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Martinlest
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Questions re. VNAV to 737 pilot please :-)

Wed Jul 15, 2020 7:45 am

Hi. If you fly the 737-800 (in particular) real world, I'd be grateful for some information to help me in my X-Plane flights! The 'Zibo' model there is supposed to be pretty accurate in the way it behaves, so...

Situation: having levelled off at say, 7000' and you're flying at 250kts, you are given instructions to climb further. What are the options commonly used to restart your climb? Specifically:

1. Would you already be in VNAV mode at that stage? When would you engage VNAV + A/P typically? Soon after takeoff? Or could you use V/S mode for takeoff? (Yes, you could hand fly too of course!).

2. Assuming you are in VNAV mode as you level off, what are the options to start the climb again when instructed? If you dial in the new altitude and VNAV mode is engaged, does the a/c start climbing straight away - or do you have to disengage and immediately re-engage VNAV (as in X-Plane)? Is it possible to use ALT INTV to restart the climb? Or do pilots use LVL CHG? Or, as I mostly do, use V/S?

Seems to me that being in VNAV mode at this stage always results in way too aggressive an increase in engine power (non de-rated takeoff - or are 737 takeoffs de-rated as a matter of course??) and therefore a very steep climb rate (the X-Plane 738 will shoot up to as much as 265kts and climb at up to 6000ft/min to bring that back down to 250 till 10,000' - a very uncomfortable ride for everyone). Does that reflect the real behaviour if you use VNAV to climb at this stage? From what I have read, it does.

Am I right then in sticking to V/S mode at this stage? If so, when would pilots (and I) most commonly engage VNAV? Seems a bit odd to use VNAV after takeoff, then disengage it (??).

Many thanks for any feedback on the various options you'd commonly be able to use here. I tend to engage VNAV + A/P at about 2000' AGL (A/Thr is on too from before takeoff) and that works OK.. it's just after levelling off and restarting the climb that the problems occur.

Martin :smile:
 
Redbellyguppy
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Re: Questions re. VNAV to 737 pilot please :-)

Fri Jul 17, 2020 7:28 am

So if you arm the vnav on the ground, you’ll have the window open and v2 set. When you take off, the window will close and the speed bug will be set by the vnav and you will be in vnav speed for your climb. Assuming you don’t have 7000 set as your cruise altitude, to continue your climb you would most logically press altitude intervene and again climb at 250 knots, at clb-1,2, or max climb thrust depending on what you choose, in vnav speed. This is functionally identical to pressing level change except that the window will be open in level change, as opposed to closed in vnav speed. The speed bug also won’t reschedule through 10,000 if you are in level change.

I don’t typically use vs in the climb unless I’m trying to shallow out a climb due to approaching traffic/tcas or trying to do a zoom climb... this is rare.

We derate every takeoff that we have the numbers for and I’ve not experienced the kind of vnav misbehavior you describe. That said, I’m typically hand flying below 18, too.
 
spacecadet
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Re: Questions re. VNAV to 737 pilot please :-)

Sat Jul 18, 2020 8:05 am

Keep in mind that X-Plane is almost certainly not realistic in how it handles thrust vs. speed and climb in its AP modes. I doubt it's doing much other than adding some arbitrary amount of power and pitching up some arbitrary amount. The logic is almost definitely not the same as real life.

I'm trained on the E-190, not the 737, so I can't really speak to exactly how the 737's VNAV or LVL CHG modes work. But I doubt the V/S mode works any differently. V/S mode is a "speed on thrust" mode where the plane is using engine thrust to try to maintain a vertical speed. This is one mode you usually do *not* want to use in climb. You will either need to constantly be adjusting vertical speed manually, or the plane will eventually just not be able to maintain the vertical speed you set and could potentially stall. There might be some kind of protection for this in the 737, I'm not sure. There definitely is not in all airliners. Even if there is protection, it's still just not a great idea to use it all the way up in a climb.
I'm tired of being a wanna-be league bowler. I wanna be a league bowler!
 
Martinlest
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Re: Questions re. VNAV to 737 pilot please :-)

Sat Jul 18, 2020 9:52 am

Thanks for the comments. So if you are cleared to, say, 7000' after takeoff and end up levelling off at that altitude before ATC clears you to ascend, which of the various options for the further climb do you most often employ?
 
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Starlionblue
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Re: Questions re. VNAV to 737 pilot please :-)

Sat Jul 18, 2020 10:16 am

Martinlest wrote:
Thanks for the comments. So if you are cleared to, say, 7000' after takeoff and end up levelling off at that altitude before ATC clears you to ascend, which of the various options for the further climb do you most often employ?


Anything but V/S is fine. It depends on the location most of the time. On the 'bus we can't use NAV (VNAV) if we're on a heading. So we use NAV (VNAV) if on the FMS path but otherwise not.

