I'm confused between the two. During descent for 737 PMDG, I usually start a descent via ALT INTV, but sometimes the plane doesn't response to ALT INTV but responds to LVL Change instead.

I need help to understand the difference.

  • $\begingroup$ Could you explain what exactly you mean with "doesn't response to ALT INTV"? If your target altitude is below current altitude in VNAV descent, than ALT INTV should always allow you to descend. Are you sure VNAV descent is active? $\endgroup$ – Bianfable Aug 4 '20 at 8:01

Altitude Intervention will start a descent only if VNAV is the active mode (and the MCP altitude is below the current cruise altitude). If you're in Altitude Hold, then VNAV isn't active, and the Altitude Intervention button won't start a descent.

On the other hand, Level Change is its own mode, and selecting it (with a different altitude set in the MCP than what you're at) will always produce a change - either climbing at climb thrust, or descending at idle. In both cases, flying at the speed set in the speed window.

VNAV is complicated, because while modes like Level Change, Vertical Speed, Altitude Hold, and GS Capture do "this," where "this" is a single easily understood thing, engaging VNAV means that we've given control to the computer logic, which can do lots of things. And we have to have an understanding of that logic to understand what VNAV will do. The Speed Intervention button and the Altitude Intervention button are inputs to the VNAV system only; VNAV also considers the altitude window and, when it's open, the speed selection. And of course VNAV uses what we've entered into the FMC as well.

For all its complexity, VNAV is one mode; Level Change is another. To start the descent, Level Change will always do it, but then you aren't in VNAV with its various features (such as meeting the speed & altitude constraints in the LEGS page). If you're level in Altitude Hold and wish to start a descent in VNAV, then you should select VNAV & observe that the FMA changes to show VNAV as the active mode. Now, Altitude Intervention will work, as well as "Descend Now" on the CDU's Descent page.

The key is knowing what mode is active - and that's on the FMA. If Altitude Hold is active, you'll have to change the mode to start descending. You might change to Vertical Speed or Level Change to start down right now. If VNAV is active, you have the tools which that mode gives you to control your path, and you can also select a different mode too (useful if VNAV isn't doing what you need the airplane to be doing right now).

In line flying, we'll generally prefer to stay in VNAV as much as possible, so if I'm at cruise altitude in Altitude Hold, I'll generally engage VNAV. Thus, I can control our cruise speed via the FMC, and when I'm given "descend at pilot discretion..." I can set the MCP altitude to the new clearance, and let VNAV manage the descent - starting it right at the calculated Top of Descent, managing the speed thru the transition from Mach to IAS, meeting step-down constraints on an arrival, etc.

There are times that the other modes are useful; for a 1,000' climb or descent, Vertical Speed at 1,000 FPM is a more gentle ride than going to full climb or all the way back to idle, and for "descend now to FL xxx and give me a good rate" then Level Change fits the bill.

Mostly, though, VNAV is what's active.

  • $\begingroup$ Many thanks. In summary, the key is to know that mode is active, if Alt Hold is active, use VS speed or Level Change. If VNAV mode is active, use ALT Intervention for descent. $\endgroup$ – Billcoke Aug 4 '20 at 11:28
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    $\begingroup$ @Billcoke Not necessarily: if ALT HOLD is active, you should press the VNAV button first, then select a lower altitude and press ALT INTV. This is the preferred way to initiate a descent in the 737. You would only use V/S or LVL CHG if you specifically need them for some other reason. $\endgroup$ – Bianfable Aug 4 '20 at 13:01

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