PDA

View Full Version : LNAV/VNAV Approach.


mach.865
13th Apr 2006, 02:46
Was watching a VOR/DME App. flown in LNAV/VNAV. The pilot was flying with speed intervention selected, he then closed the window and the a/c started accelerating to 250 knots, after which he reselected speed intervention.

Does anybody know when the approach is activated? ie distance from the initial approach point. I cannot find it in the books.

Apart from inserting a lower speed into the FMS are there any other methods from preventing this happening.

If for instance changing modes from LVCH to VNAV and the a/c is being flown at 210kts clean, once VNAV is selected the a/c again wants to accelerate to 250 kts making it look very untidy.

Would appreciate any opinions.

alatnariver
13th Apr 2006, 05:42
Well there is no such point or procedure, at least on the B744, when or where to activate the approach. Maybe you refere to Airbus by using activate in the contents of fyling an approach.

Basically you have at one point before you start the approach to change to VNAV to enable the aircraft to follow the coded descent profile of the approach. If you are flying in any other mode than VNAV, speed intervention is active and so the speed window is open. When you change to VNAV the speed window will close and the aircraft will fly the VNAV comanded speed. As you have noticed this may end that the aircraft starts accelerating to the standard VNAV speed, below FL100 this is usually 250 kts.

As you mentioned you could avoid the aircraft accellerating by inserting a speed, preferably the speed you are currently flying when switching to VNAV, on the VNAV page, this works fine, or you change to VNAV before you start decellerating, this works also fine. I would do the second method and for me this worked fine until now, even when flying on radar vectors. The other option is the one you have seen, but you have to be quick and prepared. An other option which just came to my mind, would be to disengage the A/T before changing to VNAV and thereafter reengaging the A/T. Should work as well, but a lot more of switching would be involved.

I totally agree with you that this behaviour of the aircraft when switching back to VNAV looks untidy.

junior_man
14th Apr 2006, 04:42
If this was an Airbus, it will activate the approach mode on it's own only when it has an LNAV route all the way to the runway (at least in the terminal area).

That is not very common. When you are getting radar vectors to an approach you do it by selecting perf page and selecting activate approach mode and you then confirm it. That will give you managed speeds for the approach. If you forget you will get speed 250 when you try to select managed speed.

BuzzBox
14th Apr 2006, 13:55
Sounds distinctly Boeing to me. On the B777, the FMC transitions to "on approach" under the following conditions:

the aircraft is in the descent phase and the flaps are out of up, or
a published approach has been selected in the active flight plan, and the aircraft has sequenced the first waypoint of the published approach, or
the aircraft is enroute to a direct-to or intercept-to waypoint and is within 12nm of the runway.

If VNAV speed intervention is being used and the FMC transitions to "on approach", the speed window will close and the FMC will command whatever speed was programmed at that point in the approach profile; it may be 250 knots if further out, or whatever speed constraint was programmed at the FAF if close in. There's no way around it, other than changing the speed in the FMC.

When the FMC transitions to "on approach", the IAS/MACH window can be opened, and the command speed can be set while VNAV remains in VNAV PTH. Once the window is open, VNAV will command the speed set in the window.

Boeing recommends using VNAV with speed intervention active (ie the speed window open) during the final approach. They don't recommend adding extra speed constraints to the final approach waypoints.

Old Smokey
14th Apr 2006, 16:44
BuzzBox has spelled out very well the logic used by the B777 FMC for the entry 'gates' to approach mode. During Descent and VNAV approach, the FMC VNAV functions well to maintain the profile accuretely, but makes no consideration for the Flap setting at the time.

During the pre-approach descent, speed is allowed to vary Up and Down to maintain the VNAV path accurately. Speed is allowed to vary +10/-20 knots from the programmed speed to maintain the descent profile. For example, a descent scheduled at 300 KIAS may be seen to operate between 280 to 310 KIAS (common). Not a problem at high speed, where even the lower speed limit is safe. The default speed below 10,000 feet is 240 KIAS, which, after allowing for the tolerable 10 knot increase, the nominal 250 KIAS speed limit below 10,000 feet is not exceeded. Perhaps, mach.865, this is where you have seen the 250 KIAS excursions.

The default speed during approach is 170 KIAS, which approximates the Flaps 5 manoeuvering speed. This is of course, like all other speeds, pilot programmable to a different value. The problem is that the VNAV logic does not consider the Flap setting at the time, and will, using auto-thrust, maintain the programmed speed irrespective of the Flap setting. Auto-thrust will intervene if the speed drops as low as VLS (well below manoeuvering speed).

It is not at all uncommon during training, or with a low experience pilot, to see low speed excursions resulting from leaving VNAV engaged for too long at the end of the descent and at the Initial Approach phase.

Now, if the pilot were to pre-programme all of the various speeds for the approach, 5 of them - Clean Speed, Flaps 1 speed, Flaps 5 speed, Flaps 20 speed, and finally Vapp with Flaps 30, and VNAV used, the pilot would have to take the next Flap setting when commanded by the VNAV re-scheduled speed, very messy indeed! It is infinitely preferable to pre-set the INITAIAL entry speed for passing the approach mode 'gate' (as defined by BuzzBox), taking manual speed intervention from there on, let VNAV do it's primary task of accurate profile management, and manually setting the new speeds as further Flap settings are made by the pilot.

It would be a rare day indeed if the speed / altitude / wind upon arrival was exactly as expected when the FMC set-up was made 200 miles previously.

