VNAV speed restrictions [Tel Aviv situation]

[airlinermanager]

Today I tried out the situation files, that come with PSX. So I flew the Tel Aviv to Hong Kong Departure Situation. The SID requires a right turn to NOGAH right after Take Off with a maximum speed of 170 kt. The PSX 744 VNAV does a brilliant job on that.

However, as a reader of the pprune.org forum, I stumbled across a complain by several 744 jockey back in 2008.[*] If I understand them correctly, the 744 does only respect speed limits at a waypoint. The aircraft will decelerate upon reaching the waypoint and accelerating after it. It simply ignores the restriction otherwise. As a solution for this problem, those pilots suggested to enter a speed restriction on the VNAV CLB Page.

Also I have trouble to understand the FMC setup for departure. On PERF / TAKEOFF REF Page the acceleration height was set to 400ft. I do know, that VNAV takes control over the speed from the takeoff ref page at acceleration altitude. So if the acceleration altitude would be -lets say- 1,500ft, the speed restriction could not come into effect until the aircraft reaches 1,500ft. But: Is it realistic to setup the FMC like that?

Thank you for your input,
Johannes

[*] http://www.pprune.org/tech-log/252121-when-will-744-vnav-respects-speed-altitude-restriction.html

[Hardy Heinlin]

The aircraft will decelerate upon reaching the waypoint

Define "upon reaching".


Regards,

|-|ardy

[John H Watson]

Also I have trouble to understand the FMC setup for departure. On PERF / TAKEOFF REF Page the acceleration height was set to 400ft. I do know, that VNAV takes control over the speed from the takeoff ref page at acceleration altitude. So if the acceleration altitude would be -lets say- 1,500ft, the speed restriction could not come into effect until the aircraft reaches 1,500ft. But: Is it realistic to setup the FMC like that?

400' does seem unusually low. The real world pilots can probably give an answer here, but I've seen 3000' entered in the real world if noise abatement was a consideration. If the focus is on climb rather than acceleration just after takeoff, then the higher the aircraft will be in the shortest distance (which means the people on the ground on the outskirts of airport will hear less noise). However, the gains in altitude produced by a fast initial climb rate start to diminish as time progresses. Climb rate is also dependent on speed as well as thrust. If the aircraft does not accelerate, you won't be able to retract the flaps and build up more speed. If there are mountains ahead, you  don't want to delay your acceleration too long.


Our airline's default, I recall, was 1000'.

(EDIT: Note that default values can be set on the FMC PERF FACTORS page. This page normally isn't for pilot's use, so this page has a kind of password. The word "ARM" has to be typed in the scratchpad and entered into the space provided. After this, the values on the PERF FACTORS page can be modified. When using this page, be sure to take a note of the original values in case something goes wrong. Note that these values will only be changed in one situation (situ file). You might want to save this file for future flights (if you want the same default values).

Rgds
JHW

[airlinermanager]

@Hardy:
Good morning,
as I do not have access to a real 744, I cannot answer this question. As far as I understand the issue, this could not even replicated in the Full Flight Sims at that time. Maybe the linked thread above can supply additional information?

400' does seem unusually low. The real world pilots can probably give an answer here, but I've seen 3000' entered in the real world if noise abatement was a consideration.

Hello John,
thank you for your reply. I am aware of the things you mentioned. What I was trying to say:

400ft are very low and I do not know why it has been set that low. For training purpose to avoid the following behavior?

If the acc. height would be 3,000ft, the speed target would be set by the TAKEOFF REF page rather than the VNAV CLB page. Therefore the speed restriction at the first waypoint would be ignored until the aircraft reaches acc. height. That is at least the way I understand the logic behind the TAKEOFF REF / VNAV CLB pages.

Best regards,
Johannes

[Hardy Heinlin]

400ft are very low and I do not know why it has been set that low

I created this situation. I set it to 400 to avoid an acceleration command to V2+10..25 etc., which would be useless anyway as the waypoint speed constraint is so low that flap retraction is impossible.


