PDA

View Full Version : When will 744 VNAV respects speed and altitude restriction?


kuobin
13th Nov 2006, 14:12
When we take off from HKG 7R,there is a speed restriction of 220 kt before cross PROPA,so I set speed restriction of 220 kt over PROPA and an Altitude restriction of 1500A,this should be a fairly reasonable setting,but actually I find when we started to speed up after reach 1500 ft (and before PROPA) VNAV will speed up for flap placard speed -5 kt(225 for take off with flap 20) instead of 220kt:confused: ,VNAV does not respect that 220kt restriction at all, anybody got ideal?thanks?:rolleyes:

banana head
13th Nov 2006, 17:42
Probably the aircraft is accelerating to the climb speed on your vnav climb page.

There are 2 ways around this:

1/ Input a speed limit of 220/ 5000 on the vnav climb page speed restriction line. ( I've just picked 5000 as you probably won't be above 5000 until after porpa)

2/ Speed intervene on the MCP with 220kts once the aircraft is above acceleration altitude, closing the speed window once established on track outbound from porpa.

Capt Fathom
13th Nov 2006, 21:04
Probably the aircraft is accelerating to the climb speed on your vnav climb page.

That brings us back to the original question ...
Why doesn't VNAV fly the 220kts restriction on the LEGS Page. LEG Page constraints should override the VNAV CLB & DSC Pages.

Intruder
14th Nov 2006, 03:06
I've been on the Classic for the past 18 months, but IIRC the LEGS page only enforces the speed AT the waypoint, not necessarily BEFORE the waypoint. It may accelerate then decelerate...

Dagger
14th Nov 2006, 03:54
Think Intruder might have it. Not sure that the pea sized brain of the FMC is smart enough to reverse the sequencing of the speed constraint during the acceleration phase. Did you notice whether the 220 speed restriction became the target speed once at Porpa? Although you probably had the window open by then to meet the requirement.

littlejet
14th Nov 2006, 11:07
Maybe the aircraft is near PORPA so the FMC is sequencing to another waypoint with its own constraints if any ( in case of OCEAN 2A rnav SID, TD) or in this case flap placard speed -5.
Interesting in HKG SID's is that OCEAN 2A rnav sid and OCEAN 2A sid has two different speed restrictions. first one has 220 kt at PORPA and the ssecond has 220 kt untill established on 116 track.

Phil Squares
14th Nov 2006, 11:19
Just a question before I take a stab at an answer.

1) What is your thrust reduction altitude or flap configuration?

I assume you're taking off at 20 Flaps? The reason I ask is because until there is a thrust reduction to climb thrust the takeoff ref page controls the speeds.

kuobin
15th Nov 2006, 14:21
Our thrust reduction altitude is 1500 ft and takeoff flap is 20.Tks.

alistomalibu
25th Feb 2008, 19:13
In such situations I dont like VNAV.
FLCH wins...

Lemper
25th Feb 2008, 23:56
How about this one:
220/5000 at Porpa in the FMS. After T/Off, Departure control clears you to 9000Ft (Usually). So, PM (or PF is AP is on) puts 9000 in the altitude window and pushes the alt knob; consequence, the 5000 Ft restriction is lifted, BUT THE 220 KTS TOO, as the pushing of the altitude knob cancels ALL restriction on the top LSK.

taufupok
26th Feb 2008, 00:06
Discovered a long time ago that an FMC software glich caused it to ignore the speed restriction enterred IF THE ACCOMPANYING WAYPOINT ALTITUDE RESTRICTION IS ACHIEVED. To forestall this, banana head is correct; just enter a speed/altitude( an intelligent estimate ) restriction in the VNAV climb page. I believe Boeing/Collins/Honeywell are STILL working on it. Reported it years ago to my company techies but they laughed me off, I believe they were not able to simulate in the Sim!!! I know Boeing was also alerted but with such LOW TECH complaints coming from an Oriental pilot, THEY JUST IGNORED IT!!!

BuzzBox
26th Feb 2008, 00:12
As taufopok said, once you have satisfied the waypoint's altitude constraint, in this case the 1500A you entered, the FMC legs page will dump the speed constraint associated with that waypoint.

We normally get around it by inputting 220/5000 as a restriction on the VNAV climb page. As banana head said, you could also open the MCP window during the acceleration and set 220 knots until established on track to the waypoint after PORPA.

skiesfull
26th Feb 2008, 09:13
KUOBIN
K.I.S.S. - Keep It Simple + Safe.
To minimise workload and distractions during a 'terrain' RNAV SID, arm LNAV/VNAV before take-off as per your SOP's and at Flaps 5 (or Flaps 1, depending on take-off weight) call for/ or select Speed Intervene 220kts and then when' round- the - corner' on the SID cancel speed intervene and accelerate to 250 kts. Simple and Safe!

csd
26th Feb 2008, 09:30
There are 2 ways around this:

1/ Input a speed limit of 220/ 5000 on the vnav climb page speed restriction line. ( I've just picked 5000 as you probably won't be above 5000 until after porpa)

2/ Speed intervene on the MCP with 220kts once the aircraft is above acceleration altitude, closing the speed window once established on track outbound from porpa.

My understanding is that if you just open the speed window then the aircraft will fly at that speed but the aircraft will plan to fly the turn radius calculated as if the speed window is closed. Therefore if the aircraft had planned on going faster than 220 kts (for example 250 kts), then by just opening the speed window to 220 kts will cause the bank angle in the turn to reduce in order to fly the 250 kts track.

Therefore, the FMS needs to be programmed with a restriction and the most common way that I have seen is to program the VNAV CLB page on the FMS with a speed restriction of 220/5000'. Obviously you may have to change the height depending on aircraft performance.

Regards

csd

PACIFIC BARON
26th Feb 2008, 11:09
From my experience the speed restriction on the legs page is married to the altitude requirement. If you require the aircraft to maintain no greater than 220 kts until Porpa whilst in VNAV then you have to programe an altitude that is valid until passing that point. I would suggest 220/9000B. By programming an altitude at or below that cannot possibly be made will keep the 220 kts in VNAV until passing the required point. The at or below stops the problem of "UNABLE NEXT ALTITUDE"
The real problem is the observed fact that once an altitude constraint has been achieved, the legs page drops both the altitude & speed requirements. By programming an altitude higher than that achievable & making that altitude an "at or below" ensures that the speed constraint attached to this waypoint remains until passing that point without causing any other problems. Thus VNAV will work as advertised & delete any need for speed intervention.
This has worked for me on departure from VHHH on many an occassion.
Regards
P B :)

Molokai
26th Feb 2008, 21:10
Taufupok, I guess Mr Boeing thought that you were an uneducated tree dwelling hominid masquerading as a B744 ace telling them that the FMC had an anomaly OR YOUR COMPANY TECHNIES WERE TOO CHICKEN OR INARTICULATE TO present your complaint to those guys at SEA/ORD!;)

I believe the B777 crew did receive a recent bulletin about this anomaly.

SpaceNeedle
26th Feb 2008, 22:12
More like his/her company techies and CP did not follow up with Boeing.

PACIFIC BARON
26th Feb 2008, 22:22
Honeywell Flight Management Computer Anomaly currently existing on all Boeing 747/757/767/777 aircraft equiped with the Honeywell FMC. The Boeing bulletin issued late last year states that "VNAV will incorrectly delete the speed constraint prior to reaching the waypoint IF the altitude contraint has been satisfied."
Expect a software fix sometime this year. :D

lessaspirin
26th Feb 2008, 23:00
http://www.ecacnav.com/content.asp?PageID=389:)