744 Forum

Apron => Hangar 7 => Topic started by: emerydc8 on Tue, 13 Oct 2020 15:42

Title: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: emerydc8 on Tue, 13 Oct 2020 15:42
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?

(http://www.hoppie.nl/forum/var/2020-10-13_14.39.38_jfund2-250kt-crossing.jpg)

Jon
Title: Re: 250 Knots Below 10,000
Post by: Hardy Heinlin on Tue, 13 Oct 2020 19:14
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
Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Wed, 14 Oct 2020 00:55
Quote
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.
Title: Re: 250 Knots Below 10,000
Post by: Hardy Heinlin on Wed, 14 Oct 2020 01:12
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.
Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Wed, 14 Oct 2020 02:12
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.
Title: Re: 250 Knots Below 10,000
Post by: Hardy Heinlin on Wed, 14 Oct 2020 02:25
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.
Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Wed, 14 Oct 2020 14:36
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.
Title: Re: 250 Knots Below 10,000
Post by: Hardy Heinlin on Wed, 14 Oct 2020 21:54
That's clear. I thought you were talking about altitude constraints rather than descent rate management.
Title: Re: 250 Knots Below 10,000
Post by: Mariano on Thu, 15 Oct 2020 01:50
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

Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Thu, 15 Oct 2020 02:21
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.
Title: Re: 250 Knots Below 10,000
Post by: Hardy Heinlin on Thu, 15 Oct 2020 03:52
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
Title: Re: 250 Knots Below 10,000
Post by: Mariano on Thu, 15 Oct 2020 04:39
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
Title: Re: 250 Knots Below 10,000
Post by: andrej on Thu, 15 Oct 2020 05:07
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!
Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Thu, 15 Oct 2020 10:52
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.
Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Thu, 15 Oct 2020 14:54
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
Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Thu, 15 Oct 2020 15:01

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
Title: Re: 250 Knots Below 10,000
Post by: Hardy Heinlin on Fri, 16 Oct 2020 01:09
Glad to see the theory agrees with the real world :-)


Cheerio!

|-|ardy
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: Captain_al on Fri, 16 Oct 2020 11:03
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...
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: Hardy Heinlin on Fri, 16 Oct 2020 11:16
Same on the 744 if the NG FMC is installed.
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: Mariano on Fri, 16 Oct 2020 19:42
Great trick to know. Very informative.

Best regards,

Mariano
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: andrej on Sat, 17 Oct 2020 07:04
Hi Jon,
thanks for confirming our hypothesis. 8) If anything, PSX is a must have for any pilot (especially for Boeing drivers).
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: emerydc8 on Sun, 18 Oct 2020 11:12
With regard to maintaining the last speed on a STAR, it looks like Atlanta ATC was correct.

Quote
NOTE-

Pilots are required to comply with published speed restrictions.

    A speed restriction published as part of a SID/STAR is canceled when an aircraft is vectored off, or a deviation from the SID/STAR is approved. If necessary, assign a speed in conjunction with the vector or approval to deviate.

NOTE-

The last published speed on a STAR will be maintained by the aircraft until ATC deletes it, assigns a new speed, issues a vector, assigns a direct route or issues an approach clearance.

FAA Order JO 7110.65Y - Air Traffic Control
Section 7
5-7-1 SPEED ADJUSTMENT
https://www.faa.gov/air_traffic/publications/atpubs/atc_html/chap5_section_7.html
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: andrej on Mon, 19 Oct 2020 07:10
Hi Jon,
so, at least in the US, one must always obey ATC. Thanks for confirmation. As I only fly virtual via VATSIM, I tend to obey ATC as such. However, from some YouTube videos, I have noticed that most of the time, even in Europe, pilots follow ATC's instructions (unless directly told that speed is (i) "no factor", (ii) "speed is yours", etc.).

Cheers,

Title: Re: 250 Knots Below 10,000
Post by: emerydc8 on Mon, 26 Oct 2020 07:54

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!

To answer your question, in this situation, you would hold 250 knots until the minimum distance necessary to slow to 220 by the next fix. At least that's the case in the US. The VNAV on the 767 will usually ratchet down later than it should, often times a mile or two prior to the fix, and then you get the DRAG REQUIRED message.

Jon

(http://www.hoppie.nl/forum/var/2020-10-26_07.50.34_arrivalfixspeed(1).jpg)
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: andrej on Wed, 28 Oct 2020 08:40
Thanks Jon. So, slowing down "until the minimum distance necessary" to next speed restriction can be achieved by programing the limit speed to the FMC, or manually based on PIC's experience and knowledge of the type, and/or based on decision to slow down at certain point prior to the waypoint.

Unless, one is instructed by the ATC of speed limit.

Very informative and learning experience for me. :)

Cheers!
Title: Re: Delete VNAV's 240 Knot Speed Limit Below 10,000
Post by: emerydc8 on Wed, 28 Oct 2020 09:00
Thanks Jon. So, slowing down "until the minimum distance necessary" to next speed restriction can be achieved by programing the limit speed to the FMC, or manually based on PIC's experience and knowledge of the type, and/or based on decision to slow down at certain point prior to the waypoint.

The limit speed on the STAR is usually coded into the arrival. For whatever reason, it seems that just about every descent I do that contains a crossing speed, I end up having to use speed brakes because VNAV just isn't smart enough to figure out it needs more time to slow, even when we load the descent winds via ACARS. VNAV is a total waste of fuel (at least it is on the 767), and it was sold to the airlines as a way of saving fuel. We consumed less fuel when we used V/S and a 3-1 profile. Go figure.