If all of the fore-going made no sense, let me summarise it by saying that VNAV makes no consideration for the existing Flap setting or it's manoeuvre speeds, only VLS (last chance) is considered. Manual speed intervention is very very prudent indeed.

Regards,

Old Smokey

mach.865
15th Apr 2006, 07:58
If for instance one was flying the above approach and the flap clean speed was 210kts, would you insert 210kts in the FMC at the FAF, which in this case is 10.6DME.

Would you then fly a decelerated approach in VNAV with speed intervention or would one create a WPT before 10.6 DME to be at 210 kts and then be totally configured at 10.6DME with Vapp+5.thereby flying the entire approach totally configured and stabilised.

As an aside, on some descents where one is vectored up to 30 degrees from the track I have seen that flown in VNAV speed. Is there any advantage in staying in VNAV with a diverging track?? when this occurs I normally fly it in LVCH or V/S. I would appreciate any comments.

BuzzBox
15th Apr 2006, 10:02
If for instance one was flying the above approach and the flap clean speed was 210kts, would you insert 210kts in the FMC at the FAF, which in this case is 10.6DME.

Would you then fly a decelerated approach in VNAV with speed intervention or would one create a WPT before 10.6 DME to be at 210 kts and then be totally configured at 10.6DME with Vapp+5.thereby flying the entire approach totally configured and stabilised.


I suppose you COULD fly it that way, but why bother? Boeing states: "Adding speed constraints to the final approach waypoints is not recommended because of the extra workload, no safety benefit and reduced ability to make last minute approach changes". Boeing also recommends using VNAV with speed intervention on the final approach.

It's far better to keep the process as simple as possible, rather than complicating things by adding extra speed constraints, waypoints, etc. I would fly the approach as follows:

1. Leave all speed constraints at their default values.

2. Fly the descent to the initial approach altitude in whatever mode you like - VNAV PTH, VNAV SPD, FLCH or VS. As you said, if you are being vectored off track, it might be preferable to use VS or FLCH rather than VNAV to maintain a reasonable descent profile.

3. Reduce speed as required, using speed intervention.

4. At some stage before the approach commences, select VNAV if not already selected.

5. When the FMC transitions to "on approach", the speed window will close, and the speed bug will move to the FMC speed. Be prepared, and open the speed window again and set the speed YOU want to fly. As Old Smokey said, the FMC makes no allowance for flap settings and will select a speed lower than the flaps up manoeuvring speed even if the flaps are up, so you must anticipate the speed window closing.

6. From then on, you will be in VNAV PTH with speed intervention. Make sure you have a lower altitude set in the MCP window, configure the aircraft and reduce speed on schedule using the MCP selected speed. Simple!

300ER
16th Apr 2006, 04:20
Quote:BuzzBox

Adding speed constraints to the final approach waypoints is not recommended because of the extra workload, no safety benefit and reduced ability to make last minute approach changes".
When the FMC transitions to "on approach", the speed window will close, and the speed bug will move to the FMC speed.

BuzzBox,
It's quite obvious mach.865 knows how to set up the approach, what he did ask for was a better way to do this because as he pointed out it looks untidy.

1.How long would it take to insert 210kts in the FMC, less than 5 secs and how does it reduce the ability to make last minute changes?

2. Secondly whenever you select VNAV from basic modes the window always closes, you can then select speed intervention.You don't have to be in approach mode for the window to close.

3. I can see one benefit in inserting the speed he quotes 210kts in the FMC at the initial approach altitude because when leveling off at that altitude and VNAV is selected the window closes and provided the a/c is already at 210kts there is no thrust or speed increase. Speed intervention can then be employed for speed reduction. Obviously the a/c will be levelled off a few miles before the descent point.

4. From an intersest point of view do you fly the whole approach configured at Vapp or do you fly a decelerated approach, a question he also asked. The question is" simple! "

BuzzBox
16th Apr 2006, 05:00
300ER:

Take a chill pill fella.

The information I provided was taken straight from the Boeing 777 Flight Crew Training Manual. If you don't like what I quoted, perhaps you should argue the toss with Boeing.

I was trying to be helpful by outlining how I would fly the approach. Instead of attacking me, how about providing some insight yourself?

300ER
16th Apr 2006, 06:45
BuzzBox:

Perhaps the way you expressed yourself got up my nose also ending a post with the word 'simple!" is unwarranted.

There are many ways of operating an aircraft and as the initial poster pointed out he was trying to make the approach tidier. There was no need for sarcasm.

I have found placing an appropriate speed in the FMC at the descent point works well and suits me, I'm sure others have their own points of view.

BuzzBox
16th Apr 2006, 07:07
My comments were absolutely not intended to be sarcastic. I merely suggested keeping things as simple as possible rather than getting tangled up by adding extra waypoints/speed constraints, etc for little benefit. If adding an extra speed constraint works for you, then great - personally, I don't bother.

Cheers.

alexban
17th Apr 2006, 12:21
As you know VNAV will compute it's own speed reduction point ,going from 250 kts to flaps up speed at that point.If you'll select Vnav before this point,the target speed will become 250.
FCTM recommends to select VNAV aprox 2NM before FAF (if you've used other pitch mode).
You can force the 210 kts on the FMC by selecting restriction on different waypoints or by making it the target speed,but generally it's not required.
If you fly an unrestricted vnav app,the method is to use lnav with whatever pitch mode you feel required,and 2 NM before FAF select VNAV with MDA on the MCP.Procedure it's the same for 737 classic or NG.