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B737 VNAV ALT Intervention Techniques

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B737 VNAV ALT Intervention Techniques

Old 14th Aug 2019, 18:17
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B737 VNAV ALT Intervention Techniques

Hello everyone,
I just started training on B737 and had few observation flights on B737 last week. Reading off the book and applying in real ops is not the right learning process, however there were few scenarios that conflicted with my understanding when captain manipulated VNAV ALT INTV.
ALT INTV cancels any FMC restriction during climb/descent which are lower/hihger than MCP setting. When VNAV PATH or VNAV ALT and you were instructed to climb higher or lower, you set new MCP ALT and press ALT INTV and plance will continue to climb/descend (putting this as simple as possible)
From what I have noticed was that, we were VNAV ALT and soon instructed to climb higher, captain sets the new ALT on MCP and set LVL CHNG and VNAV right away. (After plane began positve climb).
It was done in a similar way during the descent and going through FCOM and FCTM does not really relieve me of curiosity on that day.
Looking forwad to ideas and comments.
tae9141 is offline  
Old 14th Aug 2019, 18:23
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I don't see any valid reason to leave VNAV ALT in LVL CHNG and then straight back to VNAV SPD/PATH.

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Old 14th Aug 2019, 21:32
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Originally Posted by Banana Joe View Post
I don't see any valid reason to leave VNAV ALT in LVL CHNG and then straight back to VNAV SPD/PATH.
Not done on other Boeings with VNAV ALT either. Homemade procedure by that captain?
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Old 14th Aug 2019, 21:36
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Is the A/T more aggressive in LVL CHG? VNAV SPD can be preeeety lazy with thrust changes.
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Old 14th Aug 2019, 21:40
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Why not ask him in the cruise? He may have a back story to go along with it that might help you make your own choices. Always ask the question, a hidden golden rule of aviation.
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Old 14th Aug 2019, 23:34
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Originally Posted by ImbracableCrunk View Post
Is the A/T more aggressive in LVL CHG? VNAV SPD can be preeeety lazy with thrust changes.
No. As you probably know, they function the same way but VNAV SPD will respect the AT and AT or ABOVE restrictions. You can always override the thrust levers and literally slam them to idle. Also upon VNAV PATH descent capture.
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Old 15th Aug 2019, 00:18
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I'm only speaking about the speed at which N1 is changed. In VNAV SPD during a transition from level to climb, it takes VNAV forever to start a climb where LVL CHG seems to add power more quickly.
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Old 15th Aug 2019, 00:55
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My head hurts.
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Old 18th Aug 2019, 03:38
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Originally Posted by Capn Bloggs View Post
My head hurts.
Likewise!

You only need to use ALT INTV in the climb to set a new cruise alt in the FMC. For example: your FMC cruise ALT is FL380. You've leveled off at FL360 due to ATC. When they give you a climb to 380 you just set that in the MCP and the AP will go to ALT HOLD mode, when you re select VNAV it will resume the programmed CLB profile up to 380. If you later want to climb to 400 you simply set the MCP to 400 and press ALT INTV, it will automatically re cruise the FMC to 400 and begin a cruise climb at the calculated speed (Mach).

On descent INTV now has 2 functions depending on distance to TOD, at least on U11 and later software. If you're more than 50 miles from calculated TOD and set a lower MCP altitude then press INTV it will re cruise to the lower altitude and descend at 1000fpm at the lower alt cruise speed. You shouldn't do this for a large (4000 feet or more) change as the new cruise speed may be too slow for your current altitude. If you're less than 50 miles from TOD when you push INTV it will go to "early descent" mode, which means 1000fpm at the normal descent profile until you reach A: the new MCP altitude, B: a restriction in the FMC above MCP altitude, or C: the idle descent path at which point it will descend in the path until reaching A or B. Since the FMC was not re cruised in this case when starting a further descent one needs only set the new MCP altitude and re select VNAV if necessary, it will continue descending in early descent as above.