Also, we never "ascend". We climb. Because of potential confusion on the radio. ;)
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Martinlest
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Re: Questions re. VNAV to 737 pilot please :-)

Sat Jul 18, 2020 6:14 pm

Also, we never "ascend".

I know - but I teach English: one should always avoid using the same word twice in close proximity: sounds ugly. Hence 'climb', and 'ascend'... :-)
 
AABusDrvr
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Re: Questions re. VNAV to 737 pilot please :-)

Sat Jul 18, 2020 8:07 pm

Martinlest wrote:
Thanks for the comments. So if you are cleared to, say, 7000' after takeoff and end up levelling off at that altitude before ATC clears you to ascend, which of the various options for the further climb do you most often employ?


At my airline, we usually engage VNAV at 400' AFL. In the scenario you described above, level at 7000', and cleared to a higher altitude, we would set and confirm the new altitude in the MCP altitude window, and press the ALT INV button once.

If you are in VNAV, you can think of the ALT INV as a "delete" button. Each press of the ALT INV button will delete an altitude constraint. So if you are level at 7,000 and press the button, it deletes the 7,000' altitude, and the airplane will climb away from it. If you had a departure, or arrival procedure with multiple altitude restrictions, and kept pushing the ALT INV, it would eventually delete all the altitude restrictions, and the VNAV won't honor any of them.

We are strongly discouraged from using vertical speed, because there is no speed protection. You can stall, or overspeed the airplane in V/S.
 
IAHFLYR
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Re: Questions re. VNAV to 737 pilot please :-)

Sat Jul 18, 2020 9:11 pm

I'm not a "guppy" (B73) pilot, though have very good friends who are and also have quite a few hours flying in a Level D B738 sim some years back and have PMDG NGX on my computer which does seem to have quite good thrust/speed/climb/descent profiles within the A/P. From what I recall using VNAV on an RNAV SID that contains altitude restraints, setting the top altitude on the SID in the altitude window of the MCP, the plane will comply with all altitude constraints then climb after the fix at 250 KIAS if below 10,000' MSL or at whatever speed you have input into the IAS/MACH window, or at the published speed at the next fix on the SID should there be one.

LVL CHG I used when there were no altitude constraints and had selected 250 KIAS in IAS/MACH window of the MCP so stalling or over speed below 10,000' MSL was not an issue. After 10,000' MSL the speed would produce the climb speed from the performance data input into the FMC such as weight and cost index.

If my memory has left me I'm sure an NG driver will correct me. Best of luck with your issue.
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Starlionblue
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 1:15 am

Starlionblue wrote:
Martinlest wrote:
Thanks for the comments. So if you are cleared to, say, 7000' after takeoff and end up levelling off at that altitude before ATC clears you to ascend, which of the various options for the further climb do you most often employ?


Anything but V/S is fine. It depends on the location most of the time. On the 'bus we can't use NAV (VNAV) if we're on a heading. So we use NAV (VNAV) if on the FMS path but otherwise not.

Also, we never "ascend". We climb. Because of potential confusion on the radio. ;)


Re-reading this, I wrote the wrong mode. :oops:

What I meant to say was we can't use CLB (VNAV) while on a heading, so we use CLB (VNAV) if on the FM flight plan but otherwise not. And anyway avoid V/S in a climb.

The exception would be at lower altitudes to avoid a TCAS alert. You can temporarily use V/S to decrease the rate of climb.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
IAHFLYR
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 6:12 am

Starlionblue wrote:

The exception would be at lower altitudes to avoid a TCAS alert. You can temporarily use V/S to decrease the rate of climb.


TCAS adaptations in the last 10 years have pretty much eliminated the reversal situation which happened quite often with fast climbers to fast descending aircraft. Your point is well taken though.
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Martinlest
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 10:01 am

In the scenario you described above, level at 7000', and cleared to a higher altitude, we would set and confirm the new altitude in the MCP altitude window, and press the ALT INV button once.


In X-Plane, I use ALT INTV (and later the SPD INTV) in descent and it works just fine: doing that to climb from level flight below 10,000' (assuming a speed constraint of 250kts of course) causes, first, the engines to increase power markedly, which in turn causes the airspeed to rise from 250kts to around 265kts: consequently, because of the 250kts constraint in the FMC, the 737 can reach a climb rate of almost 6000ft/min in order to bring the speed back to 250kts. At that climb rate, it does this pretty quickly, so the reduction in climb rate is then also fairly severe. The overall result is, for ten seconds or so, very unauthentic-looking, almost porpoising behaviour. I have never piloted a real 737 of course, but I have travelled in one many times and would have noticed this, along with all the other passengers!

We are strongly discouraged from using vertical speed, because there is no speed protection. You can stall, or overspeed the airplane in V/S.


Quite! But in X-Plane (the 737-800 is constantly 'tweaked' by an enthusiast, and in many respects in deemed (by real-world pilots) to be very accurately modelled, I should add) that is the only way I seem to be able to get a satisfactory climb profile, using V/S. I posted this question really to verify that the programming is unrealistic here.

