit, but that should not be an issue as I met limitations.

My other question is, when to slow down to 220 kts. Immediately after passing WINTA or a few miles prior to reaching SPYSD? I managed my speed via VNAV all the way up to IAF.

Thanks and best!

[emerydc8]

You can delete the 240/10000 SPD TRANS in 3L on the DES page (it cannot be changed but it can be deleted). Then set a SPD RESTR of 250/10000 in 4L. Or set a waypoint constraint at WINTA for 250/10000 (the respective deceleration phase before WINTA will be started automatically). But the SPD TRANS in 3L must be deleted, otherwise you'll get 240/10000.


Edit: If WINTA has a 10000/8000 altitude window, then better 250/10000B8000A

In the airplane now getting ready to fly to Boston. We will delete the speed transition at 3L and add a restraint in 4L of 250/10000.

[emerydc8]

I know it's been said many times but --  you're a genius, Hardy. I did exactly as you suggested and it worked perfectly. I deleted the 240/10000 at 3L and then added 250/10000 at 4L. The guy I'm flying with can't believe you're not type rated in a Boeing. This technique will work on the CHPPR 1 arrival into ATL to keep it from slowing to 240 knots below 10000. Notice that out of 10000, the SPD REST blanks when SEL SPD displays 250. The video speaks for itself. Cheers.

https://youtu.be/OHjLGpe-ByA

[emerydc8]

My other question is, when to slow down to 220 kts. Immediately after passing WINTA or a few miles prior to reaching SPYSD? I managed my speed via VNAV all the way up to IAF.

Thanks and best!

VNAV normally ratchets down a few miles before the fix, but the 767 is also a slippery airplane and it almost always requires speed brakes. I scoured the Internet looking for something that addresses whether ATC expects you to stay at whatever speed the arrival leaves you. For instance, if the last speed constraint on the arrival is 250 knots, does ATC expect you to maintain that until otherwise assigned or cleared for the approach? I could find nothing, but like I stated above, Atlanta controllers seem to think you should not slow without permission.

Jon

[Hardy Heinlin]

Glad to see the theory agrees with the real world :-)


Cheerio!

|-|ardy

[Captain_al]

Good discussion, Boeing modified the -8 so it is an off idle descent, stays in SPD, and if you open the MCP window, like to maintain 250 instead of 240, it stays in VNAV PTH and SPD. Interestingly, not a problem in the -8...

[Hardy Heinlin]

Same on the 744 if the NG FMC is installed.

[Mariano]

Great trick to know. Very informative.

Best regards,

Mariano