Regards,

|-|ardy

[IefCooreman]

The pprune thread talks about the requirement that speed restrictions in the legs page have to be linked to an altitude constraint. So you cannot have 220/... where "..." is the fmc calculated altitude. If you try to put 220/ in there it will not accept it (this is the same on the 777, according to the pprune thread this is the same for some versions of the 747) You have to set ie 220/1500A: it needs to be linked to an altitude constraint. What they complain about is the fact that apparently the FMC doesn't consider the speed restriction anymore if the altitude restriction is fullfilled. Since 1500 in a departure is below acceleration altitude, the aircraft will accelerate to 250 kts (if 250 below 10000 is set as a restriction on the VNAV page, otherwise it's the econ climb speed or similar).

Departures in Hong Kong do require some pilot manipulation anyway. In our FMC database, PORPA shows 220/5000. Since you need to keep 220 untill the turn after PORPA is completed, the FMC is unable to handle the departure completely on its own (there is no waypoint defining the end of the turn). You always open the MCP window when acceleration starts (but the FMC commands 220 since you still are below 5000) to keep the speed 220 in the turn after PORPA. I've personally never encountered the situation described on pprune thread because you are always cleared to climb to 9000ft unrestricted before you reach 5000ft. since pushing the ALT button will delete both speed and altitude restrictions, the speed window needs to be open with 220. For an arrival it certainly is not the case. Although it always starts to decelerate too late...

400ft for the acceleration is low, but acceptable according to the basic rules (it's the minimum I guess, also for the initial turn), but never done in real life. Noise abatement procedures are standard in all companies I know. They tend to change (ICAO to JAR to EASA...). I think minimum acceleration now is at or above 800ft AGL, although many still use 1500 for the CLB thrust, 3000 for the acceleration (unless ATC wants you out of the way, than we do it earlier). Look for NADP procedures on the internet and you will read all about it.

[Hardy Heinlin]

As unusual the 400 ft height is, as unusual is the speed constraint of 170 kt for a  heavy 744, filled up for a long-range flight.

As long as the aircraft is below the acceleration height, the waypoint speed constraint is ignored, and the aircraft will do the V2+10..25 timing stuff after VR. In my situation it would jump to 180 kt, I think, and only after acceleration height down to 170 kt. That's why I set 400.

It's a rather experimental situation, but the 170 kt constraint at LLBG is (or was?) a real-life requirement.


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[airlinermanager]

@Ief and Hardy:
Thank you for your input! I think I understood everything now. Thank you.

Best regards,
Johannes

[IefCooreman]

Quickly checked: it's actually 170 or V2+10kts up to 3000ft for noise abatement, which is pretty standard and close to NADP 1 procedures. So depending on the weight, you are allowed to delete the restriction in the FMC.

LLBG is a rather peculiar airport where you always maneuver in a very tight area when you are east of the airport. Departures require an immediate turn, arrivals: the RNAV GNSS 30 is preferred, which gives you a 90° (descending if you are in VNAV) turn to a 6 mile final, which is something you don't do a lot on a widebody... (read: only at LLBG :-))

[Avi]

The charts of the SIDs for RW08 say Max 170 KIAS or V2+10 valid up to 3000 only (it is not about NOGAH).
The reason was that until less than a year ago the airport was under constructions and only one RW was in used (26 or 08 ).
When traffic was on 08, a right turn after takeoff put the departure traffic in a direct path into the arriving traffic so it was important to give them altitude as fast as possible. The speed restriction is not because of radios turn or noise problems, it was (and still is) to delay the speed acceleration and gain altitude so you need to put speed acceleration to 3000 in the takeoff page and maybe even add a speed restriction in the VNAV page.

Heavy aircraft, by the way, can't make 3000A at NOGAH so at the time when many heavy aircraft departed to the USA at night one after the other, the airport was closed for landings.
Today, with 2 runways, it is not a problem but aircraft don't fly these SIDs anymore.

Cheers,

[airlinermanager]

Very interesting information. Thank you folks!

[Hardy Heinlin]

it's actually 170 or V2+10kts

Ah, that's nice. I didn't know that; I just looked at the FMC's nav data where the 170 kt constaint is hard coded in a leg for obvious reasons.


Cheers,

|-|ardy

[Hardy Heinlin]

01 Departure 002 - Tel Aviv - Hong Kong.situ

In this situ, V2 is 169. Suggestion:

1. LEGS page: Delete the constraint 170/3000A at NOGAH

2. VNAV CLB page: Enter 179/3000 in 4L

3. EXEC

During the climb the aircraft will aim at 179, and when passing 3000 -- which happens about a mile before NOGAH -- 4L will be dashed and the aircraft will start accelerating.


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