A couple of 737 questions

[VHOJT 167]

Hi there guys:Hoping some 737 guru out there can perhaps enlighten me:I have entered a series of waypoints and altitude constrains for an RNP (required navigation performance) RNAV approach from a chart I have. However, the aircraft seems insistent upon flying the approach in VNAV SPD mode, as opposed to VNAV PTH, even although I have speed well under control and on-target. Is there some trick to VNAV approaches I am missing or is this 737 not too great at them? I haven't done many in the 744, but it seems to be fine. That's my main question - it seems to stick in VNAV SPD mode, rather than VNAV PTH.Secondly, I take it from the PMDG documentation that the aircraft's minimum landing fuel is about 5,000lbs (about 2.2 tonnes). Does this sound correct, or should I not be adding both the minimum quantity required in tanks for pump coverage as well as go-around fuel? I have seen a couple of 737-800 flight plans that seem to plan for about 2.7 tonnes remaining on touchdown, it seems like there is not a lot of room for error? Thirdly, what do I do with flap 2 for departure? Skip straight from 5 to 1? There doesn't seem to be a marker for it on the PFD. Does one skip any increments of flap on the 737?Fourthly, during engine start, do I engage fuel from cutoff to run at 20% N2?I realise some of these questions are pretty obscure, although it is mostly the VNAV approach that is the one I wonder about.Thanks for any help any 737 flyers can offer!CheersRudy

[Chock 17,769]

You are in luck, because I've just spent the past three days investigating all that malarkey for my review of the Ariane 737, and quite a surprising journey of discovery it has been too. And you will be pleased to note that you are in good company in having difficulty with VNAV concepts...It's a common misconception among pilots as to what VNAV does and how it actually works. Most pilots are under the vague impression that VNAV is a magic button that can somehow make an aircraft maintain both a certain speed and a certain flight path to hit a target in three dimensional space from another point in three dimensional space either above or below. And in still air, it might actually be able to do that, but perfectly still air is a rare thing indeed, especially spread across a depth of several thousand feet. Of course when you think about it in those terms, it is obvious that something has to give in either speed or rate of descent if one is to hit a specific point in space on either a descent or climb.And so, in your example, if you have your speed 'well under control and on target' as you mentioned, it follows that the aircraft cannot maintain a set path if you are aiming to be at a certain height at a certain point in space, if it holds your speed, it must adjust the trajectory when aiming for a specific point, especially because you are moving through air which is not stationary. Seems obvious when you examine it like that, but look at these statistics:On average 73 percent of pilots use VNAV for climb phases, but only 20 percent use it for descents63 percent of pilots complain of having issues with understanding the relationship between the vertical speed and trajectory calculations which VNAV makesApproximately 70 percent of pilots have reported that VNAV is the most disliked feature in the modern airliner cockpitHave a read of this PDF (link below) and it should further clear things up for you. When you've checked that out, look at the CDU descent page on the 737 which has FPA (Flight Path Angle) V/B (Vertical Bearing) and V/S (Vertical Speed) on it:linkHappy reading.Al

[VHOJT 167]

You are in luck, because I've just spent the past three days investigating all that malarkey for my review of the Ariane 737, and quite a surprising journey of discovery it has been too. And you will be pleased to note that you are in good company in having difficulty with VNAV concepts...It's a common misconception among pilots as to what VNAV does and how it actually works. Most pilots are under the vague impression that VNAV is a magic button that can somehow make an aircraft maintain both a certain speed and a certain flight path to hit a target in three dimensional space from another point in three dimensional space either above or below. And in still air, it might actually be able to do that, but perfectly still air is a rare thing indeed, especially spread across a depth of several thousand feet. Of course when you think about it in those terms, it is obvious that something has to give in either speed or rate of descent if one is to hit a specific point in space on either a descent or climb.And so, in your example, if you have your speed 'well under control and on target' as you mentioned, it follows that the aircraft cannot maintain a set path if you are aiming to be at a certain height at a certain point in space, if it holds your speed, it must adjust the trajectory when aiming for a specific point, especially because you are moving through air which is not stationary. Seems obvious when you examine it like that, but look at these statistics:On average 73 percent of pilots use VNAV for climb phases, but only 20 percent use it for descents63 percent of pilots complain of having issues with understanding the relationship between the vertical speed and trajectory calculations which VNAV makesApproximately 70 percent of pilots have reported that VNAV is the most disliked feature in the modern airliner cockpitHave a read of this PDF (link below) and it should further clear things up for you. When you've checked that out, look at the CDU descent page on the 737 which has FPA (Flight Path Angle) V/B (Vertical Bearing) and V/S (Vertical Speed) on it:linkHappy reading.Al