I didn't explain the use of intervention to delete FMC crossing altitudes as that seems to be understood already.

There are way too many techniques to get into, but if someone is fat fingering multiple modes in short time for no apparent reason it would seem to indicate a lack of confidence/familiarity with the automation. Which is understandable as it's not an intuitive system.
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Old 18th Aug 2019, 06:01
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Originally Posted by Sig229 View Post
Likewise!

You only need to use ALT INTV in the climb to set a new cruise alt in the FMC. For example: your FMC cruise ALT is FL380. You've leveled off at FL360 due to ATC. When they give you a climb to 380 you just set that in the MCP and the AP will go to ALT HOLD mode, when you re select VNAV it will resume the programmed CLB profile up to 380. If you later want to climb to 400 you simply set the MCP to 400 and press ALT INTV, it will automatically re cruise the FMC to 400 and begin a cruise climb at the calculated speed (Mach).
If you level off in VNAV below the FMC cruise altitude it will be in VNAV ALT. Pressing ALT INTV while in ALT HOLD does nothing, as ALT HLD isn’t a VNAV mode.
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Old 18th Aug 2019, 08:45
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Not every aircraft have VNAV ALT installed. There are several configurations available.
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Old 18th Aug 2019, 18:30
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TAE9141: What you described was/is quite common at my carrier and is generally caused by a poor understanding of Vnav. Assuming VNAV ALT works the same as the B777/B787, here is how I taught it:
Think of the VNAV ALT function as nothing more than an MCP selected vertical constraint to your FMC programmed vertical profile. Another way to think of VNAV ALT - when annunciated, you have a conflict between your FMC programmed cruise (or final descent) altitude and the MCP altitude window.

When you load the FMC as part of preflight duties, you will have entered a CRUISE ALT on your PERF page (assuming of course it’s identical to the big Boeings). The FMC will create an optimum climb IAS/MACH to get you to your FMC cruise altitude based on your weight/winds/OAT, etc. But what the FMC can never know are the ATC restrictions to your climb. So, for example, you get airborne having told the FMC that you want to cruise at (for example), FL380. During the climb, let’s say ATC clears you to FL220 due traffic, expect higher shortly. The FMC had no way to know that ATC was going to interrupt your climb, so you have to restrict the FMC VNAV Climb phase by selecting FL220 on the MCP, and VNAV ALT results. So after a few minutes ATC now clears you to your FMC cruise altitude of FL380. To clear the conflict (as described above), you set FL380 in the MCP window, but remember that whenever VNAV ALT is triggered, in our case at FL220, you’re “stuck” at FL220. All that is now needed is one push of the MCP knob to clear the MCP/FMC conflict, and the climb in VNAV will resume. It is not necessary at all to select FLT LVL CHNG, then reselect VNAV, all that captain was demonstrating was possibly a lack of AFDS knowledge.
I’ll recap: If VNAV ALT is triggered during the climb (or descent for that matter) by an ATC required temporary level off, the “conflict” between the programmed FMC altitude and the MCP selected altitude is cleared, once you’re recleared to a higher/lower altitude, by selecting that higher/lower flight level, and pushing the MCP altitude selector knob once.

Works the same in descent, except that an ATC required level off will of course put you high on your vertical profile once you’re cleared lower. So on descent in VNAV ALT when ATC has required a temporary level off due traffic, the conflict between your FMC altitude (could be an altitude on a STAR, or the IAF altitude, whatever) is cleared by selecting lower once you’re cleared, and pushing the knob once. Since you’re high on the profile VNAV SPD will be annunciated. It is not necessary to select FLT LVL CHNG. Either extending speedbrakes to increase your descent rate will descend you back onto the vertical profile, or opening up the speed window while in VNAV, and bugging a higher airspeed will also descend you onto your vertical profile. Once the aircraft is within a couple of hundred feet (as displayed on the VDEV), VNAV SPD will be replaced by VNAV PATH.
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