The basic problem. as I think I originally stated, seems to be the immediate increase in engine power as you initiate the climb, which then has the expected consequences. In a real 737NG, what increase in N2 would one expect to see in this situation?

Maybe I should always set my under 10K' SPD constraints to around 230kts in the FMC, rather than the automatically prescribed 250?

Thank you again for all your answers and feedback.

Martin
 
AABusDrvr
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 1:09 pm

Martin,

Thats a modeling issue. The real airplane makes a pretty smooth transition from level to climb when in VNAV, adding power slowly, while adjusting pitch.

All of our airplanes have a default speed of 240 below 10K in the FMC's. This may be a carrier specific thing, but I'm not certain. I was told they did that to help limit flap overspeeds, but the may be just airline legend. We also don't select flaps 1 above 240 (even though the limit speed is 250) again, to prevent overspeeds.
 
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Starlionblue
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 1:23 pm

The stock aircraft in flight simulators tend to take some liberties and shortcuts in their accuracy. Most users aren't using the sim to that level so it doesn't hurt sales. And I think if you accurately model the autoflight modes and systems, operating the thing ends up beyond the scope of casual simmers.

If you want modelling that comes close to replicating the real thing, you have no choice but to buy an add-on that typically costs more than the base program.

There's a reason Flight Factor's A320 for X-Plane costs US$90. :)
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
IAHFLYR
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 2:43 pm

AABusDrvr wrote:
All of our airplanes have a default speed of 240 below 10K in the FMC's. This may be a carrier specific thing, but I'm not certain. I was told they did that to help limit flap overspeeds, but the may be just airline legend. We also don't select flaps 1 above 240 (even though the limit speed is 250) again, to prevent overspeeds.


Yes, the descent page of the FMC on all Smiths boxes on the NG's do have 240 KIAS set as default so not to bust the below 10,000' MSL restriction as well as the Rockwell Collins boxes. The guys who I know are actual pilots of NG's and B75/6 tell me one of the first things they do is to override the default 240 KIAS speed and put in 250 KIAS in the box.

Martin, solve your entire issue simply by using LVL CHG and don't worry about the rest of it, leave that to the sim designers! :white:
Any views shared are strictly my own and do not a represent those of any former employer.
 
AABusDrvr
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 3:17 pm

IAHFLYR wrote:
AABusDrvr wrote:
All of our airplanes have a default speed of 240 below 10K in the FMC's. This may be a carrier specific thing, but I'm not certain. I was told they did that to help limit flap overspeeds, but the may be just airline legend. We also don't select flaps 1 above 240 (even though the limit speed is 250) again, to prevent overspeeds.


Yes, the descent page of the FMC on all Smiths boxes on the NG's do have 240 KIAS set as default so not to bust the below 10,000' MSL restriction as well as the Rockwell Collins boxes. The guys who I know are actual pilots of NG's and B75/6 tell me one of the first things they do is to override the default 240 KIAS speed and put in 250 KIAS in the box.

Martin, solve your entire issue simply by using LVL CHG and don't worry about the rest of it, leave that to the sim designers! :white:



That must be a carrier culture thing, I've never flown with anyone at my shop that has changed the default 240 to 250. If there are no other speed restrictions on a departure, we just climb 240 to 10K, and then accelerate to 290.

Unless we are doing an RNAV approach, during descent we almost always use LVL CHG below 10K, and select 240. But I can count on one hand the number of times I've used LVL CHG during a climb.
 
IAHFLYR
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Re: Questions re. VNAV to 737 pilot please :-)

Sun Jul 19, 2020 5:07 pm

I'm pretty sure it was their culture as you mention.

As for LVL CHG in the climb reference I made a few posts earlier, I failed to put in that VNAV climbs do an excellent job and leaving 10,000' MSL will accelerate to the FMC climb profile speed in the CLB page. My bad! :banghead:
Any views shared are strictly my own and do not a represent those of any former employer.
 
Redbellyguppy
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Re: Questions re. VNAV to 737 pilot please :-)

Mon Jul 20, 2020 3:08 am

Using vs to shallow a climb is just as a courtesy to the other aircraft by trying not to cause a climb escape if it can be avoided.
 
Martinlest
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Re: Questions re. VNAV to 737 pilot please :-)

Tue Jul 21, 2020 7:04 am

Thanks again for the continuing comments.

The 'Zibo' 738 is an ongoing project (updated every couple of weeks!).. it is quite something on the whole, and it is freeware! Some of the payware I have installed into X-Plane is equally amazing - the Toliss a319 or the FF767 for instance. I think such aircraft replicate accurately and in great detail the procedures and dynamics of the real aircraft - certainly 'feels' that way, and reports from real-world pilots of these aircraft seem to confirm that.

The climb issue with the 'Zibo' 738 is clearly a glitch that still needs to be addressed. (LVL CHG causes exactly the same behaviour, by the way).

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