Hi there AlThanks for the very interesting reply and link, I am still in the process of reading the journal you linked to - it makes interesting reading.My idea was that the 738 during an LNAV/VNAV APP should be using throttle to control speed, LNAV for direction and VNAV PTH to follow a path. Perhaps I am mistaken though? It seems that is what it was trying to do, but wasn't able to slow down enough, so kept switching (and staying in, even after speed was captured, VNAV SPD mode), even if given spoilers. I attempted the approach a couple more times, once in the 744, starting at 240 kts with no problems, and once again in the 738. I think it has to do with the fact the 738 doesn't really want to slow down very much without spoilers, and seemed to revert to FMC SPD if too "radical" a slow-down was put in (i.e. about 20 kts, from 200-180). If I start the approach at about 160-170kts (albeit the first waypoint is at 5600'), it seems to be happy to maintain the approach (as there is not much slowing down required throughout it) in VNAV PTH. The approach is in a constant turn, so it also makes it somewhat disconcerting when your plane is not doing as it should. I wonder if it is like that in real-life.The 744 flew the approach in SPD | LNAV | VNAV PTH mode, whereas the 738 seemed to alternate between ARM | LNAV | VNAV PTH and FMC SPD | LNAV | VNAV PTH. I didn't really "trust" it, and in future I think I'll fly such approaches in V/S mode and use the V/S recommendations on the FMC DES page for such an approach as you pointed out. I'm going to keep reading that article, it's interesting...Thanks againRudy

[downscc 1,327]

I prefer to use LNAV, MCP SPD and V/S controls on descents below flight levels in the 737 (the MD11 PROF abilities are amazingly better). Approaches are never flown with V/S, either approach-coupled autopilot or manual with autothrust for speed set with MCP.Being too fast is a common error to new jet pilots... you gotta stay ahead of the airplane. You can safely slow to 210KIAS in the terminal area (within 10 nm or so), you shouldn't need spoilers in most cases to do this.

[Guest fencer88]

Thirdly, what do I do with flap 2 for departure? Skip straight from 5 to 1? There doesn't seem to be a marker for it on the PFD. Does one skip any increments of flap on the 737?Fourthly, during engine start, do I engage fuel from cutoff to run at 20% N2?CheersRudy

Hi Rudy,Re your third question : I think flap 2 is rarely (ie never?) used. Maybe another simmer or RW pilot can confirm this.Re your fourth question : see link belowhttp://forums1.avsim.net/index.php?showtop...p;#entry1567136Cheers,Bruno

[michal 44]

I have entered a series of waypoints and altitude constrains for an RNP (required navigation performance) RNAV approach from a chart I have.

I think that particular PMDG implementation of VNAV was done fairly long time ago and as such doesn't have full blown RNAV capability. The upcoming NG 2.0 should be RNAV capable - at least per some recent hints on this forum.

[Chock 17,769]

Here's another good guide for the 737's systems. It's for the Classic rather than the NG, but the FMC explanations are very clear to follow and still largely relevant for NG 737s:737 guideAl

[downscc 1,327]

I think that particular PMDG implementation of VNAV was done fairly long time ago and as such doesn't have full blown RNAV capability. The upcoming NG 2.0 should be RNAV capable - at least per some recent hints on this forum.

This is an interesting subject all to its own. I have programmed the RNP approaches at locations that have them in my collection, in most cases I believe the PMDG to fly within the 0.5 range but not at the 0.1 error range. Hard to tell, but it is a moot point since the arcs are not real because we don't have a fix-to-arc definition in the procedure language. The performance on final RNP descent bugs me a little where intermediate fixes have crossing altitudes the FMS will generally descend to that altitude then start to level off instead of a continued descent... I don't know how to fix this.Anyway, the 737 will certainly fly RNP0.5 well enough with carefully built terminal procedures.

[VHOJT 167]

This is an interesting subject all to its own. I have programmed the RNP approaches at locations that have them in my collection, in most cases I believe the PMDG to fly within the 0.5 range but not at the 0.1 error range. Hard to tell, but it is a moot point since the arcs are not real because we don't have a fix-to-arc definition in the procedure language. The performance on final RNP descent bugs me a little where intermediate fixes have crossing altitudes the FMS will generally descend to that altitude then start to level off instead of a continued descent... I don't know how to fix this.Anyway, the 737 will certainly fly RNP0.5 well enough with carefully built terminal procedures.

Hi guysSorry for the late reply! Thanks for the all the useful replies that you have given. As far as I can tell the 747 seems to be consistently better at flying the approach, as someone said it could be due to the fact that the 737 is quite an "old" design for PMDG now. Still, the 737 seems to manage it okay some of the time - except as squire above mentioned the problem with VNAV "diving and driving". I won't use V/S if that isn't done in the real-world.That journal was also quite interesting.Thanks again guysRudy

[Spin737 102]

Hi guysSorry for the late reply! Thanks for the all the useful replies that you have given. As far as I can tell the 747 seems to be consistently better at flying the approach, as someone said it could be due to the fact that the 737 is quite an "old" design for PMDG now. Still, the 737 seems to manage it okay some of the time - except as squire above mentioned the problem with VNAV "diving and driving". I won't use V/S if that isn't done in the real-world.That journal was also quite interesting.Thanks again guysRudy

Another note on this topic - VNAV will overspeed the plane in VNAV PATH. This is mostly a prob above FL370. To prevent this, you can do an early DESCEND NOW. I use the formula of 4nm/1000' above FL370. Soooo, if you're at FL410 start your descent 16nm before T/D. And a HW/TW of 50knots = 1mn of correction, also.Another handy trick is to put in a fix 35nm from your destination at put in a restriction of 10k/250. VNAV down to that point, and then go into LVL CHNG or V/S.~Matt Colles